a couple of bandpass filters walk into a bar...

[garcho]

I've pieced together this dual tunable bandpass filter with LFO modulation from a few different circuits, mostly from this forum. My primary goal was to make a goofy filter pedal. My secondary goal was to learn a little about how to adapt circuits to work together. Right now all I've done is slap everything together, which works, but I am assuming there is much I could do to 'tune' the circuit, so the PS, buffers, switching, filters, and LFOs are all working together optimally. Any suggestions or advice? I'll do sound samples soon, lots of neat-o sounds with this thing.



A couple questions:

I first breadboarded this without buffering the VCC/2, and got something more like VCC/4 or 8. I couldn't figure it out, so I tried buffering it with an op amp after remembering some suggestions from Cliff Schecht and Jordan A. regarding a different project, and voila, it worked. Why is this the case?

I tried tying the two filters together, after the 27K resistors (before the op amp), and putting them to lug 3 of the blend pot, with clean signal going to lug 1. All I got was farty noise and not much signal. Why?

[Ronan]

I tried tying the two filters together, after the 27K resistors (before the op amp), and putting them to lug 3 of the blend pot, with clean signal going to lug 1. All I got was farty noise and not much signal. Why?

I can only guess that maybe it was a phase issue. The filters invert the signal, then you tried to mix the inverted output with the input. When you added the buffer you corrected the phase. Just guessing, don't know for sure.

Nice project, I'd love to hear a sound clip, circuits with parallel filters always get me going.

Edit: You asked for some suggestions, all I can think of is to try halving the cap values of say C7 and C8, and using a 50k mix pot instead of R14 and R15 so you can vary the mix of the 2 filters. Assuming this is still on a breadboard that is.

[garcho]

Good ideas, thanks. Still on the breadboard, will try those out soon. Sound samples soon, too. I tried doubling C7/8 before, think it made it a little muddy, or less narrow? Will take better notes.