Signal Presence Detectors?

[Don Tillman]

Hi.  Does anybody have a schematic for a really good signal presence
detector? 

This is a feature you can find on some quality mixers, a little green
LED that lights up when the signal level on an input is above about
-20 dB.  It's very nice to have a visual indication that a signal is
arriving at an input.

I think this would be a *great* feature on any synth module.

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.

  -- Don

-- 
Don Tillman
Palo Alto, California, USA
don at till.com
http://www.till.com