Fairfield Circuitry 20% More Slew Limiting Distortion

Explorations around a schematic found among old papers of mysterious origin, suspiciously similar to a datasheet sq/tri oscillator. All that was needed was to connect the dots.



SLEW LIMITER CORE

A slew limiter smoothes out how quickly incoming voltage can change, limiting it to a fixed v/sec. slope. since time and frequency are intrinsically linked, this acts as a steep, somewhat distorted low pass filter.

When it resonates (FEED), these fixed slopes become triangle wave oscillations, giving 20% more its unique sound. Your input signal can overwhelm this oscillation, making for pseudo-wavefolding and synthetic distortion artifacts.

This topology can result in quite a dark sound when SLEW is low, as there will not be much harmonic content present beyond the cutoff frequency. You can use BLEND to mix in a square wave signal that can help bring back some higher harmonics.

LOW PASS GATE

20% More has a slightly resonant, 2 pole, 12dB/oct low pass filter at it's output. It offers a nice counterbalance to the grit of the slew limiter, and allows tasteful reigning in of the crackling potential present in the circuit.

ENVELOPE FOLLOWER

There is a simple internal envelope, its main purpose is to allow high feedback settings while avoiding (possibly unwanted) oscillations when no input is present. Both envelope destinations achieve this through different means.

When the envelope goes to the LPG, it is a bit like a sidechain input, where your playing dynamics open up the filter and let highs pass through, giving some nice springy west coast bongos; the LPG knob needs to be set low for the gating to be effective.

When the envelope goes to FEED, the average feedback is lowered slightly, loud playing will bring it back up. As loud inputs also fight the oscillations, interesting swashes of noise arise as your notes decay.

Play with INPUT. Interactions become intuitive.

SUB OCTAVE

20% More can route a squared version of the signal to one or
two filp-fiop based clock dividers, giving sub-octave and sub-sub-octave effects. It is only heard when BLEND is up somewhat. The sub octave is summed through diodes, giving it a characteristic 75% pulse.




FULL WAVE RECTIFIER

Envelope following requires a strictly positive input voltage. To achieve this, a full wave rectifier circuit is used. It is repurposed in the audio path as a switchable octave up. This particular implementation squishes very hard when INPUT is beyond noon. Enjoy.

CONTROLS


INPUT
'mu amp' jfet preamp, lots of gain

BLEND
mixes in a square wave / sub-octave copy of the signal

VOLUME
output volume, gets very loud

SLEW
slew limiter frequency

FEED
slew limiter feedback

LPG
low pass filter

D - OCTAVE DOWN
clock division of the blended square wave,
-1 or -2 oct

U - OCTAVE UP
analog octave up, full wave rectifier

E - ENVELOPE DESTINATION
routing of the internal envelope
either added to the (L)PG or (F)EED controls


INTERNAL CONTROLS

There are two user adjustable trimpots inside the pedal; they are labeled TR3 (drive) and TR2 (sens). The two other trimmers set the jfet bias and v/oct calibration, they should not be adjusted by the user.

DRIVE
Sets the amount of signal going into the JFET clipping stage, another 20% more?

SENS(ITIVITY)
Sets the gain of the internal envelope, allowing you to attenuate or exaggerate the response to your playing dynamics.

TECHNICALS


dimensions: 4. 7" X 3.8" X 1.9"

cv input range: 0 - 5v normal ; 0 - 8v max 9VDC
power supply: 20 mA@ 9.6 VDC