[sdiy] low voltage svf with bjts

Chris McDowell declareupdate at gmail.com
Wed Apr 8 10:03:07 CEST 2020

Howdy list, 

I hope every is feeling alright! So I thought why not try to do a state variable with bjts, kind of like the early ms20. One reason why not is that I don't completely understand how to, but I have gotten this working pretty well, and it will simulate down to a supply voltage of 3.3V. 

I took a working filter I've been playing with at 3.3V, and just as a start, almost a joke, just swapped the OTAs for NPNs. the outputs surprisingly still looked like a filter, but this is kind of clearly not right.

I've left the exponential converter and the two transistors (Q7 and Q8) connected naively, as that's what I have now. What would be the right way to connect these two sections? Q9 is not delivering the ~70uA that it would into an OTA or dummy load, possibly obviously. This technically works but the range is very limited, about a 1:10 change in current / cutoff frequency. 

The point of this I suppose would an easily controllable alternative to twin-t drums, with very low parts count. Plus, JLCPCB.com <http://jlcpcb.com/> assembly service is currently out of LM13700s... so I ended up here. 


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