The modded poweramp project for Xmas- OR dooo-in it right for my little Redbox.
- Thread starter ced1
- Start date
OK a bit more progress made:The build- amp boards
Well, here we go with the before and after shots. The Redbox amp board against the Naim board:
First thing is to lay out the workspace. Fiddling round looking for parts wastes so much time, and is usually a source of mistakes/frustration so its worth having everything to hand and in order.
A good red or whisky may also be essential kit depending on mood.
Strong lighting and tools required. The tools I like to have to hand are:
snips,
snub nosed pliers,
screwdriver with various heads,
wire stripper,
pen,
multimeter and bizzarely
a file.
Then obviously
the soldering iron,
solder and
solderwick. I have had 2 solder suckers and never ever got them to work even once! I don’t know if there's a secret method, but I'm buggered if I've figured it out.
It also helps to have built up parts bins for when stuff’s missing like the legendary 'specific and odd value resistor'. I have a couple of resistor boxes, 3 caps bins, fixings, couple of transistor bins and some sundry parts cases. Even fixings such as solder pins, bolts and other little bits that don’t seem critical can stop you dead in your tracks if you don’t have them to hand. And I tend to get real ‘resistor rage’ throw-computer-out-of-window type frustrated so I think of them as ‘Daddy’s little helpers’- electronics Valium.
This is also where building a thorough Bill Of Materials (BOM) really pays dividends. You will have bought that little annoying piece that didn’t really seem part of the pcb you were building but nevertheless you need to install it. And similar cockups. I’ve resort ed to ‘the bins’ about 5 times on just the amps boards already, but hey, this is still in development. Part of the appeal of kits. Christ I hate pcb assembly!
Anyway, parts for the build include the boards (stating the obvious perhaps, old mate?), caps (electrolytic and film [polystyrene, polypropylene and polyester]), diodes of various types, fixings and wire, heatsinks, resistors (power, pots and standard) and transistors.
Then lastly but not least the BOMs and schematics. I forgot to print mine off so am working on screen instead. I’ll pass that off as eco-warrior-iness, in saving paper. Good dog.
To business.
First off is the you’re-an-edjit-if-you-don’t rule of pcb assembly- start with the smallest, lowest parts first and then build up to the biggest last. More or less. So, as no ICs, its resistors next.
As am still waiting for one more parts delivery, which is apparently somewhere over the channel right now, I am doing as much as possible on the Amp board first- Fortunately only a couple of power resistors and heatsinks still to come which are completely accessible so this won’t cause problems down the line.
A nice thing is I’ve used Vishay Dales for the amp boards and BCC (Actually Vishay BCC, like all parts these days) MRS25s for the amp boards as they’re good low noise 1% metal film resistors for regs. It makes assembly a lot easier.- The irritating bit with resistors is there are so many and they all look the same so If I can sweep half out the way for the moment, that’s a good start. It also pays to lay them out in value order. I’ll start with the vishays and the amp boards first.
I laid out my resistors in 3 rows:
1 – 1,000R,
1,001 – 10,000R and
10,001R upwards.
Mmm, absolutely fascinating fact.
And then to stuff the boards as per the BOM.
Start with the RN65s first, as they are the 0.5W (1W industrial) large ones.
Then stuff the rest of the RN60s.
Note 2 resistors go under the pcb due to layout constraints. The need to go right flush up against the pcb- no sloppy soldering allowed on these two.
resistors all done.
I always make sure the resistor values are facing upwards so that when they are inserted in the pcb, you can easily read the value. Makes 'flight-checking' and trouble shooting a world easier if a problem occurs.
Now a quirk with this RedBox is that the BOMs vary a bit depending on the voltage the amp rails run at. I, in glorious cowboy fashion, hadn’t quite got round to working out the various voltage ranges so got nailed on some resistor values. Thank god, parts bins. I’ve just done an upgrade to my big amp psu, going to a 4 winding transformer which has a different voltage output that I worked to. Allowed me to relocated the star ground back in the amp where it damn well belongs (it was a pisspoor make do before as I didn’t have the right capacitor boards for the design) which has been an great improvement and reaffirms the Mr Tibbs grounding mod. It also makes me realize just how damn good even the prototype to this build is. If the RedBox delivers as expected, it will be mindblowingly, hair-raisingly good! I’ve been listening on and off all morning to the prototype and Christ, synth bass notes sound so tight and textured they’re like shimmering, inflated balloons appearing and disappearing in front of you. You can almost reach out and caress each one they’re that vivid. And the musical access is just mad, wah. Its all there in front of you like a medieval feast. Choose a rhythm line, follow it, jump to another, ooh a little backing vocal line ever so quiet, just tucked away in the back corner, lets have a nibble at that, ahhh. Little chime tapping away to the left, god that’s so clear. Ahem, er… that might have been a bit mad. Very satisfactory upgrade anyway.
I have to say that I never used to rate poweramps as particularly important in the musical chain sound quality wise, but I have completely changed my opinion. I think my original view was largely formed from listening to various Naim poweramps which, truth be told, all sounded much of a muchness. OK, the 250 was a shade meatier and if you needed extra power for the speakers, then yes, it or 135s were essential as I found out with some Epos ES22s. But really, for the money a 140 is quite satisfactory for reasonably sensitive speakers in smaller rooms. Spend the difference on 3 weeks in Mexico for a way bigger grin.
But, that’s complete cobblers I now realise- do em right and poweramps are every bit as important as preamps!
ANYWAY…, back from that random tangent.
Basically resistor values have to be selected for the CCSinks depending on what your raw PSU rails are. For Nap140 running at around 34V (probably also applies to a Nap180 at about 40V??) raw DC, I want minimal voltage drop. With a 250, 135s, (which run at 56V raw dc due to the power regulator boards), and the higher voltage DIY PSUs (basically any stuff with rails running at over say 50V), you can afford a bit more off the max voltage the amp can swing, so can adjust for more optimum CCSs. Basically, referring to a paper by. .Walt Jung maybe, measurements showed that IC voltage references performed better than diodes as references in CCSinks. Cascodes obviously perform better too (by the numbers anyway, though sonically too in my exerience). So this means the voltage references should be swapped out at the same time as you select the resistors. This is basically what it boils down to:
CCS biasing resistors.
Refering back to the amp schematic
For 35V raw dc supplies, use R10, R11 of 30K. This should give biasing currents of about 2ma on the CCS voltage references and power dissipation of ballpark 150mW in the resistor, within both the mil and industrial max ratings though they should get warm in practice.
For 50V+ Raw DC supplies, I will use 50K resistors which should give around the 200mW mark dissipation. Higher currents would push power dissipation too high for long term reliability in these resistors I think. Current is essentially solid dc with minute ac components through these anyway so I think those values will be fine.
The VAS CCSink.
Whatever you use, the current through R15 must remain at 8.2ma
For 35V RAW DC rails on a NAP140, the regulators are going to eat a fair bit of voltage headroom at least 5V. I therefore want to keep the VAS CCS headroom as low as possible or voltage swing will be poor and the amp may run out of welly to drive speakers. That means making the D3/D4 voltage drop as low as possible.
I am using 2x1N4148 diodes in series to replace each LED (D3/D4), Like this:
Eeach 1n4148 drops 0.6v, giving a 1.2V drop when using 2 in series. Or LM385 1.2V voltage references would also probably be very nice/better. Again R15 would have to be 69R1 to give 8.2 ma through R15.
Now cascoding stiffens up the VAS CCS very considerably. In its standard form the ac component though this is around 0.2ma with a steadyish 8ma bias ie its not a very good current regulator. Casoding this reduces the a.c component to about 3uA in sims; or look at it as current regulation improving about a hundred fold at 1khz and high power. I would be surprised if this wasn’t clearly audible.
If running raw DC rails higher, (NAP250/135s come in at 56V DC for example regulated down to about 40V), there is 16V headroom to burn, nice. In this instance I would feed the redbox regs off the raw dc, let the powerregs just handle the output transistors and voila a fully, from head to toe regulated, poweramp.
