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Old 19th June 2006, 06:00 AM   #1
sangram is offline sangram  India
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Join Date: Sep 2002
Location: India
Default Computer PS Hum

Hi

How do you get rid of mains hum from a PC power supply?

I have two systems, a Delta 66/AMD643200+/Antec True Power and a brand new E-Mu 1212M/AMd sempron 2500+/cheapo-generic PSU.

Both the soundcards are connected in fully balanced mode to a BPA-200 type application using 4 LM4780 chips. Both sound absolutely fantastic, the E-Mu is a treat to listen to, but with both cards there is the slightest trace of hum (when my ear is right up against the speakers). There is also a bit of white noise, but I guess that's pretty normal given that there is technically zero attenuation from the card output to the amp input.

I think the hum is due to the power supply of the PCs. With the PCs off and the amp switched on and input connected, the system is dead silent (I hear absolutely nothing with my ear halfway into the cone).

How does one begin the process of eliminating the power supply hum? It doesn't look like a supply build quality issue, as a premium supply like the Antec and a cheap generic supply have essentially the same problem.
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Old 19th June 2006, 06:44 AM   #2
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Join Date: Mar 2006
Location: Great White North
Hi.

I've come across this a lot (as well as others here).

The very slight hiss is normal for almost every amp. If you can't hear it from 6~12", things are in good shape.

For the hummmm...

My test was to lift the ground pin on the power cable. Yes this can be potentially dangerous from a safety standpoint, but computer power supplies (monitors, and board/case design) are not really designed for audio. Typically the earth is connected to the frame, conducting to the pci card fingers, then typically also bolted to a grounded pad on the card, linked to the audio ground. Silly.

If you can test the system this way and hear no hum, it is a ground loop to earth. But since relatively unsafe, a proper solution needs to be found.

I had to separate the earth vs. audio ground in lot of my audio equipment. Earth goes to the chasis for safety, and is isolated from the audio ground. Nothing can be done about my computer, but it seems that since it is the only source to earth now, there is no sign of any hum.

Also I replaced a lot of my audio cables, using ones I made over the junk that comes with the gear you buy. Nothing fancy, but lots more reliable.

It's all a big pain. Just piece by piece, isolate and test...
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Old 19th June 2006, 07:24 AM   #3
sangram is offline sangram  India
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Join Date: Sep 2002
Location: India
Thanks, that was quick!!

The hiss doesn't worry me, it can't be heard from 4 inches away and closer than that only when my ear is level with the tweeter.

For the hum , I should have thought of the ground loop caused by the supply being directly connected to the audio ground via the case.

Maybe an isolation transformer is the only way out of a situation like this?

Thnx

Sangram

Quote:
Originally posted by bluesmoke
Hi.

I've come across this a lot (as well as others here).

The very slight hiss is normal for almost every amp. If you can't hear it from 6~12", things are in good shape.

For the hummmm...

My test was to lift the ground pin on the power cable. Yes this can be potentially dangerous from a safety standpoint, but computer power supplies (monitors, and board/case design) are not really designed for audio. Typically the earth is connected to the frame, conducting to the pci card fingers, then typically also bolted to a grounded pad on the card, linked to the audio ground. Silly.

If you can test the system this way and hear no hum, it is a ground loop to earth. But since relatively unsafe, a proper solution needs to be found.

I had to separate the earth vs. audio ground in lot of my audio equipment. Earth goes to the chasis for safety, and is isolated from the audio ground. Nothing can be done about my computer, but it seems that since it is the only source to earth now, there is no sign of any hum.

Also I replaced a lot of my audio cables, using ones I made over the junk that comes with the gear you buy. Nothing fancy, but lots more reliable.

It's all a big pain. Just piece by piece, isolate and test...
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Old 19th June 2006, 07:38 AM   #4
diyAudio Member
 
Join Date: Mar 2006
Location: Great White North
Hi,

Wouldn't jump to that point yet. Better to find the souce of the problem first... then work from there. (could always be somewhere own the line... mixer/preamp grounds, cable TV, telephone connected to surge supressor...)
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Old 19th June 2006, 08:09 AM   #5
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
Quote:
Originally posted by bluesmoke
Hi,

Wouldn't jump to that point yet. Better to find the souce of the problem first... then work from there. (could always be somewhere own the line... mixer/preamp grounds, cable TV, telephone connected to surge supressor...)
Hi,

No telephone at home, but cable TV? Of course, the TV is the monitor for this system! and connected permanently to the cable connection...

Good, that's a few things to check out after work today.

Thanks. Will report back tomorrow.

Edit: I just thought of something...

My amp requires a bridged input so what I've been doing so far is using XLR pin 1 (chassis ground) as a signal ground reference, and Pin 2 and 3 to drive each half of a bridged amplifier. Maybe this has something to do with it?
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Old 20th June 2006, 05:14 AM   #6
sangram is offline sangram  India
diyAudio Member
 
Join Date: Sep 2002
Location: India
Nope, scratch that.

After four hours of switching connections and switching things on and off, I just realised that the hum is only in one channel, and is independent of whether or not the computer is switched on. So it's one channel of the chipamp, not the computer PS.

So the quesiton goes directly to PD.
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