Craig Anderton’s Bi-Filter Follower

Featured in the July 1977 edition of Guitar Player Magazine. This uses two parallel filters To give the effects some extra umph in the funk.  We have some great modification information here by Mark Hammer.  This project is in transition and we are working on a new layout for it.  We have the schematics updated so far.

Posted By: Mark Hammer on Aron’s Stompbox Forum

Date: 2/11/02 10:41

There’s a few things you need to do to make the BFF (Anderton’s Bi-Filter Follower) go past its limitations. First, there are the requisite mods to vary attack and decay time. Second, tinker with the Q of either or both of the filters, à la Seamoon Funk Machine. Third, vary the values of the range-setting caps on the higher filter. Drop them to about 1/3 of the value shown. What you get is a kind of phase shifter sound, only envelope controlled. If you want to get wierder, tack on another op-amp mixer stage (although this goes beyond the layout, obviously), with a resistor between the input stage and the mixer, and switch the filer output between the inverting and non-inverting inputs to the mixer. This will get you notches OR peaks. Normally, you’d need 4 optoisolators to get you 2 envelope-controlled notches. By having the LDR control a bandpass, rather than all-pass section, and by spacing them appropriately, you only need 2 instead of 4 to make two notches when summed with the straight signal.

More from Mark…

Posted By: Mark Hammer on Ampage Effects Forum

Date: 3/19/2002 6:48 PM

It’s a very “clean” sounding unit, largely because the use of optoisolators reduces distortion and the overall design reduces a lot of the envelope ripple. The shifting/staggering of filters farther apart, by use of smaller cap values in the upper filter, gets a very nice phase-shift-y sound. I’m glad you saw/heard merit in it.

Other things to try:

1) Stereo outputs for the two filter sections.

2) Filter gain/Q for one or both sections (vary R15/16 from, say, 220k-700k by use of a 500k pot and fixed resistor).

3) Add another op-amp mixing stage and install level controls for each filter and straight signal, or use existing output level control to vary overall filter amount. The filters outputs are inverted relative to the straight signal. Combining the straight and filter signal at a mixing stage would provide two notches, similar to an envelope controlled phase shifter, only you have independent control of notch spacing. Invert the straight signal prior to mixing and you get straight signal with one or two (your choice) humps.

4) Wire up a 1meg pot in parallel with one or each CLM6000 LDR section, tying wiper to the ground end of the LDR (which is then lifted from ground), one lug to the other end of the LDR, and the other outside lug to ground. This will let you tweak the amount of “travel” of each filter section independently. I’m quite certain that vocal-like effects can be gotten by tuning the range, Q, and amount of travel for each section suitably.

5) Route the + side of C9 to its own input jack and modulate the filter sweep with something other than the signal source. For instance, plugging in a radio would provide a source of semi-random control signal. Miking the drum kit and feeding that to the input would modulate filter sweep with the song’s rhythm.

6) Drop R11 from 470R to some smaller value (e.g., 47R) for faster attack, and drop C10 from 100uf down to a smaller value for faster decay (though try to stay above 10uf just to keep envelope ripple at bay).

A very maleable design that permits a lot of interesting variations without much effort and with great certainty of interesting and useful outcome.

From: RickL on Ampage Effects Forum

Date: 3/19/2002 5:11 PM

I built this a couple of months ago and wasn’t really happy with it so I let it sit for a bit. I got back to it a couple of days ago, fiddled some, and now I’m happier.

I couldn’t get the Sweep/Calibrate pot to do anything, so I tried replacing R17 (from Sweep pot to envelope generator)with a 470 ohm (from 470k). Bingo, it works. Note: I used the schematic and layout from JD Sleep’s site.

I added a switch to select either or both of the filters.

I used and expanded on Mark Hammer’s suggestion to change the values of the filter caps. He suggested changing the values of the higher filter caps down to about 1/3 the value (3300p to about 1000p). I used a switch to select 1000p, 2000p or 3300p. I also did a similar mod on the lower filter.

For me this takes the BFF from an OK project to one well worth doing.