Building an Envelope-controlled LPF

[VectorHorrific]

Firstly, allow me to finally say 'Hello!' as a long-time lurker and first-time poster (I made this account all the way back in 2012)!

Now, I recently built Tim Escobedo's 9V MS20 filter filter in to a wah enclosure, with the treadle controlling the frequency sweep, and it sounds like a dream. I loved it so much, in fact, that I would love to build one where the frequency sweep is controlled by an envelope follower circuit. The only problem is that I have nearly zero knowledge of envelope filters and their circuits so I'm flying blind with this one. I thought it'd be helpful to document this somewhere that other folks in the know could offer some insight in to where I might be going wrong.

Before I continue, I need to mention that I used Fancy Lime's Quackmire as the template for my preliminary schematic on account of its relatively low parts count and how I studied it to help me develop my current understanding of envelope-controlled effects. Big shout out to them for their og circuit, as without it I might be struggling a lot more with this!



The above schematic is what I've come up with so far. I apologise if it seems messy or if I've made any serious layout faux-pas; this is my first time drawing out a diagram for anyones eyes other than my own! It is quite literally a Frankenstein job of stitching the Quackmire's buffer and envelope follower on to the original Escobedo circuit. The only changes I've made are to replace the 680pf caps in the Escobedo circuit with 470pf caps (They're all I had at the time), and I used an LM13700 instead of the original LM13600. Having built my earlier filter with these components, I know these changes work well enough.

The first big change is obviously replacing the 100k frequency pot of the original with a voltage divider featuring the other end of the quackmire's optocoupler. If my understanding of how the 100k pot worked is correct then this configuration should provide less resistance as the brightness increases, allowing the higher frequencies through as volume from the input signal increases. If I'm right, then I should be able to implement a rudimentary "direction" control by installing a dpdt to switch the 9v and ground back to front to get the opposite effect. I also believe that I could have a pot wired as a variable resistor in place of the 1k resistor on the schematic in order to create a rudimentary 'range' control by increasing the resistance to ground? Or this whole section could be based on a false understanding.

My second idea for refining this idea is to replace the input buffer with the other half of the NE5532AP chip. Since taking the original filter out of the quackmire, it's no longer being used. Using it as a buffer might help with the parts count and keep things a bit tidier.

Please let me know what you guys think. I'd appreciate any and all insight you could give me, and any ideas you might have for additional features or implementation. I'm here to learn and hopefully, in the end, contribute something you guys might find worth building yourselves.

[Kipper4]

Your D2 is arse about face.

Stick the meter on the ldr at all settings and measure the resistances bright and dark.
Then you'll have a good idea of the kinda voltages you'll get at voltage divider with r17.

I'd check the data sheet for the 13600 versus the 13700 I think they both have different Iabc input tolerances.

Welcome I Hope this helps.
Have you had it on the breadboard without the OTA ?

[VectorHorrific]

Didn't catch the D2, thank you. Also noticed that I mislabelled the resonance control. Yikes.

It's a wonder I've managed to make it this far without buying my own voltmeter, but until very recently I've mostly been making simpler circuits. Distortions and the like. I'll pick one up and breadboard it tomorrow night, I've got a few miscellaneous LDRs in my box I could test out for this.

As for the 13x00s; I've got a working example of the filter using one already so I know it's an acceptable substitute (to my ears, at least). I haven't encountered any issues as of yet.

I've not breadboarded the envelope follower yet. That's on my to-do list, next to redesigning the buffer. I'll let you know how that turns out. I appreciate the pointers!

[duck_arse]

welcome to posting, Vector.

you "can" reverse your sweep by taking the NSL32 led anode to +9V and the kathode to the R11. then as the envelope output increases, the led goes dimmer. an LM358 is good here, often used, as it will swing down to near ground. the second half can be used as a led driver - you'll need to search how to put the led in the opamp feedback loop. plenty of circuits about here will show - search for magnavibe and variants.