Stompbox Build Inquiry

[johnnyc]

Why not just use a rotary switch? Just switch in different series caps to change the frequency, the main problem being the frequency will depend on what the pedal is driving.

[brianroth]

A passive filter can be a tad tricky to design for this application.....

Bri

[gtzack4]

I know little to nothing about ciruicts but am trying to learn. But if I would try this my first thought is to use the same circuit as guitars tone know. Use a rotary switch to select the cap and and a bypass footswitch.

[fastlanestoner]

Thanks for the responses guys! I got a response from RG Keen over at DIY and I've come to the realization that passive is out. It'll load and become counter-productive. So the filter will have to be active.

I've never had to design something like thie before and I"m admittedly stumped. Can anyone link me to a schematic that is a decent starting point? A guitar tone control, or maybe something like a Big Muff tone stack would be a good jumping place?

I'm really enjoying the challenge of this design, and thank everyone for their help!

[Torea]

Just in case you find the answer elsewhere, would you mind posting it here? This is something I've been wanted to try for a while too =)

[johnnyc]

You can still use the same concept of a simple RC high pass filter with a switch changing the cap values, just with added input and output buffers. You might a few extra things to ensure no clicks and pops when switching.

[tINY]

State variable filter - Wikipedia, the free encyclopedia

You could steal the filter out of the classic Wah pedals and use the HPF output.

Sallen–Key topology - Wikipedia, the free encyclopedia

Or, you could steal the 2nd order filters out of an active cross-over. This would require switching in/out resistors or capacitors.

Or, you could just buffer a passive filter.... I'd suggest that you have at least as second-order (12dB/octave) reponse.



-tINY

[fastlanestoner]

I found this schematic:

Google Image Result for http://ok1ike.c-a-v.com/soubory/tipy/HIGH%20PASS%20ACTIVE%20FILTER.JPG

It notes that it's for a 100Hz cutoff. I could just interchange parts to allow for different cutoffs, and wire them to a rotary switch, yes?

[lame pseudonym]

I actually have one that I built a few years ago. It was for recording, not a stompbox, but about the same size and shape. I actually don't need it any more....

If you kinda know what your source and load impedances are, you can do it quite readily in a passive deal or a buffered passive deal or some kind of active thing. Look at some electronic crossover schematics -- that's actually what you're building, without the low-pass section.

Here's kind of an obvious but probably very effective one:
http://www.eeweb.com/blog/circuit_pr...gh-pass-filter

Heck, you could probably put this in a pedal -- I mean a pedal pedal like a volume pedal body. Just make the capacitors smaller if it doesn't cut high enough. If it wuz me I'd start with capacitors half the size of those in the drawing for a guitar application. You will be tailoring this at the end so make them accessible / changeable.

This is probably a senseless and redundant post but I just woke up.

LP

[fastlanestoner]

In theory I could find out the right C/R combo here:

Guitar Pedals: R-C Filter Calculator

and wire them all to a rotary switch, yes? no buffer, just have the I/O for the pedal attached to the switch/pot?

[fastlanestoner]

friendly *bump

[tINY]

Again, a passive filter like that is fine if you are sure of the PU impedance....

Assuming an input impedace of 500k or higher at the amp input is pretty safe.



-tINY