In that case use I’d at least use Green LEDs for D3,D4 and R15=162 Ohm.
Or Use LM336, (2.5V references) for D3,D4 and R15=…???- can’t remember, somewhere around 200 Ohm? Have to work it out again. I would guess the LM336s are the best sonically, but that is something for a listening test. So the VAS CCS then starts eating about 5V instead of the standard Naim1.2V of voltage headroom but that would be fine as you can run the redbox reg boards at say 45V and not affect the overall voltage swing of the amp a single iota. Very elegant.
Now, to the LTP CCSink.
R6 must pass 0.9ma whatever the parts selection for the CCSink and its value.
The collector of Q1 doesn’t drop more than a couple of volts below ground in worst case scenarios. This means we can use very good voltage references in theory with plenty of voltage headroom- better CCSs tend to require more volts to function. I would go for the trusty LM329DZs, 6.9V references. These are buried zeners which are super low noise, low dynamic impedance devices. Cascoding drops the a.c element down to about 17nAin sims! holy cow. Also significant will probably be Cob – 6.9 V is quite generous headroom but if you look at transistor datasheet s you’ll see how Cob invariably reduces as Vbc is increased- ie the larger the voltage across a transistor, the lower the ouput capacitance. Lower capacitance should= better speed, linearity and high freq performance as a general rule. A reason why its no bad idea to run highish voltages across signal path transistors when possible. Q1 which takes the brunt of voltage swings from the input diff transistors should also be nicely linear. However, despite this I’m going to start with green LEDs as I know these are stable in this cascode CCS topology. I don’t want to have to deal with too many variables at this stage.
If using LM329s for D1 and D2, R6 must be about 6K5 Ohm.
Otherwise use Green LEDs for D1 and D2, and R6=1K5.
In summary I’m doing this for a NAP 140, so I’m starting with Green LEDs for D1,D2 and 2x1n4148 each for D3,D4. R10, R11 are 30K
So after having stuffed the boards with resistors, and diodes, both normal as Light emitting, as the CCS voltage references, they’re ready to be soldered into place.
But before that, a quick double check that all the resistors are in the correct positions against the BOM. Saves a horror of trouble shooting down the line.
I started by soldering R6 and R100 on the underside of the board. These need to be completely flush with the board as they sit in the sandwich gap and will prevent proper mating of the boards if set badly. The leads must also be trimmed completely flat on the top side of the board as the feedback caps sit above them.
Though through hole plating is a pain in the butt in general, it does mean you can solder from the top of the board. The underside is a thorn bush of leads. And yes this is a bit stupid from a flux cleaning perspective, but I'm a bit lazy about that and never had any problems with corrosion. Hate flux cleaning chemicals too and they seem to create as much mess as they remove. Otherwise I'd do it immediately on all the resistors if I was going to.
I also try to make sure not to accidentally slop solder around and fill up empty plated though-holes- its murder trying to suck out solder once they’re filled. Otherwise it’s a fast process and only takes about 5 mins per board, weee.
Any leads I can’t easily get at to solder from the top, I leave.
Then flip the board and trim off all the leads. I love this, snip snip snip, scissors hands style. I Leave about 1mm of lead sticking up, no more.
Then I finish off soldering by going over each joint and soldering any that look unfilled and finishing off any leads I left unsoldered from the top side.
Double check everything again under a bright light. I’ve tracked down more than one fault to an unsoldered joint when tired and rushing.
Link
I’m going to use T03 ouput transistors so the 2 Link PTHs have to remain EMPTY. This link is only for using T0264 output transistors.
diodes
D5, D6, D7. Solder and snip.
VR1, Is the blue box in the pic above. I place a big dollop of solder on the tip of the iron, fit VR1, hold in place with one hand and solder one leg in place using the dollop of solder on the end of the iron held with the other hand. It will be a dry and faulty joint but it holds the pot in position which is the point. You can then flip the board, solder the other 2 legs and then resolder the bad joint properly. Nice snug fit. I don’t have one of those soldering sponge flip table thingys.
At this point its worth just checking all the resistors values with a multimeter to make sure they aren’t faulty and haven’t been damaged during soldering. Once capacitors go into the pcb, they screw up the readings and its often impossible to measure on the board. Takes 2 minutes.
Polystyene capacitors.
Fit (value facing upwards), solder, flip board, finish soldering, snip.
Note, C7 has to be fitted under the board and must lie completely flat.
Bears remembering that polystyrenes can’t tolerate much heat so mucho care. Don’t touch the body with the soldering iron or you can melt it and possibly damage the capacitor, and I try to be swift but sure in soldering the leads so it doesn’t heat up via conduction.
Anal-retentively I always keep a handful of snipped component legs. They always come in handy as ready tinned link wires.
I gound myself regularly by touching exposed metal on a nearby radiator (Radiators and piping are linked in to your domestic ground) to ensure no static buildup- It can blow semiconductors such as …
Small signal transistors.
Laid out as per resistors on the work table.
Stuff PCB as per BOM, I make a point of pressing firmly all the way in for minimal leg length. I recommend a dab of solder on one leg from the top of the pcb, then flip board and solder rest of legs and snip.
I usually stuff each type of transistor before moving on to the next type i.e do all the BC546s first, then the 2sc2705s etc. Its easy to get them mixed up otherwise which can become an arse-ache to check as the text on the bodies is so damn faint.
Note, the middle legs need to be pushed either forwards OR backwards on insertion into the redbox PCBs. Orientation of the body is the critical element to watch- My PCB CAD program gives you great control over the primitives on the PCB so you can move a component’s pads around in situ to suit routing which I do a lot with transistors.
PCB pins
Then came the vero pins and 4 pcb tabs. The tabs need to be pressed in quite hard, but they're rock solid then.
Almost there; the TO220 devices:
Caddock power resistors.
Again I recommend the ‘solder dollop’ technique- big dollop of solder on the iron tip,
hold the TO220 resistor in position with one hand, flip the board and dollop the solder on one leg to make a bad joint. Then solder the other leg properly, go back and solder the first leg again. I did each resistor one at a time by this technique and they stand perfectly.
Driver Transistors
Did Q10,Q9. also using the ‘solder dollop’ technique; particularly important as these parts should sit straight and even for the heatsinks.
Capacitors
And then my favourite bits, because they are easy, chunky and done in seconds. Also you have a little frisson of nearing completion because suddenly the board’s almost full.
The inductor.
Ok, inductor should be about 0.8- 0.84uH. I’m using 1.5mm enameled copper and I have a 7mm diameter pencil to hand as a winding form which works out that I need 20 turns and should make a 3cm long inductor- I can just fit it on my pcb. Otherwise I'd have to find a smaller form.
The knack is to wind on something slippery and long (fnah fnah); my synthi-pencil (it’s a weird eco pencil thingy made from recycled plastic cups and is almost kinda greasy) and wind directly off the reel so you keep a good grip on both the fixed end of copper and the winding end. Grip one end of the copper around the pencil and then keep rotating the pencil in your wrist so the copper winds round it. Perfect.
Ive used small dowels as formers before and it becomes fiddly and messy. Snip off the ends leaving about 1cm leads then hold the inductor and gently file away the enamel on the leads so you can solder onto the board.
And that’s about it.
I have 3 small capacitors, 2 heatsinks and 3 power resistors still to arrive and then ready to test
After shot; here the new RedBox boards against the old Naim ones. I sure know which look like they sound better, hah.
There are however enough variables, that for once I may use a bench supply to test this thing before hooking it up, instead of my usual gung ho approach. As I don’t have a bench supply, in true DIY spirit I was thinking of using 2 x 9V batteries to make a +/- supply which should give enough voltage to get the board up and running and be current limited- how much current can a 9V batter dump in short circuit after all, hey?
Ok for now.
