Calling all MP New Zealanders (Both of you!!) (5 Viewers)

[littlematt]

Hi all, posting here cos you guys are always helpful!
I seem to have a ground loop issue thats a bit unusual. Ive just upgraded my system to a more hi-fi set up and noticed an annoying hum coming through the speakers. I suspect its 50hz but is also coming through the tweeters so this doesnt make sense to me. I have an m-audio revolution 5.1 connected to the RCA in of the stereo amp. When I unplug this lead the hum goes away. Went I turn off the HTPC and leave the audio cable plugged in, its still there....Even with the HTPC unplugged from the wall. Unplugging the DVB-T cable makes no difference either. Running a wire from the chassis of the amp to the HTPC chassis works only while the HTPC is off, when its on its worse than ever I can hear digital 'noise' coming through the speakers that wasn't there before. Both the amp and HTPC are plugged into a double wall outlet, so will have a common earth. Both items have three pin plugs.
I know you can get audio isolators for this problem to run on your audio interconnect but dont want to reduce sound quality or spend hundreds.
I dont think its a problem with the amp itself because I have two different pioneer amps (A300x and A400) that do the same thing. The level of the hum is independent of the volume level set on the amp, and is audible with the volume right down.
I read elsewhere that its a good idea to use a volt meter to check the AC volts between pieces of gear + signal cables and gear, will try this tomorrow if my voltmeter does AC, cant remember right now and its pissing down outside so not going to the garage to look
Actually this thing is driving me crazy! Any help/advice much appreciated, hope you are all keeping warm and dry, bring on spring!

 

[mm1352000]

Hmmm. Sounds like you're investigating the right things. Common grounding is usually the place to start, but these problems can still arise when there are loops further "downstream" in the (source and amp connections). I take it you haven't got anything else plugged into the amp? I would try lifting the ground pin on either the amp or HTPC if you have an appropriate adapter. Also, if the HTPC has onboard audio I would try that too to see if it is isolated to the one sound output - I'd expect it wouldn't be but still an interesting check. Try checking the voltage on the outer "sheath" of the RCA connectors as well if you can. I have found that they can sometimes be a problem.

mm

 

[littlematt]

Heh was hoping you'd step in mm, thanks... You know I completely forgot I had onboard audio will try that after work. No, nothing else is plugged into the amp. I tried unplugging EVERYTHING from the PC including network cable + aerial, except for the audio interconnect, hum still there. Kinda weird, its not grounded, its not on, where in bloody hell is the voltage coming from? I went to 5 different stores on sunday looking for a cheater (ground lift) plug, including jaycar, havent found one. Might try corys or ja russel today if I get a chance... Will fiddle more tonight with my multimeter. Problem is the wife likes to watch TV in the evenings... Will also open HTPC case and look for dust buildup. Just thinking on the fly, what kind of PC to amp interconncects do you guys use? Ive got some nice ones, not pudney basic ones, 3.5ml to stereo RCA. Paid about 30 bucks for them.... I suspect the majority of DACing is done on the reciever side these days via spdif/toslink etc.... My amp does not have onboard DAC...

Slightly different topic, I posted as well under hardware/audio, I believe the reason to have an offboard dac is to eliminate electrical/digital 'noise' from inside the case. Surely if I just got a PCIe extender and put my soundcard in a shielded case with its own clean power supply outside of the HTPC case this would be WAY cheaper than buying an external DAC? Those things are so overpriced.... Wonder if the MB would pick it up as present if it wasnt drawing volts though...Dont know anything about PCIe protocol
Anyway better go jump in shower get my arse off to another soggy/cold day at work...

 

[littlematt]

OK went to Cories an JA Russel, no cheater plugs. But JA R was very helpful, took my number and got an AV guy from the city branch to call me, gave me a few ideas. Still no success though. However I was wrong about one thing. The PC needs to be plugged in for the hum to occur, doesn't have to be on or even on at the wall so its definitely an earthing issue, no? Have tried a different audio cable, tried the MB audio out, tried a different power lead of the PC with shielding and magnets on it.....Nope. Tried a double adapter for the PC and Amp, nope. I thought a loop had to be a circuit, with a common earth where the hell is the current coming from? I tried my multimeter but its a super cheapy and only measures in 100s of AC volts, so got a zero there.... The speaker cables run under the house so I thought maybe they were getting a bit of current induced in them so ran a short bit of speaker cable above ground, still no joy. Dag nabit, this is pixxing me off. The soundcard is earthed properly, I blew out all the dust about the only thing I can think to do now is to swap out the antec power supply with the corsair unit in my desktop..... Tried plugging my old ipod in for the source, no hum..... Also tried turning absolutely everything else in the house off at the wall....No joy of course.

 

[mm1352000]

However I was wrong about one thing. The PC needs to be plugged in for the hum to occur, doesn't have to be on or even on at the wall so its definitely an earthing issue, no?

I would say so. If it were induction (which is the other common source of hums) then I would expect moving gear around to make a difference.

Have tried a different audio cable, tried the MB audio out, tried a different power lead of the PC with shielding and magnets on it.....Nope.

