Breadboarding A Silicon Fuzz Face

2010 By Small Bear Electronics LLC

This article picks up immediately from the end of Using A Solderless Breadboard. If this is your first project, please go to the intro article before you continue and make sure that you are comfortable with the concepts and techniques. If you are already familiar with breadboarding, note that the level of detail in this article is meant to guide complete beginners, so please be a little patient.

I will walk you through the setup of a complete circuit. When you are done, you will have something that can be committed to a soldered build on perfboard or a PC board. I will post completed builds as I make time to do the layouts and testing.

Here's the basic circuit that we will set up:

This is a more complex schem than the LED demo in the intro, but it's still only a dozen parts including the battery...not too tough. I have purposely not included jacks, stomp switch or power switching in order to keep it simple; we'll add those later. The only things to note right now are:

So let's go! Trim and form the leads of a 2.2 mf. ("microfarad") electrolytic capacitor, following the convention of leaving 1/4" to 5/16" of lead length for insertion in the board. Electrolytic caps are most often polarized; on radial-case types like this one, a black band marks the negative side (Fig. 18.) I formed the leads to span five holes, but again, this is arbitrary. You may want to follow my directions and layout this time around; once you have the idea, you're free to do whatever works. Similarly, I am working from left to right on the board, but that's not engraved in stone, either. Plug in the cap with the negative side on the left, since that's where the guitar input jack will connect later (Fig. 19.) I have it located between indexes e3 and e7. You don't have to use exactly the same locations I do. However, if following my footsteps (bear tracks?) helps to build your confidence and avoid mistakes, go for it.

Now add transistor Q1. It has three leads: Collector, Base and Emitter, identified as C, B and E in the schem. I suggest either a 2N2222A or a BC108, both of which come in a metal can called a TO-18 package. In this style, the leads form a triangle, and the can has a tab next to the Emitter (Fig. 20.)

I plugged this device into indexes d9, d10 and d11 (Fig.21.) The choice was arbitrary, but it turned out to be perfectly workable as I added more parts and connections. When you get to "rolling your own," if your first choice of a location doesn't pan out, back up and try somewhere else! Now bend a short, bare-wire jumper to span four holes and plug it into indexes c7 and c10 (Fig. 22.) Fig. 23 shows the connection that you have made. Got the idea?

Now complete the connections to the Base of the transistor by adding Resistor R2 between indexes b10 and b13 (Fig. 24.) The color code for 100K is Brown, Black, Yellow, Gold. The Emitter of transistor Q1 needs to get to the Ground bus, so add a jumper from index a9 (Fig. 25.) Figure 26 shows these connections.

Resistor R1 is 33K (Orange, Orange, Orange, Gold), and it needs to connect from the Collector of Q1 to the positive power supply bus. Plug it in from there to index A11 (Fig. 27.) Transistor Q2 goes in next. Since its Emitter needs to connect to R2, that pin (the one near the tab) logically goes in at index d13 (Figure 28.) Figure 29 shows where we are.

The Collector of Q1 connects to the Base of Q2, which is easily done with a jumper from c11 to c14 (Figure 30.)

Last components before we add the pots: Resistor R4 (8.2K is Gray, Red, Red, Gold) connects to the Collector of Q2. I know it will meet with one side of Resistor R3, so I run it horizontally starting from index c15. This isn't the tightest use of space, but it does make the layout clean and easily visible. Resistor R3, 330 ohms (Orange, Orange, Brown, Gold) goes from the positive supply bus to a18. Capacitor C3, .01 mf. (marked "103") starts at e18 and should terminate at e20 (Fig. 31.) We are getting close (Fig. 32.)

Prepare the fuzz and volume potentiometers for connecting to the breadboard by soldering leads to them. A few inches of insulated breadboarding wire to each terminal is fine. By convention, we refer to the terminals of the pots as ccw (counter-clockwise), W (wiper, for the moving contact) and cw (clockwise) (Figure 33.) The connections are:

Check this out in Figure 34. Then add capacitor C2 from the wiper of the fuzz pot to the ground bus. Negative side to ground, please! (Fig. 35)

Figure 36 shows those last connections. For testing purposes, we want to install input and output jacks as shown in Figure 37.

The schematic shows mono jacks. This type has two contacts: tip and sleeve. Figure 38 shows the relationship between the schematic symbol and the physical item. Figure 39 does the same thing for a stereo jack, which has three contacts: tip, ring and sleeve. The stereo jack shown is a Switchcraft #12B; note that the arrangement of the contacts may be different on other makes.

In a typical pedal build, we would use a stereo jack for the input and connect the ring contact to switch battery power. To keep things simple here, we use only the tip and sleeve. Connect a short length of shielded cable to each jack, making sure that the shield is soldered to the sleeve. (Ordinary insulated wire will work, but shielded cable reduces some of the noise inherent in an open layout like this.) Add bare wire terminations to the ends that go to the breadboard just as you did for the battery snap.

The shields are plugged to the ground bus. Input tip goes to index c3, output tip to index c24. Things should look like Figure 40.

