Seven Fuzz Topologies of the 1960s, Pt. 4: The Octavia
Part 4 of Seven Fuzz Topologies of the 1960s, a bench-reference series. New here? Pt. 1 covers the drawing conventions and how to read the clipping diagrams.
4
Differential Pair / Frequency Doubler
Roger Mayer Octavia topology · 1967
A phase splitter makes two copies of the signal 180° apart; a diode on each copy passes only its positive half. Summed, the two halves reconstruct the absolute value of the signal, a full-wave rectifier, which contains a strong component at twice the input frequency: the octave up.
- Devices
- Silicon NPN/PNP mix (MPSA18, 2N6519 / 2N4401), 9 V
- Supply as drawn
- +9 V
- Controls
- BOOST (gain) · VOLUME
- Clipping
- full-wave rectification (frequency doubling) plus stage overdrive
Stage by stage
- Q1 is an emitter-follower buffer so the guitar is not loaded down. Q2 is a common-emitter gain stage; its emitter bypass C3 gives it enough gain that the rectifier is driven by a large, already fuzzy signal.
- Q3 is the heart: a phase splitter with equal collector and emitter resistors (R9 = R10 = 2.2 k). The collector gives an inverted copy, the emitter a non-inverted copy, same amplitude.
- D1 and D2 (germanium, low forward drop) each conduct on their own positive half-cycle into the shared load R13. Positive half from one, folded-up negative half from the other: the fundamental cancels and what is left has two peaks per input cycle. Play a note at the 12th fret and you hear the octave clearly; play chords and it turns into intermodulation mush, and that’s expected.
- Q4 buffers the rectified signal to the volume pot. Lifting one diode (the "octave off" switch on many builds) leaves a plain half-wave fuzz.
Bench notes & values
- Roger Mayer’s original and the Tycobrahe reissue use a small centre-tapped transformer (1.5 k CT : 600 Ω) as the phase splitter, with the diodes on the secondary. The transistor phase splitter drawn here is the "transformerless" equivalent and behaves the same; C5/C6 and R11/R12 are needed because the collector and emitter sit at different DC voltages.
- Values are a working starting point for a transformerless build (Q3 collector ~5.6 V, emitter ~3.4 V), not a trace of a specific unit. Matched diodes (forward drops within ~20 mV) give a cleaner octave.
- The Univox Super Fuzz and Fender Blender get the same doubling from a true differential pair (two transistors with a shared emitter resistor, bases driven in antiphase): same family, different implementation.
- Bench cost: about $4 in parts; a 42TM022 transformer adds ~$4 if you want the original coupling.
Same topologyRoger Mayer OctaviaTycobrahe OctaviaOctavia Evo 1Ace Tone Fuzz Master FM-2Fender BlenderUnicord Super FuzzUnivox Super Fuzz