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

[Marc-Olivier Barre]

On 11/28/06, Carlo Capocasa <capocasa at gmx.net> wrote:
>
> 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).
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