Electric Druid FilterFX circuit question re:add-on CV control

[Itchy Scratcherson]

The FilterFX has an optional Expression/CV input to accommodate an expression pedal/ CV source for the Frequency pot. I added an additional dedicated CV jack to control Depth as well; the build doc includes a pretty straightforward wiring diagram for doing so.


I wanted to add a mini-toggle to select control of either Frequency or Depth ...and use just one switched TRS jack (I only have one expression pedal). I almost got there, but ultimately just added a second jack and no toggle, and it works fine. One pic below shows the point at which I gave up. (Hoping someone can help me out there.)

But my more important concern/question is about the few components present in the included, optional, Frequency CV that aren't part of the suggested wiring for adding more CV jacks. I outlined them in red, and want to know if it's "best practice" to add these to any additional CV modding? Or is it actually better to omit them in additional mods? One is a "Filter for I/O Protection," and the other is just a 220r resistor between the pot and it's +5v power... My wild guess is that the resistor prevents popping when a jack is inserted(?).

I hope my question is clear!
Thank You!   ~Christopher





build doc https://electricdruid.net/wp-content/uploads/2019/02/FilterFXConstructionGuide.pdf

[ElectricDruid]

Hi Christopher,

But my more important concern/question is about the few components present in the included, optional, Frequency CV that aren't part of the suggested wiring for adding more CV jacks. I outlined them in red, and want to know if it's "best practice" to add these to any additional CV modding? Or is it actually better to omit them in additional mods? One is a "Filter for I/O Protection," and the other is just a 220r resistor between the pot and it's +5v power... My wild guess is that the resistor prevents popping when a jack is inserted(?).

The "IO filter" is there because we need a series resistor to protect the StompLFO in the event of someone feeding in a voltage higher than 5V. The 2K2 resistor limits the current that can flow to something that the internal protection diodes on the chip can cope with, for all but absolutely extreme voltages. Since we *have to* have a resistor, we might as well add a cap too to get some CV filtering and keep some noise out of the ADC. The cap is optional, the resistor is not.

The 220R to 5V is there because when you insert a jack, it can short out the ring and sleeve connections, shorting the 5V power directly to ground. That's not healthy for the regulator. So the 220R is another current-limiting resistor that prevents too much juice flowing if/when that 5V gets shorted. Since the value is small compared with 10K, it doesn't really eat into the range of the pot appreciably.

I hope that explains *why* those parts are included and what they're there for.

If you want one CV input socket that can be switched between Frequency or Depth, that should be possible. It's slightly easier doing CV alone, since you don't have switch so many connections like you would for expression pedal - only the pot wiper with the voltage on it really matters. Whether a DPDT can switch between two pot wipers and one CV socket and route everything to the correct CV inputs on the StompLFO, I'm not sure. I can't see a way currently, but I'm not very awake just yet

[Itchy Scratcherson]

Thank You very much! In my OP I'd meant to include this pic... today I'll modify my idea and possibly get back here with a couple more schematic snippets and a question or two for confirmation.

[Itchy Scratcherson]

Would this approach be close to the mark re: adding a wet/dry blend «passive crossfade from the input buffer output to the filter output»?

[ElectricDruid]

I'd guess that would work pretty well, since both ends of the pot are direct low-impedance outs from an op-amp (whichever filter output you choose, that's still true). I'd probably try a 10K linear pot, to avoid it being heavily loaded by the C4/R13 output components.

Good luck! Let us know how it goes!

[antonis]

In an ideal world there should be zero DC offset between IC1.2 and IC2.2 outputs, hence absolutely silent (from crackling point of view) blend pot..

[Itchy Scratcherson]

I'd still like to try tapping the dry signal from a couple different places to hear the difference when applying that to a wet/dry mix... but...
for the moment, instead of mod'ing the signal flow of the pcb, I decided to use JMK PCB's "Panner" as a daughterboard blender, and I'm astonished at the results: never considered a wet/dry mix on a filter pedal before.

If you haven't yet considered it, I can't recommend more strongly adding a "mix" pot to your next revision of the FilterFX. Adding some dry signal to the filtered sound brings all the body back, and dialing-in only a hint of the filtered sound just floored me with the possibilities for subtle modulation I didn't hear before with this circuit.

JMK's "Panner" calls for a B10K blend pot, and it takes some getting used to, but I was able to tune more or less the mix I was hearing. I was so impressed with the results I'm planning to keep this as one of my main chorus-y phasey modulation pedals. It was a solid, clean, and versatile Filter circuit yesterday; it's the bee's knees today!