SCHLAPPI ENGINEERING
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SPECIFICATIONS
SCHLAPPI ENGINEERING
•Quadrature Sine Wave VCO / State Variable Filter
-with four phase related outputs: 0°, 90°, 180°, 270°
-or lter response outputs: LOW PASS, BAND PASS, HIGH PASS, INVERTED BAND PASS
•HIGH range from 10Hz to over 20kHz
•LOW range from 0.3Hz to over 600Hz
•Sine wave outputs are 5Vpp (+-2.5V)
•Grind section is a voltage controlled waveshaping section
-or pole mixing nonlinear feedback network
-GRIND and lter outputs are not limited and can hit 22Vpp (+-11V)
•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
•Grind section is a voltage controlled waveshaping section
-or pole mixing nonlinear feedback network
-GRIND CV inputs are unipolar 0 - 5V with bias from sliders
•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
Please address questions or issues to: eric@schlappiengineering.com
PANEL CONTROLS
SCHLAPPI ENGINEERING
Angle Grinder is a quadrature sine wave oscillator, lter, and waveshaping eect.
The SPIN section is a quadrature sine wave oscillator.
The GRIND section compares each phase against input signal, then subtracts the result 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 lter (of sorts).
Lorem ipsum
GRIND SLIDERS
Mixes the amount of signal to grind
from the associated SPIN output
Feedback playground
IN
Insert audio or cv here
OUT
Output from GRIND
INJECT
Direct input to SPIN
AC coupled on header
for soft sync-like eect
SPIN
Coarse tuning control
RANGE SWITCH
LOW 0.1 Hz to 500Hz
HIGH 10 HZ to over 20kHz
GRIND -> SPIN
Feeds the output of GRIND
into SPIN (lter/osc)
FINE
Fine tuning control
DAMPING
Counteracts oscillations
V/OCT
Volts per octave cv control
over SPIN
FM 2
Exponential CV control over SPIN
GRIND CV
CV control added to associated GRIND SLIDER
*FM1 is normalled to GRIND OUT for easy noise making
but you will want it turned down for tracking pitch if there
FM 1
CV attenuator
switchable betwean linear
and exp by header on rear
SPIN OUTPUTS
Four phase related outpus
0°, 90°, 180°, and 270° if oscillating
LOW PASS, BAND PASS, HIGH PASS, and INV BAND PASS if ltering
is no cable plugged into it
HOW IT WORKS
V/OCT
OUT
IN
IN
SUM
(mixer)
square waves are subtracted
from input signal
- - - -
GRIND
GRIND -> SPIN
DAMPING
(FILTER IN)
INJECT
AUDIO
INPUTS
(OSC/FILTER CORE)
FILTER
OSC
FM2
FM1
SPIN
(COARSE)
FREQUENCY INPUTS
SPIN
OUTPUTSMODE
LOW PASS HIGH PASSBAND PASS
0º 90º 180º 270º
FINE
INVERTED
BAND PASS
COMPARATORS
osc/lter outputs are compared
against an external input
sliders sum with cv inputs
to provide voltage control
over vcas
creating four square waves
ALL PASS
(PHASE SHIFT)
TWO PARTS: SPIN & GRIND
SPIN
This is a quadrature sine wave oscillator. That means it is as oscillator that outputs four sine waves
ninety degrees out of phase with each other as shown:
Internally the circuit is very similar to a state variable lter except there is an always-on positive
feedback path optimised for clean oscillation. We can cancel the positive feedback with a negative
feedback path (this is what the DAMPING knob does) and stop it from oscillating.
When enough of the grind signal is fed into the SPIN section it overwhelms the oscillations and
starts to lter (it will do both at the same time sometimes.) The SPIN outputs then become the
familiar LOW PASS, BAND PASS, HIGH PASS, and an INVERTED BAND PASS.
GRIND
This section consists of four comparators, four vcas, and a mixer. Each SPIN output is compared against the
input and depending on which one is higher in value a square wave is created. These square waves are fed
into the VCAs and their amplitude is controlled by a sum of the GRIND CV INPUTS and the GRIND SLIDERS.
These signals are then subtracted from the input signal creating waveforms like these:
As the GRIND -> SPIN knob is turned clockwise these forms paths will transform into voltage controlled
nonlinear feedback paths and by mixing them together unpredictable shapes are formed.
