[sdiy] LFO VC Skew?

ackolonges fds ackolonges at hotmail.com
Tue Feb 18 09:08:35 CET 2020

Thanks so much everyone for your input.

Those that mentioned digital options - yes indeed it's quite straightforward to implement such a thing via digital means, but I would like to try for the analogue route.

Thanks a lot for that link to the SNICster Ian, that's a heck-of-a waveshaper!

It would indeed be great to find out how the Electrik Multiwaves implements this function, maybe if Vinicius see this he might be willing to let us know his approach.

Also a big thanks to Ben for alerting us to the Wave Morph website. They mention that the VCS3 uses such a circuit, but I was unable to find info on that because the link was broken. He has a hand-written sketch by Don Tillman, that I can't fully parse (does anyone know if anyone ever built that one?), and mentions that Sam Houshu had a design of such a function but that link is also broken. His Wave Morph looks to do just what I'm after, but uses the RC4200, a long out-of-production chip. I'd much rather use the LM13700 OTAs that I have plenty of, or a modern drop in replacement for the RC4200 if anyone knows if such a thing exists. He mentions an OTA version, but alas, another wrong link.

So, I'm waiting with baited breath to see the magic that David has in mind!

From: David G Dixon <dixon at mail.ubc.ca>
Sent: Tuesday, February 18, 2020 6:53 PM
To: 'ackolonges fds' <ackolonges at hotmail.com>; 'SDIY List' <synth-diy at synth-diy.org>
Subject: RE: [sdiy] LFO VC Skew?

Nevermind.  I think I might have figured it out.  And the beauty of it is that I think I can do it by adding one very simple component to a linearized 2164 VCA circuit.

The trick is that if the log of X is log X, then the log of 1/X is –log X.  Look at Mike Irwin’s article and see what I saw, and how it can easily be done.

Now I feel even more stupid for not seeing that three years ago, when I was struggling with this.  Now I should have a design ready by tomorrow.  Stay tuned.


From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of David G Dixon
Sent: Monday, February 17, 2020 6:37 PM
To: 'ackolonges fds'; 'SDIY List'
Subject: Re: [sdiy] LFO VC Skew?

I’ve struggled with this mightily and come to no solution in the analog realm.  The problem is that there seems to be no way to implement the magic of a potentiometer fed on both ends with VCAs.  I’ve been through the math, and it seems impossible.  The original circuit (two diodes and a potentiometer in the loop of a tri-square oscillator) is so simple and elegant, and yet so elusive for voltage control, other than physically rotating the damn pot with a servo.

It’s easy to get saw-to-tri-to-ramp with VCAs, but the trick is to maintain the same frequency when you do so.  That’s the impossible part.

It is probably easily done digitally, but where’s the fun in that?

Unless I’m dumb.


From: Synth-diy [mailto:synth-diy-bounces at synth-diy.org] On Behalf Of ackolonges fds
Sent: Monday, February 17, 2020 6:02 AM
To: SDIY List
Subject: [sdiy] LFO VC Skew?

Hi All,

I'm a big fan of the 'skew' (sweep from sawtooth, through triangle, over to ramp wave) function of some analogue LFOs and find it indispensable. I would love to be able to CV the skew function though it generally doesn't seem to be an option. Some circuits seem to control skew by having a pot control the balance between current flowing each direction via two diodes with opposite polarity (eg. the MFOS VCLFO circuit, or the Nicolas Woolaston LFO circuit), which seems like a particularly tricky thing to be able to voltage-control.

I was wondering if anyone here knows of any existing circuits which provide that functionality, or have any ideas as to how one may go about making it happen?

Thanks a lot for any input 🙂
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