I was one of those guitarists who spended more time with his effect pedals, then actually playing. I ended up exchanging my guitar for synths when it comes to making music after a while. Not really a surprise there, but my love of piling effects together into a huge unpredictable pile is still there, and somehow, even with a bunch of 19″ racks I haven’t been able to scratch that itch. You could use a computer these days, but nothing beats a bunch of wires and noisy electronics to spark creativity. Guitar pedals are nice and all, but do lack in certain areas when you try to use them with synths.

So, why not design some FX for my modular, designed to use with synths and handle those levels. There are quite a few out there already, most of them digital, but I rather want the ‘battered up pedal quality’ then ‘crispy clean studio quality’, those I already have in software or 19″ racks. It’s not that hard to get nice crispy clean effects these days, but unless you hook up those old effect pedals or spend a sizeable amount of computer power on a well designed plugin, it’s hard to get one which has that character you’ll find in a good guitar pedal.

I got plenty of ideas but I didn’t want to make a very complex one as a first build as I had a bunch of other design considerations to tackle. I’ve build some effects before which haven’t made it into my modular yet, so I thought to revive one of those designs first. I ended up choosing a ‘bit crusher’, or rather, a sample rate reducer. It’s not a novel idea in any way and you’ll find some existing schematics online with ease. It comes down to triggering a sample and hold circuit quickly enough, turning your audio signal into a ‘stepped’ version of itself. Lowering the ‘trigger’ oscillator frequency makes the effect more extreme as steps between ‘samples’ get bigger. It’s not a very complex circuit, and has a very straightforward implementation. In short, loads of fun for the whole family.

But first, I needed to have some kind of performance control. An effect shouldn’t be just ‘on’ all the time, a bit of control would be nice. A foot-switch as found on guitar pedals can take a beating, but isn’t all that ideal for use in a modular. An on/off switch is easier to operate by hand, but probably can’t stand up to the abuse. There’s currently an Electrix MoFx residing in my studio and while I’m not terribly fond of the sound, it does has interesting performance controls for live use or generally goofing around. It uses a combination of a push button toggle switch and a push button momentary switch. When the toggle switch is off, the effect is bypassed but turns on as long as the momentary switch is pressed. When the toggle switch is on, the effect of the momentary switch reverses, turning the effect off for as long as it is held. A nice big led indicates status. It’s intuitive to use but the idea needs some modifications to use in a modular, since, apart from manual control ,we also need to be able to control the on/off status through CV. So, time to dust of those logic IC’s I’ve been keeping around and see how it can be implemented.

Turns out a simple XOR does the trick. Switching the audio signal according to a voltage proved to be a whole other issue though. In the end I settled for a DG419 analogue switch. It’s expensive, but I couldn’t find any other way to cleanly switch a 10vpp signal without jumping through a lot of hoops. As an afterthought I also added a ‘gate-out’ to the circuit, which follows the effect status. It gives a way to control something else when the effect is engaged. I’m pretty curious on how it will turn out when in use and whether the UI ideas I had for this effect will be as much fun as intended.

Audio demo

A simple beat through the effect (on/off, triggered by hand). I’ll add a better demo when I get around to recording one.

Schematics

Build

For this one I ordered some PCB’s instead of using my trusty prototype board. It sure looks a lot nicer that way. I got a few things wrong on the PCB, but none that couldn’t be fixed easily. I should check schematics against the breadboarded version more thoroughly to avoid these stupid mistakes next time. I might build some with the extra PCB’s I have and put them up for sale if someone is interested. Right now I’m still looking for the right momentary switch to use, with the right feel and right size. One that could take some abuse.

Tweaks

You can put a different capacitor in C10 if you want a different range for the internal oscillator. Something like a 470pF will give you a range up to 20kHz or so making the effect almost inaudible at low settings. The 3n I’ve put in gives you a range up to about 8kHz, which IMHO gives a better result but you might want to play a bit with this if you desire a wider/smaller range.