Schlappi Engineering Angle Grinder

Angle Grinder is a quadrature sine wave oscillator, filter, and distortion effect.

The SPIN section is a quadrature sine wave oscillator. The GRIND section compares each phase against the input signal, then subtracts the resulting square waves from the input signal. If the spin section is either damped enough or enough signal is fed into it from the GRIND section then it will stop oscillating and become a state variable filter.

The Angle Grinder tracks volts per octave and can be used as a sine wave oscillator with voltage control over harmonic content available at the GRIND OUT. With no input it’s four phase related sine outputs (0°, 90°, 180°, and 270°) remain clean from 0.1Hz to over 20kHz and are suitable for quad panning or XY applications.

With an input signal applied the GRIND out ranges from quadrature pulse width modulation (at LFO rate) to sync-like metallic tones (at audio rate). When GRIND is fed into the SPIN then it becomes a nonlinear feedback network with the faders and corresponding CV inputs providing instinctive control over phase cancellation, oscillation, and distortion. The four phase related outputs will become low pass, band pass, high pass, and inverted band pass filter outputs.

The function of the Angle Grinder depends on both the position of the knobs and the presence of an input signal.

Quadrature Sine wave VCO with four phase related outputs: 0°, 90°, 180°, 270°
HIGH range from 10Hz to over 20kHz
LOW range from 0.3Hz to over 600Hz
Tracks 4+ octaves of volts per octave at the V/OCT input
FM1 is selectable linear or exponential FM via jumper on back
FM2 is additional exponential V/OCT jack
With no input GRIND section provides voltage controlled harmonic content
With input GRIND section compares each phase against GRIND IN signal
Each slider subtracts the compared signal from the input signal, creating complex waveshapes
When GRIND is fed into the SPIN section the outputs become a state variable filter
DAMPING counteracts the oscillation and allows an external signal through
When filtering the four phase related outputs are: LOW PASS, BAND PASS, HIGH PASS, and INVERTED BAND PASS
When filtering the GRIND section is a nonlinear feedback network for harsh and unpredictable timbres
The four phase feedback paths can be separately voltage controlled
The INJECT jack can be used as a second input for either audio or CV, bypassing GRIND
All analog OTA based design
18 HP
Power requirements :+12V 81mA, -12V 78mA