Trying to understand peak limiter circuit.

[armstrom]

I ran across this peak limiter circuit and was trying to understand how to control the max output voltage. The author gives guidelines on adjusting the attack and decay times but not the limit voltage.

http://sound.westhost.com/project67.htm  (circuit in figure 2)

My ability to understand what is going on in this circuit is severely limited so I was hoping someone here could shed some light on how the limit voltage is set. Does it have anything to do with the forward conduction voltage of the diodes? I would like to be able to change the max output voltage to something closer to 1V RMS rather than the 1.65V RMS that it currently has. Of course, I could always take the easy way out and put a voltage divider on the output to get the max voltage down where I want it to be, but that seems like a bit of a hack
-Matt

[Enzo]

WHy is that a hack?  That woud be exactly what we would do after a gain stage in a guitar amp to control the amount of signal.  In fact we usually put them on the panel and label them.

D2 and unity inverter U1B with D1 form a full wave rectifier that turns the signal into a DC voltage lightly filtered by C5 as an average signal level indicator.  That voltage in turn runs the FET.  All the FET does is ground off the incoming signal through C2, at least to the extent that it is turned on.  SO you want the FET to turn on earlier - at a lower signal level.  This is not my forte, but looking at it, it seems to me you could massage the value ratio between R4 and R6 to set the operating point of the FET.

[J M Fahey]

Agreeing with Enzo and adding: put a 10K trimmer pot in that output and adjust it to the *exact* point-just-before-clipping of your power amp, which I assume is your interest. As a bonus, whenever anything changes (speaker type/impedance, outlet *real* voltage, etc.) you'll be able to adjust it instantly, no soldering involved.
Please build it and tell us how it worked.
Juan Manuel Fahey