[sdiy] low voltage svf with bjts

Richie Burnett rburnett at richieburnett.co.uk
Wed Apr 8 12:34:45 CEST 2020

Neat!  Do you get much control feed-through at the BP and LP outputs of the 
filter due to the BE currents of the transistors feeding into the 


-----Original Message----- 
From: Chris McDowell
Sent: Wednesday, April 08, 2020 9:03 AM
Subject: [sdiy] low voltage svf with bjts

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 

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


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