BSRI Audio Hydrazine

The BSRI Audio Hydrazine is a fuzz pedal from BSRI Audio, described by Delicious Audio as an emerging Baltimore-based builder. It is a modern take on the Harmonic Percolator fuzz circuit, built with period-correct NOS germanium and silicon transistors and germanium diodes¹. Pedal of the Day reviewed it in April 2026 as the Hydrazine Harmonic Percolator Fuzz, in a black and gold enclosure².

The circuit it comes from

The original is the Interfax HP-1 Harmonic Percolator, a two-transistor fuzz that used a PNP germanium transistor paired with an NPN silicon transistor, allowing it to run in a standard negative-ground configuration³. Guitar Pedal X notes the original featured 2N404 germanium and 2N3565 silicon transistors⁴. Unlike most germanium fuzzes, the Harmonic Percolator uses clipping diodes³, which is central to how the design behaves.

The Hydrazine follows that blueprint with its own NOS germanium and silicon transistor pairing and germanium diode clipping². Pedal of the Day describes it as "a modern interpretation of the Interfax Harmonic Percolator with updates to give players a wider range of tones"².

Controls

The Texture knob is the Hydrazine's distinguishing control. Delicious Audio's Paolo De Gregorio wrote that it "allows to vary the clipping threshold, producing a wide range of tones 'between transistor overdrive and compressed full diode clipping distortion'"¹. In practice that means one knob sweeps the pedal's character from a softer, transistor-driven edge to a harder, fully clipped distortion, which is a wider span than the original circuit offered on its own.

The Tone knob is an addition rather than a restoration: "The Tone, knob, not present in the original, delivers basic tone sculpting"¹. The core amplifier circuit has also been modified for improved clarity and punch¹.

How it sits among other Percolators

The Harmonic Percolator has attracted several modern builders, and the Hydrazine's approach of NOS parts with circuit updates is a common one²¹:

• Fredric Effects builds a Harmonic Percolator that "uses the correct vintage transitors (NOS 2N404 Germanium and 2N3565 Silicon)"⁵, and also offers a silicon version using an East German diode array paired with NOS silicon transistors.
• Bowman Audio Endeavors makes a Harmonic Percolator that, "Based specifically off of the original... uses the same NOS NPN and PNP transistors as the vintage models"⁶.

What sets the Hydrazine apart within that field is the Texture control's continuous clipping-threshold adjustment and the modified core amplifier aimed at clarity and punch².

Videos

Click to load

Click to load

Questions