Princeton Chorus Limiter circuit

[Mark Hammer]

Interesting idea.  You can see it here: http://www.fender.com/support/amp_schematics/pdfs/Princeton_Chorus_DSP_Sc9A4.pdf

The limiter portion is in zone D6/D5.  Ignore the optoisolator in there for the moment, and you can see that it is essentially an inverting clipping stage with a 2+2 diode complement in the feedback path (although note that the diode pairs are not oriented the same way; more in a moment).

During one half cycle, if the output of that op-amp is high enough, there is a path provided by R6, R3 and the LED portion of the optoisolator (one schematic lists it as a vactrol VTL5C4).  During the other half cycle, the path is provided by D5, the LED and D4.  Note that each of those two 3-diode combinations allows signal to flow when it reaches the level of their combined forward voltage.

When that LED glows on either half-cycle, it results in the resistance of the LDR dropping.  Note that the 1meg "Limiter" pot is essentially a second 1M resistance placed in parallel with the 1M fixed resistor.  The way the LDR is connected to the pot we have a "nested" parallel resistance.  So, imagine that the pot is linear and set midway.  One side of the wiper is 500k and the other side is 500k.  If the LDR resistance is in the megohms, then the LDR plus the right side of the pot will still provide roughly a 450-500k combined parallel resistance.  That, in tandem with the 1M fixed resistor and the other side of the Limiter pot gets us a total feedback resistance of around 500k.

If the LED lights up and reduces the LDR resistance, though, the overall parallel resistance of the pot and LDR will drop.  If the wiper is pushed over to the right on the diagram, then most of the pot resistance is to the left and whatever reduction in LDR resistance occurs will have little impact.  Move the wiper to the left on the diagram, though, and now the LDR is placed in parallel with the full resistance of the pot, and whatever happens to the LDR will have an effect on the combination of pot+LDR resistance.  That, in turn, will drop the overall feedback resistance, and by consequence, the gain of the stage.  So, hit a note hard and the gain drops momentarily, which is what a limiter does.  Note that it does not really do anything to increase the level of low signals other than provide a steady gain.  Also note thatthe attack/decay of the limiting is set by the properties of the LDR.

Fender's own user manual indicates that the limiter will really only kick in when the gain is up.  I suppose the gain could be kept more modest if D3/4/5/6 were germanium or Schottky diodes with a lower forward voltage.

Interesting circuit.

[Eb7+9]

neat circuit block Mark,

there's a FW bridge nestled inside the loop of an inverting op-amp with virtual ground on neg input ... so you have a FW bridge loaded by the LED with one side of the bridge virtually grounded and the op-amp operating as a trans-resistance amplifier - so an input "current" signal is injected into the neg op-amp node ... nice way to combine both functions around one op-amp ...

in essence what Fender is calling a sufficient "gain" really alludes to a signal threshold and the limiter control is really providing a "rough" ratio control ... if you preceded this circuit block by a follower circuit (with low enough Zout) and a variable resistor in series the resistor would act as a threshold control relative to the turn-on voltages of the diodes and the LED (something around 3v total) and you'd have a stand-alone limiter circuit with thresh/ratio controls ...

as long as output of op-amp lies under +/-3v swing you won't get any clipping from the diode strings - and the LED has roughly a 200mV useful swing range before you get hard clipping from the whole thing - that's way late in the game ... in essence, you would set the voltage-current converting resistor against the strength of your instrument signal and the turn-on voltages of your rectifiers - hence you can use whatever rectifiers you wanted and set the "threshold" resistor accordingly ... but the way it's done here you end up with enough signal at the end so you can scale back down using a volume pot - hence, you have enough signal to give you "built-in" post-gain ...