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Old 3rd November 2012, 10:37 AM   #1
mcd99uk is offline mcd99uk  United Kingdom
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Default Three bigger transformers or Nine smaller

Hello all,

I have a question. I am currently creating a new USB / SPDIF DAC. The basic design of the PSU is as follows. Each IC has it's own supply split over (analog and digital if necessary). Each power rail (for the DACs and interface Ics) is being regulated by an LT1763 regulator. Each Regulator is being fed from its own rectifier and smoothing caps (2x5600uF).

Now the question is, is it going to be an advantage to give each rectifier it's own small transformer or just use less larger transformers and split over Analog, Digital and Output stages?

I know that to go for a no compromise PSU then going for individual transformers is the way to go but in the real world would this make any difference? As I am not a rich man I have to consider cost as well.

Many thanks

Paul
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Old 3rd November 2012, 11:09 AM   #2
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by mcd99uk View Post
As I am not a rich man I have to consider cost as well.
IMHO a good compromise would be to use two transformers, one for the digital circuits and one for the analogue circuits.
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Old 3rd November 2012, 11:26 AM   #3
mcd99uk is offline mcd99uk  United Kingdom
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Originally Posted by jitter View Post
IMHO a good compromise would be to use two transformers, one for the digital circuits and one for the analogue circuits.
Thank you for the reply.

Hopefully with all the smoothing and decoupling cap + regs there would be enough separation between the rails for noise not to spread too much. Given what I have learnt through my CD67 project noise is an important factor.

A couple of 50VA transformers would do the job?
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Old 3rd November 2012, 11:39 AM   #4
DF96 is offline DF96  England
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You will need to connect grounds together, in order to get the signal from here to there, so you need to do proper grounding anyway. Any PSU connection carries the risk of injecting noise. Better to have one connection near the output where signals are bigger than one per stage. Separating digital and analogue PSUs makes sense. Any further could create more problems than it solves.
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Old 3rd November 2012, 12:56 PM   #5
jitter is offline jitter  Netherlands
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Quote:
Originally Posted by mcd99uk View Post
A couple of 50VA transformers would do the job?
Should be more than enough. I'm with DF96 on the grounding/noise issue, pehaps in this respect the two-transformer approach is also the best compromise...
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Old 3rd November 2012, 01:10 PM   #6
mcd99uk is offline mcd99uk  United Kingdom
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I'm struggling to get my head around additional supply rails causing more problems with noise. Is it that each reg makes some noise itself and it all could add together? Or is that the grounding system needs to be of a star type and use hefty conductors? Otherwise, currents in the grounds could add to the problems?

Thanks for the help. My knowledge in this area of audio is very limited at present. Before doing the CD67 I hadn't even heard of I2S and the only power supplies I ever made were big and basic for driving amplifiers.
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Old 3rd November 2012, 04:14 PM   #7
DF96 is offline DF96  England
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Currents in grounds always cause problems, because no ground has zero resistance and zero inductance. Therefore you have to make the problems as small as reasonably possible. Using a star ground blindly can cause as many problems as it solves. Using
hefty conductors can be an admission that the ground scheme is wrong, unless they are solely present to carry high currents or fast (i.e. digital) edges.

All these additional supply rails also have additional grounds. If you know how to connect all these together properly then you won't need separate PSUs anyway.
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Old 3rd November 2012, 04:27 PM   #8
dmills is online now dmills  United Kingdom
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For stuff that is mostly signal level rather then power, and especially if it is fast (Digital) signalling, a simple solid ground plane on a buried layer is often as good as anything, and a great deal better then trying star grounding with Mhz rate square waves.

What you tend to find is that high speed current usually wants to return along a path directly under its signal conductor so as to minimise energy in the magnetic field, and a solid ground plane helps a lot here. Split planes can be trouble as any signal crossing the split has to be very carefully thought out.

'Star ground' is pretty much a way of saying 'loop area' and that really starts to hurt as edges become fast.

Regards, Dan.
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Old 3rd November 2012, 07:38 PM   #9
mcd99uk is offline mcd99uk  United Kingdom
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This is going to make my cerebal hemispheres hurt.

So a separate ground point is a bad idea due to loop area. This explains twisted pairs. So to each regulator you would run a twisted pair to the IC you are powering and attach to the ground plane locally to that IC. This would minimise area between supply and ground and make the star point centre the DAC PCB. Or am i going in the wrong direction?

All conductors associated with digital circuits need to be reasonable hefty and short as possible.

I have much to learn! Need to stop thinking about steady state conditions.

Thank you again for the help.
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Old 4th November 2012, 08:26 AM   #10
jitter is offline jitter  Netherlands
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Whaddayaknow... I found some pics of my old Micromega DAC1 and it uses a three-transformer PSU (but in a different way from what you intend).

I'm not entirely sure of the PSU-layout, but it looks like the two smaller transformers (2*15 VAC) each power one channel in the analogue section of this DAC (from analogue parts of the DAC-ICs to the output opamps). The bigger transformer (2*9 VAC) takes care of the digital circuits (logic, processing and digital part of the DAC-ICs).
Pretty much each section gets its own voltage regulator (16 in total). The PSU for the opamps is based around the LM317/LM337 (4). The others are 7805s (12).
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