[sdiy] 3340 VCO question - setting the lowest frequency

[Tom Wiltshire]

Ok, an update - whatever the effect was that I was seeing, it went away, and hasn’t come back. I call this “prototype woes”.

I’ve now got 7Hz with no CV input. The only changes from the data sheet design is upping the 1M5 on the Expo convertor pin to 2M2 (as per Prophet 5/T8) and increasing the 360K on the sum input to 390K. Either of these push the lowest frequency lower, although as mentioned, you can force it too far and make it stop, or start to distort the waveform.

Most circuits that use the 3340 as an LFO do just bump the cap up to a larger value (which then enables you to run the expo convertor in the range where it’s happy), and they also rarely care about perfect tracking, mostly abandoning the high frequency trim and similar niceties.

Thanks,
Tom

==================
       Electric Druid
Synth & Stompbox DIY
==================

> On 12 Sep 2017, at 20:22, Mattias Rickardsson <mr at analogue.org> wrote:
> 
> On 10 September 2017 at 13:56, Tom Wiltshire <tom at electricdruid.net> wrote:
>> 
>> 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!
> 
> Oh, come on, Tom - do it! :-D
> 
>> But maybe none of those synths went this low? (It *is* sub audio, after all…)
>> Does anyone have any experience to offer?
> 
> Not exactly experience, but the Memorymoog comes to mind... IIRC it
> has 19 VCO chips where one is used as the LFO. Looking for the full
> schematics (but actually not finding it, does anybody have it?) I
> stumbled across this instead:
> 
> http://www.synthdiy.eu/files/Memorymoog%20Voice%20Modulation%20(from%20Tech%20Service%20Info%20p%2021).png
> 
> Oscillator 3 can apparently be switched down to low-frequent duties
> using a larger capacitor, and the service documentation describes how.
> 
> 
> /mr