Little Phatty overdrive

I can't remember if I've written about this before, but I've certainly looked at it.

EDIT: Turns out I did, in great detail! https://atosynth.blogspot.com/2024/01/moog-overload-circuit.html

I'm trying to retrofit overdrive to my ladder filter, so I had a closer look at how the little (and slim) phatty does it. The slim phatty schematics are available online but the circuit is spread across multiple pages so it's a bit harder to see what is going on.

Here is a simplified schematic:

 

There are a few surprises here:

- there is no feedback from the filter output to input, which is the way the minimoog achieves overdrive.

- it uses a pretty standard soft clipping circuit with CV-controllable clipping - the overdrive of the minimoog filter, if I recall correctly, happens in the transistors of the differential amplifier at the end of the filter.

- the output of the overdrive is fed back to the oscilllator mixer and returned to the soft clipping circuit

- the mixer is not a normal inverting summer op amp - it's just an op amp buffer, with all inputs connected to the positive input. This is possible because the OTA outputs are current, not voltage outputs

- the differential amp of the filter is realised using op amps, which won't give soft clipping when overdriven. 

- the filter has an additional output gain OTA.

It's a pretty neat circuit, and all three OTAs - feedback, distortion and post-filter gain - are driven from the same CV. 

 All CVs are biased in various ways, to +5 or -5, I haven't studied exactly how they work in conjunction. Neither is it clear to me if the post filter OTA contributes to the overdrive in any way or if it just makes up for lost gain during overdrive.

 

Major take-away

Distregarding the post filter gain, all distortion happens before the filter, just as I'm currently doing in my synth. I don't have the additional feedback OTA though, whatever that does. 

 

Moog overload circuit

Since my previous posts I've gotten some feedback from the Modwiggler forums about the breakdown I see when overdrive/feedback is high. The answer is that this is normal for positive feedback, which I guess is ok. I've since chosen to abandon the per-filter overdrive in place of my own pre-filter distortion.

I've taken a closer look at the Little Phatty (LP)/Slim phatty overload circuit to see how the professionals do it, and to see if the circuit exhibits the same behaviour.

The LP overload is a combination of two things - first, a voltage controlled distortion that puts an OTA in the negative feedback of an op amp, basically acting as a voltage controlled resistor. This is the same solution I'm using for my distortion. But then it also has a second OTA that feeds the output back to the oscillator/source mixer. This is positive feedback, much like what I'm doing on the filters.

Overload circuit uses two OTAs

In addition to the two OTAs in the overload circuit, the overload CV controls a VCA after the filter, more about this towards the end.

LP CV generation

The CVs on the LP is generated by a DAC8581. This is a bipolar DAC that outputs +/- a ref voltage. The reference voltage is 4.096V. The voltage is then fed through an op amp gain circuit, which has a trimmer in the negative feedback. I've simulated this, and at the extremes the trimmer has an output of around 4.7 to 5.5V. Thus, I assume the trimmed output is supposed to be +/-5V, at least this makes it easier to reason about the later parts of the circuit.

 

Oscillator VCA

Each oscillator has a VCA controlling the level into the source mixer. At +5V VCA CV the output sees approximately unity gain.

OSC1 VCA cv -5 to 5V vs output

OSC1 VCA input vs output, approximately unity gain

Distortion

The distortion in the circuit is done using two diodes in the feedback of an op amp. This is a very common scheme and gives soft clipping. An opamp controls the feedback amount. 

 

Approx 3 x gain from OTA in distortion op amp, feedback to mixer disconnected

-5 to 5v overload CV. Unity gain in distortion circuit when CV is -5, feedback to mixer disconnected

 

 

Feedback

The output from the distortion circuit is fed back to the source mixer (and then gets distorted again and again and... You get it). This increases the distortion and also the amplitude of the signal fed into the filter.

-5 to 5v overload CV, feedback to mixer connected

Breakdown

As for the big question - do we see the same issue here as in my filter-overdrive circuits? Yes we do! At small input amplitudes, < +/-0.6V input, we get the same breakdown/railing. It's kind of comforting to see that there's not something magical going on in the Moog overload. The reason it doesn't show up earlier is, I assume, that the amount of feedback is not as high so the effect appears much later.

0.4Vpp sine is railing

5Vpp sine is ok

Post filter VCA

As mentioned at the beginning of the post, the overload CV also controls an output VCA. At -5V overload CV the VCA has a gain of 1.6. Increasing CV to 5V gives a gain of 2.3, adding some additional oomph to the signal. Not sure exactly why they do this. Also, when looking at the overload in a previous post I did, it does not look like overload increased the signal amplitide by much. I really expected this VCA to attenuate the signal when overload increased, but it doesn't seem to be the case

Filter output VCA - from 1.6 x gain to 2.3 x

Distortion circuit mostly ready for production

I've finished v1 of the distortion circuit PCB:

I still have to double check the schematics, I've done so many silly errors lately...

The board has switchable soft/hard clipping, and CV adjustable distortion and output gain.


Little/slim phatty distortion/overload

Before I send this one to production, I will breadboard and try out the distortion circuit from the little phatty:

It looks fairly similar, but the output, labeled audio_sum, is actually connected to a point BEFORE the input, labeled mixer_out here. This means it actually feeds the result back to the start, not only distorting it (there is only buffer and voltage divider between though). Also, the overload_cv controls an additional output VCA located after the filter. I can't tell from the circuit diagram what the gain / level is at various points of the circuit so I will simulate this.

Little Phatty overload is just soft clipping

From the time I studied the Little Phatty to build my moog filter, I have wondered if the LP does pre-filter distortion of the signal or if it uses feedback for its overload.

From the Little Phatty schematics it is clear that overload is just soft clipping, and the circuit is extremely similar to what I did for my distortion with in-feedback-loop OTA :-D

My circuit: http://atosynth.blogspot.com/search/label/Distortion

The LP:

There are a couple of differences: The moog circuit uses both inputs of the OTAs, and also the same CV (though inverted) for both increasing the amount of distortion and reducing (?) the output gain. A nice trick to keep output at the same level I assume.

The overload CV also controls gain/attenuation of the filter output it seems.

UPDATE: I missed something - in addition to the soft clipping, the output from the distortion is fed back to the audio mixer, thus the distortion also has feedback.

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...