How to make a filter to feed into an octaver?

[Muzz]

Hey guys.

I want to make a filter to dial in what frequencies are let through this pedal i want to make so i can feed a dry chord that is being played and cut out as much of the higher strings as possible to get as clean a possible bass note coming out of the end of an octaver running into a bass amp. Obviously this is to fill out a guitar with bass working as an accompaniment.


I'm not entirely sure what circuits i should be looking at to get around this.

For reference, the electro harmonix multiplexer and the boss oc-3 have pretty much what I'm looking for built in where you can dial in what frequencies get fed through, but those pedals don't feature amazing tracking, so i would rather make a filter and run it through a micropog as i feel I'll get better results from that.

If this wont work the other option is to mod a cheap guitar (which i would rather use my normal non modded guitar live) with a pbass pickup on the lower strings, going through the octaver to give a clean version of what i would be attempting to filter out here.

Any help would be appreciated!

ps. I have the stuff in the mail to make my first pedals.... So by the time we work this out i should have a few pedals built .

Edit: using reaper i recorded a rhythm track and used the refir effect (essentially a super eq letting you cut off frequencies) and it worked exactly as planned. I can upload the audio if you want to hear it.

[PRR]

90 dB per Octave filters are really-really tough to do in analog circuitry.

A 36dB/Oct filter at 250Hz may do what you want and is not THAT hard to contrive.

[Muzz]

Just tested it out, that would work fine!

Just curious why you are calling it an octave filter? I'm just looking to make a circuit to cut out higher frequencies and then run into a micropog, wouldn't that just be called an eq, or a high-pass filter?

[Derringer]

don't think of it as an "Octave Filter"
what PRR said is, "90 dB per octave" filter

and by that he means that per octave (doubling of frequency, like in your picture going from 400 Hz to 800Hz) the filter removes 90dB of content from "point x" (about 400 Hz in your picture) on up in frequency

it's a "rate of frequency attenuation"


"EQ" is a general term and could refer to any kind of selectable frequency boosting or attenuation

the filter that you are talking about would be a "low pass filter" because it filters/attenuates the high frequencies and allows the low frequencies to pass

[PRR]

Analog filters come in certain rates of fall.

A simple single R-C filter falls at *6dB per Octave*.

An L and a C can give 12dB/Oct.

One opamp with 2 R and 2 C can also give 12dB/Oct.

One opamp with 3 R and 3 C 'can' give 18dB/Oct, but you probably need a buffer in front.

Two opamps (and probably a buffer) with 6 R and 6 C can give 36dB/Oct.

Don Lancaster's Active Filter Cookbook is a must-have. There is also a lot of bumph on the interwebs, some of it reasonably true and clear.

For trial, I'd aim at "Unity Gain Sallen Key" with "Butterworth" (Q=0.7) response, both at 250Hz. Two Butters in cascade is not the sharpest knee around, but your digi-filter shows a fairly roundy knee.

[Muzz]

Thanks guys, that's exactly what i needed, I'll get to reading and researching then, and i'll be back with my results.

[merlinb]

Also look up 'mutiple feedback filter' or 'MFB filter'. This the name for the general circuit configuration used to make Butterworth and some other filters.