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Problems with Envelope Follower Filter Section
Problems with Envelope Follower Filter Section
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Old 1st February 2019, 04:45 PM   #1
SlunkBoy is offline SlunkBoy  United States
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Join Date: Jan 2007
Default Problems with Envelope Follower Filter Section

I am playing around with building an envelope filter/follower/auto-wah for bass guitar and have run into some issues with the filter section.

The follower section comprised of a peak detector illuminating two LEDs responds beautifully to my playing intensity with a nice, gradual taper. The LEDs were subsequently paired with LDRs and encapsulated with heat shrink tubing to form opto-isolator units.

I am being somewhat stubborn in avoiding the traditional state-variable filter section common to Mutron and Q-Tron commercial units, instead trying to implement Sallen-Key low or high-pass filters (as I am more familiar with these from building active crossovers).

The opto-isolators are paired with sensible resistor values to pad their values down to hundreds of kOhms (from their muliti-mOhm dark resistances). I can't seem to get a moving filter response from either low or high-pass arrangements with broad changes to capacitor values. The low-pass circuit sounds more like an all-pass circuit, whereas the high-pass is thinner and distorted (I can't account for the distortion). I know regular wah pedals typically use band-pass circuits for their vowel sounds, but devices like the Mutron III have separate low and high-pass settings that sound preferable to my ears.

Is the Sallen-Key topology totally unsuited to this task? Should I just shrug and build a state-variable filter like everyone else? Am I missing something?
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Old 2nd February 2019, 08:31 AM   #2
OrbitalLex is offline OrbitalLex  United States
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Join Date: Sep 2014
Is phase correct? Using non-inverting op-amps?
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Old 4th February 2019, 02:59 PM   #3
SlunkBoy is offline SlunkBoy  United States
diyAudio Member
Join Date: Jan 2007
The phase is correct and I am only aware of non-inverting Sallen-Key configurations.

I went ahead and prototyped a state variable filter and it just worked. I was trying to save a couple of op-amps for simplicity, but it does offer some worthwhile flexibility.

I suspect that the Sallen-Key filter is "working", but that the lack of resonance is making it too subtle to be noticeable for this application.

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