low pass filter with no caps or inductors

[Mick Bailey]

I've been experimenting a lot lately with low-pass filter designs for audio (mainly for guitar synth use), especially something with a low parts count. The main problem I've come across is unwanted distortion on clean signals but I came across a design based on the Polivoks filter that does not use any inductors or capacitors and instead uses a TS271 programmable opamp. I breadboarded this yesterday and it's really quite remarkable and I'm building this into my guitar synth over the next couple of days. I thought it would make the basis for a guitar filter pedal if housed in the shell of an old wah. The resonance operates really nicely, going from nothing and right through to self-oscillation.

You'd expect to find plenty of music applications for this IC, but apart from the use in Polivoks clones I can't find anything.

The only downside I hear is some slight bleedthrough of HF when the filter is in cutoff. I think this is a manifestation though in me feeding it with a PWM signal rather than straight audio - there's a lot of hash in PWM that's difficult to get rid of. I don't think this would be a problem for regular use.

Here's the article; https://www.google.com/url?sa=t&rct=...v&opi=89978449

[tedmich]

Thats a cool circuit Mick!
And the old TS271 (although a SOIC-8) is only $.75

[The Dude]

That is awesome. I might have to build one for our band's bass rig. We use bass guitar split into a Sonus G2M (https://www.sonuus.com/products_g2m.html) MIDI converter. It then runs a sampler via MIDI. While the system works pretty well, harmonic content will easily make it mis-trigger. I've tried different methods of cutting everything but actual note information feeding the Sonus (eq's, crossovers, etc.) with some degree of improvement. This might just do the trick. Thanks Mick!

[Mick Bailey]

I got six DIP packaged ICs from a decent UK company for £0.90 each (about $1.14). I'm impressed with how well this filter works and the slope is excellent. I uses a J113 Jfet to invert the CV response for use in my synth when running it from the envelope generator, but for most other uses using pots the inverted response doesn't matter.