Project 9 - Hemmos Bazz Fuss
The Hemmos Bazz Fuss is one of the best sounding, yet easiest to build fuzz circuits in existence. The original design, by Christian Holmberg (aka Hemmo P), is so basic that it only requires 2 capacitors, 1 transistor, 1 resistor and a single diode. Together, these components create a rich, synth style fuzz that is loved by pedal builders, guitarists and bassists alike.
Over the years the original circuit has been re-imagined and modified countless times; and because of its simplicity, it makes a great project for unseasoned pedal builders like myself.
I wasn't able to find the original schematic for the Bazz Fuss (which supposedly uses a 2N3914 transistor). Instead, I decided to work with this schematic from Beavis Audio...
So how does this circuit create such nice fuzz? Well, as you can see in the schematic, this design has a diode between the transistor's base and collector. This diode feeds the base with current, while limiting the voltage by way of a diode drop (aka forward voltage drop). This clips the audio signal resulting in beautiful fuzz distortion.
About Diodes
Until now I have only used Zener diodes, which I add to help protect a circuit from the accidental use of a centre-positive power supply (like those used to power a Germanium Fuzz Face built with PNP transistors).
In addition to Zener diodes, there are Schottky Diodes, Shockley Diodes, Laser Diodes, Light Emitting Diodes (or LED's), and many others.
Diodes have been around since the early 1900's and were originally made of mineral crystals such as galena. Today most diodes are made of Silicon, although you can still buy Germanium and even Selenium diodes as well.
For guitar pedals, diodes are often used as a type of switch or valve, allowing current to pass in only one direction (such as the Zener diodes I mentioned earlier). They are also commonly used to generate distortion by "clipping" the signal wave - the more you clip, the more distortion you hear.
Clipping
There are generally two types of clipping - soft-clipping and hard-clipping.
Overdrive pedals typically have soft-clipping; which clips only a small portion of the signal wave.
Distortion pedals on the other hand typically have hard-clipping; which clips a larger portion of the signal wave giving it a slightly more square appearance (when viewed with an oscilloscope).
Fuzz pedals, well I guess they can only be described as super hard-clipping since they clip more of the signal wave than the other two pedal types.
Asymmetrical vs Symmetrical Clipping
There is another important thing you should know when using diodes to clip an audio signal and that's Asymmetrical vs Symmetrical clipping.
Adding a single "clipping" diode to a circuit will result in single-sided asymmetrical clipping. This is where only half of the signal wave is clipped.
Adding a second diode (or any even number of diodes), in parallel and in opposing direction to the first diode, will give you symmetrical clipping. This is where both the positive and negative portions of the signal wave are clipped.
If you want something really crazy you can use any odd number of diodes (for example, two in one direction and a third in the opposing direction) to get double-sided asymmetrical clipping. This is where both the positive and negative portions of the signal wave are clipped, but to varying degrees depending on which types of diodes are used.
So what's the difference in sound between symmetrical and asymmetrical clipping?
Asymmetrical clipping tends to sound less compressed with improved clarity, whereas symmetrical clipping will usually have a bit less volume (because both halves of the signal wave have been clipped) but more compression, sustain and distortion.
From what I've read, Asymmetrical clipping offers a more "natural" and "tube-like" sound, similar to what you find in a traditional tube/valve amplifier.
Back to the Build
After first testing the circuit on my breadboard, I couldn't resist the opportunity to make some simple modifications to the design; here's what I did...
First, I used a MPSA13 NPN Darlington transistor instead of the 2N5088. Both sound good, but I found the MPSA13 sounded better to my ears.
I then added a 100K Audio pot to the Output signal to serve as a volume control. I also added a 1K Linear pot to the transistor's emitter, allowing me to control the amount of gain being produced.
Finally, I replaced the 1N4148 diode by adding a 1P4T rotary dial that allows me to switch between a single 1N914 diode, a pair of 1N914 diodes (in parallel with reversed polarity to each other), a Green LED and 1N34A diode (in parallel with reversed polarity to each other), and a single Blue LED. Using the Component Switching Device I created (read about it here), I was easily able to test out several different combinations of diodes before settling on these four.
For the indicator LED (that tells you when the pedal is switched On), I used a blue Fender style jewel, which is lit using a standard white LED.
A look inside and you will see I decided to go with mostly red and black wiring (solid core), which I think gives it a rather nice appearance...
Well there you have it. I hope you have found this project interesting.
For my next build, I have something special - a slightly modified version of the legendary Pro Co Rat Distortion pedal. This is perhaps the coolest pedal I've built and features a useful option that allows users to customize how it sounds. Oh, and there will be a video for this one as well.
As always, thanks for reading.
-M-
