Voltage controlled LFO: Flat Version

Voltage controlled LFO front view
Voltage controlled LFO

This is the flat version of my VC-LFO I’ve build this flat version to minimize the depth of the module and avoid the wiring for the potentiometers. A VC LFO with multiple synced output waveforms is a very useful and versatile module. You can’t have enough of them. They can add a lot to sounds making them more animated. This one provides triangle, ramp up, ramp down pulse. square and sine wave output (-5V to +5V). The frequency range is easily adjusted to your needs from some minute per cycle up to 700Hz. I started with the VC LFO design form Ray Wilson MFOS but changed the exponentiator and pulse adjust schematic completely. I have added a range switch and a linear FM input as well.

Specs and features

  • Synced triangle, ramp up, ramp down, pulse, square and sine wave output
  • Output -5V to +5V
  • log and lin CV input
  • Temperature compensated
  • Voltage controlled pulse width
  • Range switch
  • Coarse and fine frequency setting
  • Runs on +/-15V and +/-12V
  • Power consumption below 10mA each rail

The documentation and the Gerber files for downloadcan be found in my website.

Voltage controlled LFO schematic
Voltage controlled LFO schematic back PCB
Voltage controlled LFO schematic
Voltage controlled LFO front PCB

C1 and associated components comprise a linear voltage to log current converter. IC1A sums the control voltages. IC1B provides the temp compensation realized with KTY81-110. TR3 adjusts the V/Oct characteristic. Q1 and Q2 forms the log converter with IC1D as constant current source. IC1C scales the control voltage for the linear FM input. The transconductance of IC1OTA1 controls the frequency of the oscillator. IC2C, C1 and associated components comprise an integrator. When current flows into IC1OTA1 output the integrator ramps up, when current flows out of IC1OTA1 the integrator ramps down. When the integrators output goes above the threshold of comparator IC2D its output goes high. The output of IC2D is fed to the non-inverting input of IC1OTA1 OTA through D1, D2, R1, R2 and TR1. TR1 balances the current flowing during the high and low periods of IC2D. With TR1 you can adjust the symmetry of the triangle. While IC2Ds output is high current flows out of IC1OTA1 OTA and the integrator ramps down until the voltage at the input of IC2D goes low enough to overcome the hysteresis provided by R13 and its output goes low. When this happens the comparator starts to ramp up again and thus we have a triangle wave at the output of IC2C. The bias of the comparator IC2D is controlled by the current generated by the linear voltage to log current convertor. This controls the current that flows in and out of IC1OTA1 and thus the frequency of the oscillator.

The sawtooth is created by mixing portions of the original triangle wave and an inverted version of the triangle wave. N-FETs Q1 and Q1 are used as analog switches.

Voltage controlled LFO screenshot waveforms: ramp up, pulse, square
Voltage controlled LFO screenshot waveforms: ramp, pulse, square
Voltage controlled LFO screenshot waveforms: triangle ramp down, sine
Voltage controlled LFO screenshot waveforms: triangle ramp down, sine
Voltage controlled LFO back view
Voltage controlled LFO back view
Voltage controlled LFO side view
Voltage controlled LFO side view