Proper grounding with more then one PSU - diyAudio
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Old 13th April 2012, 10:00 AM   #1
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Location: Jönköping, Sweden
Default Proper grounding with more then one PSU

I think I've read most of the threads out there regarding grounding and I still can't make my system noise free.

In my current system I'm using 3 PSUs powering a total of 5 Amps, with active filtering from a MiniDSP.

There's four 4780 boards with each pair sharing a PSU, powered by a 300VA toroid transformer.

There's also one TA3020 based two channel amplifier, powered by a A1000SMPS from connexelectronics.com

My problem is that when I connect everything together there's a quite loud hissing sounds from the speakers. If I disconnect the SMPS it goes dead quiet. It does not matter if the amp is connected or not. However, when it is connected and there's larger currents drawn from it (It's powering two subwoofers) I can clearly hear crackling sound on both the mids and highs. It becomes more noticeable the higher the volume.

This is how my current wiring looks like:
Click the image to open in full size.

Not in the picture: Output ground taken at the Star ground. There's also one more PSU and amp but they look exacly like the upper one in the picture so I didnt draw it.

What I have tried:
Lifting safety earth from SMPS.
Lifting safety earth and having ground going straight from SMPS to amp module, not connected to star ground.
Reverse live/neutral

Any suggestions? I'd like to have the SMPS in the same case as the others but if there's no other way...

Thanks in advance
// Max

Last edited by Maxhax; 13th April 2012 at 10:22 AM.
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Old 13th April 2012, 12:36 PM   #2
AndrewT is offline AndrewT  Scotland
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Location: Scottish Borders
take the SMPS out and place it on the ground just outside the garage door.

Ask the wife to go and buy the groceries, "Oh and remember to take the car".

I can guarantee that your amplifiers will not hiss any more!

Earth, or more correctly, Protective Earth (PE) is connected to chassis permanently, not to your Audio Ground.

Post pic and schematic of the analogue PSU and amp. We are interested in what you have for filtering of interference !!!!!!!
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Last edited by AndrewT; 13th April 2012 at 12:41 PM.
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Old 13th April 2012, 03:07 PM   #3
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Quote:
Originally Posted by AndrewT View Post
take the SMPS out and place it on the ground just outside the garage door.

Ask the wife to go and buy the groceries, "Oh and remember to take the car".

I can guarantee that your amplifiers will not hiss any more!

Earth, or more correctly, Protective Earth (PE) is connected to chassis permanently, not to your Audio Ground.

Post pic and schematic of the analogue PSU and amp. We are interested in what you have for filtering of interference !!!!!!!
Andew, you don't really like SMPS, do you?

My PE connection to chassis is my star ground, isn't that how it is supposed to be?

I cant access the amplifier right now so I'll post a picture in a few days.

But its Peter Daniels LM4780 kit, connected in parallel with a total of two boards on each PSU.

However I'm using 10000uf Elna for audio filter caps in the PSU and then I've got 2200uf ROE soldered directly on the wires between amps and PSU.

10000uf is snubberized using 3n3 and 0.1R
2200uf is bypassed with 0.1uf WIMA

All wires are tightly twisted and the analogue boards are placed about 20cm from the SMPS.

There's also a EMI filter installed in the chassis, however both the transformers and the SMPS are connected to the "secondary side" of it. Maybe it would be a good idea to put the SMPS in front of the filter?

Hope that you understood me, I'll provide the picture in a few days.
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Old 13th April 2012, 05:51 PM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Maxhax View Post
...........My PE connection to chassis is my star ground, isn't that how it is supposed to be?..........
No,

Chassis ground/PE is for Safety and Safety alone.

Audio Ground is there to allow the amplifier to work properly.

Only when you have the audio side properly connected and properly working do you install it in a box and make all the exposed conductive parts safe as well as the chassis. That final Safety connection may degrade the Audio. Now your work really starts to find how to make good audio, that is also safe.

Initially, treat safety and audio as completely separate issues.
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Old 13th April 2012, 06:08 PM   #5
sbrook is offline sbrook  Canada
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Grounding (Earthing) and noise and hum have to be some of the biggest cans of worms ever invented.

The goal is to provide the lowest possible impedence path to ensure that induced noise in the power rails is filtered to ground.

So, we came up with the idea of the star ground. Great, except that we then use the lightest gauge wire to the actual power supplies.

Then the filtering from the non-common positive / negative rails to ground is lacking.

Then we forget to the need for the star on connections also applies to those positive and negative rails and to ensure that more sensitive devices have the absolute lowest impedence path to the filter point go ground.

I see the layout of circuit boards time and again where the power rails dance all across the board with no filtering and so noise and hum are easily induced into the more sensitive circuits.

Noise reduction on paper is easy. Noise reduction in practical circuitry is a specialized art form! There are folks whose job in life is to design physical circuits to eliminate noise and they do a fantastic job while we who know the "rules" struggle time and again!

The best info I can give you is to ensure that your filtering is adequate and your common mode power cabling is large enough.
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Old 17th April 2012, 12:45 AM   #6
mjona is offline mjona  New Zealand
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Location: Upper Hutt
Hello Max,

You could be taking a risk with a mixed supply system in one case.

The four earthing points at the corner mounting holes of your SMPS module probably earth an on board ground plane to prevent emi radiation.These rather than an earth loop may be the cause of your problem.

For safety reasons I recommend that you avoid experiments until you have consulted your supplier.

Michael Jonassen
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