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
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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Open circuit cable is perfect for picking up hum, even balanced cables pick it up and the input cmrr is not infinite meaning that some it will get through.. Connect the + and - together at the far end or connect to a source that is powered. Should be much quieter.
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.
The XLR shell on the cable is not connected to anything because of possible accidental contacts in the field.
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