To the metal

Here’s an easy project from Quincas “SynthDIYGuy” Moreira:

He builds a module that serves as an interface to an expression pedal, for no-hands variation of a control voltage. It uses two jacks, one potentiometer, two resistors, and an LED. Simple.

Except it gets less simple when we dig into it. The problem is that Quincas’s expression pedal is built around a 100kΩ potentiometer. That’s unusual. Most expression pedals use a potentiometer of about 10kΩ. I have an M-Audio EX-P pedal, whose schematic apparently is this:

Using the low limit pot the input resistance to ground can be as high as 62kΩ but as low as 12kΩ.

For such pedals the voltage divider, consisting of a 10kΩ fixed resistor and 100kΩ potentiometer, which Quincas uses for his interface won’t work very well. With 12V into the voltage divider (and the low limit pot all the way down) we won’t get more than about 5V out, and it’ll be pretty nonlinear. To get more reasonable behavior we’d have to lower the voltage divider resistances correspondingly, say a 1kΩ fixed resistor and 10kΩ potentiometer.

That means it’d draw more current. With no pedal plugged in, we’d have 12V going through 11kΩ to ground, so it’d draw 1.1 mV — not a huge current, okay, but ten times higher than with Quincas’s resistance values.

All right, but now let’s stir up more trouble. Suppose we put an input jack on the front panel, a switched jack; connect the bottom of the 10kΩ resistor to the switch terminal and the tip terminal to the top of the potentiometer. Now we can use the pedal not only to create a 0–10V control voltage but to attenuate an input control voltage.

Worthwhile improvement? I think so. But… now the combined resistance of the voltage divider potentiometer and the expression pedal, which is at most about 10 kΩ, becomes our module’s input impedance, and we’d like that to be higher.

And while we’re thinking about impedances, the output impedance can go as high as 24 kΩ. We’d like that to be lower.

To get out of these difficulties we can use an op amp as a buffer between the interface’s voltage divider and the pedal. And given that op amps tend to come in twos or fours — I don’t have any TL071s at the moment — we might as well put another buffer after the pedal. Then we can use a 100kΩ potentiometer after all, for a good input impedance, and we’ll have a low output impedance too. A 1kΩ resistor in series on the output to protect the op amp and we’re in good shape, on paper anyway.

The project’s gotten less simple. We’ve added a jack, and an op amp, and that means we need to use ±12V, and we should add filtering capacitors. Now our interface looks like this.

Plus the usual power header with diodes and capacitors. Still not at all a complicated circuit, just more complicated than Quincas’s.

Stripboard layout:

I built this in Kosmo format, though it’d easily fit behind a Eurorack panel too:

I’ll pretty up the front panel at a later date, but the rest of it’s there, and working. Couple of firsts here. This is the first synth module, and one of the first anything, I’ve built on stripboard, and it’s the first time I’ve used Molex hardware. Which really is slightly too tall in this configuration with the 2.5 cm wide panel and perpendicular circuit board, but I can live with it. Anyway, now I can make Molex connectors.

The mounting to the panel was done using a piece of thin sheet aluminum, cut out with snips and bent in a vise, with holes for the pot and topmost jack, and small holes for screws to secure the stripboard.

The red stuff is tape to insulate against shorts.

And now I can play synth with my feet — well, one foot.

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