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

[Vince Werber]

The individuals 'hums' can cancel each other out when mixed iff..  both 
hums are 180 degrees out of phase with each other and are of the same type 
and intensity...

The terms 'singing capacitor' has been keeping me out of this 
discussion...  perhaps the correct phase should be 'microphonic' 
capacitor?  The only time I have  known a capactitor to sing is via 1,000 
plus volts and the  cap  is dried out and  is well  into a  need for 
replacement...  the term explosion works here... :-)

A 'microphonic' capactor is noticable from vibrations from an outside 
source that causes the cap to 'ring'

In any sense though both conditions come from inferrior components,,, 
ie... dried up  floor sweepings...

vince



ka1iic at prexar.com

> 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
>