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Old 12th March 2006, 01:51 PM   #1
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Default Grounding in a Passive Line Stage

I needed a volume control to hold me over until i can get my Passive Line stage built. Digging around my junk piles i found a P&G pot that
I slapped into a project box with a Cheapo RCA Cable cut in half for In/Out. this works, but there is a bit of minor background hum.

I never even thought about the ground loop that would be created by tying the In L-R and the out L-R grounds together. but sure enough HUM.

Ok, so now, to avoid that in the construction of the Passive Line stage. How should i ground the I/O?? I will be using isolated panel jacks. I would imagine i need to somehow break the loop on the output side of the attenuator? I thought i read someplace that a small value resistor to ground would be enough to break the ground loop, yet still provide enough of a return path. But will this then add to the loss through the box??

So if we have the Input of the box going to pins 1 & 3 of the pot, and the output from the Wiper and then what should we do with the output ground? 100 ohm resistor to Pin 3 of the pot and the output jack ground???

Please Advise
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Old 12th March 2006, 03:17 PM   #2
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Default Re: Grounding in a Passive Line Stage

Quote:
Originally posted by Zero Cool
tying the In L-R and the out L-R grounds together.


Why do you have to do that?
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Old 12th March 2006, 03:43 PM   #3
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Default Re: Re: Grounding in a Passive Line Stage

Quote:
Originally posted by analog_sa




Why do you have to do that?
To complete the signal path.
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Old 12th March 2006, 04:02 PM   #4
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don't get it. no need for any connections between L & R - you can treat the attenuator as separate mono attenuators for each channel. Or am i not getting your words right?
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Old 12th March 2006, 04:21 PM   #5
AndrewT is offline AndrewT  Scotland
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Hi,
keep your signal returns separate from each other and separate from the enclosure. Insulated RCAs sound right.

Try using screened 2core for each channel with the screen connected at one end only.
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regards Andrew T.
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Old 12th March 2006, 04:54 PM   #6
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Sorry i didnt make it as clear as i should have. Yes the Left and right sections are treated seperatly. The left input ground is tied to Pin 3 of the pot and the output ground.
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Old 12th March 2006, 05:08 PM   #7
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Then you don't have a ground loop. Seek the explanation for the hum elsewhere.

Btw, AndrewT makes a good point about the jacks - are they really insulated? Is there hum if only one channel passes through the pot?
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Old 12th March 2006, 05:19 PM   #8
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At the moment i dont even have jacks, i just have a cable running in and out of the box. and the hum is still there if i run just one channel.

When i got my C-J amp, it had a ground lift adapter on the power cord. The ground loop seems to be cause by the grounds of the audio gear and the audio cables. lifting the ground of the amp makes it go away.
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