[sdiy] 3340 VCO question - setting the lowest frequency

Terje Winther terje.winther at wintherstormer.no
Sun Sep 10 15:08:36 CEST 2017

> However, I’d like to get it to go another octave lower if possible,  
> so that I have room to tune the CV to reach MIDI note 0 (8Hz). My  
> thought was that increasing the 360K resistor on pin 15 would reduce  
> the “default” current going into the CV summer and do what I need,  
> but in fact what happens is that the oscillation becomes unstable  
> and then stops altogether if that value is raised even a little.
> I’m reluctant to increase the cap value although in some ways that’s  
> the obviously thing to do. Every single design I’ve ever seen for  
> this chip uses the 1n value for the cap, and I’m not feeling  
> confident enough to go against the opinion of every synth designer  
> in history! But maybe none of those synths went this low? (It *is*  
> sub audio, after all…)
> Does anyone have any experience to offer?

If you look at the digisound modular, module 80-2 is the VCO while  
80-3 is the LFO, using the same circuit, PCB and 3340 core. There are  
only 5 component changes from VCO to LFO, including change of the  
timer cap from 1nF to 10nF. The don´t think the design is not meant to  
go subsonic unless you set it up as an LFO.

Terje Winther
terje.winther at wintherstormer.no

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