Ced
Well, here we go with the before and after shots. The Redbox amp board against the Naim board:
First thing is to lay out the workspace. Fiddling round looking for parts wastes so much time, and is usually a source of mistakes/frustration so its worth having everything to hand and in order.
A good red or whisky may also be essential kit depending on mood.
Strong lighting and tools required. The tools I like to have to hand are:
snips,
snub nosed pliers,
screwdriver with various heads,
wire stripper,
pen,
multimeter and bizzarely
a file.
Then obviously
the soldering iron,
solder and
solderwick. I have had 2 solder suckers and never ever got them to work even once! I don’t know if there's a secret method, but I'm buggered if I've figured it out.
It also helps to have built up parts bins for when stuff’s missing like the legendary 'specific and odd value resistor'. I have a couple of resistor boxes, 3 caps bins, fixings, couple of transistor bins and some sundry parts cases. Even fixings such as solder pins, bolts and other little bits that don’t seem critical can stop you dead in your tracks if you don’t have them to hand. And I tend to get real ‘resistor rage’ throw-computer-out-of-window type frustrated so I think of them as ‘Daddy’s little helpers’- electronics Valium.
This is also where building a thorough Bill Of Materials (BOM) really pays dividends. You will have bought that little annoying piece that didn’t really seem part of the pcb you were building but nevertheless you need to install it. And similar cockups. I’ve resort ed to ‘the bins’ about 5 times on just the amps boards already, but hey, this is still in development. Part of the appeal of kits. Christ I hate pcb assembly!
Anyway, parts for the build include the boards (stating the obvious perhaps, old mate?), caps (electrolytic and film [polystyrene, polypropylene and polyester]), diodes of various types, fixings and wire, heatsinks, resistors (power, pots and standard) and transistors.
Then lastly but not least the BOMs and schematics. I forgot to print mine off so am working on screen instead. I’ll pass that off as eco-warrior-iness, in saving paper. Good dog.
To business.
First off is the you’re-an-edjit-if-you-don’t rule of pcb assembly- start with the smallest, lowest parts first and then build up to the biggest last. More or less. So, as no ICs, its resistors next.
As am still waiting for one more parts delivery, which is apparently somewhere over the channel right now, I am doing as much as possible on the Amp board first- Fortunately only a couple of power resistors and heatsinks still to come which are completely accessible so this won’t cause problems down the line.
A nice thing is I’ve used Vishay Dales for the amp boards and BCC (Actually Vishay BCC, like all parts these days) MRS25s for the amp boards as they’re good low noise 1% metal film resistors for regs. It makes assembly a lot easier.- The irritating bit with resistors is there are so many and they all look the same so If I can sweep half out the way for the moment, that’s a good start. It also pays to lay them out in value order. I’ll start with the vishays and the amp boards first.
I laid out my resistors in 3 rows:
1 – 1,000R,
1,001 – 10,000R and
10,001R upwards.
Mmm, absolutely fascinating fact.
And then to stuff the boards as per the BOM.
Start with the RN65s first, as they are the 0.5W (1W industrial) large ones.
Then stuff the rest of the RN60s.
Note 2 resistors go under the pcb due to layout constraints. The need to go right flush up against the pcb- no sloppy soldering allowed on these two.
resistors all done.
I always make sure the resistor values are facing upwards so that when they are inserted in the pcb, you can easily read the value. Makes 'flight-checking' and trouble shooting a world easier if a problem occurs.
Now a quirk with this RedBox is that the BOMs vary a bit depending on the voltage the amp rails run at. I, in glorious cowboy fashion, hadn’t quite got round to working out the various voltage ranges so got nailed on some resistor values. Thank god, parts bins. I’ve just done an upgrade to my big amp psu, going to a 4 winding transformer which has a different voltage output that I worked to. Allowed me to relocated the star ground back in the amp where it damn well belongs (it was a pisspoor make do before as I didn’t have the right capacitor boards for the design) which has been an great improvement and reaffirms the Mr Tibbs grounding mod. It also makes me realize just how damn good even the prototype to this build is. If the RedBox delivers as expected, it will be mindblowingly, hair-raisingly good! I’ve been listening on and off all morning to the prototype and Christ, synth bass notes sound so tight and textured they’re like shimmering, inflated balloons appearing and disappearing in front of you. You can almost reach out and caress each one they’re that vivid. And the musical access is just mad, wah. Its all there in front of you like a medieval feast. Choose a rhythm line, follow it, jump to another, ooh a little backing vocal line ever so quiet, just tucked away in the back corner, lets have a nibble at that, ahhh. Little chime tapping away to the left, god that’s so clear. Ahem, er… that might have been a bit mad. Very satisfactory upgrade anyway.
I have to say that I never used to rate poweramps as particularly important in the musical chain sound quality wise, but I have completely changed my opinion. I think my original view was largely formed from listening to various Naim poweramps which, truth be told, all sounded much of a muchness. OK, the 250 was a shade meatier and if you needed extra power for the speakers, then yes, it or 135s were essential as I found out with some Epos ES22s. But really, for the money a 140 is quite satisfactory for reasonably sensitive speakers in smaller rooms. Spend the difference on 3 weeks in Mexico for a way bigger grin.
But, that’s complete cobblers I now realise- do em right and poweramps are every bit as important as preamps!
ANYWAY…, back from that random tangent.
Basically resistor values have to be selected for the CCSinks depending on what your raw PSU rails are. For Nap140 running at around 34V (probably also applies to a Nap180 at about 40V??) raw DC, I want minimal voltage drop. With a 250, 135s, (which run at 56V raw dc due to the power regulator boards), and the higher voltage DIY PSUs (basically any stuff with rails running at over say 50V), you can afford a bit more off the max voltage the amp can swing, so can adjust for more optimum CCSs. Basically, referring to a paper by. .Walt Jung maybe, measurements showed that IC voltage references performed better than diodes as references in CCSinks. Cascodes obviously perform better too (by the numbers anyway, though sonically too in my exerience). So this means the voltage references should be swapped out at the same time as you select the resistors. This is basically what it boils down to:
CCS biasing resistors.
Refering back to the amp schematic
For 35V raw dc supplies, use R10, R11 of 30K. This should give biasing currents of about 2ma on the CCS voltage references and power dissipation of ballpark 150mW in the resistor, within both the mil and industrial max ratings though they should get warm in practice.
For 50V+ Raw DC supplies, I will use 50K resistors which should give around the 200mW mark dissipation. Higher currents would push power dissipation too high for long term reliability in these resistors I think. Current is essentially solid dc with minute ac components through these anyway so I think those values will be fine.
The VAS CCSink.
Whatever you use, the current through R15 must remain at 8.2ma
For 35V RAW DC rails on a NAP140, the regulators are going to eat a fair bit of voltage headroom at least 5V. I therefore want to keep the VAS CCS headroom as low as possible or voltage swing will be poor and the amp may run out of welly to drive speakers. That means making the D3/D4 voltage drop as low as possible.
I am using 2x1N4148 diodes in series to replace each LED (D3/D4), Like this:
Eeach 1n4148 drops 0.6v, giving a 1.2V drop when using 2 in series. Or LM385 1.2V voltage references would also probably be very nice/better. Again R15 would have to be 69R1 to give 8.2 ma through R15.
Now cascoding stiffens up the VAS CCS very considerably. In its standard form the ac component though this is around 0.2ma with a steadyish 8ma bias ie its not a very good current regulator. Casoding this reduces the a.c component to about 3uA in sims; or look at it as current regulation improving about a hundred fold at 1khz and high power. I would be surprised if this wasn’t clearly audible.
If running raw DC rails higher, (NAP250/135s come in at 56V DC for example regulated down to about 40V), there is 16V headroom to burn, nice. In this instance I would feed the redbox regs off the raw dc, let the powerregs just handle the output transistors and voila a fully, from head to toe regulated, poweramp.
