All Pass Filter...

[Silver Blues]

Okay hmm...

It hit me just now. One of my favorite guitar fuzz pedals, the Malekko Plus Ultra 213, includes a foot-switchable all pass filter (which sounds badass, by the way). I'm just in the finalizing stages of building a Muff clone, and I got this crazy idea that I could build a standalone all pass filter to add to other effects. I found this schematic on Wiki:



But it has no values on the components and the Vin and Vout is a little confusing. It doesn't even list what kind of op-amp to use. Is this even possible? What would it end up doing (esp. to a clean signal? ) I would assume I could replace some of the resistors in that diagram with pots to allow a frequency sweep change (like one would do with an expression pedal and the 213). Any insight?

This is my first/second attempt at effects really. I don't use effects on my guitar (I have a Mustang III handling that) and I've never used effects on bass... and now I almost have a Muff and am planning this. (I do have pretty good electronics knowledge so I'm not a total butt.) Be kind to me

--Silvie

[Taylor Livingston]

Vin and Vout are the input and output, respectively, as you probably figured. For most opamp filters, and really just about any audio stuff besides envelope followers and headphone amps, any opamp will work just fine. TL072 or 4558 are popular dual "jellybean" opamps.

An all-pass filter combined with clean signal forms a single stage phaser whose frequency is stuck at one position. Two of these create a notch filter. Sweeping the freq around will sound like a really weak phaser.

The R to ground is the one you want to replace with a pot to vary. Check out schematics for the Phase 45 for examples of the other values. Hope that helps!

[Silver Blues]

Wow thanks a lot, Taylor. I'll be sure to do that.
It doesn't seem like a very complex circuit. I'll give it a shot after my Muff is done.

--Silvie

[Taylor Livingston]

Keep in mind that that image is for a bipolar supply. To run it on 9v you will need to make a reference voltage to bias the input and add input and output caps. You may know that already since you have some electronics background; just adding it to be sure.

[Silver Blues]

Oh right, yeah forgot about that. What cap and res values am I looking at for that conversion? Can't think, it's late

--Silvie

EDIT - found an MIT document on how to bias an op-amp, might read a little through that but any help still appreciated.