All good things to try. I think by doing that you rule out the cable and sound card.

I thought a loop had to be a circuit, with a common earth where the hell is the current coming from?

Technically all it requires is a difference in impedence to earth. Or at least that is my understanding. If the grounding is poor it can also be a problem.

The speaker cables run under the house so I thought maybe they were getting a bit of current induced in them so ran a short bit of speaker cable above ground, still no joy.

Also a good check to confirm earthing issues.

...I blew out all the dust...

Can you or have you done the same for the amp (if relevant)?

...about the only thing I can think to do now is to swap out the antec power supply with the corsair unit in my desktop.....

That is a very good check. To simplify, you could also potentially switch the PCs completely - may or may not be easier.

Tried plugging my old ipod in for the source, no hum.....

iPods aren't really grounded, so not surprising to me.

The fact that when you unplug the audio lead the problem goes away suggests that the earthing is bad in either the amp or PC. Current will be flowing from one to the other on the sheaths of the RCA connectors... that is the link. Get a decent MM if you can and measure the DC level relative to the power socket ground level on the outer part of each RCA socket with the cable unplugged. This may/should tell you whether the PC or the amp is the problem (ie. you'd expect both to be flat at 0V, but I think you'll find at least one connector that is not). Then you can take it from there.

That's my 2c anyway.

 

[littlematt]

Thanks again mm, appreciate the extra brain cycles you are putting into this for me
Haven't had much of a chance to fiddle around yet due to life/wife/work etc, however...
Tired of looking for a 'cheater pulg' or earth lift, I made one with a double adapter and a pair of pliers heh... Broke off the earth prong. Using this on the amp solves the problem. Wont leave it there as it's dangerous. Doesn't make a difference on the TV or PC. Tried the desktop with the corsair PSU, still hums. No dust in the amp.
Found this site which was very interesting. Also has interesting reading on cable 'snake oil' merchants. In the conclusion part it talks about switched mode PSUs ie PC PSUs not really being grounded, in fact devices with SMPS's chassis 'float' at around 50% of line in current if I read it correctly. Not sure why this doesn't cause instadeath whenever I touch my PC, but I will never ever remove/insert the signal cable with the amp or pc on.... If this was the case, I would have thought EVERYONE with an HTPC and analogue signal leads would have issues, but search hits are rare.
One other interesting thing I found out which is why I got confused in my first post... I get a loop (hum) via the audio interconnect between the PC and Amp when the amp and PC are earthed through the same wall plug and nothing else is plugged in. If I unplug the PC from the wall, hum is gone, but if I then plug the VGA lead to the TV into the PC and the TV is earthed, hum returns.
As in my first post I tried connecting the chassis of my amp and PC together with some hook up wire. Was fairly light gauge and only made the problem worse, introducing lots of digital noise to the hum. I forgot about electrons being lazy, so will try this again with some heavy cable from my old car audio set up, will also ensure sound card has an extra ground from mounting plate to chassis. I will remove the VGA lead to the TV when I do this (cant remember if I did last time). Am going to muck around with different locations for this grounding wire as well, next to PC PSU, straight off sound card plate etc. My amp already has a grounding point for a turntable, so can assume this is good to use. Was also wondering if I grounded one of the line in RCAs on the amp to earth if this would help.
All this mucking around with electrons does make me nervous as it involves 240V AC but I reckon as long as Im only touching the earth side of things I should be okay.
I had a chat with a mate last night who reckons most modern appliances (my TV, maybe PC) are earthed through neutral rather than ground. If this is the case, it could explain the porblem, but then why does my TV have a three pinned power plug?
Oh well, will keep fiddling on and update as results permit....

 

[kiwijunglist]

have you tried using a simple power surge protecter, i've had these remove electrical hum in the past.

 

[littlematt]

have you tried using a simple power surge protecter, i've had these remove electrical hum in the past.

Yep, sure have

 

[kiwijunglist]

somnething like this not sure if they good or not
http://www.radioshack.com/product/index.jsp?productId=2062214

 

[littlematt]

Those things apparently filter out any 50-60hz signal, including bass I think, so degrade the signal, therefore not really an option Im afraid. Thanks for the suggestion though....
However...
Heavier cable tying chassis together, no difference. On a hunch I tied the exterior of an RCA input to the chassis of the amp, wife says its gone (I cant listen up close, holding the wires on by hand). Out came the mulitimeter again...Went over all the earth points and there's continuity between the RCA ground and power plug ground so I figure it cant hurt. Make a permanent solution out of an old RCA lead, tie signal ground to chassis. Plug everything in, power up, very slight hum from driver and hiss from tweeter up real close but marked improvement.
So what is going on here? I pulled the amp apart looking for obvious dry solders, checked all the earth connections to PCB, chassis and line inputs against plug earth, seems fine (zero resistance at lowest setting on mm).
SO I guess Im happy to put up with the end result, but is there any way this could be harmful to the amp? Paid a fair bit for it, dont want the magic smoke to come out...
But if I can get rid of the hum entirely I would be much happier....
Are ground loops potentially harmful to equipment anyway?
Does the fact that what I did seems to help mean there is a poor connection within the amp?