Radical Conversion | Discrete Opamp Distortion

In the depths of the Covid-19 pandemic, when everyone was stuck at home making sourdough and wiping down groceries with Clorox and picking up new hobbies out of boredom, I spent a couple months tracing and building clones of all the analog Boss distortion pedals in my collection. I wanted to pick them all apart and see what made them do what they do while also doing many hours of relaxing PCB design work and a whole lot of soldering. What came out of that project was a deep interest in simple discrete opamps like the architecture Boss has used in roughly a dozen different products starting with the OD-2 in 1985. It has appeared in hits like the BD-2, OD-3, and FZ-2, as well as more obscure cult classics like the MZ-2, PW-2 and MD-2. It is one of the fascinating elements that has tied a wide range of disparate Boss products together over the last 40 years.

Radical Conversion uses multiple discrete opamp architectures, including the classic Boss-design, to create a completely discrete, 27-transistor alternate universe homage to the Metal Zone, including the semi-parametric active EQ circuit. The titular Radical Conversion from integrated circuits to discrete opamps started primarily as a flex, because its crazy to make a discrete Metal Zone, but also because it sounds really good to create harmonically-rich drive sounds with lower performance opamp designs. All the subcircuits and filtering have been fine tuned for modern preferences and Radical Conversion adds a trio of preamp voicings to shape the overall voice of the pedal.

Volume - Sets the output volume of the pedal. Crank this one up.

Gain - Sets the amount of distortion in the primary gain stage. EVERYTHING in this design is contributing harmonic complexity, but this control sets the overall saturation level.

Preamp voicing (toggle)
I - The traditional MT-2 preamp voicing adds a cutting midrange boost before the primary gain stage. Often disabled in popular mods, this boost is the classic midrange snarl and works really well for adding pseudo cocked wah edge to riffs and leads. You’ll hear it mostly on the plain strings.
II - Tight preamp boost for a razor sharp low end response in the gain stage. Excellent for players in low tunings who don’t want any flub.
III - Fat preamp boost feeds more low end into the gain stage for added compression. Beef up single coils or a bright guitar or really lay into the riffs for sludgier tones

Bass - Peaking low end boost/cut

Treble - Shelving treble boost/cut

Mid Freq - Semi-parametric frequency control for the midrange band. CCW is lower mids down to ~200 Hz and CW is higher mids up to around 4K Hz

Mid Gain - Boost/cut control for the semi-parametric mid band

Activate - True bypass soft-click footswitch to turn the effect on and off

Power supply - Always use a high quality regulated 9V DC negative tip power supply with your BSRI Audio pedal.

Many thanks to Dr. John Snyder of Electronic Audio Experiments for his encouragement and for pointing me in the right direction a couple times in this project. Without his advice, I probably wouldn’t have solved a key design problem in making the more current-hungry active filter stages work.