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