Uploaded February 2017 | Updated September 2026, 1 week ago
Rough video... maybe I'll make a better one later.
This is the transistor and IC incarnation of my previous vacuum tube synth oscillators. There are two RF oscillators, one is a fixed 120kHz reference, provided by a signal generator, and the other is a variable frequency Colpitts oscillator. The VFO is controlled by a control voltage input run through an exponential converter.
The problem faced was trying to get the output to sit at zero Hertz when the frequency input is at zero Volts. The VFO can be higher or lower than the reference frequency and either case makes a sine wave. The further away from the reference frequency the VFO is, the higher the pitch. If the frequency isn't nulled, the tuning will be out, because there's a linear frequency offset on all notes.
My solution is to have a button that does two things. It drives the CV input to a negative voltage which sets the exponential output to zero and secondly it connects the audio output through a sample and hold circuit back to the linear frequency input. This creates a phase locked loop. When the loop is opened to play a tone, the offset voltage is stored in the S&H. It's not perfect (as can be seen) but it means I might be able to make more than one oscillator and not have to null them all individually.
Rough video... maybe I'll make a better one later.
This is the transistor and IC incarnation of my previous vacuum tube synth oscillators. There are two RF oscillators, one is a fixed 120kHz reference, provided by a signal generator, and the other is a variable frequency Colpitts oscillator. The VFO is controlled by a control voltage input run through an exponential converter.
The problem faced was trying to get the output to sit at zero Hertz when the frequency input is at zero Volts. The VFO can be higher or lower than the reference frequency and either case makes a sine wave. The further away from the reference frequency the VFO is, the higher the pitch. If the frequency isn't nulled, the tuning will be out, because there's a linear frequency offset on all notes.
My solution is to have a button that does two things. It drives the CV input to a negative voltage which sets the exponential output to zero and secondly it connects the audio output through a sample and hold circuit back to the linear frequency input. This creates a phase locked loop. When the loop is opened to play a tone, the offset voltage is stored in the S&H. It's not perfect (as can be seen) but it means I might be able to make more than one oscillator and not have to null them all individually.
