Kinda OT: really cool capacitorless VCF

[Tim Escobedo]

http://m.bareille.free.fr/modular1/vcf_polivoks/vcf_polivoks.htm

This idea came from a old Soviet-era analog synth. While I'd seen the schem before, I'd never been able to figure out what was what, not being able to read Russian. Enter Marc Bareille, with a SDIY posting of a updated version. Turns out I have a couple LM4250 op amps, and gave it a quick-and-dirty try.  I'll be damned...it works. Though it seems better suited to VCO signals than guitar. Uses no caps (except any input/output caps, like I had running off 9V). Pretty clever idea, and not too difficult as far as VCFs go. LP/BP outputs.

[mattv]

I was just looking at that schem too. Glad to hear it works okay.

[R.G.]

Actually there are capacitors. They're just hidden inside the opamps. The caps are the ones that limit the opamps response. The bias current support changes how fast these caps can be filled/emptied, and that controls the bandwidth, and hence the opamp response.

It's a clever way to not have to put an external cap on the board, though.

[ExpAnonColin]

Actually there are capacitors. They're just hidden inside the opamps. The caps are the ones that limit the opamps response. The bias current support changes how fast these caps can be filled/emptied, and that controls the bandwidth, and hence the opamp response.

It's a clever way to not have to put an external cap on the board, though.

My thoughts exactly.  

PLPOA

The problem, though, is that he's using coils on the power supply, and man oh man, do I hate coils.  

-Colin

[Tim Escobedo]

The inductors on the power supply aren't absolutely necessary, though good practice, particularly for a bipolar supply that most likely won't be coming from batteries. I'm guessing they could just be a loop around a ferrite bead. Nothing fancy.

[Nasse]

Thanx for the nice circuit link and info. I like simple and easy circuits.

[Dan N]

Cool site! His Coron clone just went to the top of my "things to build" list!

Thanks!

[jrc4558]

Just for the future:

I do speak, read, write, and translate Russian.

If you need any translations, whatever with Russian - PM me and I'll do it.