Ground Loop Isolation

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Mike
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Ground Loop Isolation

Post by Mike » Thu Jul 28, 2005 7:21 pm

Took a trip to RadioShack today, and, as always, choked. I'm trying to hook up my computer to my surround amplifier.<p>First, because the card is a Sound Blaster Audigy, it's got a digital coax output through a special adapter cable (1/8" mono to RCA). The ouput works perfectly with my newer Yamaha reciever, but I can't get it to work with this older Denon. At any volume on the reciever (except mute), there is a humm at the same volume no matter where the volume control is at, and there is no sound from the computer.<p>So, I tried using an analog connection, and there is a horrible ground loop problem, where to be able to even tolerate the sound, I have to completely turn off the subwoofer, otherwise there is just an annoying humm that's almost as loud as the sounds themselves.<p>Anyway, because of that, I figured I'd go get a ground loop isolator. How much could they be? 5 bucks? MAYBE $10? Nope! $20. What a ripoff.<p>So, why not build my own. Does anybody know how to build a ground loop isolator?<p>Thanks,
Mike

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sofaspud
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Re: Ground Loop Isolation

Post by sofaspud » Thu Jul 28, 2005 7:32 pm

That $20 RS ground loop isolator is probably a 1:1 transformer.

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Chris Smith
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Re: Ground Loop Isolation

Post by Chris Smith » Thu Jul 28, 2005 7:35 pm

I made one, one time using a isolator transformer, for the same thing. <p>It is isolated but it also dumps the hum through the transformer to ground. <p>It wasn’t perfect, but it was acceptable. With a little more work, a few caps and possible chokes, Im sure you can do it.

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jwax
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Re: Ground Loop Isolation

Post by jwax » Fri Jul 29, 2005 3:22 am

There's also the lossy 120 VAC/24 VAC transformer to another same type wired 24 VAC/120 VAC.

cato
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Re: Ground Loop Isolation

Post by cato » Fri Jul 29, 2005 4:08 am

Ground loop? Is any of this equipment actually grounded? Consider grounding the computer and not grounding the other equipment. That should open any loop there might be.

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sofaspud
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Re: Ground Loop Isolation

Post by sofaspud » Fri Jul 29, 2005 5:01 am

If it's at all practical, you might try plugging all the equipment into the same outlet strip.

Mike
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Re: Ground Loop Isolation

Post by Mike » Fri Jul 29, 2005 6:58 am

I've tried all of that. The computer does plug into the same surge protector, but now that I think about it, Only the computer has the third ground prong in the power plug, not the amplifier.<p>But wouldn't the ground of the audio signal be connected to ground in the computer and also to ground in the amp?<p>Either way, I will try connecting a ground wire between the chassis of the computer and amplifier.<p>I just wish that the digial output would work on there, then I would have true 5.1 surround sound. But Maybe, if I'm lucky, it just needs a ground to function, and get rid of the humm. I will have to try grounding the entire setup.

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Re: Ground Loop Isolation

Post by Newz2000 » Fri Jul 29, 2005 7:26 am

Are you sure the noise is not coming from the computer? If you used the digital output, there would be no noise, but if reverting to the analog output results in noise, it could be because of the computer. You have a good sound card, but maybe you have a flaky power supply? Also, this sounds bizarre, but there's a coil or capacitor in my computer that's bad, located right near the CPU. It causes a hum that changes pitch when the computer has to do more processing. Maybe yours could be similar?<p>What frequency (aprox) is your hum? Is it 60Hz? If not, I doubt its ground related.<p>How long are your RCA/Headphone cables? Are they shielded? Do they pass by some other noisy device?<p>I used to work with car stereos and its amazing how adding an amp can cause an otherwise inconspicuous noise to suddenly become painfully obvious.

Mike
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Re: Ground Loop Isolation

Post by Mike » Fri Jul 29, 2005 8:00 am

Now that I think about it, my motherboard is an ABIT KT7-RAID, which is one of the ones with supposably bad capacitors on the motherboard and there is a huge lawsit against ABIT for. They would replace the board, but unfortunately I do not qualify since I did not purchase the board and do not have original paperwork.<p>Maybe that could cause it. But I dobut it because those caps would cause the computer to crash, and I've got to say this computer has not crashed for no reason once the entire time I've had it. It's probably the only system I've had like it.<p>The thing is, when the computer is not connected to the stereo it's conncted to the set of speakers I've made (2.1 setup w/ 2 x LM3886 for full range speakers, and 2 x OPA549 bridged for the sub), and there is absolutely no humming sound.<p>Even if I use analog cables, I can't get the same problem through my Yamaha receiver, even though that is not grounded.<p>The analog cables I'm using are NXG 1/8" stereo to dual RCA cable. It's shielded, is about 4ft long and uses gold plated ends, so I dobut it's the cable. The digital cable is just one from radioshack, it's a basic shielded 1/8" mono to single RCA, but it caused no problems with my yamaha. I even used a coupler and a 15ft cable I made using coax cable and Neutrik RCA connectors to extend it to the yamaha, and still no problems.<p>The power supply is a very good one. It's a 350W Mad Dog Surepower supply. Although Mad Dog is not considered high-end, their products really are. I use one of their video cards in this system, a sound card in another computer, their 500W power supply in my A64 system, and a DVD-ROM/CD-RW in this machine. The only problem I've ever had was with the DVD Drive - it wouldn't burn CDs but a firmware upgrade fixed it. So I dobut it's the power supply. Just in case though, I checked the voltages:<p>VCore - 1.86V
3.3V - 3.38V
5V - 5.13V
12V - 12.54<p>To me, nothing appears to be odd there.<p>
I would guess the hum to be about 60hz, because it comes through the sub loud and clear, and the receiver crosses over the sub at 80hz, so it's got to be lower. I've got a test CD, so I will compare them sometime (humm on the computer to 60hz frequency on the CD).<p>Does anybody have any Ideas though on how to get the digital output working on this card? Could it be the ground problem?

