13/07/2015The Technology of the Fuzz Face The Technology of the Fuzz Face Copyright 1998 R.G. Keen All Rights Reserved. The term "Fuzz Face" is a registered trademark of Dunlop. [Picking Transistors] [A Peek at the Circuit] [Mods] Introduction The Fuzz Face remains an enigma today, perhaps three decades after it first hit the market. This simple, two transistor circuit remains for many the very definition of the guitar effects stomp pedal. What makes such a simple circuit the stuff of legend? For one thing, it's got a reputation. There can't be many guitarists that don't know that a lot of Jimi Hendrix' songs were cut with a Fuzz Face. But Jimi had, above all else, a good ear. If it didn't sound good, he wouldn't have used it, and furthermore, it didn't have a rep when he picked it up. A reputation is good reason to look at it today, but not in the beginning. A Peek at the Circuit Let's look at the circuit. This is simple almost beyond belief for distortion devices. Two transistors, four resistors, three capacitors, and two controls make all that tone. The first transistor is set up with the simplest of arrangements - input through a DC blocking capacitor directly to the base, emitter grounded, and a single collector resistor; the driven load is a second transistor base, directly coupled. Ordinarily this would be the maximum possible gain arrangement, except that there is that resistor coming from the second transistor's emitter what's that do? For DC purposes, the second transistor acts as an emitter follower. The voltage at it's emitter must follow the first transistor's collector, although it is lower by the amount of the base emitter drop of the second transistor. To the first transistor, that means that the 100K resistor from the emitter of the second transistor to the first transistor's base is effectively a feedback resistor, passing a current proportional to the first transistor's collector voltage to the first transistor's base. This arrangement is called the "voltage feedback biasing" circuit, and some of the Fuzz Face's unique properties stem from this. The Voltage Feedback (VF) circuit has the properties that it offers the potential for the highest possible gain from any given transistor - good for a distortion device. It also has a very low input impedance, which means that it heavily loads anything trying to drive it. This will be an important point later. Finally, it can't really hard saturate like many other transistor hookups. When driven with a large signal on the base, the collector voltage moves toward the emitter. This lowers the bias voltage through the 100K biasing resistor, and steals some of the input signal. Saturation is mushy - again, very good for a musical sounding device. Was this intentional on the part of the designer? Probably not. It's likely it was a happy accident, as the VF circuit was common in the early days of circuit design to wring more gain from poor transistors. http://www.geofex.com/article_folders/fuzzface/fftech.htm 1/7 http://www. There is a secondary effect in that the AC gain of the FIRST stage is also set by the AC feedback from that pot through the 100K resistor. As the second stage is driven harder. The gain of the second stage is approximately determined by the ratio of the AC load in the collector to the AC load on the emitter.in this case a 1K pot. a 100K should sound somewhat brighter. and the emitter load to AC is the portion of the 1K pot not "shorted" to ground through the 20uF capacitor. a common Euro type at the time the FF was designed. as the 100K acts as a load on the collector resistor. there is still plenty of room for the second transistor to have some collector-emitter voltage left over for a linear swing.htm 2/7 . so the clipping begins softly for smaller signals (and gains) and then transistions to hard clipping with harder playing . It seems likely that the AC 128 was picked for it's cost and availability (in retrospect) rather than any super sound qualities. Because the first transistor biases up so close to ground. as well as determining the lowest frequency that gets out.the thing gets touch sensitivity! The split collector load resistor of the second transistor acts like a volume pot permanently set to a low value. The AC collector load is just the sum of the collector resistors. this is why the FF sounds different with different batteries and with the same battery as it gets run down . it cuts more lows out by it's interaction with the output coupling cap.the impedance of the battery can change the effective value of the smaller resistor by an significant amount. The output volume control is an entirely ordinary volume pot. The base is directly coupled to the collector of the first transistor. Eric Johnson supposedly favors 100K over the stock 500K. it can hard clip in cutoff on the same polarity that the first stage clipped softly on. essentially. so when the control is turned full down. The Fuzz Face has asymetrical clipping designed into it! The second transistor serves a couple of purposes. divided by the 1K resistance. This is because the power supply is effectively at AC ground because of the low AC impedance of the battery. so there is a lot of room upwards. not much larger than the input signal to keep the (relatively!) huge amount of signal available from overdriving the input of the amp following it. This could have some effect. The amount of signal tapped off at the junction of the two resistors is deliberately small. The pot forces the static current through the device to be equal to the first collector voltage minus the base emitter voltage of the second transistor.com/article_folders/fuzzface/fftech. The earliest FF's were made with AC128's. More importantly. so the effective gain goes up. the bias current then is largely set by the value of the emitter resistor . hit cutoff on the other polarity. and is less subject to stray capacitive loading cutting highs than the 500K. That same current. The output capacitor blocks the DC level from saturating any device following the Fuzz Face. is not as constrained as the swing towards saturation. As the control is turned up more. Incidentally. So the gain can vary from a low of about 8 to as high as the transistor's basic internal gain when the pot is maxxed out. the feedback reduces the first stage's gain most. less AC is fed back to the input. The input stage will first hit mushy saturation on one polarity of signal and then if driven hard enough. there is a whole mythology that has grown up around those transistors.geofex. Making this cap bigger will let more low frequencies out. What sounds good in there? We've just hand waved the transistors