60Hz hum problem, please help

wl336

Member
2011-04-07 12:45 am
I got a strange 60hz hum problem in my newly finished preamp. The preamp remains very quiet (a few uV of noise) with input open until I just plug the xlr cable in, with nothing connected at the other end, I got a big 2mV 60hz hum. XLR pin 1 is connected to signal ground and outer shell contact connected to chassis ground, two ground are connected through a CL60 thyristor. Could anyone help to identify what might be the problem? Thanks

debug list:
1. no cable connected, open input ---- quiet (I guess this means internal layout is fine)
2. various xlr cable connected, other end open ---- 2mV 60hz hum at output, channel specific (i.e. left channel input cable only cause left channel output hum)
3. various connected to prism dscope signal generator output (50R output impedance) --- hum persists, almost no change in amplitude ( originally the chassis connector housing is left floating, so I ground them to chassis ground, but this does not change anything
4. xlr cable connected, other end all short ---- quiet
5. xlr cable connected, pin 1-2 short ---- hum drop half
6. 1-3 short ---- much quieter, around 200uV
6. xlr cable connected, pin 2-3 short --- still 2mV 60hz hum
 
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The XLR connector pin #1 is not part of the audio circuit and is not connected to the circuit ground (at least directly). The pin #1 should be connected only to the chassis and that's wire a very shout wire.
The XLR shell on the cable is not connected to anything because of possible accidental contacts in the field.
 

wl336

Member
2011-04-07 12:45 am
The XLR connector pin #1 is not part of the audio circuit and is not connected to the circuit ground (at least directly). The pin #1 should be connected only to the chassis and that's wire a very shout wire.
The XLR shell on the cable is not connected to anything because of possible accidental contacts in the field.

connecting pin1 to chassis only worsen the problem