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Old 4th February 2011, 12:49 AM   #1
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Default Balanced output confusion

I have a slight 60hz buzz in my new preamp. It is just loud enough to drive me nuts, but not bad really. It is a differential circuit that I am running into a single ended amp. I have been toying with the idea of going to a true balanced amp. How do I test my preamp to make sure the change to a balanced amp will remove what I hope is a common node buzz. The more I think about it the more I get confused. I have a computer based two channel oscilloscope and another balanced preamp. Any suggestions would be appreciated.

John
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Old 4th February 2011, 01:14 AM   #2
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Could be an earth loop problem.
Try adding an earth connection to the ground line.
Or try removing the earth from the ground line.
Dont disconnect the earth from the chassis.
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Old 4th February 2011, 10:28 PM   #3
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It really depends on the input and output circuitry and how you have them connected. Are you using one phase and ground from the preamp or tying one of the output phases to ground at the amp. You could always use a trafo at the amp input.
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Old 4th February 2011, 10:49 PM   #4
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The easiest way is to unplug everything from the input to the amp, if the noise goes away then it's probably a ground loop and going to full balanced should cure it.
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Old 6th February 2011, 03:46 PM   #5
DF96 is offline DF96  England
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Can you see the buzz on the output with your 'scope? Is it common-mode or differential-mode? Does the preamp have a true balanced output, or just a rough approximation such as a phase-antiphase output pair? Can you ground the other output phase instead?
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Old 7th February 2011, 02:10 PM   #6
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by DF96 View Post
Is it common-mode or differential-mode?
How do we test for this?
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Old 8th February 2011, 09:33 AM   #7
DF96 is offline DF96  England
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Put the scope on both outputs simultaneously. See if buzz signal is in phase or inverted on one of them.
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Old 10th February 2011, 10:43 PM   #8
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Well I put both channels on the scope and they were in phase. I hooked it up to my other pre amp that does not have the same noise issue and it was gone when I hooked it up balanced but there with SE. I am 90% sure that it is an issue involving poor placement of the transformers. I am going to do some shielding experiments and see if it helps. The pre is a true balanced design, assuming that an ARC ref3 is a balanced design.

Thanks all
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Old 11th February 2011, 11:20 AM   #9
DF96 is offline DF96  England
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If it is in-phase (i.e. common mode) then probably caused by grounding. If there is an output transformer then grounding is about the only possible cause. If the balanced out comes from op-amps etc. then it could be PSRR within the circuit, or internal grounding issues.

Common-mode noise will always disappear when connected to a true balanced input.
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