Easyvibe LFO?

[nordine]

Hey there,
by this point im completely convinced what makes or breaks a modulation circuit is the quality of its LFO,
having that in mind... i was tinkering with the incredible Easyvibe (where is John Hollis these days?) ..and tried to modify the waveform, particularly its symmetry.. but failed to do so...what kind of LFO is this? ...and how to alter its symmetry?

heres the circuit:



cheers

[R.G.]

what kind of LFO is this? ...and how to alter its symmetry?

It's an integrator plus a Schmitt trigger. The opamp with the 0.1uF feedback cap is the integrator. The opamp fed through the 47K from the integrator is the schmitt trigger.

It work by the schmitt trigger banging against either the most positive or most negative power supply. This output is fed through the 470K to the integrator. The integrator's + input is fed from (about) half the power supply, so both most positive and most negative force it to ramp the opposite direction from the voltage its fed. The Schmitt trigger stays latched up until the integrator ramps enough to make it flip the other direction. This inverts the input to the integrator, which then ramps the other direction again.

The symmetry is determined by the symmetry of the Schmitt trigger's output with respect to the reference voltage on the integrator's + input. Perfect symmetry is when the reference is exactly in the middle.

You can force it to do asymmetry by replacing the 470K resistor with a 1M pot and two diodes. The diodes connect in series across the outside lugs of the 1M pot, and the input to the integrator connects to the junction of the diodes. The output of the Schmitt trigger connects to the wiper. The diodes force the input to the integrator to go through two different resistances on positive ramps versus negative ramps instead of a single 470K pot both ways. This forces the integrator to ramp faster on the side of the pot that has the lowest resistance and slower on the other.

[nordine]

That was very informative and useful, thanks a lot R.G.