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Old 2nd July 2006, 08:56 AM   #1
Nuuk is offline Nuuk  United Kingdom
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Default SMPS noise problem

I have recently built a system consisting of a Monica2 DAC, pre-amp based on OPA134 opamps and stepped attenuator, and a Charlize amp. These are all powered by Skynet 8080 SMPS.

This is how it is all wired together. There is a master mains switch with the centre SMPS. The connections from the SMPS are +12v, 0v, +5v, 0v for the DAC and Charlize, and and extra -12v for the preamp. All pairs of wires are tightly twisted together. The 5v supplies are for the LEDs.

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There is a problem with a whining noise coming mainly from the DAC, but also to a lesser degree from the preamp. The noise is coming from the SMPS. Can anybody suggest a solution?

Here is how it looks.

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Old 2nd July 2006, 03:32 PM   #2
infinia is offline infinia  United States
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Hi
What kind of noise are you having problems with and can you describe it? Measured and/or audible?
I suspect ground loop issues and/or beat frequencies of SMPS oscillators. You need to trouble shoot the system in order to localize problem areas. Start with pwr amp first and use a clean source. Then add next in chain one at a time, etc.
You will need to look at a star grounding scheme with an isolated signal ground. Also common mode noise (big) of the SMPS will get converted at some point using single ended connections. Differential or optical connections between stages would probably fix it, but might not be realistic at this stage. May have to repackage design together to fix it easier, but can be done either way.
So re-evaluate ground scheme by taking into account it can be spoiled with signal ended connections between stages. Think about adding common mode filters on each supply before new star ground connection.
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Old 3rd July 2006, 01:01 PM   #3
Nuuk is offline Nuuk  United Kingdom
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This morning, I isolated the mains ground on the SMPS modules from the signal ground and the noise has gone!

I have not had to do this before, but this is the first time I have put this sort of system into metal housings!

Now it sounds as good as it looks!
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Old 3rd July 2006, 01:20 PM   #4
John C is offline John C  United Kingdom
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Obviously the drainpipe is not correctly finished

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Old 3rd July 2006, 02:02 PM   #5
infinia is offline infinia  United States
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Quote:
Originally posted by Nuuk
This morning, I isolated the mains ground on the SMPS modules from the signal ground and the noise has gone!
Hi,
BTW, you never said how you isolated the signal ground.
You still have safety ground still to chassis, right? With single ended connections there are always paths for noise currents from signal to chassis ground even if it's at a higher impedance.
If it was my system, I would add common mode chokes on all of the SMPS inputs and outputs (Its real easy to do with found toroids). If you look with a scope or spectrum analyzer you will see a big improvement and that can't be bad for the sound. But if you are happy with it, then all is good for you.
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Old 3rd July 2006, 02:59 PM   #6
Nuuk is offline Nuuk  United Kingdom
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There is quite a choice of ferrite cores when I search the Farnell catalogue . What should I actually be looking for here?

John C - you may have a new ISP but the humour hasn't improved any!
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Old 3rd July 2006, 04:01 PM   #7
infinia is offline infinia  United States
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Generally I prefer high Mu powdered iron because it's wideband losses are good (acts more like a snubber), but the trade is it's lower perm. or inductance. Mostly higher inductance is preferred so ferrites are best here for a given size. To do it right you need to think through your whole system grounding/wiring scheme and use common mode chokes in conjunction with ceramic shunt caps at the output and Y caps at input. This network forms a filter with a target goal of about 60 dB attenuation realizing 40dB across whole stop band. Without getting too technical and not able to get your answer. (because of the unknowns to me of your system). I would suggest scavenging for big toroids from PC SMPS. Take one for the output (including ground returns) and wind a few turns and then reconnect to your PCB. Repeat for the input for suppressing EMI outside. Think of this as isolating or keeping noise within the SMPS. Sometimes I use both ferrite and powder iron toroids together for best advantage of both.
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Old 3rd July 2006, 06:18 PM   #8
Nuuk is offline Nuuk  United Kingdom
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Nothing in my PC spare parts box unfortunately but I will try some of those toroidal ferrite cores from Farnell.

Quote:
BTW, you never said how you isolated the signal ground.
I followed the instructions here (as he sometimes knows what he is talking about).
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Old 4th July 2006, 02:02 PM   #9
infinia is offline infinia  United States
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Quote:
Originally posted by Nuuk
I followed the instructions here (as he sometimes knows what he is talking about).
Hi
Checked out the link. That guy knows just enough to be dangerous
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