Simplest VC-LFO ever?

macumbista · Posted: Sat Sep 26, 2009 5:28 am Post subject: Simplest VC-LFO ever?

I was checking the thread on the Buchla 208 Random PCB, and thought it would be good to have a built-in VC-LFO in the same module. So I'm curious what people might recommend for the simplest LFO of all time.

Here's what it would need to have:

Maybe the voltage controlled 40106 that got posted recently?

Dave Kendall · Posted: Sat Sep 26, 2009 2:13 pm Post subject:

Hi.

Maybe something based around a CD4046? like in the CGS31. Look at the top section of the schemo.

http://www.cgs.synth.net/modules/cgs31_digital_noise.html

The output is a square wave. The 10n sets the freq range. I added a 3-way toggle to change the cap values as a range switch.

I also added a simple non-inverting summer made from 2 x opamps before the 10K resistor, to give manual control and 2 x CV inputs, and did not include the 1K to +15V at the front end, as it was now redundant.

This could possibly replace the voltage follower....?

just a thought...

cheers,
Dave

de_engineered · Joined: Jul 16, 2009 Posts: 18 Location: USA

There was a Lunetta (or Lunetta-ish) 40106 VCO design somewhere on here - used a diode as current controlled resistor. Might fit your bill - uses the 40106, a 2N3906, the diode, and a cap... think that's about it. Add your necessarily pots and you're good to go.

de_engineered · Joined: Jul 16, 2009 Posts: 18 Location: USA

Oh royal brainfart, you already found it, doh!

Well, count this as my approval of the design then!

macumbista · Posted: Sat Sep 26, 2009 7:47 pm Post subject:

de_engineered · Joined: Jul 16, 2009 Posts: 18 Location: USA

Another possible solution: I wouldn't be able to tell you the exact values for building one, but you could do a discrete Schmitt Trigger with two transistors, and treat that as a functional Schmitt block, and have a pot and cap in a feedback loop, and you'd have your pulses. Can't say how 'square' it would be, as within a 40106 itself, per channel it's actually three inverters and a Schmitt trigger in a row - really square!

http://en.wikipedia.org/wiki/Schmitt_trigger#Schmitt_trigger_with_two_transistors

This amounts to *more* parts but with *less* area. Again though, I can't vouch for how sharp the output would be.

Good luck!

bugbrand · Posted: Sun Sep 27, 2009 4:28 am Post subject:

You can actually also get an almost ramp-wave off the 4046 -- if you buffer (non-inverting follower) straight off either side of the timing cap and then combine the signals.

'tis true!

DGTom · Posted: Sun Sep 27, 2009 7:30 am Post subject:

... & all of a sudden the only part of the Wogglebug #3 schem. that didn't make sense to me... does! Very Happy

Thanks Tom! You rule!

macumbista · Posted: Sun Sep 27, 2009 8:46 am Post subject:

bugbrand · Posted: Sun Sep 27, 2009 1:01 pm Post subject:

Oh - I hadn't looked enough at the Woggle to see that either! I just started trying such things out quite by chance, but then afterwards realized that THenry has something going half-way there in his XR2206 part of the VCO chip cookbook to get a rampoid wave (ie he only taps from one side of the timing cap)

I think I did try it initially just with diodes, but thought it didn't work out quite right -- was better with a buffer on each side and then diode mix before a non inverting opamp (to give a little gain and offset the signal to standard +/-5v)

DGTom · Posted: Sun Sep 27, 2009 8:50 pm Post subject:

as soon as I read your post above I had a total 'head meet desk' moment! I should've realized alot sooner what was going on there as I'd tapped into a timing cap pin on another PLL type chip before to steal the internal oscillator signal.

Very cool tho, can't wait to try it out on a 4046 linear VCO type cct.
square & ramp waveforms off a 50c chip with half a dozen passives and an op-amp or 3 sounds like a good deal to me!!

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

I believe Tim Servo posted a pretty simple vc-lfo here some time ago - also with some nice trapezoid waveshaping add-on-stuff...

C

Edit: Oh - and this one of course!
http://electro-music.com/forum/topic-34550.html&sid=3cb459053c73c11fe5d6dab7b4b33ce5

Tim Servo · Posted: Wed Sep 30, 2009 3:44 pm Post subject:

Actually, I posted an LFO with a wide frequency range and a couple of add on waveshapers (including the trapezoid / TSQ waveshaper), but my LFO design was not voltage controlled. However, at the start of the same thread, the very prolific Nicolas3141 had posted a simple VCLFO. Nic's design runs off a single supply, but he said it should work with a dual supply. Looks like a cool design:
http://electro-music.com/forum/topic-32372.html

Tim (captain trapezoid) Servo

Coriolis · Joined: Apr 11, 2005 Posts: 616 Location: Stilling, Denmark

Ah..so that's how it was. Sorry about that!

C