In that case use I’d at least use Green LEDs for D3,D4 and R15=162 Ohm.
Or Use LM336, (2.5V references) for D3,D4 and R15=…???- can’t remember, somewhere around 200 Ohm? Have to work it out again. I would guess the LM336s are the best sonically, but that is something for a listening test. So the VAS CCS then starts eating about 5V instead of the standard Naim1.2V of voltage headroom but that would be fine as you can run the redbox reg boards at say 45V and not affect the overall voltage swing of the amp a single iota. Very elegant.
Now, to the LTP CCSink.
R6 must pass 0.9ma whatever the parts selection for the CCSink and its value.
The collector of Q1 doesn’t drop more than a couple of volts below ground in worst case scenarios. This means we can use very good voltage references in theory with plenty of voltage headroom- better CCSs tend to require more volts to function. I would go for the trusty LM329DZs, 6.9V references. These are buried zeners which are super low noise, low dynamic impedance devices. Cascoding drops the a.c element down to about 17nAin sims! holy cow. Also significant will probably be Cob – 6.9 V is quite generous headroom but if you look at transistor datasheet s you’ll see how Cob invariably reduces as Vbc is increased- ie the larger the voltage across a transistor, the lower the ouput capacitance. Lower capacitance should= better speed, linearity and high freq performance as a general rule. A reason why its no bad idea to run highish voltages across signal path transistors when possible. Q1 which takes the brunt of voltage swings from the input diff transistors should also be nicely linear. However, despite this I’m going to start with green LEDs as I know these are stable in this cascode CCS topology. I don’t want to have to deal with too many variables at this stage.
If using LM329s for D1 and D2, R6 must be about 6K5 Ohm.
Otherwise use Green LEDs for D1 and D2, and R6=1K5.
In summary I’m doing this for a NAP 140, so I’m starting with Green LEDs for D1,D2 and 2x1n4148 each for D3,D4. R10, R11 are 30K
So after having stuffed the boards with resistors, and diodes, both normal as Light emitting, as the CCS voltage references, they’re ready to be soldered into place.
But before that, a quick double check that all the resistors are in the correct positions against the BOM. Saves a horror of trouble shooting down the line.
I started by soldering R6 and R100 on the underside of the board. These need to be completely flush with the board as they sit in the sandwich gap and will prevent proper mating of the boards if set badly. The leads must also be trimmed completely flat on the top side of the board as the feedback caps sit above them.
Though through hole plating is a pain in the butt in general, it does mean you can solder from the top of the board. The underside is a thorn bush of leads. And yes this is a bit stupid from a flux cleaning perspective, but I'm a bit lazy about that and never had any problems with corrosion. Hate flux cleaning chemicals too and they seem to create as much mess as they remove. Otherwise I'd do it immediately on all the resistors if I was going to.
I also try to make sure not to accidentally slop solder around and fill up empty plated though-holes- its murder trying to suck out solder once they’re filled. Otherwise it’s a fast process and only takes about 5 mins per board, weee.
Any leads I can’t easily get at to solder from the top, I leave.
Then flip the board and trim off all the leads. I love this, snip snip snip, scissors hands style. I Leave about 1mm of lead sticking up, no more.
Then I finish off soldering by going over each joint and soldering any that look unfilled and finishing off any leads I left unsoldered from the top side.
Double check everything again under a bright light. I’ve tracked down more than one fault to an unsoldered joint when tired and rushing.
Link
I’m going to use T03 ouput transistors so the 2 Link PTHs have to remain EMPTY. This link is only for using T0264 output transistors.
diodes
D5, D6, D7. Solder and snip.
VR1, Is the blue box in the pic above. I place a big dollop of solder on the tip of the iron, fit VR1, hold in place with one hand and solder one leg in place using the dollop of solder on the end of the iron held with the other hand. It will be a dry and faulty joint but it holds the pot in position which is the point. You can then flip the board, solder the other 2 legs and then resolder the bad joint properly. Nice snug fit. I don’t have one of those soldering sponge flip table thingys.
At this point its worth just checking all the resistors values with a multimeter to make sure they aren’t faulty and haven’t been damaged during soldering. Once capacitors go into the pcb, they screw up the readings and its often impossible to measure on the board. Takes 2 minutes.
Polystyene capacitors.
Fit (value facing upwards), solder, flip board, finish soldering, snip.
Note, C7 has to be fitted under the board and must lie completely flat.
Bears remembering that polystyrenes can’t tolerate much heat so mucho care. Don’t touch the body with the soldering iron or you can melt it and possibly damage the capacitor, and I try to be swift but sure in soldering the leads so it doesn’t heat up via conduction.
Anal-retentively I always keep a handful of snipped component legs. They always come in handy as ready tinned link wires.
I gound myself regularly by touching exposed metal on a nearby radiator (Radiators and piping are linked in to your domestic ground) to ensure no static buildup- It can blow semiconductors such as …
Small signal transistors.
Laid out as per resistors on the work table.
Stuff PCB as per BOM, I make a point of pressing firmly all the way in for minimal leg length. I recommend a dab of solder on one leg from the top of the pcb, then flip board and solder rest of legs and snip.
I usually stuff each type of transistor before moving on to the next type i.e do all the BC546s first, then the 2sc2705s etc. Its easy to get them mixed up otherwise which can become an arse-ache to check as the text on the bodies is so damn faint.
Note, the middle legs need to be pushed either forwards OR backwards on insertion into the redbox PCBs. Orientation of the body is the critical element to watch- My PCB CAD program gives you great control over the primitives on the PCB so you can move a component’s pads around in situ to suit routing which I do a lot with transistors.
PCB pins
Then came the vero pins and 4 pcb tabs. The tabs need to be pressed in quite hard, but they're rock solid then.
Almost there; the TO220 devices:
Caddock power resistors.
Again I recommend the ‘solder dollop’ technique- big dollop of solder on the iron tip,
hold the TO220 resistor in position with one hand, flip the board and dollop the solder on one leg to make a bad joint. Then solder the other leg properly, go back and solder the first leg again. I did each resistor one at a time by this technique and they stand perfectly.
Driver Transistors
Did Q10,Q9. also using the ‘solder dollop’ technique; particularly important as these parts should sit straight and even for the heatsinks.
Capacitors
And then my favourite bits, because they are easy, chunky and done in seconds. Also you have a little frisson of nearing completion because suddenly the board’s almost full.
The inductor.
Ok, inductor should be about 0.8- 0.84uH. I’m using 1.5mm enameled copper and I have a 7mm diameter pencil to hand as a winding form which works out that I need 20 turns and should make a 3cm long inductor- I can just fit it on my pcb. Otherwise I'd have to find a smaller form.
The knack is to wind on something slippery and long (fnah fnah); my synthi-pencil (it’s a weird eco pencil thingy made from recycled plastic cups and is almost kinda greasy) and wind directly off the reel so you keep a good grip on both the fixed end of copper and the winding end. Grip one end of the copper around the pencil and then keep rotating the pencil in your wrist so the copper winds round it. Perfect.
Ive used small dowels as formers before and it becomes fiddly and messy. Snip off the ends leaving about 1cm leads then hold the inductor and gently file away the enamel on the leads so you can solder onto the board.
And that’s about it.
I have 3 small capacitors, 2 heatsinks and 3 power resistors still to arrive and then ready to test
After shot; here the new RedBox boards against the old Naim ones. I sure know which look like they sound better, hah.
There are however enough variables, that for once I may use a bench supply to test this thing before hooking it up, instead of my usual gung ho approach. As I don’t have a bench supply, in true DIY spirit I was thinking of using 2 x 9V batteries to make a +/- supply which should give enough voltage to get the board up and running and be current limited- how much current can a 9V batter dump in short circuit after all, hey?
Ok for now.
