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
I also ran into some problem with my laptop. Those things are known to emit a lot of EM waves which easily get caught by cables transmitting asymmetric signals. When I switched all my connections between my sound card (RME) and my pre-amp (TL-Audio) to symmetric XLR, the hum disappeared.
__________________
Marc-Olivier Barre,
(Markinoko).