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haklesup
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Re: Ground Loop Isolation

Post by haklesup » Fri Jul 29, 2005 1:12 pm

To have a ground loop, usually both the sound source and final amplifier have to share the same DC source (AC wall supplies are isolated multiple times on the way to DC). A PC and A sterio reciever would not share the same supply unless you have an interesting setup. In many cases the "ground loop" is actually a DC offset (or transient DC offsets A.K.A. AC noise) which overdrives the amplifier<p>Since the sound comes through on the yamaha there may be a problem with the Dennon inputs. Do they work with other sound inputs. Could there be an impedance mismatch (phono vs tape inputs for example).<p>If you have an oscilloscope, diagnostics would be straightforward.<p>Try reversing the polarity on your adapter. The Yamaha reciever may be more compliant about single ended signal sources while the Dennon requires ground to be ground.<p>Try decoupling the audio input with a capacitor. if the hum disappears, the source may be a DC offset, if it persists, it is coming from the cable.<p>I get a hum like you describe if I touch the inputs to my reciever with my finger. Not sure how that correlates to your problem though.<p>The ground prong on the power cord is really not ground. Think of it as an extra neutral wire in case the chassis gets shorted to hot. only appliances with metal cases will have a ground prong and it has nothing to do with sound quality.

Mike
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Re: Ground Loop Isolation

Post by Mike » Fri Jul 29, 2005 4:59 pm

No, this is the only source that does it.<p>I've got three devices hooked up to it - a DVD player with optical input, a VCR with analog input, and the computer - also with analog input. The VCR is plugged into VCR1, the computer into VCR2, and the DVD player into OPTICAL 1. The VCR has no humm at all, and nor does the DVD player. Just the computer. I've even tried plugging the computer's audio through the VCR's front inputs, and the humm is just as bad.<p>So it's gotta be either something not right with the ground, or not right with the computer. But since my computer speakers work fine, I don't see why the stereo wouldn't. And, yes, the ground wire on the power supply is connected to the case and the ground circuits on the motherboard, so, the chassis is grounded. I should be able to just connect a plain wire from the chassis of the computer to the chassis of the amp, and see if it helps. I'll also try the capacitor inline with the computer's output.<p>Also, I found out why the digital didn't work. The connector on the Denon is broke, because I took a coax audio cable, disconnected the optical from the DVD player, and tried a coax output from the DVD. No sound came through, and it didn't even pick up a signal. Odd thing here is there was no sound at all. With the computer hooked up to the digital, even though the input is broke, it still has that same humm at a constant low volume.<p>This is getting really odd, but hopefully somebody can help me make sense of what's going on with this setup!

hp
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Re: Ground Loop Isolation

Post by hp » Sat Jul 30, 2005 12:56 pm

Take a look at this article: http://www1.electusdistribution.com.au/ ... umloop.pdf . I don't know if it will help you, but it looks interesting.<p>Harrison

Mike
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Re: Ground Loop Isolation

Post by Mike » Sat Jul 30, 2005 2:36 pm

Thanks. That was pretty interesting. It's amazing how easy and common this problem is.<p>Since my optical inputs work, and the coax and optical data is the same, can I just buy an RCA connector and optical audio transmitter (the little case with an LED in it), and wire the coax output from the PC directly to the optical transmitter so I can use the digital inputs on the stereo?

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Edd
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Re: Ground Loop Isolation

Post by Edd » Sat Jul 30, 2005 3:36 pm

Mike:
Have you tried just working with one right or left channel at a time plugged in at the computer proper at seeing if the hum is present ….instead of having both of them plugged in.
And then if that one channel has the hum, how about unplugging that lead and twisting it such that you can have ONLY the ground male and female ground shells touching, with no signal transfer involved since those pins aren’t connected…and then see if that hum is still present back at the amp , then if not, hold the connection and also do the other channels ground the same way.
Waiting for a comeback on the above……
Also, you wanted to build an audio isolation……but….unfortunately 10k to 10 k transformers aren’t as common and popular anymore, such as they once were.The best I can see being done with available…..since the ‘60’s….. stuff is the use of some old Calrad stock.
http://www.calrad.com/calrad/45series.shtml
Two of their 45-708 or 45-712 hooked back to back would give a 10kZ to 10kZ isolated match. Which would require two xformers for each channel. And of course each one is as cheap as I know of at ~$5….so that is back to the $20 RS …pricing range!!!!
This is the hook up manner for isolation with the exception of the use of two of them back to back to get matching..
http://www.tkk.fi/Misc/Electronics/circ ... olator.gif
Final question is to check your”Junque Boxe” and tell us what xformers you currently have in it. <p>73's de Edd
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:D<p>[ August 01, 2005: Message edited by: Edd Whatley ]</p>

Robert Reed
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Re: Ground Loop Isolation

Post by Robert Reed » Sat Jul 30, 2005 5:44 pm

Edd
I thought all computer sound was at or about 600 ohm impedance ,since they refer to it as line level, I assumed this was so. Are they really using 10k impedance on these jacks and if so--Why?
Once you have sound source converted and entered into any solid state circuitry, theres no reason to even approach these levels, much less output it as such.I am also thinking that if his hum level is that horrific it may be something beyond ground loops.
Ouch--I just felt a pin prick or something!<p>[ July 30, 2005: Message edited by: ROBERT REED ]</p>

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