Bullfrog synthesizer review

By Lucy Hattersley. Posted

Bullfrog is a collaboration between Erica Synths and Richie Hawtin (the legendary DJ and music producer from Detroit) and features a Raspberry Pi RP2040 microcontroller at its heart.

Raspberry Pi is used to help power this subtractive synthesizer (a device in which tones are created by removing unwanted frequencies). RP2040 is used to take care of MIDI implementation and in a Sampler/Looper voice card.

The Bullfrog itself an analogue synthesizer that uses Eurorack patch cables to connect various sound modules together. It can be used as a professional sound tool, but its primary purpose is educational, to learn how synthesized sounds are created: “Our goal is to both nurture a passion for electronically produced sounds and promote fun,” says Richie.

The front of Bullfrog is a smorgasbord of dials with instant appeal to anybody who loves tweaking and the feel of hands-on analogue technology. There are three main sections: VCO, VCF, and VCA/Delay (corresponding to the three elements of sound: pitch, timbre, and amplitude). To the right of this are envelope generators and a Sample&Hold section, while at the top sits a blue cartridge socket. This is where the (included) voicecards slot in. Voicecards patch the internals of Bullfrog and quickly expand the sounds to create a variety of noises. The kit comes with three voicecards: an acid bassline, sampler-loopers (that can record and playback any sound), and a sequencer. There are also three blank voicecards that you can patch yourself by soldering the points together with wires.

Around the back of Bullfrog we see input and output including CV and MIDI control inputs

To the rear are CV (control voltage) and MIDI (Musical Instrument Digital Interface) ports, phone and audio out, plus power sockets and config buttons. There’s a speaker set into the device itself, or you can use headphones.

Wired for sound

The 77-page manual is where things come to life. It walks you through sound generation, pitch, waveforms, overtones and harmonics, plus virtually every aspect of sound synthesis. Far more than just how to use the equipment, it covers the science behind sound. If there’s any criticism, it’s that it gets a little stuck in the weeds before getting you to patch together the components and start making noises. But this is nitpicking on what is a wonderful educational resource. Girts Ozolins from Erica Synths has made a YouTube video that explains the Bullfrog project that also includes a patching guide.

Taking things further

Bullfrog is more fun with a CV (controlled voltage) keyboard, and the manual mentions an Arturia Keystep or a MIDI keyboard. These enable you to turn the synthesized sounds into notes. It’s also possible to use Raspberry Pi to expand on the music abilities and learning. Either by using Pico to create a CV generator or by attaching a MIDI HAT to Raspberry Pi (see this OSA tutorial). Both of which could add programming aspects to this sound generator.

Girts Ozolins running a workshop in Hamburg

Erica Synths is using Bullfrog as an educational tool, and to that end has been running workshops using an XL version of the kit that also features an oscilloscope. They are hoping to get it into educational environments around the world.



An innovative educational resource that takes you through sound creation and is a fully working subtractive synthesizer to boot. We loved testing this one out.


Features: Analogue design, 8-octave voltage controlled oscillator (VCO), voltage controlled amplifier (VCA), voltage controlled waveshapes with pulse width modulation (PWM, voltage controlled amplifier (VCA)

I/O: DIN5 MIDI input, USB connector, CV (controlled voltage), phones out, audio out

Voicecards: cid bassline, sampler-looper, sequencer, 3 × black voicecards are included

From The MagPi store


Subscribe to the newsletter

Get every issue delivered directly to your inbox and keep up to date with the latest news, offers, events, and more.