Ced
Yeah, the input cap. I know lots of people don't like em and its a fair comment. but personally its the only place where I wouldn't not have one. CD oupt, preamp, in or out is fine by me on split rail supplies. Its dc at the poweramp that kills speakers after all though, not the preamp or cd player. In my view if you want to get rid of it safely you need a well thought out and very reliable dc servo and ditch the feedback cap while your'e at it. A very valid approach i'd like to try some day. Sure the feedback cap knocks dc down to unity gain so in practice a few mv at the input show up as a few mv at the output which is fine most of the time but the amp becomes very vulnerable to any glitches. I know me and it would just be a question of when, not if. If you're very careful you'd probably be ok long term, but I don't know.
To be honest a big drama is made about input caps in the signal path. Yeah they're audible but a half way decent one is not the weakest link in the audio chain by a long long way. Or put another way, your system would have to be utterly mind meltingly astonishing for the cap to become the limiting factor on overall performance.
and yep 2 x 22uf. The mkp4s in feedback are simply the best there is, period. I hate bypassing caps in general- it hardly ever works right and certainly not in the signal path. Just sounds weird imo. However, I have had mixed success paralleling identical caps, generally ok. in this case the currents are minute and I suspect will get away with it handsomely. I have a 33uf one which i have been running for years with which to make a direct comparison so there will be no doubt if it works or not- Any weirdness will be audible.
cheers c
To be honest a big drama is made about input caps in the signal path. Yeah they're audible but a half way decent one is not the weakest link in the audio chain by a long long way. Or put another way, your system would have to be utterly mind meltingly astonishing for the cap to become the limiting factor on overall performance.
and yep 2 x 22uf. The mkp4s in feedback are simply the best there is, period. I hate bypassing caps in general- it hardly ever works right and certainly not in the signal path. Just sounds weird imo. However, I have had mixed success paralleling identical caps, generally ok. in this case the currents are minute and I suspect will get away with it handsomely. I have a 33uf one which i have been running for years with which to make a direct comparison so there will be no doubt if it works or not- Any weirdness will be audible.
cheers c
Ref the input cap - my thinking was if there's an output one on the preamp there's no need for one on the poweramp input or visa versa. The comment was as simple as that. Is 1uf not a little low??
Has anyone else tried 2 x 22uf MKP4s in the feedback loop? Btw where did you get them from or were they samples from wima?
Is there any chance of the regulator schematics ??? I've seen the patent but it does not list parts. I'm very interested in your implementation.
Excellent thread though.
Has anyone else tried 2 x 22uf MKP4s in the feedback loop? Btw where did you get them from or were they samples from wima?
Is there any chance of the regulator schematics ??? I've seen the patent but it does not list parts. I'm very interested in your implementation.
Excellent thread though.
Hi midcm (do you have a nickname that runs off the tongue a bit easier?),
Pleased you had a look at the end of the patent- it really is informative when you digest it. It does actually list parts and values though which is quite unusual, just way its written takes a bit of getting used to- long hand text. I copied it out and then 'translated' it to normal notation. On reflection i don't want to post my take on the schematic yet as i've not really finalised it. The original design was developed in the 80s so i've used different parts and tweaked the design a bit for low dropout but will probably have to play around to get it to my satisfaction.
Re 1uf cap, yep what you say is correct. For me more sensible to have it on the poweramp though and not the preamp output. Reason is smaller caps are cheaper, generally better (slew rate is higher and inductance is lower in smaller sized boxes- check a wima film datasheet) and if you want to try exotic boutique caps, then the again size, availability and price are a winning factor. It is after all the poweramp is where DC becomes a potentially expensive problem. 1uf is fine value- no bass problems. I've used various pitches on the board so its easy to swap out to smrs, polyesters, electrolytics or whatever.
Drop me your email and i'll send you a copy for personal use only if you want though as you are curious.
those MKP4s are, yes, 'difficult' to get. I actually had to buy in a large quantity. I guess if it works out and someone is interested in boards I could supply a few sets of those caps too as they are the only part not readily available.
cheers
C
Pleased you had a look at the end of the patent- it really is informative when you digest it. It does actually list parts and values though which is quite unusual, just way its written takes a bit of getting used to- long hand text. I copied it out and then 'translated' it to normal notation. On reflection i don't want to post my take on the schematic yet as i've not really finalised it. The original design was developed in the 80s so i've used different parts and tweaked the design a bit for low dropout but will probably have to play around to get it to my satisfaction.
Re 1uf cap, yep what you say is correct. For me more sensible to have it on the poweramp though and not the preamp output. Reason is smaller caps are cheaper, generally better (slew rate is higher and inductance is lower in smaller sized boxes- check a wima film datasheet) and if you want to try exotic boutique caps, then the again size, availability and price are a winning factor. It is after all the poweramp is where DC becomes a potentially expensive problem. 1uf is fine value- no bass problems. I've used various pitches on the board so its easy to swap out to smrs, polyesters, electrolytics or whatever.
Drop me your email and i'll send you a copy for personal use only if you want though as you are curious.
those MKP4s are, yes, 'difficult' to get. I actually had to buy in a large quantity. I guess if it works out and someone is interested in boards I could supply a few sets of those caps too as they are the only part not readily available.
cheers
C
Agree CED on the Wima MKP4 I used the 22uF in my Starfish they were easily the best in feedback position Got mine as frr from Wima but they do popo up on E-Bay.
I would be interested in some boards especially as I have CAP's lying around doing nothing.
Would be interesting to compare with my NCC200's which I love.
Cheers Geoff
I would be interested in some boards especially as I have CAP's lying around doing nothing.
Would be interesting to compare with my NCC200's which I love.
Cheers Geoff
Well its been a ghastly couple of weeks with many wobbles at work. However time to do a bit more on the amps. So, finishing off the amp build:
Arse got a little smacked on the heatsinks for the drivers. Caught off guard by these and they are a bit longer than a TO220 transistor pushed down to its roots. So the lesson is:
Fit the transistor to the heatsink before you solder the transistor to the board. Had to cut them off and replace with new drivers done correctly- what a pain.
Anyway, fitting the heatsink. It’s a bit fiddly as these are tapped not drilled. So the knack is either screw the transistor direct to the heatsink with a dab of heatsink compound, and accept that the heatsink will be electrically live.
Or insulate it with a silpad
As the heatsinks are tapped the insulating washers wont fit. The trick is to fit the washer from behind a transistor and then using a very sharp blade, trim off the excess by cutting towards you safely. The body of the transistor will stop the blade edge slipping and cutting you and the washer is easily trimmed to a perfect fit. Then fit in place using silpad and screw. Test using a multimeter to make sure no continuity between heatsink and transistor body. Voila all done.
Last of the parts fitted including some decoupling caps, the VBE transistor and then slap the heatsinks and output trannies back on.
Voila, it lives, though it is not yet live.
With heatsinks fitted again. All ready to go.
On pondering, have decided to get the regulators up and running first before testing the amp boards. On their own the amp boards would need to have the rails linked up and the decoupling is not really suited to running them without the regulator boards- the bulk decoupling is more on the reg boards. I’d have to bodge them with some decoupling on the rails as I'm not sure they'd be happy/stable without. Also, in terms of plain amp boards it’s a bit of a must sound wise so would be difficult to compare these against my proto boards sonically. I Must add the Martin Clarke rail decoupling mod to these boards so they can actually run independently of the reg boards. I'm sure they'd make a stunning alternative to Naim boards in their own right, and I’d expect them to pip NC200 boards nicely. Listening tests later though.
All for now. More on the regulator boards tonight after a quick voltage test. I think these things are beginning to look like they might actually damn well work as planned.
cheers
Ced
Arse got a little smacked on the heatsinks for the drivers. Caught off guard by these and they are a bit longer than a TO220 transistor pushed down to its roots. So the lesson is:
Fit the transistor to the heatsink before you solder the transistor to the board. Had to cut them off and replace with new drivers done correctly- what a pain.
Anyway, fitting the heatsink. It’s a bit fiddly as these are tapped not drilled. So the knack is either screw the transistor direct to the heatsink with a dab of heatsink compound, and accept that the heatsink will be electrically live.
