Percussor
Digitally-Controlled Analog Kick Drum With Tap Tempo
Overview
Have you ever seen one of those one man band performers with the bass drum on their back? It's kind of cool with how the bass drum helps keep the beat and can really flesh out a solo performance. If you have something that will do it for you, it also helps keep time and sounds way cooler than a boring metronome click. So I started looking around at analog kick drum circuits and found one that had some great sound shaping capability and decided I had to make a tap tempo kick drum pedal.
How It Works
The concept for the Percussor pretty straightforward. It has three main parts: guitar audio path, trigger generation, and the kick drum circuit. The guitar audio path allows for the kick drum sound to be mixed in with the guitar signal so that you can send it all to an amplifier and play it together. It also has the ability to send the kick drum signal out on it's own, acting as a standalone kick drum generator if you want to send it to its own channel or something like that. As you can see below, the guitar audio path is simply an inverting input buffer and a summing amplifier, so that overall the output is in phase with the input.
Percussor Guitar Audio Path
The kick drum circuit actually isn't mine, I found it third hand on a blog post here. It's a pretty clever design and only uses a single TL074 quad opamp package to provide controls for frequency, decay, drive, and tone, along with a two position attach switch. I did need to convert it to single supply voltage and I made a few small adjustments to taste, but it's certainly not an original design on my part.
​
The first opamp is used for conditioning the input pulse so that it will work well with the following stage.
Percussor Pulse Conditioning Stage
The second opamp stage is a twin T filter where the actual kick drum sound is created from the pulse by filtering the trigger. Changing the two different filters allows for changing the frequency and decay of the kick drum sound, while the attack switch allows for a faster and slower setting.
Percussor Twin T Filter
The third stage is a simple soft clipping opamp stage to add some drive/grit to the kick drum sound. While the clean kick drum sound is great, sometimes you want something that is a little more modern or edgy, and the drive can do that.
Percussor Drive Section
The final stage is a tone control, buffer, and volume control. The tone control is useful for rolling off some high end if you have a very bright amplifier or lots of drive on the kick drum sound. The buffer adds a little gain and allows for driving the volume control well, and the volume control allows you to adjust the output level so that it sits just right with the guitar.
Percussor Tone Section
While the kick drum circuitry is cool and all, it doesn't do anything without a trigger pulse repeated regularly to create the pattern. To do this, I used a microcontroller to produce 2 ms wide pulses according to a tapped in tempo. This allows you to tap in the tempo you want and then the drum will repeat during that period indefinitely. Additionally, I put in some press and hold code for the tap tempo switch so that it also has a manual mode where the kick drum will only sound when you tap the TAP footswitch, making it function like a higher tech foot stomp pedal. I did have the though that, with all the unused pins, I should allow for different patterns, or put in a snare drum circuit, too, but I decided that those were outside the original scope and could be added later.
Percussor Trigger Generation
If you would like to make one of these yourself, you can find everything need here. Now go form the next great one man band!