ETI Hyper Fuzz is strange Fuzz from the 1988 Electronics Today International Guide to Making Music. I found it here. I wrote more about this here.
This is my modification to the original circuit. I replaced what looks like an over complicated input stage. The original was also not true bypass and a buffered bypass. I simplified this to a simple single transistor stage and true bypass.
I replaced the original input stage with an LPB-1 CE amp with an additional gain control instead the volume. I did this sort of paint by numbers, and the original values are not very well chosen. I have been trying to improve my understanding of electronics theory by writing this zine. With some study I came up with some better values. Luckily this didn’t require any jumpers or added parts. The original design was good, i just needed to adjust the bias, and collector and drain resistors.
CE Amp Design
Here is what I learned about designing a CE amp. Start with the assumption of 0.1mA collector current Ic, and an emitter voltage Ve of 1V. Everything else builds on this foundation.
Find R1-4
Start with Collector current: Ic = 0.1mA
Shoot for Vc (collector voltage) of 4.5V, use OHMs law to calculate the voltage drop across R3:
4.5V / 0.1mA = 45K –> Use R3 47K
Look for Ve (emitter voltage) of 1V. Use OHMs law:
1V / 0.1mA = 10K –> R4 10K
Vb (bias voltage) is Ve + 0.6V:
1V + 0.6V = 1.6V
Find the base current Ib. Assume Beta β = 450 (2N5089) We need ≥ 10 * 0.1mA / β
0.1mA / 450 = 222.nA
222nA * 10 = 2.2µA (We need at least 2.2µA divider current)
Find the Bias Divider, for a base Voltage (Vb) of 1.6V
Find the ratio of the divider resistors: Vb / VCC (9V)
1.6V / 9V = 0.178
Calculate a divider current of 10µA (Well above the min 2.2µA calculated above)
R1 + R2 = 9V / 10µA = 900K (OHMs Law , gets the resistance for the current)
R1 = 900K * 0.178 = R1 160K
R2 = 900K – 160K = 740K –> R2 750K
Calculate Gain
At minimum gain is calculated as R3 / (R4 + re). re is the minimum internal resistance of the transistor. re is not fixed, it changes with collector current, assume 26mV / Ic in Ω.
re = 26mV / 0.1mA = 260
Minimum gain is:
R3 / (R4 + re) = 47K / (10K + 260) = 4.5
With Gain pot the other extreme re is the only resistance. Max gain:
R3 / re = 47K / 260 = 180
Testing
I put the whole thing together, plugged it in, and tested the LED, which didn’t work. I tested the audio, and that was working. I took a closer look and realized the LED was just very dim. I checked the schematic and I marked the LED as 1M (must have copied and pasted that resistor, and not changed the value). I swapped that out and everything was working. I wish every error was this easy to resolve.
The sound is interesting, so good on the first test, I awarded this pedal a pair of solid aluminum knobs.
For some reason, the PCB in Eagle has the upper corners cut on the diagonal, but the PCBs, I received from OSHPark.com are square. Looking closely I noticed that some of the traces are different on the PCBs. I must have updated board in Eagle, after I exported everything. I had to move holes down about 5mm. it all worked out in the end, and it seems like its working.
Conclusion
Sounds great, super fun. The sound is super distorted/fuzzy. The distorted sound is good and sounds good with chords.
The three positions of the switch sound similar. The differences are noticeable mostly in volume. The differences in sound for each mode is there but the sounds are not widely different.
With the switch up, I think this is the mode that engages all of the diodes, there is a low frequency thumping. You might call this “motor boating”? Not sure where this is coming from. Could be some instability between the two inputs on the op-amp?
On close inspection of the original schematic. It looks like the original input stage has a gain of about 1 to 3. My modifications give the input stage a gain of 4.5 to 180, which is much higher than the original. The sound is pretty good, so it might be worth keeping the higher gain. There may be lower gain sounds available that I am missing.
I disconnected the emitter leg of the Gain pot and put a 2K2 resistor between it and the emitter. This give me a gain of about 3 to 19, much lower, and closer to the original. There seems to be a little more range in distorted sounds. The “motor boating” is gone when the gain is turned up.
Should you build this?
Maybe. Needs a little more work. Has anyone else built this?