Jupiter 6 All pass filter

This post

https://www.muffwiggler.com/forum/viewtopic.php?t=137105&start=all&postdays=0&postorder=asc&sid=3ef1fe17aeeff7d7fd87444b7188ac69

mentions that by mixing the input with 2 x the BP filter, you'll get an all pass filter. The same does this pdf:

https://www.analog.com/media/en/training-seminars/tutorials/MT-223.pdf

I tried this with some success. I had to disconnect the 33k resonance resistor, but after this I got a response with a slight 667 mdB notch.

I will try breadboarding this later.
The MT-223-pdf also mentions an alternative way of doing a notch filter:

Which gives this notch:

As opposed to the notch we get by mixing LP and HP:

Jupiter 6 filter - overdrive and multiple output variations

Overdrive

With a 3.9k input gain feedback resistor I could easily pass a 20V p.p. wave through the filter without distortion. But I wanted to see if I could get a distortion similar to the moog filter, and yes, I could.

Swapping the 3.9k resistor with a 33k makes the filter overdrive close to 10V p.p, quite similar to the Moog.

This is of course pre-filter amplification. I have seen people talking about the minimoog doing feedback of the original signal through the external input jack, and this sounding better, so I'll try that next. I also need to come up with a good way to control overdrive, one that is not so dependent on input amplitude.

Input (green) vs output (blue). 33k input resistor, 50k output pot

Output variations

This being a state variable filter means it can produce a multitude of filter variations at the same time - low pass, band pass, high pass and notch. It is also two filters after one another, which means we can get various falloff. I've played around with this and come up with 10 variations that are more or less usefull:

12dB LP
24dB LP
12dB HP
24dB HP
6dB BP + 12dB LP
6dB BP + 12dB HP
Notch (first SVF)
Notch + LP
Notch + HP
Notch + BP

I am not sure of the usefullness of all these but the cost to add them all is very little. Here is how I indend to wire them, with resistor values giving the following 'plateau' gain.

Constant current inputs

Just like with the Juno and Moog filters, I've swapped the resonance and vca gain CV circuits for my own, linear designs. The VCA gain  control is exactly the same as for the Juno (but with slightly different part values), and has a similar deadband. The resonance on the other hand, is different. The resonance circuit works opposite of the one in the Juno, increasing the resonance OTA gain reduces the amount of resonance.

Because of this, increasing CV must decrease the output current. Also, when doing exponential conversion in software later, we must generate a negative exponentially decaying signal instead of an exponentially increasing one, which is too bad as it means that we cannot have a common control system for all filters. I will have to look closer into this.

CV vs I_abc for one resonance OTA, original Jupiter 6 circuit. With a linear response, a similar CV curve must be calulated in software.

Oh, and because of the way I did the linear control, we don't get a deadband.

State variable filters, some quick resources

I found these resources while trying to understand how resonance defeat works in a state variable filter:

A very good general description
https://www.electronics-tutorials.ws/filter/state-variable-filter.html

Some more about gain and stuff
Says that passband gain should not be affected by Q (resonance)
http://sound.whsites.net/articles/state-variable.htm

All pass and equations
https://www.analog.com/media/en/training-seminars/tutorials/MT-223.pdf

Musical Applications of Microprocessors
Then I found this on a forum (which I unfortunately forgot to bookmark):

I first read about how SVFs work in Hal Chamberlin's book, Musical Applications of Microprocessors , where there's a lovely diagram of the filter's structure. I later spotted that same diagram in an article about an analogue computer. Apparently an SVF and a physical model of a spring have the same structure! Well, they're both dynamic systems that resonate, so I shouldn't be too suprised if they share the same mathematics.

Incidently I have this in my bookshelf, so I checked it, and it did indeed have a good diagram. But even better - it had an implementation using CA3080, which is almost exactly identical to the one in the Jupiter 6!

It's the first time I have come across that implementation, it even says that the capacitor-to-ground between the OTA and the buffer equals the configuration with the capacitor in the feedback loop of an opamp that I have seen elsewhere. Oh - and the book is from 1980, three years before the JP6 came on the market.

This one has both BP and band reject/notch. Oh, and it is missing the resistor i talked about in a previous post and has no change in gain for frequencies below the resonance frequency.

Matrix 12
I also looked at the Matrix 12 filter. It is NOT a state variable filter, it is a multimode filter, which mixes the output from four poles in various gain/variations to get 15 different responses. This matches well with what MT-223 above says about mixing different gains.

Cem chip used in the Matrix 12, CEM3372

https://pdf1.alldatasheet.com/datasheet-pdf/view/95160/ETC/CEM3372.html

On bandwidth vs Q:

https://www.rane.com/note170.html