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Old 1st Jan 2012, 9:58 pm   #8
Marconi_MPT4
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Join Date: Mar 2011
Location: Dorset, UK.
Posts: 519
Default Re: AKG C60 valve mic

Hi Chris,

If the hum was nearer 60Hz then maybe look for one open circuit diode in one of the bridge rectifiers or an earth loop with connected equipment?

Now, if hum is really 40Hz then maybe some form of instability is occurring, however the valve is just configured as an impedance match so would not normally expect this type of problem.

Does the hum change when the LF gain switch is moved to each position?

Is there a switch on the PSU to select local ground (earth) or equipment earth?

Cheers

Rich
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