The ASM-2 VCLFOs are basically low-frequency function generators that produce two waveforms.
The basic oscillator circuit consists of the two op-amps U502A and U501C connected respectively as an integrator and a Schmitt trigger. When the output of U501C is positive the positive voltage output of U501C is applied to P503, so a current (dependent on the wiper position of P503) flows in to the integrator through R508. The output of U502A ramps negative until it reaches about -5V, when the voltage on the non-inverting input of U501C will fall below the voltage on the inverting input (zero volts) and the output of U501C will swing negative. The negative output voltage of U501C is applied to P503. Current will flow out of the integrator through R508, and the integrator output will ramp positive until it reaches about +5V, when the voltage on the non-inverting input of U501C will rise above zero and the output of U501C will swing positive. The whole cycle then repeats.
The output from U502A is thus a triangular waveform with a peak-to-peak voltage of about 10V, while at the output of U501C a square-wave of the same frequency is available.
P503
is used to adjust the frequency of the VCLFO by varying the voltage
applied to the integrator input, which alters the integrator input current
and hence the rate at which the integrator ramps positive or negative.
The value of R507 determines the minimum integrator input voltage that
can be set by P503, and hence the minimum frequency of the VCLFO. The
value of R507 must not be chosen too high or the minimum frequency of
the VCLFO will be too great. On the other hand it should not be chosen
too low, or the minimum integrator input current at the minimum setting
of P503 will be comparable with the input currents of U502A. This will
result in unreliable operation of the oscillator at low frequencies.
The optimum value for R507 is 47 ohms, but the user is free to experiment
with this for best results.
The squarewave at the output of U501C has an amplitude of +/-15 volts, so the resistor attenuator network R512/R515 has been included in the output path to provide the standard +/-5 volt level.
Voltage control of the LFO is introduced by applying a control voltage to a half-wave rectifier circuit built around U501A and U501B and then taking the resultant output and injecting it as an offset voltage to the integrator input voltage.
An LED Indicator circuit is built around U501D and provides a visual indication of the operational state of the VCLFO. The triangle output is fed to this circuit resulting in the LED intensity following the amplitude of the VCLFO output.