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...

Both orders from JLCPCB has arrived!

Finally, after waiting for over two months, the second order from JLCPCB arrived yesterday. I now have loads of circuits to test:

- JP6-style SVF
- Juno-style OTA filter
- Moog-style ladder filter (but this requires some soldering)
- 4ch mixer
- 4 way VCA
- Three types of CV/sample and hold
- Noise
- VCO, CEM3340 w/waveshaper
- DCO
- Waveshaper
- Led ring

The quality is overall very good, though I have found at least one solder bridge and and one of the led rings appear to have been slightly reworked (oh, and I've already broken a led, but that was my own mistake).

Some pics:

4ch mixer and VCA, with room for v2164 or similar

Top: Waveshaper. Bottom: Ring modulator and 4ch CV buffer

VCO, DCO, two versions of S&H for CV buffering, DAC.

Moog filter with overload

Not much to say really, added overload feedback w/vca. Specified single/multiturn pots. Almost ready for computer control.

CA3046 - it's a trap!

I spent the last few days breadboarding the moog filter, and got it mostly working yesterday. But cutoff did not work as expected. On the scope I could see that the wave amplitude got slightly smaller when turning the pot counter clockwise, but at the same time the centering of the wave dropped (adding a negative dc component) and the wave got distorted.

After a lot of debugging I switched what transistors in the CA3046 I used for the bottom pair of the ladder, the one with the common emitter. I had chosen not to use the internally connected pair because of the way I breadboarded the circuit.

This fixed things. Studying the datasheet for the CA3046 I discovered this:

Pin 13 is connected to the substrate, and should be connected to the most negative part of the circuit!

This means that the transistor between pins 12-13-14  CANNOT be used as a normal transistor after all. Switching it for 6-7-8 worked well. Instead, I tied 13 to -15V. I am using two more CA3046s on the breadboard and have to rewire them too to free up pin 13. I am having an issue where the DC component of the signal increases as the cutoff drops, not sure if that's related.

Anyway - I remembered that I have intended to use a different dual transistor for the Xonik VCO. Looking through my parts box tonight I found these:

BCM847BS
BCM857BS

They are matched dual transistors. They are however in tiny SOT-363 packages. For the 'production' version of the filter they pose no problem but they suck for prototyping. Still, they may be a good replacement for all transistors in the filter.

Update: They also come in SOT-666 and SOT-457/TSOP-6:



TSOP-6 seems a little easier to handle so perhaps I'll look for that instead.

Moog ladder filter revised

After all the work I did on improving the range and controls of the Juno filter, I decided to have a closer look at the Moog filter. I have improved the following:

• VCA CV is now similar to the Juno - it has an adjustable deadband at the start and a summing point for multiple CVs

• Resonance CV is similar to the Juno in the same way

• Cutoff CV summer has been altered to give a 10Hz cutoff from a 0V CV, with +/- 5.6 octaves trimability, fairly similar to the Juno.

• I have added a tempco resistor to the exponential converter.

• I have added an option to tap the audio output at 12dB/oct instead of 24. Resonance is still tapped at 24dB to get a nice high resonance. No switch has been drawn but either an electronic or mechanical DPDT switch will do.

• I replaced the output buffer with a transinductance circuit and instead flipped the inputs to the VCA OTA to keep phase. This fixed the maximum cutoff frequency similarly to the Juno.

• I added a linear FM input to the cutoff frequency.

In addition to all this, I've completely changed the signal attenuation/gain to enable overdriving.

Overdrive 

The Memorymoog schematics indicates that a 40mV p.p. at the transistor base of the bottom ladder transistor is the 'norm'. My simulations show severe distortion at this level. I could see distortion all the way down to 10mV p.p. I have suggested three component values that will work for 5, 10 and 20V p.p. input (with a 15mV p.p. base voltage at those values). This gives a slight distortion at the highest inputs, and further distortion should the input exceed the specification. For example, a 10V p.p wave maxes out at about 16V p.p even with much higher input.

How the overdrive sounds in practice is still unknown.

Input (blue) vs output (green), input is 8V to 30V p.p

Computer calibration

It would probably be possible to tune most parameters of the filter using lookup tables in the computer control. Here are some thoughts on that:

- If no deadband is needed (i.e. 0V input to the VCA CV gives an 'off' output), the U17 trim pot and R41 may removed. Keeping R41 but connecting it to -15, and changing the value to 1.5MOhm will give a 100mV deadband.

- The U13 VCA gain potentiometer can be replaced with a fixed resistor that gives a more-than-unity gain at max CV. By doing this, the VCA CV can be calculated in software to give a correct curve with a end point that results in unity gain.

- Similarly, U20 can be chosen in such way that it results in a resonance higher than what is wanted, and CV then corrected in software for max resonance.

- I would probably still keep the tracking potentiometer, but R36 can be replaced with a lower value, giving a lower frequency as a starting point. Then you could replace R32 with a 25k resistor to get 20 octaves of CV control, enabling proper calibration of lowest and highest cutoff.

- It IS also possible to replace the tracking pot U9 and resistor R23 with a 100k resistor. Tracking could then be adjusted in software.