Signal Presence Detectors?

[Martin Czech]

:::I'm a stickler for audio quality though, and I'd be interested if
:::there are known good ways to do this without creating any noises,
:::distortions or spikes on the power supply lines.  So running the input
:::signal through a gain stage, rectifying it, and directly drving an LED
:::would not be a good approach.

why? As long as you don't allow the gain stage to saturate (clamping)
the supply current should only change very little with audio
signal. So the gain stage allone should be no problem.
Or any other op amp in your circuit will give you the same problem
(ok, say gain stage, for all op amp haters ;-=>)

However, as soon as you run a comparator this will be the case.
The largest current thump will be of course by the LED.
Do you remember the days of ECL logic? It is switching, but supply
current is almost constant. Because it's differential.
This would mean: make the LED driver symetrically, a dummy
LED and the real one, one of them is always active.

OTOH you could choose a soft turn on/ turn off circuit,
it is not the current that bothers you, it is di/dt.

But certainly separate power and ground for the detector will
be the best. If you really have problems you could use an opto
coupler, linearity is no requirement here.

There are basically three ways of interference:

-galvanic: separate all lines, ie. power and ground
-electric: relatively easy to shield with metal, or use slow slopes
-magnetic: difficult to shield low frequency, so distance or slow slopes

Sorry for beeing academic.

m.c.