Friday, August 30, 2013

Super Simple Linear Asymmetric Portamento -- Glissando -- Slew Limiter -- Glide

FYI, this is not the schematic for the Attack Decay Gate controlled portamento I just built, I will be posting that build/schematic very soon.  Thanks! (that schematic incorporates aspects of this schematic, so if you're at all into understanding analog circuitry, this post will help you understand the upcoming post)

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.

For those who don't know:  Portamento, glissando, glide, and slew limiter are all different names for the same musical concept. It's basically sliding between notes. Since Modular synthesizers are analog, their "musical note information" is represented by a voltage between 0 and 5v instead of a digital number.

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!


  1. Hello.
    Thank you for posting the portamento schematic. I have a 2 quick questions if you could answer them for me.

    1) Do pins 4 and 8 get hooked up as normal on the op-amp? Pin 4 to ground and power to pin 8?

    2) Where in the circuit does this get placed? I am using a CD40106 to produce square waves and momentary buttons to trigger them.

    Thanks in advance!

    1. I have the pins numbered as though you would be using half of a quad op amp. So, yeah, Pins 4 and 8 should be connected to the rail supply. For me, that is +15v and -15v but I think it will work with any power supply. This module is used to modify control voltage. lets just say control voltage is pitch information, among other things, so the circuit would normally be placed between a CV source like a sequencer/keyboard and an oscillator. I don't have all the information for your set up, but you want to use the circuit to modify whatever voltage you are sending to the oscillator as pitch information.

    2. Thank you for the quick reply!

      Understood. My current setup is super simple. I'm using all 6 oscillators on a CD40106 chip with small momentary push buttons to trigger them (rather than always on) and potentiometers to control the pitch of each.
      Here is what the schematic basically looks like but times 6 and with the push buttons I inserted between the potentiometer legs effectively breaking the circuit until pushed.(not shown)

      Thanks in advance for your help.

    3. cool, fun stuff. I understand the set up. sounds like this portamento circuit won't work for that set up though.

    4. I am realizing that now. Your circuit is to glide between the notes of one oscillator. I own many analog synths but just starting to mess with what's inside.
      Thanks again!