Friday, August 10, 2012

Exponential Portamento (Slew Limiter) Module

My friend Joel--a fellow synth geek--built this circuit for his Roland Modular System and I thought I'd give it a try.  Basically, this is what's called an RC Lag Circuit.  I played around with a few different combinations of potentiometers and capacitors to see what worked best for me.  This circuit is for exponential portamento--meaning it is time constant no mater the distance between notes--and the potentiometer determines the length time.  I ended up with a 10K pot and a 10uF cap--it fit the range and sensitivity I liked.

There's almost nothing to the circuit.  I breaded it up in about 2 minutes and spent about 10 minutes changing out the values of the 2 variable components to see what I liked.  I'm using and LM324 which is a quad op amp IC.  The circuit only uses 2 op amps so I figured I might as well double it and create 2 portamento circuits--maximizing my space and moving towards my eventual goal of having the ability to have 4 separate synthesizers paths in one modular rack.
LM324 -->

I put it all together on a perf board and tested both circuits out.  They seemed to work well but there's something I wasn't noticing, that will come into play later...

I got a drill press about a month back and it's really made panel production much cleaner, easier and faster.  I drilled my holes for my 1/4" jacks, potentiometers, and mounting screws.  Something you can't see in this pictures is a stepper bit for cutting holes in metal, it's very handy.

So, after I got this thing all mounted up and plugged into my system, I noticed something I hadn't noticed when I breaded it--there is a small amount of portamento--even when the knob is all the way left. I asked Joel about it, and he said his does the same thing and that's why he built in a "true bypass switch"--something I had overlooked.  So I went ahead and wired one of those in for each circuit.  I used 2 DPDT switches in a way to disconnect the circuit completely from the signal path.

Here it is all racked up in my case.  The portamento is the 3rd module from the left.  The bypass switches are above the potentiometers. An interesting artifact happens when you switch on the portamento while sound is coming out, there's this cool little ping sound, like a ray gun charging or something.  These sort of cool anomalies, I guess is why it's fun to build your own stuff.

Up next, I have a noise circuit and a LFO circuit I bought from MFOS.  I am also working on a buffered signal splitter type thing too.


  1. How does this portamento circuit connect into what you already have? Is it just on the output, or is it more involved than that?

  2. it has an input and an output. the input is control voltage which is basically Note information, which the oscillator uses to determine what frequency or "pitch" to produce. The Portamento module fits between what is creating pitch information and whatever is receiving pitch information. The portamento circuit allows you to adjust the rate at which it changes from one control voltage to another control voltage. For this module to work, the pitch information needs to be communicated through voltage (it won't work with midi or any digital communication). I'm sure this basic principle could be applied to much more than modular synths.