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Old 21st June 2012, 08:38 AM   #1
diyAudio Member
Join Date: Aug 2006
Location: Copenhagen, Frederiksberg
Default Potmeter to balanced amp= Hum; Help!

Hello gentlemen,

I need to attenuate my balanced power amp: ATC SPA-200, and I have tried inserting a potmeter between the unbalanced pre and the power amp which has an rca-to-XLR adapter.

The result is HUUUuummmmmmm...

The path is like this:

Pre(rca) -> potmeter -> rca_to_XLR adapter -> power amp

There is a description of the setup and problem here also:
Help(tri-amping): Too much gain in my ATC power amp

I really hope someone here can help out.

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Old 21st June 2012, 09:07 AM   #2
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Join Date: Apr 2011
Location: Seaside
Hi Kris,

Could you please include a drawing of how you inserted the potmeter?

An FFT of my life would show a couple of peaks and some low level harmonics, but mainly random noise.
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Old 21st June 2012, 11:57 AM   #3
AndrewT is offline AndrewT  Scotland
diyAudio Member
Join Date: Jul 2004
Location: Scottish Borders
PIN1 of your Amp input is almost certainly connected to chassis. That is where PIN1 should be.

Be careful your adaptor cable and your pot don't try to connect signal ground to Chassis. That will almost certainly be the cause of your HUM.

If PIN1 is not connected to chassis, then download all the info you can find on the PIN1 Problem and decide whether to modify your balanced Power Amp input to the now recommended wiring.
regards Andrew T.
Sent from my desktop computer using a keyboard
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Old 21st June 2012, 12:56 PM   #4
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Join Date: Aug 2006
Location: Copenhagen, Frederiksberg
Default PIN1 issue

Thanks for the preliminary input.

AFAIK, ATC has a 47 ohm resistor between PIN1 and the chassis, but I am not 100% certain of that.

I will revisit the layout of my potmeter and adapter cable.

Question: Would a balanced l-pad (U-pad) be the way to go? I might need ~ 9 dB attenuation and then I can dial in the other two amps with their respective pots. (Be aware that the pre is unbalanced)

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