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Old 9th July 2006, 10:32 PM   #1
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Default Another grounding problem-need advice.

Hi

I have some problems when driving a power amplifier directly from PC's sound card.
Currently I am using 10R parallel with two diodes side to side 'ground dividers' between both physical earthing and power ground and between power ground ans signal ground.
This removes any hum from looping, but causes sound card to severly distort, probably because of high current in ground (PC is earthed of course).
Removal of earth-to-power 'ground divider' makes distortion disappear, but brings the hum back.

Now, can I remove power-to-signal parallel diodes? Or do you have any better options?

regards
Adam
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Old 9th July 2006, 10:36 PM   #2
poobah is offline poobah  United States
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Try a differential amplifier?
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Old 9th July 2006, 11:40 PM   #3
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Default Grounding to the PC won't help, need isolation

ALL computer power supplies are dirty, noisy beasts. Large switching PC power supplies have barely enough capacitance to get the job done for computer CPU, RAM and drives, etc. .... thus any direct analog metal conductor connection to the input side of your amp or pre-amp will probably be noisey and full of hum, etc.

Best advise: for digital music outta da PC is to get a USB to SP/DIF Toslink optical interface to your pre-amp / amp. (FireWire to SP/DIF Toslink optical is even better, but few WinPCs have FW ports.)

2nd best: for digital music outta da PC is to get a USB to SP/DIF Coaxial to your pre-amp / amp. (FireWire to SP/DIF Coaxial is even better, but few WinPCs have FW ports.)

The above are relatively inexpensive = about the same costs as a "good" plug in sound card ... but the above assumes your pre-amp / amp has the SP/DIF (Toslink or Coaxial) connections.

3rd best but more $$: (actually this can be the best of the best, but mucho denaro will have to be laid down) a USB to Quality DAC. The better breed here all have electronic isolation and very good filtering ... to get the noise offa the PC data line.

Sources: Roland (Edirol) and M-Audio ... semi pro and professional digital to analog converters (DAC) and digital to digital converters .... There are also quite a goodly number of really fine USB and FireWire to DAC makers. (http://wavelengthaudio.com is but one.)

The important thing is that that $100 analog sound card is really not up to snuff when it comes to keeping PC noises outta you sounds ... This is the reason those new Apple MiniMacs and MacBooks sound so great = they have built in Toslink optically isolated audio.

I know, I know you were going to say that your plug in sound card already has SP/DIF output ... BUT the sound card is plugged into the PC chassis and the SP/DIF coaxial cable has a metal shield ... which carries all that chassis / PC power supply / ground looping noise all the way to your input on your amp = poor electronic or optical isolation between the PC and sound system = no way to get around it except to buy very expensive addons or ... start at the top and read this all over again = junk your sound card and connect by USB or FireWire ... :>( ...

Disclosure: I work for an M-Audio distributor/online outlet ... http://3dotaudio.com ... http://usbstuff.com ... http://firewirestuff.com ... http://industrialcomponent.com/mudio/index.html ... http://3dotaudio.com/maudio/transit.html = ~$100
... BUT the Roland stuff is just as good! = http://rolandus.com/products/product...3&ParentId=114 = ~$100
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Old 10th July 2006, 08:20 AM   #4
MikeB is offline MikeB  Germany
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Hmm, skip earthing the power amplifier ?
Or drive PC and amp from the same mains outlet.
Skipping power-to-signal parallel diodes might burn the 10ohms.
Have you measured the voltage difference between the 2 grounds ?

Mike
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Old 10th July 2006, 07:17 PM   #5
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Default start fresh?

MikeB: ... " skip earthing the power amplifier ? ... "

Well it really won't do much good as the noise from the PC will still be generated. You will probably see some improvement, but not enough to warrent the effort.


" ... Or drive PC and amp from the same mains outlet. ..."

This is effect will also help some, but you will still have a) a dirty supply side ripple and hum ... and b) the possiblity of added ground loops (quite common).

" ... Skipping power-to-signal parallel diodes might burn the 10ohms. Have you measured the voltage difference between the 2 grounds ? ..."

Actual voltage measurement with volt meter (DC or AC) may prove to not tell the whole picture as noise like "hum" and ground loops may be in a higher frequency range than the DVM can read ... although certainly if you have any DC or AC component differential between ground points, you will have noise ... or even worse.

(I still stand by my recommendation to go digital interface and let the pre-amp do the translation and noise filtering = USB to Digital and/or USB to Analog sound.)
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Old 10th July 2006, 07:46 PM   #6
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Hello,

Don't these little "ground lift" circuits normally include a cap??