SPIN
-With GRIND->SPIN and DAMPING fully CCW
the four SPIN outputs will have phase-related
sine waves
-These all come from the same oscillator
and will be at the same frequency
-This will track v/oct (check that FM1 is down)
-At audio rate you can process them seperately
to create stereo eects or mix them together
-At LFO rate you could use them for quad panning
by sending one audio signal to four VCAs and
control each vca with one phase
0°
90°
180°
OSCILLATE
KNOB POSITIONS
IN FULL CW
GRIND -> SPIN FULL CCW
DAMPING FULL CCW
FM 1 FULL CCW
270°
ALL TOGETHER
GRIND - OSCILLATE
-With GRIND->SPIN and DAMPING fully CCW
-With nothing plugged into the input*
-Turn GRIND IN clockwise to feed the SPIN
section into the GRIND section
-Try listening to the GRIND OUT
and exploring the sliders one at a time
-The 90° output is normalled through an ALL
PASS (phase shift) lter to the GRIND IN
-These oscilloscope traces were taken at 1kHz
but the wave shapes will vary by frequency
-These traces are representative of the
waveforms that you can nd as the outputs
are compared against and subtracted from
the input
GRIND OUT
NO SLIDERS UP
GRIND OUT
ONLY FIRST SLIDER UP
OSCILLATE
KNOB POSITIONS
IN FULL CW
GRIND -> SPIN FULL CCW
DAMPING FULL CCW
FM 1 FULL CCW
ONLY SECOND SLIDER UP
GRIND OUT
GRIND OUT
ONLY THIRD SLIDER UP
*you must turn up the GRIND IN
to get any signal out of the GRIND section
ONLY FOURTH SLIDER UP
GRIND OUT
GRIND - OSCILLATE
-Same patch as previous page but raising
more than one slider at a time
-Notice how the “one slider up” waveform
is dierent than the one on the previous
page?
-This is because these shots were taken at
a dierent frequency
-As more sliders are brought up we
converge on a pyramidal shape
GRIND OUT
NO SLIDERS UP
ONLY FIRST SLIDER UP
FIRST TWO SLIDERS UP
GRIND OUT
GRIND OUT
OSCILLATE
KNOB POSITIONS
IN FULL CW
GRIND -> SPIN FULL CCW
DAMPING FULL CCW
FM 1 FULL CCW
FIRST THREE SLIDERS UP
GRIND OUT
GRIND OUT
ALL FOUR SLIDERS UP
GRIND - EXTERNAL
-With the SPIN section oscillating in LFO mode
plug in an external wave form and listen from
the GRIND out
-The waveforms to the right show a triangle input
in yellow and GRIND out in blue
-Each phase of the internal oscillator is compared
against and subtracted from an external signal
-If the internal oscillator is at a low frequency
the output with resemble a supersaw
-If the internal oscillator is at audio rate various
metallic overtones can be created
GRIND
KNOB POSITIONS
IN FULL CW
GRIND -> SPIN FULL CCW
DAMPING FULL CCW
FILTER
-With a external signal in the INPUT
and the GRIND->SPIN knob turned CW
the external signal will disrupt the
oscillations
-The outputs will become
LOW PASS, BAND PASS, HIGH PASS,
and INV BAND PASS
-GRIND sliders are now voltage controlled
non-linear feedback paths
Experiment with all controls
-Four outputs responding to a triangle
input at various freuencies are shown to the right
FILTER
KNOB POSITIONS
IN FULL CW
GRIND -> SPIN 75%
DAMPING 25%
INJECT
-The INJECT jack bypasses the GRIND section
as a direct input to the oscillator/lter core
-It is header selectable to be DC or AC coupled
on the read of the module
-AC is selected by default
-The AC mode is a high pass set to turn an
incoming signal (square or saw preferably)
into spikes to reset the oscillator
-This can act as a soft sync
-The top set of scope traces shows this:
YELLOW: input
BLUE: Grind out
PURPLE: Low pass out
BLUE: Band Pass out
-You can see spikes on the band pass on the
each reset
-Bottom set of scope traces shows DC coupled
with a very large input signal
-The input signal will add to any other signal
present and can mix with or clip depending
on amplitude
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