ARE YOU READY?? Connect your amp and connect a battery. Turn up the level control and turn up the fuzz pot till you hear distortion. Try playing with it a bit.

Doesn't work? Go back through the pics and figure out what isn't connected right. Also be sure that:

I have built exactly as shown, and the instructions have been vetted by my assistant as well, so we are sure that your build will work if everything matches the instructions.

It works, But It Sounds Terrible!

I know...The distortion is harsh, and you only hear it in the last 10% of the rotation of the fuzz pot. Fuzz Faces had a deserved reputation for being variable in tone quality, and what you hear is exactly what many guitar players heard when trying them out back in the day. They would try many pedals till they found "the one" that sounded good. Since then, numerous people who are way smarter than I am (Thank You, R. G. Keen, Mark Hammer, and many others!) figured out exactly How the circuit works, and how to tweak it so that every build sounds consistently good.

Fixing The Fuzz Control

"Taper" in potentiometers is the relationship between percent rotation and percent change in resistance. The 1K fuzz pot in the original Fuzz Faces was linear taper; a 10% change in rotation gives about a 10% change in resistance. Over time, a number of people noticed that an audio (left-hand logarithmic) taper gave much better control of the fuzz. But the direction of control was reversed, and so not intuitive; the greatest distortion was at the counterclockwise end. What is needed to completely straighten things out is a pot with a reverse audio taper. A few years ago, I asked the Taiwan Alpha company to make this variant, and I am happy to say that it has become a popular mod part for Fuzz Faces. Change R5 to a 1K reverse audio (SKU 1007 in my Stock List,) and try the control again. Control of the fuzz is much better now, right? But the tone is still harsh...

Choosing and Substituting Transistors

Transistors are as individual as snowflakes; no two are precisely alike. Even among devices that have the same type number, a variation in gain of three to one or more is not unusual. The BC108, for example, which I have seen specified in many old schematics for the silicon Fuzz Face, might have a gain of anywhere from 80 to over 200 depending on what factory made it and when. The problem is similar with any of the other NPN silicon devices that are commonly substituted for it.

Without going into the math that describes exactly why, it turns out that, for the standard resistor values that you will usually see in the Fuzz Face circuit, most people like the resulting sound when Q1 has a gain of roughly 70 to 90 and Q2 a gain of roughly 120 to 140. If both devices are very low in gain, the circuit is likely to sound lifeless; if both devices have gains in the 200s, it's going to sound harsh. Part of the key to getting smooth clipping from the Fuzz Face circuit is selecting transistors that are in the right gain buckets.

The factory-specified gain range for the 2N2222A that I used above is 100 to 300. So that makes it likely to be a good choice for Q2, but its typical gain is likely to be way too high to work well for Q1. Rather than trying to find a 2N2222A with an unusually low gain, I fished for a different NPN transistor that typically runs lower in gain according to the factory specs. One that I found I like is the 2N4123; I tested half a dozen samples from my stock using the gain scale of my multimeter, and they all measured between 80 and 90.

So what do you do? SBE offers a kit to accompany this article, and it includes suitable transistors that have been sorted. If you already have a breadboard, the good news is that many transistor type numbers will work in this circuit. They are also very inexpensive and easily available from many sources, both local and mail order. If you buy a few of several types, you are likely to find at least two devices that will work well together. The basic specs are:

The part numbers I suggest are: BC108, 2N2222A, 2N3904, 2N4123, 2N4401. You'll find other recommendations in previous threads at diystompboxes.com. Try a search on "silicon fuzz face."

If your multimeter has a scale that will read gain directly--and most do these days--you will be able to sort your devices in a few minutes. If not, check out the method for testing gain in my article about testing Fuzz Face transistors. It refers specifically to germanium, but the method is equally applicable to silicon. The only difference is that you won't see any leakage current when testing silicon devices.

Try the circuit with a couple of devices whose gains you have measured. If the distortion was harsh when you started, you'll be surprised at the difference that this makes. Pay attention to the pinouts! Figure 41 shows a 2N4123 installed as Q1. This case style is called a TO-92. For the 2N4123, Emitter is on the left, Base on middle, Collector on right. Note that other type numbers may be in the same case style but have a different pinout.

It Still Doesn't Sound Quite Right

Most likely the bias needs to be tweaked. For best results, this circuit wants the collector of Q2 to be at about half the supply voltage, or roughly 4.5 volts. See figure 42. To make this measurement, set your meter to its low-voltage scale. You don't have to have the guitar connected, do need the battery plugged in. Connect negative side of the meter anywhere on the ground bus, positive side to collector of the transistor. Figure 43 shows what I saw.

This is way too low, and it's no wonder that the sound is limp and unfocused. To put the collector voltage where it needs to be, we want to reduce the value of R4. This will cause more of the supply voltage to drop across the Collector-Emitter of Q2. I started substituting standard resistor values, and Figure 44 shows what things looked like with 4.7K in place of the original 8.2K.

When you have the bias close to 4.5 volts, try playing through circuit again. Much better, right?

