[linux-audio-user] Pre-Amp hum (related to jack, somehow)

[Carlo Capocasa]

The irony of this is killing me. I had a singing capacitor in my
notebook computer, which I was able to solve using a load generator at a
low scheduling priority. However, that same capacitor is what had
triggered me to get a higher-end sound card in the first place.

Now this higher end sound card has quite a loud hum when the phantom
power for its mic pre-amps is on. Interestingly, when I mix the signals
from both pre-amps by connecting them both to the same outputs in JACK,
the noises from the Mics cancel each other out.

The load generator has no influence on the hum; however, the sampling
rate does. The higher the sampling rate the higher pitched the hum is.
The inverse is true for buffer size: The larger the buffer, the lower
pitched the hum.

My current theory is this: Jack causes a surge of power in the CPU
through an interrupt. This surge gets transformed into current in the
grounding cable through electromagnetic induction, which in turn induces
a current in the microphone pre-amps, which causes the hum. The hums can
cancel each other out because... uh, not the slightest on that one.

Help appreciated.

Carlo