Chuck D. Bones
Circuit Wizard
I know I said next up was the Baby Blue OD, and I will get to that one soon, but I ran across this high gain JFET Fuzz on Luciferstrip's FTP site and had to give it a whirl. I have a pile of Ge diodes so I thought this would be a good circuit to try out some of them.
This is the schematic I found:
I started with this as the boilerplate design and then made mods from there. Since I used PN4393s, I had to adjust the source resistors to get the biasing right. I don't like having the GAIN control before everything because it can be too noisy. I put the GAIN control between the 1st & 2nd stages. I deleted the other gain control between the 2nd & 3rd stages. The signal level across that last pair of Ge diodes is pretty small, so I added a 4th stage to bring the signal level up and then stuck a TONE control after that. Most of the caps are film. The pF caps are silver-mica. The 4.7uF & 22uF caps are tantalum. The 100uF caps are aluminum. Here's the more-or-less finished product.
This design has some interesting qualities. It has a shit-ton of gain and compression. The three JFET stages have 24dB gain each. The Ge diode hard clippers can be fairly leaky without degrading the circuit performance. The hard clippers clamp the drain voltage, so none of the JFET stages can saturate. Because of that, the drain voltages can be set lower without running into headroom problems. The diodes make most of the distortion and compression. At high GAIN settings, the 2nd & 3rd stages are driven into cutoff (drain current goes to zero) for asymmetric clipping. The MOSFET 4th stage does not add any distortion, it's just there as a boost.
I had a bit of trouble with this at first; it wanted to oscillate when GAIN was above noon. I cleaned up the layout, power distribution, grounding and wire routing; now it's stable. The 1N128s are lower leakage than the D2Es. With all of the gain on tap, it's easy to get feedback going, even at low volume.
This is the schematic I found:
I started with this as the boilerplate design and then made mods from there. Since I used PN4393s, I had to adjust the source resistors to get the biasing right. I don't like having the GAIN control before everything because it can be too noisy. I put the GAIN control between the 1st & 2nd stages. I deleted the other gain control between the 2nd & 3rd stages. The signal level across that last pair of Ge diodes is pretty small, so I added a 4th stage to bring the signal level up and then stuck a TONE control after that. Most of the caps are film. The pF caps are silver-mica. The 4.7uF & 22uF caps are tantalum. The 100uF caps are aluminum. Here's the more-or-less finished product.
This design has some interesting qualities. It has a shit-ton of gain and compression. The three JFET stages have 24dB gain each. The Ge diode hard clippers can be fairly leaky without degrading the circuit performance. The hard clippers clamp the drain voltage, so none of the JFET stages can saturate. Because of that, the drain voltages can be set lower without running into headroom problems. The diodes make most of the distortion and compression. At high GAIN settings, the 2nd & 3rd stages are driven into cutoff (drain current goes to zero) for asymmetric clipping. The MOSFET 4th stage does not add any distortion, it's just there as a boost.
I had a bit of trouble with this at first; it wanted to oscillate when GAIN was above noon. I cleaned up the layout, power distribution, grounding and wire routing; now it's stable. The 1N128s are lower leakage than the D2Es. With all of the gain on tap, it's easy to get feedback going, even at low volume.