Assuming the mic preamps are actually balanced(not all mixers with XLR
inputs have balanced preamps!) since you said you used two different
mixers, then it is most likely cheap mic cable.

In a past life I did engineering in 24 track studios and saw every
possible problem.

Cheerful regards,

Bob


Mark E. Skeels wrote:
> Greetings, PICsters.
> 
> I have a friend who has a small personal recording studio consisting of 
> a rack of various preamps and effects devices, and a smalll TASCAM 2 
> channel all in one hard drive recording unit.
> 
> He lives in a multi-unit apartment building; maybe 6 to 8 apartments in 
> a two story residential structure.
> 
> Some time ago he began to experience periodic 60 Hz hum in his 
> headphones and on his recordings.
> 
> I stopped by last night to take a look.
> 
> We disconnected the rack units and all other outputs/inputs so that only 
> the mics were connected directly to the recording unit for test purposes.
> 
> We isolated it to the compressor cycle of a window air conditioner in a 
> nearby apartment.
> 
> It occurs ONLY when the compressor cycles on, not when the AC fan runs 
> alone.
> 
> It occurs ONLY when a mic is plugged into the unit, not when the inputs 
> are unused (no source connected).
> 
> It does not occur with any of the other window air conditioner in any 
> other apartments, as far as we could tell.
> 
> Moving the AC power line plug to other outlets in the apartment does not 
> improve the situation.
> 
> The AC hum can also be experienced with a Yamaha powered mixer and with 
> any mic, either dynamic or condenser.
> 
> It does not occur in another location, ie, at my house, say.
> 
> The hum is not a small background hum; it's presence is significant and 
> can easily be heard along side the desired input signal.
> 
> BTW, the mics are professional, balanced low impedance types using the 
> balanced XLR inputs.
> 
> I am guessing maybe a loose screw on an outlet or some other undesired 
> resistance maybe in the neutral?
> 
> Maybe some kind of fault with the air conditioner itself.
> 
> Do you have any advice for troubleshooting this problem or any ideas 
> about the failure mechanism?
> 
> Thanks,
> Mark
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