Tweaking With a Trimpot

Sometimes you want to be able to tweak a little, play a little, etc., to see what difference a small change in bias makes. Trimmer potentiometers ("trimpots") are good for this. You can get them in a variety of case styles and a wide range of resistances. Figure 45 shows top and bottom views of one of my SKU 1015 cermet types (10K resistance) along with its schematic symbol. Figure 46 shows the trimpot set up to replace R4. When installing it on the breadboard, it does not matter whether you connect to pins 1 and 2 or 3 and 2. Start with the adjusting screw at about the halfway point before applying power.

Some builders just tweak the trimpot till they like the result; others connect the multimeter to measure the bias as before, set the trimpot to get exactly 4.5 volts, and then experiment from there. You might then measure the resistance of the trimpot using your multimeter and substitute the closest value fixed resistor, 1% tolerance if you want to get really anal. Or leave the trimpot in. It's your build...

To my ears, this thing is now plenty good enough to commit to a soldered board. However, I can already hear it from someone out there:

I want More Bass/Treble...

Add a tone stack! Lots of recipes and schematics are available on-line. An easy and effective one is the tone circuit from the Big Muff. Figure 47 shows what I lifted from the schematic of that pedal. Figure 48 shows it married into the setup on the breadboard.

Can you work out how to add this mod? To begin, remove from the breadboard the output jack, the level pot and the .01 mf. output capacitor C3. I replaced C3 with a 1 mf. electrolytic, + side toward R3-R4. This worked fine, though it might be higher value than needed. Any value from .1 mf. up should work, poly film or electrolytic. I chose to replace the 500K audio taper level pot that was the standard in the original Fuzz Face with the 100K linear that's part of the BMP tone stack. Here is how the breadboard looked when I was done (Figure 49.)

I like the result! I still recognize it as a Fuzz Face, but the tone palette is far wider...lots of nuances of sharp or smooth sound.

A very similar tone stack, but lifted from the Boss DS-1 and others:

The tone pot can be 25K linear taper, but you'll get more subtle control with a 20K "W" taper pot, SKU 1007 on my Stock List.

Another tweak, this one from Phil Bryant's Fuzz Central: a small capacitor, from 100 to 500 pf., between the Base and Collector of Q2. Tames raspiness but doesn't decrease definition. I'm not photographing these, because you should be able to add them on your own now.

I know that someone will ask me about adding a diode loop for more distortion. Absolutely, it can be done. However, regular silicon diodes need at least .6 volt across them to clip, and the output of this circuit is not that high going into the tone stack. I stuck a back-to-back pair of 1N5818 Schottky diodes (SKU 2215A) from the input of the tone stack to ground as in (a) in Figure 51:

While the result is very good to my ears, it sounds to me like a completely different palette. I would want a switch to take the diode pair in or out as in (b). Call this the "More Fur" mod?

Where To Go From Here

Clearly, we want to turn this idea into a finished pedal. That really needs another article, which will follow soon. I hope you enjoyed learning to use the breadboard, and that you will use your new skills to hack into other designs and try out your own ideas.

A Parts List

Here is everything needed to do all of what I have described, including the LED demo in the intro article and the "wrong way" scenarios for the Fuzz Face.

Quantity Description SBE Stock List SKU
  Resistors - All 1/4-watt 5% Carbon Film  
1 10K 0900, 0901, etc.
1 330 ohms  
1 8.2K  
1 100K  
1 33K  
1 39K  
1 47K  
1 2.2K  
2 6.8K  
     
  Potentiometers and Trimmers  
1 1K Linear 1005A
1 1K Reverse Audio 1007
1 500K Audio 1005A
1 100K Linear 1005A
1 25K Linear 1005A
1 10K Cermet Trimmer 1015
     
  Capacitors  
1 1 mf. 16 Volt Radial Electrolytic 1400
1 2.2 mf. 16 Volt Radial Electrolytic  
1 22 mf. 16 Volt Radial Electrolytic  
1 .01 mf. 50 Volt Polyester Film 1101B or 1150
1 .022 mf. 50 Volt Polyester Film  
2 .1 mf. 50 Volt Polyester Film  
1 .22 mf. 50 Volt Polyester Film 1103
1 .47 mf. 50 Volt Polyester Film 1105
1 220 pf. Ceramic 1311
     
  Transistors and Diodes  
2 2N2222A 2000
1 2N4123 2008
1 LED 5mm High-Brightness Red 2302
2 1N5818 2215A
     
  Wire and Tubing  
  Bare Tinned Copper Wire, #22 or #24 0509
  Insulated Tinned Copper Wire, #22 or #24 0508M
  Shielded Cable 0510
  1/16" heat shrink 0500
     
  Jacks, Fittings  
1 Mono Jack, Switchcraft #11 0600
1 Stereo Jack, Switchcraft #12B 0602
1 9-Volt Battery Snap 0619
     
  Tools  
1 Breadboard or Breadboard Strip 2700, 2700A, 2700B
1 Multimeter 2701, 2701A