Adcom GTP-400 ground loop - diyAudio
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Old 24th August 2006, 01:49 AM   #1
zlast is offline zlast  United States
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Default Adcom GTP-400 ground loop

I have an Adcom GTP-400 connected to the soundcard in my pc via mini stereo "Y" to RCA cable. THe GTP-400 connects to a leach amp. This has created a ground loop and some really enjoyable hum which increases with volume level. THe real offender is the PC to Adcom connection. THe Adcom to leach is totally silent if I disconnect the PC "Y" to RCA . I temporarily connected the PC to the UNSWITCHED AC output on the back of the Adcom, this allmost eliminated the hum. The problem is this eliminates the AC ground to the PC (outlets on the back of the Adcom are only 2 prong). The Adcom GTP-400 does NOT have a ground wire on the AC plug, and the AC plug is hard wired into the chassis. Can the Adcom GTP-400 be modified to accept a standard 3 prong (USA 110 volt) plug and have the chassis grounded to AC ground? Are there other options to eliminate the ground loop ? Thanks for the input!

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Old 24th August 2006, 01:59 AM   #2
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try cutting the shield on the signal wire from the PC to the Adcom. I DO NOT recommend cutting the grounds on your AC cords. The AC corded grounds will provide a telescoping (Faraday shield) shield for your signal wire, but the ground currents travelling down the shield will be eliminated. It's those ground currents (due to a difference in ground potential) that is causing your hum ........unless it's in the source. You can test if it's a loop by just connecting the chassis with no signal connection and see if you get the hum. If no, the hum is most likely in the source. If yes, it's a ground loop.
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Old 24th August 2006, 02:01 AM   #3
anatech is offline anatech  Canada
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Hi zlast,
If you ground the GTP-400, you will only create yet another ground loop. It will not normally solve your problem. Computers don't play nice with audio systems as a rule.

A couple 1:1 balancing transformers between your computer and GTP-400 will break the ground connection, and kill the noise. Running balanced will not help because you will still have a ground current in the shield. That is your problem.

-Chris
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Old 24th August 2006, 02:33 AM   #4
zlast is offline zlast  United States
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"try cutting the shield on the signal wire from the PC to the Adcom"

I tried removing the Shield on the RCAs between the Adcom and PC, this INCREASED the hum noise level.

"If you ground the GTP-400, you will only create yet another ground loop."

Would it not be advisable to 1) have a ground on the AC cord since there currently is no ground 2) if both the Adcom and PC are connected to the SAME AC outlet would this not decrease the potential for ground loops? Anyone know why the Adcom does not have a ground...I do not believe it is double insulated...


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Old 24th August 2006, 02:47 AM   #5
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Quote:
"try cutting the shield on the signal wire from the PC to the Adcom"

Wow!? Is the Leach 3-wire grounded? ...and connected to the Adcom when you get the increased hum with a cut shield? And just checking, but you mean't you just cut the shield from one end, not "removed" the shield..... ....right? If this is all true, Anatech's suggestion Xformer coupling might be the best suggestion for your setup.
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Old 24th August 2006, 02:55 AM   #6
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.....and just for grins. Would you mind taking an ohmeter reading between your PC's input ground (RCA shell) on the Adcom GTP400 (AUX, Tape, CD, or whatever) and your Adcom's pre out ground (RCA shell). You said something about the hum getting louder when you turn up the volume control. I've seen some Adcom's get into a ground loop situation, then frost their ground trace on the board. I just want to make sure your starting out with a ground.
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Old 24th August 2006, 04:42 AM   #7
clem_o is offline clem_o  Philippines
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Where I live, outlet grounds are not mandatory, and few houses have them. We generally don't have problems with ground loops too. Despite the fact that power here is 220VAC (and both lines are 'hot'!), on good equipment there seems to be very few occassions where there's enough leakage to make touching the equipment a 'hair raising' experience... :-)

Except when it floods...

Cheers!


ps: why not try an isolation transformer on the power line to the offending equipment (i.e the PC).

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Old 24th August 2006, 05:05 AM   #8
AJT is offline AJT  Philippines
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Default [b]isolation transformer [/b]

like this one.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.
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Old 24th August 2006, 05:35 AM   #9
zlast is offline zlast  United States
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Default Ohm meter reading

From the PC RCA shell (ground) to the Adcom RCA output shell (ground) I measure 0.00...below the capability of my meter evidently (which isnt saying much ...
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Old 24th August 2006, 05:41 AM   #10
clem_o is offline clem_o  Philippines
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Default Re: [b]isolation transformer [/b]

Quote:
Originally posted by Tony
like this one.
Nice one!

Cheers

ps: I have a digital multitester just like that one... :-)
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