Fuzzy Balls

Envelope Filter

Courtesy/Thanks to: Marjan Systems/Marjan Urekar 1999.-2001

Description: This is modified Electro Harmonix Bass Balls Envelope Filter Pedal, with some design inputs from EH Dr.Q and Jack Orman's Dr. Quack. Basically BB has two bandpass filters, tuned to emulate human vocals, with center frequency controlled by envelope derived from input signal. All the controls were Sense (amount of envelope), and select fuzz/clean signals as filters input. Dr.Q is basically the same, with single filter stage, without input buffer (lame!) and with Normal/Bass response selection.

I liked it's funny sound, and while it's guitar pedal, it sounds very interesting on vocals and synth bass.
If you take a look at it's schematics, you'll find it easy to add many modifications with ease. While you can use all my mods, you can also discard some of them and fit it to your needs, and tweak some part values to your taste. It may seem too complex, but it has very good potential for such a simple cct.

How it works:

Switching On is standard, inserting jack in Input for battery or DC In jack for ext supply. Bit of filtering and diode
protection, I've skipped standard 7809 as I have nice & clean external PSU. True bypass, based on Jack's ideas, with effect-on indicator. Input is buffer/amplifier, with gain adjust trimmer (x1-x50). I need this, as I use various external signals from processing, maybe your guitar won't need it (stock BB doesn't have it), but my practice
is to amplify guitar signal to line level (S/N ratio is then better). Set it to your taste. Input 22u caps make cheap bipolar cap - just in case of signals with bias (this may be too much for guitar use).

Next stage is amplifier/rectifier. Set trimmer to get largest envelope out from your signal. It feeds diode rectifier to extract envelope which controls filter. Envelope follower has Sense pot to select how "sensitive" envelope follower will be to input signal, it also has switch with three "response" ranges set by different caps, variable attack (lag to input signal) and decay (time needed to "recover" from last signal), simple buffer with 2 pots controlling amount of envelope feeding each filter. You can choose to control them separately, or to control both of them with single pot (SYNC2->1). Also, there's manual control of center freq, simple pot, routable to first or both filters. Default is DC but inserting FILTER CV jack makes it controlable by any ext voltage source (CV, pedal, LFO...). Those strange diodes in there protect cct from inserting too large/negative voltage.

There's nice feature borrowed from Dr. Quack with LED lifting ref. voltage at 2-3V for TL072 follower, which also can be nice external POWER ON indicator. LED in series with ATTACK control is envelope level indicator.

U3A is V/2 reference needed for correct opamp supplies. Filter stages are pretty the same, standard cookbook
bandpass filters (as in wah-wah), with different ranges. Each has 3 way range switch, to cover broad spectrum
of frequencies. Tweak those caps to get octave response for your specific guitar. By inserting signal in different
points, you acheive different responses (NORMAL/BASS MODE). First stage has lower frq range so I made it that
it can filter the same signal as the second one (PARallel mode, as in BB), or to use signal from second stage's
output for some strange effects (SERial mode, hipass like). Transistors are used as VC resistors here to tune
the filter. Not perfect, but simple and cost effective for such cct. 100p caps prevent self-oscillatons in opamps,
those sole 100n caps should be close to each IC (or soldered beneath) across the rails. Envelope follower section
is also used as fuzz generator, with two modes - standard BB one, no clipping (1) and mode (2) with Ge diodes
used to clip signal in standard Rat fashion. You can select it as SOURCE for filters, or to use clean signal.

Outputs from both filters, clean boosted, and fuzz section are mixed via level pots and large 10u caps (you