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New PCB for shoebox size NAC preamp part I

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nickcase said:
This would hopefully clean up some of the crud from the laptop psu before the starfish. I noted also that Carl started off using VBE after his traco's but has since switched to the CLCLC passive filter and the mysterious 'ov regen thingy'. Was this an improvement Carl? What did the output look like on the scope?
Click to expand...

Because the noise on mine is 2 superimposed 300KHz waveforms, it's really horrible-looking on the scope. I've got a really quiet upstream supply from the tracos to try and filter as much crap as possible before them, and I think that it's better than a laptop psu.

Even though I've got ceramic caps tight on the output of the Tracos, followed by common and differential chokes additionally decoupled by 3u3 films and 10u tants, I still see about 5-10mV of noise @ 300KHz on the scope at the output of the gain boards... That's a 10-fold reduction of noise from what's at the Traco output pins!

The good thing is that this junk is way out of the audio band. I'm still not entirely happy that the noise is there because I worry about interference and harmonic effects, but it sounds so good that the lid has been on for the last few weeks and I've not touched it!

The 0V regen mod is absolutely superb and I wouldn't go back to a standard 0V hookup now.
 
IMG_4928.jpg
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Pleasure to build so far, caps and transistors in tonight.
 
Nice one Mark :) Great to see another couple of builds coming along now! Keep posting as you go along, this is such a cool project.

I see you've got C0G caps across b/c of the appropriate transistors in the gain stage. To (mis)quote Jim, those are the spawn of satan and have no place in an audio circuit! Silvered mica are, by all accounts, a better choice :)
 
Darn it, I just stuck my order in as well, :D COG you no like? How did they come across, grainy, spitty, thin, shouty?
 
Not sure, as soon as I mentioned them to Jim he was pretty emphatic that they're best out of audio circuits...
 
Actually, I've just found some 10pf 'styrenes, not sure if they'll slow things down too far, unless somebody's tried it I'll suck it and see.
 
Excellent - nice little write up that Martin, thanks :) Have you considered fixing a re-match bout between a vbe and a passive CLCLC filter?

At this point a pair of PFM vbe's between laptop PSU and Starfish traco input will do nicely I feel :)
 
Carl - Martin's vbe/regulator shoot out had vbe's knocking 55dB of noise from an SMPS.

You started off using vbe's on the output of your Starfish traco's - what was the reason for the switch to CLCLC?
 
For C108 and C208 the 22pf Film cap RS part code in the BOM links to a 22pf COG - I have 25 now :mad:

How about a 22pf polystyrene RS: 113-229 instead?

I've used Wima films mostly here but I'd like to switch to polystyrene's. They are £1.10 each from RS and bought in packs of 10? So that'll be £55 for just 12 caps - am I missing something?

Nick
 
When will we get the details of the 0V regen option - don't want to start ordering/building and then have to change some of it for this mod.

Jo
 
I've used Wima films mostly here but I'd like to switch to polystyrene's ... So that'll be £55 for just 12 caps - am I missing something?
Click to expand...
I really wouldn't bother, IME the Wima MKS2s are excellent. If it's differences you want, trying things like Jim's CFP current source mod would be far cheaper /dramatic to try.
 
Wow! go away for a day and loads of stuff appears!

I'll have a go at answering the questions posed:

-25V oscons:
Should be fine in a symmetric setup as each reg is only putting out 12V wrt power ground.
Could be considered a little close to the bone in an asymmetric setup as the reg output is 24V. Maybe consider dropping the reg out a little?

-Traco preference:
I currently run a single 15-0-15 wide input traco (means I can power it from a 9V wall wart).
It runs fairly hot (I can hold the back of a finger on it, but it's uncomfortable) - but my current (undocumented) setup draws a bit more current than usual...
More investigation is required here...

-27R resistor or inductor:
Have a look at the particular part I have specified in the BOM for the 27R resistor - It is a wirewound power resistor, and it is spec'd to be inductively wound. This is what I currently run.
I will play around with inductors and other options here soon, but the wirewound 27R is a good+safe choice - forming a slight pi filter on the output of the reg.

