Simple Pitch LFO

By Scott R. Gravenhorst, 06-23-95, Updated 05-25-96

This FatMan modification adds a simple LFO function to modulate the pitch of a VCO. Two of these circuits may be built to add LFO pitch modulation to both VCOs. Modulation is implemented using the 555 timer's modulation input on pin 5. This method completely isolates the modulating signal from the main pitch control voltage.

No trace cutting is required.

CAVEAT: Due to the use of a simple, but brute force filter for wave shaping in this circuit, there is interaction between the LFO rate control and the effect level. That is, as the rate is increased, the level control may have to be increased to compensate. Also the filter cap switch will act similarly. I am currently working on a voltage controlled (or manual) LFO that will not have a filter, but rather will provide either square or triangle outputs and will not exhibit the phenomenon described above. The new circuit, will of course, be more complex.

The Effect: Assuming that one VCO is producing a constant pitch, when mixed with a VCO modulated by an LFO, the phase relationship between the VCOs changes constantly. A little lead, then a little lag, etc. This causes a nice 'rotating' flange effect that can thicken the sound produced. At higher LFO frequencies, a warbling effect is produced. A level pot is provided to control the 'swing' and a frequency pot is provided to control the rate. Two switches control the waveshape from a square wave to a near sine wave. These these controls and the switches in combination with the pitch and pitch offset controls determine the particular effect. There is also an LFO cut-out switch.

How It Works:
The LFO is a 555 timer IC wired as an astable multivibrator (oscillator). The square wave output on pin 3 is then fed to a very low frequency lowpass filter to provide a near sine wave output. This 'sledge hammer' technique was used to simplify the circuit as well as attenuate the output of the LFO 555 timer. Varying the component values will produce other waveshapes. The filtered signal is converted to AC by passing through a large in- line capacitor and applied to the modulation input of the VCO 555 timer IC. The changing voltage on pin 5 of the VCO 555 timer causes the trip point to change and thereby changes the period of the oscillator and thus it's pitch.