I just finished designing a far more comprehensive portamento module but I'm waiting on a few parts in the mail to complete it. While that's brewing, I thought I'd take the time to show an extremely simple and awesome linear portamento circuit. It's similar to a portamento modules I worked out in an earlier blog post, this one has the proper resistors to give it a 100k input impedance and a 1k output impedance which I find ideal for modular synthesizers.
An overly simplified example: a C note might be represented by 1v and a D note might be represented by 1.5v. If we wanted to slide between these notes, we would alter the rate at which it the circuit could move from 1v to 1.5v. We do that with an RC Circuit. The R and C are resistor and capacitor.
The two main types of portamento circuits for modular synthesizers are exponential(logarithmic) and linear. For linear portamento, the slide time is dependent on the difference between the voltage(note) values. There is a longer slide time to get from C to G than from C to D.
In exponential portamento the slide time is not dependent on the difference between voltages(notes). The slide rate is based on internal resistance, so it is consistent no matter the distance between voltages(notes).
The circuit below is linear portamento. There is a potentiometer labeled 1M in the middle of the circuit. This is the main R in the RC circuit and determines the amount of portamento. The more resistance we add with this Potentiometer, the more portamento we get. By adding a switch and diodes, we give can voltage a path through a diode and around the potentiometer. The diodes allow either positive or negative voltage through dependent on it's orientation. This gives us the ability to have slide only on ascending notes or only on descending notes
The left side is the input and the right side is the output.
2 X 100k resistor
2 X 1k resistor
1 X 1m potentiometer
2-3uF non-polar capacitor
2 X diodes 1n914 variety or similar will work
1 X spdt (center off) switch
2 X op amps. either 2 X 741's or a single TL072 or half of a LM324 or whatever
I've tested this circuit on a breadboard and it works great but I didn't build a module based on it. It's simple enough that it could be easily done on prototyping board. ENJOY!