I guess you may have already covered this one but have you eliminated 'earth
loop hum' This can occur due to multiple earths on the audio section. We
often found this was due to an earth on the input to a power amp as well as
the main star point within the mixer. It can also be intermittent and
illusive when you are trying to trace its source.

Steve

-----Original Message-----
From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf Of
Dave Tweed
Sent: 06 September 2008 16:12
To: piclist@mit.edu
Subject: [EE] recording studio hum

Mark E. Skeels wrote:
> 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.

It sounds like you've covered most of the obvious paths. Now you need
to look for the more subtle things. I once attended an AES presentation
by Bill Whitlock of Jensen Transformers about fixing problems like these
in audio systems, and he really knows his stuff. He explains it better
than I can -- check out some of his white papers here:

   http://www.jensen-transformers.com/apps_wp.html

Some aspects of your story are puzzling -- for example, the hum only
appears when a microphone is plugged in, but occurs identically (I presume)
with any type of microphone, any type of cable, and any physical position
of the microphone and cable (?).

This indicates that the power line is the source, but the coupling occurs
somewhere in the microphone preamp, or perhaps in the phantom power supply?
Something is unbalanced somewhere.

It it truly a hum, or more of a buzz? In other words, is there a
significant 60 Hz component, or is it mostly higher harmonics?

My guess would be that the air conditioner controls the compressor with a
triac, and this is producing some high-frequency harmonics on the power
line. The compressor motor itself should be an induction motor, and not
producing harmonic distortion. A brute force low-pass filter on the AC
line, or maybe a ferroresonant transformer, would help confirm this.

-- Dave Tweed
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