Common Base Confusion

Well, as you say, it's a common-base configuration. These are used as current buffers and voltage amplifiers, having a relatively low input impedance and little current gain.

In this case, it looks to me as if it's being used as part of a level-shifting circuit, taking the output of the A7 flip-flops and shifting it to the voltage levels needed to turn JFET Q on and off. When A7B's Q output is high (TTL-level), this will turn Q4 on; most of the current flowing through A6B Q and R44 will flow through Q4, pulling Q4's collector up and raising Q5's gate voltage upwards, switching Q5 on. When A7B's Q output is low (TTL) and is at ground voltage, Q4 switches off, and R51 pulls Q5's gate down to the negative rail voltage and turns Q5 off.

This wouldn't work without Q4 (i.e. connecting A7B Q more directly to Q5) because A7 can't pull its Q output down to a negative voltage level (and would probably malfunction if a passive pulldown to the negative rail were included.)

All of this allows Q5 to be turned on and off by the octave divider, altering the behavior of the feedback loop around A2B, changing the gain and perhaps even the phase of this op amp's output.

The Wikipedia article on common-base amplifiers may give you some further (or more correct) hints.

s.

Output of the CMOS gate is 0V or 12V (my assumption is that the CMOS is run from a +12 supply)

They want a signal going into the FET that's a negative voltage.

The PNP common base stage does this pretty nicely.

Tim.

A level shifter... for an unknown audiophool reason. ...Jim Thompson

Thanks very kindly for your explanation.

I was a little confused on why they had the "octave down" signal switching the non-inverting input of the op-amp on and off with the JFET instead of just summing the two signals - I believe it's because they want the volume of the sub octave to track the input level and that's how they go about it. Cheaper than a tracking VCA.

The similar circuit here:

has an interesting circuit for tracking the input frequency before going into the octave divider using precision rectifiers as peak detectors. Still, there are complaints that it doesn't always track the fundamental of a guitar signal very well.

If I were to design such a circuit in analog I think I'd comb filter the input first, and then use a tracking VCF followed by a limiter to really lock on to the fundamental.

Here's how I (in 1979) captured the "volume" (envelope)...

This worked so nicely that even classical music was improved by inserting the missing sub-low-end.

In the early '80's I installed one of these in Bobby McGee's...