the 4 identical simple filters I just slapped on there, I only wanted to test the feedback part of it all, and give it vaguely similar surroundings to the original.
It is supposed to be for a voltage controllable filter, which usually do use such identical simple filter sections and control them in parallel, but it was quicker for me to do what I did there.
The feedback and even self oscillation are intentional effetcs for this kind of stuff.
I was just curious to try this out, even though I'm a hobbyist with very fractioned knowledge, even to me this looked crude, yet fascinating because cheap, low parts count, etc.
This 2 transistor feedback thing was used in Roland's budget polyphonic synth JX-3P. Its VCA uses one NPN + 1 opamp - that "budget" :-D
It did use some IC for the filter, with external caps and all, but the feedback circuit, to get controllable amount of resonance, was done the way like depicted here.
And although primitive, it was, nontheless, used in a commercial product which sold quite okay, end even sounded quite okay.
Thanks for the detailed explanations, and hints what tests to do.
That will be some homework, as I only know some of the things you mentioned, and the depth of understanding may not be enough.
The pot was just a quick way to make a resistor divider that I could manually change.
Just today I discovered the step param command in ltspice ;-)
But yeah, I probably will try to use on OTA - as soon as I get the LM13700 subcircuit working in ltspice...
I also though about using something like a CD4051 analog mux + 8 resistors to build a DIY digi pot, would that be advisable?
It can do only 8 steps if A,B,W connectors are needed as I see it, but that could be enough for this task.
All this analog synth stuff uses quite high voltages, and usual digi pots don't, those which do get quite expensive.