The problem originated from a less than ideal earth, too high an impedance... too many neighbours... all kinds of noise on the old earth line, can even measure up to 100mVdc on it with a cheap DMM! That comes in the form of spikes when turning on/off switches, fans, appliances, sure, it does it when the neighbours do the same. Even with the soundcard's outputs muted, since it has a direct connection to the dirty earth via the ground on the computer (WHYYYYY?????) it manifested itself as anything from eyepopping crackles and pops to sustained buzzing.

This appears to be an extremely common problem with soundcards, even if the ground loop current is minimal (non audible) very few people have an ideal earth or have the capability to do anything to improve on it.

Some may recommend the use of a UPS to power the computer with, some say that won't work because most of the time they're connected to mains anyway, and I wouldnt' be shocked to find out they're always earthed as well.

You "could" invest in a signal transformer as a means of galvanic isolation.. to me it isn't an option, part of the reason I spent the money I did was because I didn't want to the hear the sound of a coupling cap, so why would I turn trade that for the sound of a transformer.

I have cured it temporarily by doing something I can't recommend, like cutting off the ground lug to the PC and ensuring nothing else that's earthed is plugged into its power bar. That resulted in a wicked hum. My amp is earthed, I like it that way, and so with the PC supply earth left to float, the only point of true earth ended up being where its best, right at the input stage of the amp.

To fill in any blanks here.. with the PC earthed I broke the ground loop by only having one end of the shield connected. The loud buzzing that ensued after I floated the earth of the PC was due to the soundcard losing it's point of reference, therefore warranting the connection of both ends of the shield. This worked well, however I found it to be noisier than with one end open. Why?

It comes back to those damned switchers. The SMPS of the computer has Y caps which would be filtering noise right to my soundcard I'd imagine. So my next move was to replace them with a MOV and a good X2 cap right across the lines. I haven't done it yet, but I may still.

However it's since come to my attention that the monitor is actually the worst offender. I wasn't willing to float that as well, so I plugged it into the wall directly, as opposed to into the PC power bar.

Needless to say this ignores the fact that there's undoubtedly a ground wire in the signal cord for the monitor connecting to the PC and so that must form some sort of ground loop... oddly enough it doesn't really seem to, no audible hum anyway, but the switching noise from it is incredible.

So what now... There's still the possible solution of using a massive ISO transformer to power the computer with... but if I'm to get one of those, I'd much rather have it front of the amp, and this would still not at all begin to cure the switching noise, only addressing the ground loop issue in a safer way than floating it all.

So I'm looking for a better, more robust cure, cheap yet effective and maybe even safe too.

Let's just "say" I have an ISO transformer for the computer, and nothing is left to dangerously float, perhaps the switching noise could be attenuated with added PI filtering, the premade modules that seem cheap enough.

Then again, maybe it's time to get off my butt, and go for the all out cure, hack the hell out of both my soundcard and audiodock (EMU 1820M) and implement auxiliary supplies for it.

So that's where I'm at at this stage of the game: the best, most robust, safest, quietest!!, cheapest, most true to the sound cure, does seem to be the auxiliary supplies. Any comments on that?

Sorry if this seems to be hijacking your thread, but it doesn't seem to be very different from your problem, and I think some of this can help you too.

Regards,
Chris
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Old 10th July 2006, 07:55 PM   #7
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Default PC = dirty earth ground?

classd4sure: " ... Even with the soundcard's outputs muted, since it has a direct connection to the dirty earth via the ground on the computer (WHYYYYY?????) ..."

'Cause that is the cheapest, easiest and least complicated for digital engineers to accomplish ... and understand ... digital computer engineers are usually more concerned with what goes in, not what comes out and most of them don't know audio from apple butter. And their level of concern is what goes in should be cheapest way to accomplish.

"Why would anyone want to listen to a computer? ... its just modem noise." - HP engineer at an engineering conference.
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Old 10th July 2006, 08:06 PM   #8
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Yeh really.

BTW I just had a minute to think about this, the aux supply might be a good cure, but that goes with the assumption that one could get away with floating the aux. supplies from the computer.. which doesn't seem very likely??
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Old 10th July 2006, 08:23 PM   #9
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Thank you guys very much

I should have added PC as a sound source is just temporary solution.
What I am really thinking about is that grunding scheme of amplifier itself is faulty if it cannot do well with grouded source!
Should I add a 'lifted-grounded' switch? (so that I could follow Mike's advice whenever I want)
I saw such a switch in numerous PA amplifiers, but I have kind of gut feeling there is a simpler solution.

Mike
Quote:
Or drive PC and amp from the same mains outlet.
Already the same mains outlet.
Quote:
Have you measured the voltage difference between the 2 grounds ?
No, what should I expect? +-0.65V p-n junction drop...


thanks and regards
Adam
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Old 10th July 2006, 08:33 PM   #10
poobah is offline poobah  United States
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These really do work...
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