FXBus 1 Module 4 - a Fuzz Face style clipper 

Copyright 2000 R.G. Keen. All rights reserved.

Module 4 is a Fuzz Face style transistor clipping stage - that is, it does distortion by a pair of transistors in a classic distortion circuit. The Fuzz Face is, of course, a unique circuit. It's usually best when done with germanium transistors. For an in-depth discussion of the Fuzz Face circuit and its operation, see "The Technology of the Fuzz Face"

I have actually provided two layouts, one for NPN and one for PNP transistors. The NPN one runs from the normal V1 (usually +9V) power supply of the FXBus 1. 

The PNP one has provision for running from either the V2 auxiliary power supply in FXBus 1, or from an externally supplied V3 which can be applied to the V3 pad near the top of the board. If V3 is used, you must install the indicated optional jumper and cut the thinner trace on the PNP layout that leads to the V2 pad on the bus connection to prevent feeding -9V back onto the V2 line of the bus, assuming that you didn't want -9V there to begin with.

Since NPN FF style circuits are sometimes made with silicon NPN devices, I've inserted an optional input resistor (the only one with no reference "Rx" label) on the NPN layout. If you want to use this, you must cut the thinner trace directly under the resistor, and insert a resistor from about 4.7K up to 50K to your taste. This softens the clipping noticeably. Again, see the FF technology article for explanations.

Here's the layout:


Here's the parts list and what each part does (Note - caps in uF unless noted; pots are audio taper unless noted):

Part Value Notes
R1 33K Q1 load resistor
R2 100K DC feedback resistor; sets Q1 bias level interactively with R1
R3 8.2K Q2 load resistor
R4 470 to 1000 Adjusts relative output level; original was 330 or 470; hot-rodded pedals go up to 1K
C1 2.2uF Input capacitor; larger values let in more bass, smaller values keep bass cleaner, fuzz treble
C2 0.01 to 0.05 Output capacitor; larger values let more bass through, smaller values emphasize distortion
C3 22 Shunts AC to ground to allow distortion control to vary gain
Pot 1K Linear Distortion control
Q1 Germanium - 2N388A (NPN) or 2N527A (PNP); gain 75-100
Silicon - 2N5088, MPSA18, BC108C, BC109C, BC209 (if you can find one!)
Q2 Germanium - 2N388A (NPN) or 2N527A (PNP); gain 100-150
Silicon - 2N5088, MPSA18, BC108C, BC109C, BC209 (if you can find one!)