Triple LF-VCO

You can get the schematics PDF here.

This module sports three independent low frequency voltage controlled oscillators or LF-VCO’s that can also be synced and cross-modulate. Three LEDs give visual feedback of the activity of the three LFOs. The modulation inputs of all VC-LFO’s are normalized at the input connectors in such a way that everything can crossmodulate and sync.

LF-VCO A provides a Sine wave similar to that of VCO A of the Rungler. The Sine has no corners, which gives very smooth modulation. It has a wide frequency range going from quite slow to well into the audio frequency band, so it can be used as a sound source as well as a modulator. It’s inspired from the 3rd oscillator of the EMS Synthi.

This LFO is tied internally to LF-VCO C, the latter will modulate the former in FM and in combined FM-AM with the Fluct control (similar to that found on the Rungler). The “fluctuation” modulation is a combination of AM and FM witch softsyncs to the harmonics of the modulating signal. Therefore the modulation depth ‘feels’ more natural even when the frequency of the modulated waveform is increased. This results in an extremely natural sounding vibrato, much gentler than through a normal LFO.

Pitch is controlled manually. By mixing LF-VCO A & LF-VCO A Fluct, at a certain point, one can only hear the amplitude modulation which is interesting when modulating the pitch of an oscillator.

LF-VCO B has a triangle output and a pulse output. The triangle output can be modulated from ramp to triangle to saw, just like the MS20. The wave shaping is set manually. LF-VCO B frequency range is lower than LF-VCO A. The outputs are unipolar.

LF-VCO B has three operating modes, selectable with a switch :

  1. MODE : LF-VCO B is free running
  2. SYNC : LF-VCO B is synced to LF-VCO C or to an external signal
  3. HALT : LF-VCO B oscillates at a frequency set by its Rate control during the Rise of the Triangle wave of LF-VCO C and will “freeze” during the Fall of the Triangle wave of LF-VCO C. LF-VCO B will restart with the next cycle of LF-VCO C. When “frozen”, LF-VCO B will output a fixed, momentary voltage. It can be use to create swing. Halt mode works best on pitch.

The frequency of LF-VCO B can be modulated internally by LF-VCO C or by an external signal. This LFO is useful for clocking a sequencer or envelopes with the pulse. The triangle output can then be used as envelope.

LF-VCO C has a triangle and an pulse output. Rate goes from several minutes to 100Hz. LF-VCO C produces triangle waves in a frequency range set similar to that of LF-VCO B but lower, for really slow oscillations. The LF-VCO C Mod slows down the rate of the downgoing slope of the triangle. The rate of the upgoing slope remains the same. So, when e.g. speeding up the LFO and opening the C MOD knob the waveform will take on the form of an fast attack and slow decay envelope. The pulse output is high when the slope is going up and low when it is going down, so the pulse length will remain the same but the time between the pulses will increase.

EOF