[sdiy] 3340 VCO question - setting the lowest frequency

[Tom Wiltshire]

Hi All,

I’ve having a play with one of the new V3340 oscillator chips. I haven’t had an opportunity to use a 3340 oscillator before, so this is all good.

I’ve built up the data sheet circuit on a strip board for experiments:

http://electricdruid.net/wp-content/uploads/2016/08/DatasheetCEM3340VCO.gif

Now, my problem is that I can’t get it to produce low enough frequencies, e.g. the reference frequency with no CV is too high. Initially it was around 160Hz, but I altered the 1M5 resistor on pin13 that sets the current to the expo convertor and that helped a lot. It has now dropped down to about 11Hz. Incidentally, I notice that the Prophet 5 and Prophet T8 do the same thing:

http://electricdruid.net/cem3340-vco-voltage-controlled-oscillator-designs/

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?

Thanks,
Tom

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