Grove Audio GMS-782b DUAL LFO/VCA User manual

GMS-782b DUAL LFO/VCA
PRELIMINARY OPERATION MANUAL
July 10, 2021 –Rev. 1
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INTRODUCTION
The GMS-782b Dual Voltage Controlled Low Frequency Oscillator/VC Amplifier provides low frequency
triangle, sine or square waves for use as modifiers or triggers for complex evolving synth patches. Two
functions, commonly used together are combined in each channel to produce a versatile voltage
controlled waveform generator. The LFO has a wide range from subsonic to low audio in three ranges
and provides a trigger function to reset the oscillator to a known state. The voltage controlled amplifier
has a selector switch allowing one of the three oscillator waveforms to be selected as its input. The switch
has a fourth position that allows selection of a front panel input to be routed to the VCA. The VCA can
then be used completely independently of the LFO. The output of the VCA is a function of the front panel
level control summed with the voltage control input.
BASIC FUNCTIONS
Generate
Scale
Wave Shape
OSCILLATOR- Oscillator frequency is set by a variable potentiometer and a range toggle switch. If
the frequency control voltage input is used, the final frequency control voltage value is a combination
of the voltage value at the control input and the position of the frequency control. The control voltage
affects the oscillator frequency causing a change of approximately one octave per volt of change.

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The sine, triangle and square waveforms are also supplied to a panel rotary switch that selects which
oscillator output is to be sent to the voltage controlled amplifier signal input. Each oscillator has a bi-
polar LED to give a visual indication of the oscillator frequency.
TRIGGER- When a control signal with a rising edge is presented to the Trigger input, the oscillator is
reset to 0 Volts output and the waveform starts rising in the positive direction. The trigger input signal
should be a 0 to +5 Volt signal. The length of the trigger pulse is not important. A continuous +5 Volt
signal on the trigger input will hold the oscillator in reset. When the trigger input causes a reset of the
oscillator waveform, the oscillator output will be reset to 0 Volts and if the trigger signal is released,
the oscillator output voltage will start the output signal on the positive phase.
AMPLIFIER- The internal voltage controlled amplifier in each oscillator section allows the waveform
selected on the WAVE selector switch to be modulated by an external control voltage input. At a
control input of 5 Volts the amplifier produces a gain of X1. The input voltage range of the control
voltage is 0-10 V so the approximately 10 V PP output of the oscillator can be raised to as much as
20 V PP at the VCA output.
BLOCK DIAGRAM

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TECHNICAL SPECIFICATIONS - GMS-782b
Frequency Range –High
Range
0.015Hz to 400Hz
Frequency Range - Low
Range
0.0015Hz to 10Hz
Frequency Range –Ultra
Low Range
0.00015Hz to 1.0Hz
Waveform Voltage Range
10 Volts Peak-to-Peak
Frequency Control Voltage
Standard
Exponential 1 Volt per Octave
Amplitude Control Voltage
Standard
Linear 1.0 V per Volt
Envelope Control Voltage
Range
0 - 5 Volts
Gate Control Voltage
Range
0 - 5 Volts (1.8 Volt Threshold)
Typical Input Impedance
100K Ohms
Typical Output Impedance
1K Ohms (short circuit
protection)
Module Power
+15 Volts 50 mA., -15 Volts 50
mA.
DOTCOM MTA-100 6 pin
connector
Module Size
Panel height - 8.75 inches (222
mm); Panel width –2.125
inches (54 mm); Depth (behind
panel) 2.1 inches (53 mm)