Or insulate it with a silpad
As the heatsinks are tapped the insulating washers wont fit. The trick is to fit the washer from behind a transistor and then using a very sharp blade, trim off the excess by cutting towards you safely. The body of the transistor will stop the blade edge slipping and cutting you and the washer is easily trimmed to a perfect fit. Then fit in place using silpad and screw. Test using a multimeter to make sure no continuity between heatsink and transistor body. Voila all done.
Last of the parts fitted including some decoupling caps, the VBE transistor and then slap the heatsinks and output trannies back on.
Voila, it lives, though it is not yet live.
With heatsinks fitted again. All ready to go.
On pondering, have decided to get the regulators up and running first before testing the amp boards. On their own the amp boards would need to have the rails linked up and the decoupling is not really suited to running them without the regulator boards- the bulk decoupling is more on the reg boards. I’d have to bodge them with some decoupling on the rails as I'm not sure they'd be happy/stable without. Also, in terms of plain amp boards it’s a bit of a must sound wise so would be difficult to compare these against my proto boards sonically. I Must add the Martin Clarke rail decoupling mod to these boards so they can actually run independently of the reg boards. I'm sure they'd make a stunning alternative to Naim boards in their own right, and I’d expect them to pip NC200 boards nicely. Listening tests later though.
All for now. More on the regulator boards tonight after a quick voltage test. I think these things are beginning to look like they might actually damn well work as planned.
cheers
Ced
Then you shall not be dissapointed binnie.
The Build- the RedBox poweramp regulator system
or PRS for short as that’s too much of a mouthful. Sounds quite ‘neat’ too as Texans would say, heh. 'PRS'. Well, back to the boards again. There are a blizzard of resistors to solder which will be a headache. Mein Gott, zo many reziztorz.
So from the start again.
First, before I forget, solder the power link pins.
the two halves of the power rails feeding the filter and collector input regs need to be linked unless, as my plan is, you have a 12 x raw dc rails supply but that’s for another rainy Sunday. Not necessary.
Then time to do a bit of maths, sigh.
So the general schematic of the shunts again in a bit more detail:
Note the PRS is composed of a gyrator as a pre-regulator to feed the CCS and shunt preregulator which feeds the output gyrators. That is 3 layers of regulation along with the current source. Damned if anything is going to get through that lot!
Choosing a build version.
Plugging in a few preliminary numbers, it looks like the PRS does need to be customized, depending on the raw DC rail voltages. Simply because the PRS eats voltage headroom.
Mulling it over, I’m seeing the options divided into 4 possible builds, depending on the parts to be used.
1) super-low voltage build
2) low voltage build
3) standard voltage build
4) high voltage build
I’m going to start with the Low dropout build to get a feel for how the numbers will pan out with target raw dc rails of 34V, as per the NAP140. Doing the maths, these are the figures I reckon I can get away with: Dropout at 6-7V is a bit more disappointing than I had worked out on the back of a fag packet, but I think doable. will see on power up. So lots of juggling numbers and I have the values I want. This actually took HOURS to get real world numbers that I think will function. A whole world of difference between sims and nominal parts values and then thinking about real life UK mains on the ground. Reason being mains vary nominally +/-10% iirc which means you actually need to work within parameters for a wide range of possible input voltages and as these boards are eating more voltage that I was counting on its not so easy. Also keeping within good power dissipation limits is a constant balancing act.
Anyway I have now more or less got a spreadsheet together to work out the values for any input voltage, output voltage and the margins.
It sounds horrible but when its all been done, it boils down to measuring your raw dc rail voltage with a multimeter and then selecting specific values for 5 parts accordingly. simple.
Figures to suit a NAP140 are as below.
Low dropout build.
V raw in = 34V, V regulated out = 26V (headroom = 8V)
gyrator pre-shunt regulator = yes
R53 = leave empty (or very low diameter capacitor on UNDERSIDE of board)
D1, D2 = 2 x 1n4148 each
R4, R5,= chose for a couple of ma of current draw
R3 about 30-40 ohm for about 20ma current output
R11 = 2k output voltage set
R12 = 6k output voltage set
this will set the shunt to about 27V ouput and the output gyrators should therefore run about 0.7V lower. Think thats right- haven't got the numbers to hand right now. It’s the shunt reg where you set the PRS output voltage accurately anyway.
Ideally Id like to see the PRS with more headroom in the10-15V range to allow the use of theoretically better parts, particularly low noise parts for D1, D2 e.g LEDS or better and allow greater margins of voltage drop for the prereg and between the CCS output and shunt voltage. But it should work handsomely with lesser margins.
For the Redbox amp boards, same restrictions basically apply as for Naim boards or indeed NC200s. So I wouldn’t go much above 40V dc rails, 75W amp output depending on the output transistors of choice. Les W says the NC200s run very nicely on 50V, which would be fine too if you have chunky amp ouput transistors such as his recommended BUV20s and don’t run very low impedance speakers.
Onwards.
So first parts to go in are the Build version selection parts.
I’ve had to improvise by putting resistors in series to get the right values, yeah yeah, because I didn’t work all this out exactly beforehand. Actually quite embarrassing how many values I’ve had to revise on the fly. The shame is completely visible for all to see- It’s taken a fair while to juggle operating points, work out values and then solder together the correct value resistors. Ideal is to work em out properly before hand and then buy the correct values from the start, hmmm?
Also as there are so many resistors on the underside, the PRS really does need a silk screen below the board as well. Always a learning curve on the first go.
Finally, also realized I have a missing pcb pad, which fortunately is easy to bodge, but still annoying. Corrected on my pcb files now.
Then in with the rest of the standard parts. Resistors first as before.
and finally the rest of the parts for the gyrator pre pre-regulator and the shunt regulator, +ve half.
tested those. A mild panic when the output voltages were completely wrong and nothing blew. Then back to the drawing board and doh, realized my test 1K load resistor was too low in value. It was eating more current than the ccs was set to deliver wit the result that the voltage dropped. Raised the load resistor to 5K and bingo bang on 27V output voltage.
Both + and –ve shunts. Biggest worry over.
In with the rest of the transistors:
the electrolytic caps. A necessary evil, heh heh.
the critical film caps for the oupupt gyrators.
And its basically all there now.
Time for final testing so in with the dummy load resistors.
the output gyrators don’t really work without a bit of a load current draw so you need these to give accurate output voltages and ensure they are working correctly.
Though the pcb is designed so that these load resistor are easy to install for testing purposes, there is another reason. A look at any small signal transistor datasheet will indicate that ft reaches its maximum with around 8-10ma of current draw…. Something to bear in mind. So a question to answer is; how might sound quality be affected by increasing ft (regulator bandwidth). My thoughts are it should improve significantly. However dumping extra current to ground and opening up direct pathways between regulator outputs and ground might also result in increased ground current noise and intermodulation between the many output regs. Did I say that right? I know what I’m getting at though it might be bollocks and sounds like a lot of buzz words thrown together. Another thing to test anyway.
And happily both boards tested and all voltages as expected after 1 glitch which turned out to be a hidden dry solder joint on the second +ve shunt. Had to track that down by measuring a few voltages but not too difficult. The superficial trouble shooting has been remarkably simple.
I’m actually slightly amazed as was convinced I would have made some fundamental, obvious but overlooked, error but all looks great so far.
So on towards install and first active and listening tests.
Next in are soldering the power pins that mate the PRS and amp boards.
the sandwich is taking shape.
And there’s a little knack to make it fit under the standard naim heatsinks by sliding in from the side at an angle but it does just as planned.
Still can’t quite believe seeing the plan all come together. No real major hiccups as yet and almost there. Still need to file those baby PRS heatsinks down by a mm and the T0220s are annoyingly a fraction too tall. I can bodge them by bending them back slightly which is fine but a niggle. I have again amended my pcb files for bigger pads to allow them to be pushed right down eliminating the problem completely. Really so far only a couple of minor tweaks haven’t anticipated and accounted for.
