So, with xmas fast approaching, there are 5 weeks left to get pcbs delivered in time for a little modding over the winter solstice break. Crikey.
So what to do with a couple of free and self indugent days?
Well I intend to finally get the Naim (style) poweramp well and truly sorted. near enough.
Some background: the current poweramp prototype is a fine mess and all over the place. Its needs to be rebuilt and while we're at it, slip in -on the sly- the latest preamp and regulator mods which are rather nice in that application. Though it looks nothing like it, its actually just a Naim poweramp circuit with regulation. I just build my own boards in a big chassis so theres bags of space to mod. One day I'll have enough, put it in a nice chassis and settle down to rear turkeys or something.
http://www.naimmods.com/pix poweramp project/poweramp prototype top view 1.jpg
http://www.naimmods.com/pix poweramp project/poweramp prototype side view 1.jpg
http://www.naimmods.com/pix poweramp project/poweramp PSU.jpg
Dunno if can hit that deadline (I should be finishing some architectural plans tonight but sick of it), but a little distraction is welcome right now.
Anyway, tried a lot of mods over the last 20 months and the prototype is a mess. Certainly not fit to move, nevermind transport (zaaap/kabooom)- all wobbly wires and shorts just waiting for a light breeze to happen. Bye-bye cat.
However it’s really the amp and regulator boards I want to tidy up and redesign in light of some nice results on the preamp too. Though got to throw in a couple of new bits at the end as well or it'd be no fun.
Besides, its been years since there was any real update on the Naim pages on Acoustica. Apart from a little poweramp teasing, and a big one’s been on the cards for ages so thought I’d run a preview.
Now, the current chassis’ above are ‘rather large’, but I thought I'd make the ‘xmas Redbox’ (got to give it a name- first law of missions) a bit more general, and so able to fit in my baby NAP140 chassis too.
http://www.naimmods.com/pix poweramp project/The Nap140.jpg
(Am I alone in thinking EVERY application Google develop is complete and utter shite? Like the free resources; damn near unusable though.)
In fact the Nap140 will be the test bed. That means there will be some compromises involved, though I expect them to be small.
And when I think about it, I kind of like the idea of an, innocuous-on-the-outside looking, ‘Stealth NAP 140’, heh.
So, to the the Design Brief, in vague order of importance:
1) Give the Nap500 a run for its money, or beat it, sound quality wise? Not power obviously. Lets make a stab at it. I expect the answer to be yes, but ****, who knows.
2) Boards to be a straight replacement able to fit into a Nap 140. That means will fit a 250, 135, 180 also. No rebuilding desired- I don't want to fiddle with the existing psu of the 140 (this is really a proof of concept).
3) Full multi-regulation of all elements except the output transistors. That means regulation of front end, VAS, driver and biasing strings.
Sorry, but the output transistors are probably the least important part of the Naim amps in terms of regulation. Anyone who's tried front end regulation will realise this. Either that or the 250 regulators are real cows but 250s left me pretty unimpressed. Either way it would also require double the chassis real estate which is impossible to do and still meet criteria 2 so that’s for another project. As I said, there are some compromises but they are small. Also I wouldn't necessarily go for straight regulation of the output stage anyway as there are other tricks to try.
And yes, the regulation will be exceptional.
4) Spitzenclasse components. All top-of -the-line parts within my current knowledge. And yes, there are better parts than the MMKs for feedback (if the sound is to your taste).
5) Budget, maybe £250-300 for the whole shebang- amp boards, regulators, parts; both channels. This is not a psu project so we live with the power supply we already have.
6) Ability to experiment a bit with grounding, parts selection, topologies and output transistors. Got to learn something new with each build and have the space to stretch a bit.
7) Extreme ease of assembly and mod-ability. To help 6. above along. And rebuilding does get tedious after a while so anything to cut down on that time is desirable.
So lets have a look at the design criteria in a bit more detail.
There are 4 main elements to consider:
1) amp circuit
2) layout and chassis assembly
3) Regulation circuits
4) parts and components
5) pcb layout, routing, wiring and grounding
I put chassis layout as number 2 advisedly as the regulation could spread like bindweed so chassis space available will be a major constraint on design and will demand very tight layout.
1) The Amp Circuit
So lets have a look at what circuit we're going to use. Nothing very exciting going to happen here. Base building blocks are going to be the classic old Naim circuit:
http://www.neilmcbride.co.uk/output-amp2.pdf
Aaaandd lets learn a few lessons from the Avondale NC200 circuit on the way. I'm going to be quite conservative as these are marvellous sounding circuits and I am, after all, a Naim aficionado.
So lets start with the NC200.
http://www.naimmods.com/pix poweramp project/1 NC200 v1.pdf
Well, as I’m going for very thorough levels of regulation won't need the (apologies Les Worstenholm) pretty crude front end regulation. Basically on the NC200 its an RC filter to protect the the sensitive front end input and VAS sections from the worst excesses of the high current driver and output stage supply demands which make the rail pretty noisy. The added diodes prevents current suckout on big output transients.
So out goes (c2, c11, r11, d3 and their -ve rail counterparts) and we're left with:
http://www.naimmods.com/pix poweramp project/2 NC200 V2 (stripped rails).pdf
Now its no longer a NAP and its no longer an NC200. It’s turning into the ‘Redbox’, named after my first ever silver bumper NAP140 with its lone little red LED. A completely undramatic and un-earth-moving change. Ahh, fond audiogeek memories though.
Anyway, we’re basically back to the NAP circuit with 2 differences:
1) Emitter degeneration on the input differential (the 100R resistors R5 and R8).
2) the output inductor and its damper resistor (L2 and R29).
Now I remember reading somewhere that the operation of the NAP circuit is deceptively complex and works not how it seems (It certainly simulates very freakily). If I remember, the guy on diyaudio.com analysing it said that emitter degeneration would be pointless in the circuit but I didn't understand his reasoning at the time and haven't bothered to really analyse the circuit yet to try and understand it. So, I am going to leave them in. Emitter degeneration generally makes bipolar transistors more linear at some loss of gain so why not? The very respected Leach amp uses emitter degeneration so another ‘yes’ to the idea.
Also, most critically, I'm working on the principle that you can leave parts OUT if you don't want to use them, but its a nightmare to add parts in afterwards without drilling holes, rewiring, jumpers and other horrible bits. So the plan is to work it into the design if you can leave it out easily. Layout is going to be VERY tight in this project so there will be little scope for hard core bodging after the event. I shall call this principle DUMPING. There is much scope to try things out and customise to tastes and prejudices.
I'm also going to leave in the full zoble network as I want the opportunity to eventually try out different cables without worrying about stability and blowing up the amp. Again it can be DUMPed if not wanted.
Ok that’s the legacy ideas over with. Why does the Avondale NC200 sound better than a standard NAP? My guess would be largely due to the good quality Mil spec tantalum capacitor it used to use for the feedback loop (polyesters now I believe?) and to a lesser extent the crude front end regulation. No great magic when you know what works, though Les W might decide to disagree with my sorry ass.
OK, back to the circuit. The final part is to add something a bit extra. Now I don't want to do anything fancy. I could try cascoding in the signal path but in my experience so far that is 'intense', and I'm not comfortable enough in that area yet in terms of musical presentation and balance; don’t intend to trade hyper resolution for musical involvement.
...and this writeup is actually going to take a 'tad' longer than I thought, so more in the next couple of days.
Cheers
Ced
So what to do with a couple of free and self indugent days?
Well I intend to finally get the Naim (style) poweramp well and truly sorted. near enough.
Some background: the current poweramp prototype is a fine mess and all over the place. Its needs to be rebuilt and while we're at it, slip in -on the sly- the latest preamp and regulator mods which are rather nice in that application. Though it looks nothing like it, its actually just a Naim poweramp circuit with regulation. I just build my own boards in a big chassis so theres bags of space to mod. One day I'll have enough, put it in a nice chassis and settle down to rear turkeys or something.
http://www.naimmods.com/pix poweramp project/poweramp prototype top view 1.jpg
http://www.naimmods.com/pix poweramp project/poweramp prototype side view 1.jpg
http://www.naimmods.com/pix poweramp project/poweramp PSU.jpg
Dunno if can hit that deadline (I should be finishing some architectural plans tonight but sick of it), but a little distraction is welcome right now.
Anyway, tried a lot of mods over the last 20 months and the prototype is a mess. Certainly not fit to move, nevermind transport (zaaap/kabooom)- all wobbly wires and shorts just waiting for a light breeze to happen. Bye-bye cat.
However it’s really the amp and regulator boards I want to tidy up and redesign in light of some nice results on the preamp too. Though got to throw in a couple of new bits at the end as well or it'd be no fun.
Besides, its been years since there was any real update on the Naim pages on Acoustica. Apart from a little poweramp teasing, and a big one’s been on the cards for ages so thought I’d run a preview.
Now, the current chassis’ above are ‘rather large’, but I thought I'd make the ‘xmas Redbox’ (got to give it a name- first law of missions) a bit more general, and so able to fit in my baby NAP140 chassis too.
http://www.naimmods.com/pix poweramp project/The Nap140.jpg
(Am I alone in thinking EVERY application Google develop is complete and utter shite? Like the free resources; damn near unusable though.)
In fact the Nap140 will be the test bed. That means there will be some compromises involved, though I expect them to be small.
And when I think about it, I kind of like the idea of an, innocuous-on-the-outside looking, ‘Stealth NAP 140’, heh.
So, to the the Design Brief, in vague order of importance:
1) Give the Nap500 a run for its money, or beat it, sound quality wise? Not power obviously. Lets make a stab at it. I expect the answer to be yes, but ****, who knows.
