switch current noise

[Martin Czech]

I bought a AKG "studio headphone" which is indeed much better than my old
Sennheiser. Now I hear more nasty effects.

In my Roland M480 mixing console and Peavy ProfexII there are input level
indicators (LED). They are made so stupid that as the signal fades away (e.g. piano)
there is some switching point, where the LED-current is PWM modulated
by the signal. The result is a krrkrkrzzzz noise in the fading signal,
very annoing espacially if there are only few partials to mask the
undesired sidebands. 
I'll put a switch into the peavy, and switch the led off.
But in the M480 ?

I'll learn from this experience for my modular.

In the future  I will always limit the maximum frequency of LED-current
down to ~10Hz, by lowpass filtering the rectified input signal
before feeding the LED.
I'll take great care that huge current drains like LED, electromagnetic
switching devices (relais ?) etc. will have extra GND & supply lines
with extra regulation. 

So no reference coupling into the audio signal via parasitic gnd-line resistance,
and no ac coupling via parasitic capacitance.

m.c.