Saffari: vibrato circuit

The vibrato circuit for the Saffari was originally based on this one from Transistor Electronic Organs for the Amateur:

reference vibrato circuit diagram

I couldn’t get it to work (maybe my substitute transistors weren’t close enought to the original?) but it looked very much like a Phase-shift oscillator so I tried out some variations on that circuit with the 2N3904 transistors I had and pretty quickly had a beautiful sine wave on the oscilloscope.

Here’s what I ended up with:

actual vibrato circuit diagram

I picked the capacitor values by ear, looking for a reasonable vibrato speed. Putting the resistor and capacitor values into a RC oscillator frequency calculator I get a frequency of about 3Hz (3 cycles per second), which sounds about right. When you turn the vibrato speed knob (RV18) down to zero it gets maybe 5-10 times faster, which is plenty fast for an organ vibrato.

Then I had to figure out how to apply the sine wave to the oscillator circuit to make the square-ish wave into a beautiful vibrating organ tone. That’s for next time.

January 17, 2023