Voltage control for CGS bandpass filter?

nofrets

Hi -

I'm trying to realize, in hardware, a dronebox I knocked together in SynthEdit. It's a simple additive synth, using resonant BP filters as oscillators. How would one add voltage control to the CGS bandpass filter? Alternately, if that's not the best design to start with for my purposes, can y'all recommend a simple resonant bandpass-only VCF?

Thanks!
Patrick

andrewF · Posted: Mon Nov 03, 2008 4:18 pm Post subject:

Hi
this question has come up before on the CGS yahoo forum.
Just in case you aren't a member, here are some of Ken's responses -

>> I'd suggest building a different sort of filter if you want voltage
>control.

and

>>The usual trick is to use OTAs instead of the op-amps to vary the >>frequency,
>>and as OTAs don't use the same physical or electrical configuration, it
>>would get ugly. Now if you are simply wanting to do the triple resonant
>>filter and don't care about accurate tracking, some possibilities do open
>>up. Basically, you replace the dual 100k pot with a pair of vactrols in
>>parallel with a couple of fixed 100k resistors. The current through the >>LEDs
>>will now control the frequency.

So, not impossible, but possibly not as controllable as you might like.
hope this helps

Cat-A-Tonic · Joined: Mar 24, 2008 Posts: 42 Location: Yokohama, Japan

The Moog MuRF does something similar to what you might be looking for.
It is 8 fixed frequency bandpass filters (or 7 with 1 low pass if you go with the bass version).

It has an internal sequencer, an LFO, an envelope, and lots of voltage control.
It is very nice in a feedback loop.
I've got the regular version, but would rather have the bass version.

nofrets · Posted: Tue Nov 04, 2008 4:20 am Post subject:

@andrewF - No, I'm not a member of the Yahoo group, so thanks for digging those resposes up!

@Cat-A-Tonic - The MuRF is one of many things on my GAS list, but for this particular project I'm looking for something more DIY/lo-fi.

Thanks for the responses - I think I'll go fishing in the main DIY forum for BP VCF designs.

Cheers,
P

zthee · Joined: Feb 20, 2008 Posts: 414 Location: Stockholm

I was planning on doing this aswell! So thanks for starting a thread about it Smile

ericcoleridge · Joined: Jan 16, 2007 Posts: 889 Location: NYC

Don't forget about the Bridechamber VC Resitor/Vactrol PCB:

http://www.bridechamber.com/bridechamber.com/VC%20Res%20PCB.html

nofrets · Posted: Sun Nov 09, 2008 6:40 pm Post subject: