Moog Sonic Six

An Unusual 1V/Oct Exponential Converter

I drew just the linear CV -> 1V/Oct exponential converter circuit of the Sonic Six to compare to other converters.

The tuning procedure for a Sonic Six involves 3 internal trims in addition to setting the panel controls.

Well, the "C-A" trim is quite odd, as we are simply adjusting op amp offset. Hmm. I don't quite agree that this should have anything to do with tuning. It should simply be set for no offset or ignored.

The "HI-A" trim would be labelled "SCALE" by most designers.

The "LO-A" trim might be called "RANGE" or just "TUNE" as it is simply an internal coarse frequency adjustment.

Next time I service one of these, I am going to try simply tuning this keyboard like I would any 1V/Oct keyboard with just the SCALE and RANGE adjustments.

The actual "HI" adjustment would be if you could vary the 10M feedback resistor. I may try swapping that out for a smaller resistor plus a series trim pot and calibrate like any "HI FREQ" compensation trim.

The last Sonic Six I tuned up went very well. I was quite satisfied. The repair started with a failed -15V regulator (or was it the +15?), and moved on to a couple of the OTAs, and then it was calibration time.

I have read or heard stories that these are "notorious" for not tuning and/or being difficult to work on. I'm not sure I agree with this, but I have only seen maybe a half dozen of these so far.

It does use a µA726 transistor pair (µA is pronounced "micro amp") which has become unobtainium since it was discontinued decades ago. I can see that being a bummer if you suspect yours is broken or faulty. I haven't had that issue yet.

It might be feasible for someone smarter than me to come up with mods to convert a Sonic Six expo converter to a hand matched BJT + tempco type circuit. Might get a little ugly on the board, but I think it could be kludged in. Something to ponder anyways.