cgs58 lfo cv->speed?

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark

hey all,
I was hoping to find this mod described in this forum but no luck, apparently...

is it possible to inject CV into this circuit to control the speed of the LFOs?
Perhaps mixing the wiper output from the Speed potentiometer with incoming CV? From what I understand that pot. is basically acting as a voltage divider here, right?

Any help would be awesome. I know I should probably experiment with low-voltage DC here, but I really don't want to fry up shit on a whim like in my circuitbending days Wink

Cheers
andreas

andrewF · Posted: Sun Mar 29, 2009 5:51 am Post subject:

Hi
this question is asked regularly, but I cannot recall anybody offering a solution....except perhaps building a VC LFO from another source. This cct was never designed to be CV controlled.

I don't think injecting a CV signal to the wiper of the 'speed' pot will help. This pot is processing the output of the Schmitt trigger stage, not a CV or DC voltage.

A possiblity would be a LDR/LED combo or vactrol, with the LDR connected between the speed pot's wiper and the node of the speed pot and the 4k7 resistor or maybe to pin 8? There are plenty of ccts out there for driving a vactrol with a CV signal.

It would be hard...impossible... to get precise control and the degree of control from the CV signal would be affected by the setting of the pot.

have fun Very Happy

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark

Cheers, Andrew.

You know, I would have thought so too, but for the life of me I can't find the threads for those mods? Confused

so weird.

Yeah, I already have some (four, I believe) vactrol circuits from Bridechamber ready to roll, so that's always a possibility - I'm not looking for precise control in any shape or form, just a nice way to freak the LFO out, really.

I have two Doepfer 147 vcLFOs, which crossmodulate in the nicest of ways - lfo out->cv in->lfo out->cv in, that sort of thing. Really simple, actually.

Righto, I'll build these up, then go nuts with the DMM/scope to see what I can do with this.

electang · Joined: Dec 20, 2007 Posts: 15 Location: fr

Yeah, I already have some (four, I believe) vactrol circuits from Bridechamber ready to roll, so that's always a possibility - I'm not looking for precise control in any shape or form, just a nice way to freak the LFO out, really.

You 'll be satisfied. Smile

I have tested this and it does vc control the frequency but at the expense of a tremendous and continous drift making it useless as vc clock Crying or Very sad

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark

good to know! Very Happy

And thanks for the info re. stability under vactrol control - I'll be sure to add a switch to punch it in or out.

synthmonger · Posted: Mon Mar 30, 2009 1:50 pm Post subject:

electang · Joined: Dec 20, 2007 Posts: 15 Location: fr

[quote="synthmonger"]

synthmonger · Posted: Tue Mar 31, 2009 1:20 am Post subject:

Did you use the vactrol he sells? That's a slow resistor you may wanna up it to a VTL5C4 i think thats the fastest.

electang · Joined: Dec 20, 2007 Posts: 15 Location: fr

otherunicorn · Joined: May 11, 2008 Posts: 136 Location: Australia

Quite frankly, I wouldn't bother trying to voltage control oscillators using this core. That is why I called it the utility LFO i.e. plain, minimum options. You really need a ca3080 at the core instead of plain old op-amps. One day I''' get around to it, but don't hold your breath. You are better off just using a VCO with a divided output, or running really slow.

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark

andrewF · Posted: Mon Apr 06, 2009 4:38 pm Post subject:

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark

bugbrand · Posted: Tue Apr 07, 2009 2:05 am Post subject:

Hey - check out the CA3080 cookbook from Thomas Henry (available from Magic Smoke, I think?) - the VCO design is basically the standard utility LFO design but with an OTA replacing the manual speed control. You could, of course, make a double using an LM13700.

Actually the THVCO1 is this sort of design:
http://mypeoplepc.com/members/scottnoanh/birthofasynth/id33.html
...though it uses a discreet transistor based comparator section (opamp in the standard utility LFO design)

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

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

For me, I would have more use with a sync/reset input on the utility LFOs; I'm wondering if there would be a 'simple' way to implement sync? I know that I've accomplished a reasonable reset on my MFOS LFO by just inserting a gate signal at the timing capacitor.

Of course there's tons of LFOs out there that are designed with sync, VC, etc. But, I particularly like these Utility LFOs, for a number of reasons. They remind me of the LFOs on the MS20.

wetterberg · Joined: Jun 27, 2008 Posts: 48 Location: denmark