2) Boards to be a straight replacement able to fit into a Nap 140. That means will fit a 250, 135, 180 also. No rebuilding desired- I don't want to fiddle with the existing psu of the 140 (this is really a proof of concept).
3) Full multi-regulation of all elements except the output transistors. That means regulation of front end, VAS, driver and biasing strings.
Sorry, but the output transistors are probably the least important part of the Naim amps in terms of regulation. Anyone who's tried front end regulation will realise this. Either that or the 250 regulators are real cows but 250s left me pretty unimpressed. Either way it would also require double the chassis real estate which is impossible to do and still meet criteria 2 so that’s for another project. As I said, there are some compromises but they are small. Also I wouldn't necessarily go for straight regulation of the output stage anyway as there are other tricks to try.
And yes, the regulation will be exceptional.
4) Spitzenclasse components. All top-of -the-line parts within my current knowledge. And yes, there are better parts than the MMKs for feedback (if the sound is to your taste).
5) Budget, maybe £250-300 for the whole shebang- amp boards, regulators, parts; both channels. This is not a psu project so we live with the power supply we already have.
6) Ability to experiment a bit with grounding, parts selection, topologies and output transistors. Got to learn something new with each build and have the space to stretch a bit.
7) Extreme ease of assembly and mod-ability. To help 6. above along. And rebuilding does get tedious after a while so anything to cut down on that time is desirable.
So lets have a look at the design criteria in a bit more detail.
There are 4 main elements to consider:
1) amp circuit
2) layout and chassis assembly
3) Regulation circuits
4) parts and components
5) pcb layout, routing, wiring and grounding
I put chassis layout as number 2 advisedly as the regulation could spread like bindweed so chassis space available will be a major constraint on design and will demand very tight layout.
1) The Amp Circuit
So lets have a look at what circuit we're going to use. Nothing very exciting going to happen here. Base building blocks are going to be the classic old Naim circuit:
http://www.neilmcbride.co.uk/output-amp2.pdf
Aaaandd lets learn a few lessons from the Avondale NC200 circuit on the way. I'm going to be quite conservative as these are marvellous sounding circuits and I am, after all, a Naim aficionado.
So lets start with the NC200.
http://www.naimmods.com/pix poweramp project/1 NC200 v1.pdf
Well, as I’m going for very thorough levels of regulation won't need the (apologies Les Worstenholm) pretty crude front end regulation. Basically on the NC200 its an RC filter to protect the the sensitive front end input and VAS sections from the worst excesses of the high current driver and output stage supply demands which make the rail pretty noisy. The added diodes prevents current suckout on big output transients.
So out goes (c2, c11, r11, d3 and their -ve rail counterparts) and we're left with:
http://www.naimmods.com/pix poweramp project/2 NC200 V2 (stripped rails).pdf
Now its no longer a NAP and its no longer an NC200. It’s turning into the ‘Redbox’, named after my first ever silver bumper NAP140 with its lone little red LED. A completely undramatic and un-earth-moving change. Ahh, fond audiogeek memories though.
Anyway, we’re basically back to the NAP circuit with 2 differences:
1) Emitter degeneration on the input differential (the 100R resistors R5 and R8).
2) the output inductor and its damper resistor (L2 and R29).
Now I remember reading somewhere that the operation of the NAP circuit is deceptively complex and works not how it seems (It certainly simulates very freakily). If I remember, the guy on diyaudio.com analysing it said that emitter degeneration would be pointless in the circuit but I didn't understand his reasoning at the time and haven't bothered to really analyse the circuit yet to try and understand it. So, I am going to leave them in. Emitter degeneration generally makes bipolar transistors more linear at some loss of gain so why not? The very respected Leach amp uses emitter degeneration so another ‘yes’ to the idea.
Also, most critically, I'm working on the principle that you can leave parts OUT if you don't want to use them, but its a nightmare to add parts in afterwards without drilling holes, rewiring, jumpers and other horrible bits. So the plan is to work it into the design if you can leave it out easily. Layout is going to be VERY tight in this project so there will be little scope for hard core bodging after the event. I shall call this principle DUMPING. There is much scope to try things out and customise to tastes and prejudices.
I'm also going to leave in the full zoble network as I want the opportunity to eventually try out different cables without worrying about stability and blowing up the amp. Again it can be DUMPed if not wanted.
Ok that’s the legacy ideas over with. Why does the Avondale NC200 sound better than a standard NAP? My guess would be largely due to the good quality Mil spec tantalum capacitor it used to use for the feedback loop (polyesters now I believe?) and to a lesser extent the crude front end regulation. No great magic when you know what works, though Les W might decide to disagree with my sorry ass.
OK, back to the circuit. The final part is to add something a bit extra. Now I don't want to do anything fancy. I could try cascoding in the signal path but in my experience so far that is 'intense', and I'm not comfortable enough in that area yet in terms of musical presentation and balance; don’t intend to trade hyper resolution for musical involvement.
...and this writeup is actually going to take a 'tad' longer than I thought, so more in the next couple of days.
Cheers
Ced