Ok, so its taken a couple more days than planned but thats been due to making this write up too. I reckon I could build the whole shebang, Amp and PRS boards, from scratch in a weekend with a bit of focus. I think I will have a couple of hours free this weekend to do final checks and tests and get everthing up and running for first listening tests. Can’t wait.
OK for now.
Ced
The Build- the RedBox poweramp regulator system
or PRS for short as that’s too much of a mouthful. Sounds quite ‘neat’ too as Texans would say, heh. 'PRS'. Well, back to the boards again. There are a blizzard of resistors to solder which will be a headache. Mein Gott, zo many reziztorz.
So from the start again.
First, before I forget, solder the power link pins.
the two halves of the power rails feeding the filter and collector input regs need to be linked unless, as my plan is, you have a 12 x raw dc rails supply but that’s for another rainy Sunday. Not necessary.
Then time to do a bit of maths, sigh.
So the general schematic of the shunts again in a bit more detail:
Note the PRS is composed of a gyrator as a pre-regulator to feed the CCS and shunt preregulator which feeds the output gyrators. That is 3 layers of regulation along with the current source. Damned if anything is going to get through that lot!
Choosing a build version.
Plugging in a few preliminary numbers, it looks like the PRS does need to be customized, depending on the raw DC rail voltages. Simply because the PRS eats voltage headroom.
Mulling it over, I’m seeing the options divided into 4 possible builds, depending on the parts to be used.
1) super-low voltage build
2) low voltage build
3) standard voltage build
4) high voltage build
I’m going to start with the Low dropout build to get a feel for how the numbers will pan out with target raw dc rails of 34V, as per the NAP140. Doing the maths, these are the figures I reckon I can get away with: Dropout at 6-7V is a bit more disappointing than I had worked out on the back of a fag packet, but I think doable. will see on power up. So lots of juggling numbers and I have the values I want. This actually took HOURS to get real world numbers that I think will function. A whole world of difference between sims and nominal parts values and then thinking about real life UK mains on the ground. Reason being mains vary nominally +/-10% iirc which means you actually need to work within parameters for a wide range of possible input voltages and as these boards are eating more voltage that I was counting on its not so easy. Also keeping within good power dissipation limits is a constant balancing act.
Anyway I have now more or less got a spreadsheet together to work out the values for any input voltage, output voltage and the margins.
It sounds horrible but when its all been done, it boils down to measuring your raw dc rail voltage with a multimeter and then selecting specific values for 5 parts accordingly. simple.
Figures to suit a NAP140 are as below.
Low dropout build.
V raw in = 34V, V regulated out = 26V (headroom = 8V)
gyrator pre-shunt regulator = yes
R53 = leave empty (or very low diameter capacitor on UNDERSIDE of board)
D1, D2 = 2 x 1n4148 each
R4, R5,= chose for a couple of ma of current draw
R3 about 30-40 ohm for about 20ma current output
R11 = 2k output voltage set
R12 = 6k output voltage set
this will set the shunt to about 27V ouput and the output gyrators should therefore run about 0.7V lower. Think thats right- haven't got the numbers to hand right now. It’s the shunt reg where you set the PRS output voltage accurately anyway.
Ideally Id like to see the PRS with more headroom in the10-15V range to allow the use of theoretically better parts, particularly low noise parts for D1, D2 e.g LEDS or better and allow greater margins of voltage drop for the prereg and between the CCS output and shunt voltage. But it should work handsomely with lesser margins.
For the Redbox amp boards, same restrictions basically apply as for Naim boards or indeed NC200s. So I wouldn’t go much above 40V dc rails, 75W amp output depending on the output transistors of choice. Les W says the NC200s run very nicely on 50V, which would be fine too if you have chunky amp ouput transistors such as his recommended BUV20s and don’t run very low impedance speakers.
Onwards.
So first parts to go in are the Build version selection parts.
I’ve had to improvise by putting resistors in series to get the right values, yeah yeah, because I didn’t work all this out exactly beforehand. Actually quite embarrassing how many values I’ve had to revise on the fly. The shame is completely visible for all to see- It’s taken a fair while to juggle operating points, work out values and then solder together the correct value resistors. Ideal is to work em out properly before hand and then buy the correct values from the start, hmmm?
Also as there are so many resistors on the underside, the PRS really does need a silk screen below the board as well. Always a learning curve on the first go.
Finally, also realized I have a missing pcb pad, which fortunately is easy to bodge, but still annoying. Corrected on my pcb files now.
Then in with the rest of the standard parts. Resistors first as before.
and finally the rest of the parts for the gyrator pre pre-regulator and the shunt regulator, +ve half.
tested those. A mild panic when the output voltages were completely wrong and nothing blew. Then back to the drawing board and doh, realized my test 1K load resistor was too low in value. It was eating more current than the ccs was set to deliver wit the result that the voltage dropped. Raised the load resistor to 5K and bingo bang on 27V output voltage.
Both + and –ve shunts. Biggest worry over.
In with the rest of the transistors:
the electrolytic caps. A necessary evil, heh heh.
the critical film caps for the oupupt gyrators.
And its basically all there now.
Time for final testing so in with the dummy load resistors.
the output gyrators don’t really work without a bit of a load current draw so you need these to give accurate output voltages and ensure they are working correctly.
Though the pcb is designed so that these load resistor are easy to install for testing purposes, there is another reason. A look at any small signal transistor datasheet will indicate that ft reaches its maximum with around 8-10ma of current draw…. Something to bear in mind. So a question to answer is; how might sound quality be affected by increasing ft (regulator bandwidth). My thoughts are it should improve significantly. However dumping extra current to ground and opening up direct pathways between regulator outputs and ground might also result in increased ground current noise and intermodulation between the many output regs. Did I say that right? I know what I’m getting at though it might be bollocks and sounds like a lot of buzz words thrown together. Another thing to test anyway.
And happily both boards tested and all voltages as expected after 1 glitch which turned out to be a hidden dry solder joint on the second +ve shunt. Had to track that down by measuring a few voltages but not too difficult. The superficial trouble shooting has been remarkably simple.
I’m actually slightly amazed as was convinced I would have made some fundamental, obvious but overlooked, error but all looks great so far.
So on towards install and first active and listening tests.
Next in are soldering the power pins that mate the PRS and amp boards.
the sandwich is taking shape.
And there’s a little knack to make it fit under the standard naim heatsinks by sliding in from the side at an angle but it does just as planned.
Still can’t quite believe seeing the plan all come together. No real major hiccups as yet and almost there. Still need to file those baby PRS heatsinks down by a mm and the T0220s are annoyingly a fraction too tall. I can bodge them by bending them back slightly which is fine but a niggle. I have again amended my pcb files for bigger pads to allow them to be pushed right down eliminating the problem completely. Really so far only a couple of minor tweaks haven’t anticipated and accounted for.
Ok, so its taken a couple more days than planned but thats been due to making this write up too. I reckon I could build the whole shebang, Amp and PRS boards, from scratch in a weekend with a bit of focus. I think I will have a couple of hours free this weekend to do final checks and tests and get everthing up and running for first listening tests. Can’t wait.
OK for now.
Ced
Outstanding.......
Can't wait for the listening tests.
Couple of questions....
Have you listened to the amp boards on their own? Thoughts ?
What prototyping had you done earlier? Are all of the benefits known or have you built straight from the initial concept? By this I mean have you heard the benefits of
i) regulated front end supply (any design)
ii) use of split supplies to the gyrator (collector and filter)
iii) the gyrator design (not published yet ??) is it optimum?
iv) use of the shunt regs, better than a LM317 ?
Sorry so many questions....and more to come......excellent work though, excellent thread.
Can't wait for the listening tests.
Couple of questions....
Have you listened to the amp boards on their own? Thoughts ?
