Transistor compressor here

[StephenGiles]

http://www.spi-electronics.nl/products/schematics/19379800.pdf

[gez]

Thanks for the link, that's interesting.

[R.G.]

I believe you can replace Q1-Q7 and associated circuitry with an opamp. The gain change is done by changing the current through Q9 by D7-8 acting as a halfwave doubler, feeding charge into C9 and then having Q8 buffer that voltage into Q9.

The envelope generator is similar to half of the Dynacomp, although the bipolar as a gain change element is different.

I suspect that you have to watch the audio level pretty closely. Current dependent diode resistances (this is one) have to keep less than about 25mv of signal on them to keep distortion to acceptable levels.

[gez]

Isn't there some -ve feedback when Q9 is turned on and isn't it this that causes the attenuation?

Probably totally wrong here, but that's what I found interesting about the circuit.

[R.G.]

Isn't there some -ve feedback when Q9 is turned on and isn't it this that causes the attenuation?

I don't believe so. The signal goes through the two 220nf caps, is shunted to ground to some variable degree by the diode resistance of Q9. There is no path from this Q9 voltage back into the signal path any other way. The rectifier/filter/Q8 circuit doesn't pass signal back into the audio path that I can see.

The input path goes to the non-inverting input of the equivalent opamp, and the feedback path for the equivalent opamp has no connection to the Q9 circuit. Looks like a standard attenuator in front of an amplifier, with the amplifier output driving the attenuator to attenuate harder with bigger signals.

The opamp runs at a forward gain of 150 (15K/100r) to make up for the attenuation of the input 22K/Q9 diode.

[gez]

OK I've got it now, thanks.