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Circuit Notebook

Interesting circuit ideas which we have checked but not built and tested. Contributions from readers are welcome and will be paid for at standard rates.

Fuzz box
for guitars

Click for larger image

This fuzz box uses low-cost discrete components rather than op amps. It’s designed for use only with amplified guitar signals.

While it may appear to be a conventional differential op amp amplifier configuration with Q5 as the long tail for the two input transistors (Q1 & Q2), it actually uses positive rather than negative feedback. It clips symmetrically and the loading of the following amplifier or mixer has no effect on its clipping symmetry.

Q4 is connected as a grounded base stage with base bias provided by three diodes: D2, D3 & D4. Q3 drives the emitter of Q4 and the positive feedback signal to the base of Q2 comes via the 4.7kÙ resistor from the collector of Q4. The three diodes also set the bias for Q5 which sets the operating current for the input pair. Trimpots VR1 & VR2 are used to set symmetry and to zero the DC output voltage.

Craig Kendrick Sellen,
Philadelphia, USA. ($45)

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