What prototyping had you done earlier? Are all of the benefits known or have you built straight from the initial concept? By this I mean have you heard the benefits of
i) regulated front end supply (any design)
ii) use of split supplies to the gyrator (collector and filter)
iii) the gyrator design (not published yet ??) is it optimum?
iv) use of the shunt regs, better than a LM317 ?
Sorry so many questions....and more to come......excellent work though, excellent thread.
Hi Dom,
I'm planning all listening tests this weekend. This will be the real clinch, however nice everything seems to have worked out so far on the build.
I'm reasonably confident though, as yes; it is all based on established concepts I've tried, tested and listened to, though its a new implementation and some minor to mid new bits for me so one never knows.
The split feeds to filter and collectors on the gyrators are what I use on my current preamp. Its one of the few ways I know of getting a significant performance boost out of gyrators.
The use of a high quality feedback regulator to run mulitiple gyrator filters- also been tested- I use ALW superregs in my current preamp. Much much better sounding than another gyrator as preregulator to the filters.
And don't even mention LM317/337s- they are laughable soundwise- Won't touch 'em for regulation when driving audio circuits. Never mind the fact they are only rated to 40V (though you can get HV variants or rig them to run at high voltage supplies as long as you respect the 40V in-to-out difference).
The actual shunt is untried for me- I'll be comparing them to ALWs jury rigged into the preamp to get a feel for how they rate relatively. I'm expecting similar or better sonic performance though its kinda irrelevant as ALW superregs won't run to poweramp voltages due to the AD825 Opamps maxxing out at 36V. You can redesign them completely to run at high voltages or play with specialist high voltage opamps but I abandoned that idea a while back. I like the idea of going discreet- its cheaper and way more flexible in principle.
Regulated front end on poweramp? Been running for years. And yes, its a huge improvement. Kind of the point of this project from my pov, heh. I want the absolute best regulation I can think of.
Optimised output gyrators? I don't know. The numbers work for me and are again based loosely on what I run on my preamp, but whether there are better values of transistor, resistor and capacitor than what I've selected, I don't know. My playing around with them leads me to say they are pretty stable soundwise as long as parts values are sensible but that's far from exhaustive experimentation. I also have a couple of extreme spec parts in mind I will want to try at some point, but that sort of tweaking is part of the pleasure. If it works as planned it should sound fantastic but thats not to say judicious tweaking couldn't improve on that. It might be a flop too, but I've put a lot of time and money into it because I'm reasonably sure of what I'm doing. Frantic wood touching here now.
Ok for now
ced
I'm planning all listening tests this weekend. This will be the real clinch, however nice everything seems to have worked out so far on the build.
I'm reasonably confident though, as yes; it is all based on established concepts I've tried, tested and listened to, though its a new implementation and some minor to mid new bits for me so one never knows.
The split feeds to filter and collectors on the gyrators are what I use on my current preamp. Its one of the few ways I know of getting a significant performance boost out of gyrators.
The use of a high quality feedback regulator to run mulitiple gyrator filters- also been tested- I use ALW superregs in my current preamp. Much much better sounding than another gyrator as preregulator to the filters.
And don't even mention LM317/337s- they are laughable soundwise- Won't touch 'em for regulation when driving audio circuits. Never mind the fact they are only rated to 40V (though you can get HV variants or rig them to run at high voltage supplies as long as you respect the 40V in-to-out difference).
The actual shunt is untried for me- I'll be comparing them to ALWs jury rigged into the preamp to get a feel for how they rate relatively. I'm expecting similar or better sonic performance though its kinda irrelevant as ALW superregs won't run to poweramp voltages due to the AD825 Opamps maxxing out at 36V. You can redesign them completely to run at high voltages or play with specialist high voltage opamps but I abandoned that idea a while back. I like the idea of going discreet- its cheaper and way more flexible in principle.
Regulated front end on poweramp? Been running for years. And yes, its a huge improvement. Kind of the point of this project from my pov, heh. I want the absolute best regulation I can think of.
Optimised output gyrators? I don't know. The numbers work for me and are again based loosely on what I run on my preamp, but whether there are better values of transistor, resistor and capacitor than what I've selected, I don't know. My playing around with them leads me to say they are pretty stable soundwise as long as parts values are sensible but that's far from exhaustive experimentation. I also have a couple of extreme spec parts in mind I will want to try at some point, but that sort of tweaking is part of the pleasure. If it works as planned it should sound fantastic but thats not to say judicious tweaking couldn't improve on that. It might be a flop too, but I've put a lot of time and money into it because I'm reasonably sure of what I'm doing. Frantic wood touching here now.
Ok for now
ced
Very nice work there ced
Good point and often neglected.
Electrostatic discharge is a real silent killer. Anything using FET technology is highly susceptible to ESD damage, and that in particular includes anything from FET input opamps to otherwise near-indestructible output devices. And to make matters worse, ESD can zap things in a way that renders them still operational, but somewhat under-performing. Then later (anythings from hours to years later) they just suddenly die without warning.
Bipolar-based amps are pretty safe in this respect (though doing as ced suggests is good practice), but CDPs and DACs are prime candidates for damage. Here, a wristband is a minimum requirement, and preferably also an anti-ESD mat like this for when the PCB is removed from the case;
Mr Tibbs
Good point and often neglected.
Electrostatic discharge is a real silent killer. Anything using FET technology is highly susceptible to ESD damage, and that in particular includes anything from FET input opamps to otherwise near-indestructible output devices. And to make matters worse, ESD can zap things in a way that renders them still operational, but somewhat under-performing. Then later (anythings from hours to years later) they just suddenly die without warning.
Bipolar-based amps are pretty safe in this respect (though doing as ced suggests is good practice), but CDPs and DACs are prime candidates for damage. Here, a wristband is a minimum requirement, and preferably also an anti-ESD mat like this for when the PCB is removed from the case;
Mr Tibbs
Ah, Dom,
been rather hectic last few weeks so no real time to piddle about with this.
Anyway, took a couple of hours out today to try to sort this little bastard out as per troubleshooting thread (http://www.pinkfishmedia.net/forum/showthread.php?t=75158) and finally cracked it. A dud MJE15031 driver. In fact not just 1 dud, but the whole damn batch!!! So of course, when I swapped it out, still no joy as just replaced a bad one with another dud. Am I pissed off with On-semi??!! The only reason I picked it up was measuring hfe and getting a reading of 300 which was just so far out of the ballpark it had to be wrong. And was. So having shredded one amp board (and my sanity) trying to find the fault, now fixed up the other and it measures as sweet as a lollipop. I was really beginning to doubt my sanity as I just couldn't figure out what I was beginning to believe was a design fault, having stripped the damn thing right back to the bones. Happily sanity now fine and design fine.
So now remains to fix up the first board, and get this project back on the road again when I can grab a spare hour. It will be a couple of weeks yet with my diary, but something to look forward to.
cheers
C
been rather hectic last few weeks so no real time to piddle about with this.
Anyway, took a couple of hours out today to try to sort this little bastard out as per troubleshooting thread (http://www.pinkfishmedia.net/forum/showthread.php?t=75158) and finally cracked it. A dud MJE15031 driver. In fact not just 1 dud, but the whole damn batch!!! So of course, when I swapped it out, still no joy as just replaced a bad one with another dud. Am I pissed off with On-semi??!! The only reason I picked it up was measuring hfe and getting a reading of 300 which was just so far out of the ballpark it had to be wrong. And was. So having shredded one amp board (and my sanity) trying to find the fault, now fixed up the other and it measures as sweet as a lollipop. I was really beginning to doubt my sanity as I just couldn't figure out what I was beginning to believe was a design fault, having stripped the damn thing right back to the bones. Happily sanity now fine and design fine.
So now remains to fix up the first board, and get this project back on the road again when I can grab a spare hour. It will be a couple of weeks yet with my diary, but something to look forward to.
cheers
C