TIL: The little phatty has an analog signal path from pots to synth circuts

I read a reddit thread this morning about a guy that is doing a synth controller front panel. He had the novel idea of switching between using the pots directly to feed an analog voltage to the synth circuit (filter cutoff cv etc) and using voltages from a DAC to do the same. By simultaneously reading the voltage from the pot for patch storage, it would then be possible to load a patch and have a dac generate the required voltage to control the synth, but switch to direct-from-pot cv when the user turns the pot to prevent stepping.

The original poster was immediately beaten down by the knowitalls of Reddit, saying that this was overly complicated and that noone would do it like this.

He countered by claiming that he had looked at the circuit board of a Moog synth and seen analog multiplexers close to each pot, and rationalised that these did what he was thinking about. He also claimed that a video told him that this was the way Moog did it.

Another poster then proceeded to post the schematics for the Slim Phatty, explaining how it was clear to see that the circuits were indeed digitally controlled.

I had a closer look at the schematics and discovered that the original poster was indeed right (though possibly not about exactly how it was done) and the one posting the schematics wrong - the Slim Phatty (and by extension Little Phatty) DO actually have the ability to switch between direct control and digital control. Have a look at this:

The filter pot. Its value is buffered by an opamp (U29), which is then both tapped directly (FILTER_CV) and sent to an ADC through AINO.2 for digital processing/storage.

Digital control: CV from the DAC is buffered by a sample and hold circuit and output as FILT_CUTOFF_BUF

A switch then selects between the CV tapped directly from the pot (FILTER_CV) and the digitally controlled voltage (FILT_CUTOFF_BUF). Notice how FILTER_CV is sent to multiple outputs, depending on what mode is currently selected for the pot.

The selected voltage is sent to the board connector as FILT_CUTOFF...

...and received on the voice bort as FILT_CUTOFF (pin 33)

There, it is mixed with CVs from other sources (modulation etc) into the filter cutoff CV FILT_CONTROL

Finally, FILT CONTROL controls the cutoff frequency of the ladder filter. Thus there is a completely analog signal path from pot to filter.

When I went back to post my findings on Reddit, the original poster had already done so. He pointed out that this is called RAC (tm) in the Slim Phatty manual - "Real Analog Control".

The discussion went on to saying that this must be a marketing trick more than having real value, and funnily enough saying that it had do be something only the phattys use because "the memorymoog and prophet 5 does not do it". I call bullshit on this.It may be true that it is only found on the phattys, but comparing it to 25 years older synths is no way of proving it...