MXR/Ross Compressor Diode Swap

[Khas Evets]

Would swapping the Si diodes to Ge diodes increase maximum compression by lowering the threshold? My thinking is the lower diode threshold would result in quieter signals passing to the 10uf cap.

[Kesh]

Would swapping the Si diodes to Ge diodes increase maximum compression by lowering the threshold? My thinking is the lower diode threshold would result in quieter signals passing to the 10uf cap.

It should lower the threshold, but this doesn't mean more compression. After all a volume control is a compressor with zero threshold.

The greatest perception of compression probably occurs when the threshold is right in the middle of the signal's range of levels.

[Mark Hammer]

Would swapping the Si diodes to Ge diodes increase maximum compression by lowering the threshold? My thinking is the lower diode threshold would result in quieter signals passing to the 10uf cap.

The outputs of the phase splitter (Q2 with the 10k resistor on each end) are combined at the base of Q5 to form the control voltage (and ultimately, control current) that governs the gain cell (CA3080).  Each of those phase-splitter outputs is half-wave rectified via a diode to ground, such that the half-cycle you want to keep passes along, and the half-cycle you don't want to keep is dumped to ground via the diode.

If the diode was in series with the signal path, then yes, your instincts would be somewhat right and more signal would be "preserved".  In this instance, the amplitude of the retained signal portion remains the same, largely regardless of diode type, because the rectifying diode is going to ground.

Even IF the diode was in series, keep in mind that the combined outputs are averaged by the 10uf cap to ground that ties the collectors of Q3/Q4 and the base of Q5.  So, you wouldn't see much increment to envelope signal once all that averaging kicked in.

But it was a good question to ask, though, since it helps to learns about when such things matter and don't.

[Kesh]

Would swapping the Si diodes to Ge diodes increase maximum compression by lowering the threshold? My thinking is the lower diode threshold would result in quieter signals passing to the 10uf cap.

The outputs of the phase splitter (Q2 with the 10k resistor on each end) are combined at the base of Q5 to form the control voltage (and ultimately, control current) that governs the gain cell (CA3080).  Each of those phase-splitter outputs is half-wave rectified via a diode to ground, such that the half-cycle you want to keep passes along, and the half-cycle you don't want to keep is dumped to ground via the diode.

If the diode was in series with the signal path, then yes, your instincts would be somewhat right and more signal would be "preserved".  In this instance, the amplitude of the retained signal portion remains the same, largely regardless of diode type, because the rectifying diode is going to ground.

Even IF the diode was in series, keep in mind that the combined outputs are averaged by the 10uf cap to ground that ties the collectors of Q3/Q4 and the base of Q5.  So, you wouldn't see much increment to envelope signal once all that averaging kicked in.

But it was a good question to ask, though, since it helps to learns about when such things matter and don't.

Only that part of the signal above the diode's forward voltage is dumped to ground. Signal below that is just recombined, and cancels to zero, so the diodes do operate as a threshold.

[Khas Evets]

That's what I was thinking. The 10uf cap's voltage is only affected when one of the diodes' forward voltage is crossed. So a lower forward voltage on the Ge diode would cause a greater voltage swing on the cap than say an LED diode. I think that would result in a lower threshold for the compressor. I'm not sure if that is any different than adjusting the sustain pot. I'm new to this type of circuit, and I may be missing the point by not looking at the entire feedback loop.