-Powering the beast.
I currently use an 800mA 9V wall wart to power a 'wide input' traco.
Be carefull if using a SMPS to power a SMPS, their noise signatures could 'fight' and produce unexpected effects. Basically, I'd suggest putting a damn great snubbing circuit between the two...
I built a StarFish that was powered directly (no onboard traco) from two 15V SMPS laptop power supplies - sounded pretty decent too! If you want to try this, make sure the SMPS have floating outputs...

-Miller cap choices.
I personally would not get a ceramic cap anywhere near an audio circuit - the dielectric non-linearity is out of this world, and they are microphonic to boot!
I have tried both silver-mica and polystyrene in that position, both sounded far better. I think I bought some of the RS 22pf polys for my last built.

-0V regen mod.
I'll try to get some details up asap. For those of you wanting to order parts, order up an extra lm337, 120R, 1K5R and a couple more 10uF tants.




Progress is a little stalled at the moment - StarFish has been developed on the living room table and has gone as far as I can take it with simple tools ( multimeter only :D )
I'm in the middle of renovating a room which is ear-marked for audio development (and general office type things) and will be able to get out some decent test equipment then.
First order of the day will be sorting out all the passive filtering - at the moment it's very much guess+ear work.
There is much more to come I am sure :)

Jim
 
Here it is:

filter-1.jpg



And this is what it looks like:

filter-pic.jpg



Can be implemented by pulling out the 0V differential choke from the Symmetric CLCLC filter arrangement shown in the manual, and then soldering in the 337 circuit onto the left hand side of the remaining differential chokes - see photo...

Three things to note:
1) Use at your own risk. I dont promise it wont do nasty things. Limited debug so far.
2) Works ONLY with a traco'd symmetric local-regulated etc etc StarFish.
3) It looks simple, but it's not doing what it looks like at first glance...


Jim.
 
Jim, that describes it nicely. My brain still hurts thinking about this mod!

My first attempt used a 337 / 120R / 10K multiturn trimmer and a 47u silmic to decouple the adjust pin:

DSCF0657.jpg

DSCF0658.jpg


I later settled on 120R / 2K2 which gives me pretty much exactly -24V:

1.25 x (1+(2200/120)) = 24.16V

My symmetric rails run at +/- 24V, a 48V potential difference, so a 337 set @ -24V is really 0V depending on where you're measuring it from.

The 337 0V-regen gubbins are connected across the +/- rails:

DSCF0659.jpg


The output pin of the 337 is soldered directly to the power-ground 0V star point, giving the shortest possible 0V path:

pfm0901.jpg


You can see the big heatsink strapped to the 337... it's dropping 24V and was too hot for my liking. The heatsink just gets a little warm.

No decoupling of the input/output pins is needed and in my case strapping a 10u tant across the output caused oscillations when checked with the scope.

I hope this is of use to someone!

Cheers,
Carl
 
Hi Jiim!

Many thanks for your reply! For the next revision of your build manual, you might need to clarify some aspects about the common mode and differential chokes.

Page 19 of the latest build manual states that it is critical the number and direction of turns is as per the diagrams in order for the common mode choke to function as intended.

I took your drawing literaly and wound the exact number of turns as per your sketch. But it now looks like you and Carl have done it differently instead ;)

Also, i did not understand what was meant to be done for the differential filters. It is only when i saw Carl's pictures just now that i understood how the inductors were meant to be built. Nothing like the common mode choke at all which the current manual somehow made me think it was

I hope it does not seem like i am in any way nit picking. Just want to give some feedback that the next manual revision can be improved by being slightly more clear for thickheads like me on how exactly you need to make the common mode choke and how to make the differential chokes. I got a little lost along the way. ;)

starfish3.jpg


Regards

mark
 
Elsewise, Jiim...she is really coming along great! Just think....all your babies are starting to grow up all around the world! How many countries did you ship to?

I will be ordering the capacitors next week for it. So maybe a few more shots then eh! Jiim...do you reckon there are enough turns on my choke?

starfish4.jpg


Cheers

mark
 
Hi Mark,

Don't pay too much attention to my differential chokes - they're just aircored, hand-wound jobs. You'd get much better filtering by using the ones Jim has wound on a toroid.

For the common mode choke (and the differential, in fact) you should get as many turns on the toroid as you possibly can, just make sure the windings are the same as the pictures Jim posted.

Good job so far, just get some more windings on that toroid! Keep us posted and keep posting pictures.

Cheers,
Carl
 
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