as being there. The value of the output level control has been the subject of some debate. Fuzz Faces naturally tend to bias with only about half a volt on the collector of the first transistor. when the input is trying to turn the first transistor off. and the voltage across the two collector resistors is equal to the static current times the sum of the resistances. flows through the collector resistance of the device.13/07/2015 The Technology of the Fuzz Face The voltage swing in the other direction. If you allow combinations of one high and one low gain device. identical-as-peas-in-a-pod devices like we have become accustomed to. The tech community went to silicon for transistors because germanium is prone to leaking (can't turn the thing off!) and heat damage (solder too long and it dies). They just put a metered amount of base current in and look for how much collector current comes out.com/article_folders/fuzzface/fftech. reliable. how do you tell which are going to sound good and which will not? To a first order. At highest gain.13/07/2015 The Technology of the Fuzz Face It's hard to remember for people who were not into electronics 30 years ago. The semiconductor industry was new back then. but then. The AC128 originals were not particularly stellar. I've done a lot of circuit simulation on the FF. The lesson here is that you can soften the sound of a FF even further by connecting a 10-100pF capacitor from the collector to base of the transistors. maker of the Fulltone "69" pedal. Silicon is much better about both of these things. twiddling the values of the transistor gains. Reliable reports from people who actually bought and sold volumes of FF's in the "golden years" say that out of a case of fifty units. they all distorted all right. but only a few would sound really good. I have measured several hundred AC128's for gain. posted his preferences for Fuzz Face transistor gain to the usenet news groups. This seems to be borne out in practice. but transistors were not all that good or reliable. It was HIGHLY variable in gain. like listening to razor blades. Most guitarists (there's that personal preference thing again) seem to want the rough edges smoothed off. the range widens out to 70 or so on the low end and perhaps 130 on the high end. We can understand this today. There is a definite sweet spot for musical sounding clipping at transistor gains of about 80-110.geofex. The AC128 has parasitic capacitances typical for then-current germanium devices. (Mike also prefers transistors with only certain colored epoxy sealant. and modern processes turn out closely clustered gains and very high frequency responses routinely. you can just gain select them in a DMM that has a transistor checking range. I say probably because the measured gains actually turn in numbers up to 500 or so. and they fall right in this range. Fuzz Faces per se do not necessarily sound good without tweaking and transistor selection. With germanium's inherently http://www. not high gain devices. who knows?) The frequency response of the transistors matters. which I can't see making any difference except coincidentally.htm 3/7 . all modern DMM's assume that the transistor being testing has no leakage at all. This almost certainly has something to do with the softened edges of the sound. and looking at the clipping waveforms and resulting harmonic spectra. too. any amplifier exhibits frequency response limited by the device characteristics. this also turns out to be a happy combination. I could play with the parasitic capacitances of the simulated devices. In simulation. and had not developed the technologies that let us make cheap. Mike Fuller. Keeping in mind that preferences for distortion tone are definitely a matter of personal taste. The fact is that the AC128 is not a super high quality transistor. It was common for guitarists to make deals to presort a batch to get a good one. with Eric Johnson's tech supporters tracking down and interviewing FF's to find the few "magic" ones. This practice continues today.well. the range of gains for unselected AC128's in this circuit would produce some really clunky-sounding devices. and they vary from a gain of 16 to probably 180. right out of the box. as we now know that clipping with razor edges sounds -. which are clearly leaky devices. The variation in gain is important because it is crucial to the tone of the FF. Picking transistors for FF Clones If you have a batch of germanium transistors. This dodge was used in later silicon versions of the FF. meaning that it was working hard to do the full audio range. and wasn't at the time. It is a moderate gain (90 typical) device with only modest frequency response. However. He noted that he feels that he can affect the relative amount of symetrical versus asymetrical distortion by selecting for non-identical gains in the two positions. and found that adding high junction capacitances from collector to base made for softer distortion. press the switch that connects the 2. get one each 2.13/07/2015 The Technology of the Fuzz Face higher leakage. What happens when we test? We chuck the thing in the socket. Let's say the device really leaks 93uA. The transistor multiplies this by its internal gain. 200 happens pretty often. although it will differ a bit. If you're going to do much of this. and the discussion on the values. stick the transistor in the socket.472 volts per milliamp.2M and 2472 ohms. If the transistor has a gain of 100 and no leakage. So the leakage is making the meter believe that there's a "gain" of almost 25 with no current into the base at all. but recognize that the accuracy will be less. The resistor will convert any leakage current from the transistor into a voltage that you can then read on your meter. Pretty neat. get several 2. You may find one that's closer to 2. get a 2. any transistor that shows more than a few micro amps of leakage is suspect. To get the real gain.30 if you get them from Mouser.htm 4/7 .2M resistor to the base.which is almost exactly 1/ 100 of the actual gain. let's say that the device leaks 100uA to start with.com/article_folders/fuzzface/fftech. A 2472 ohm resistor is 2. To test the total gain. This causes a touch more than 4 microamps of base current to flow in the base. It makes the final numbers on your DMM come out about right . How much leakage is too much? 100uA is common. so any transistor that does that is not going to be useful for a FF. If you really want to do this. better. and read the DC voltage across the 2. If you are satisfied with an indication of gain but are willing to settle for lower accuracy. and that's the gain.472 volts to display.1.229V. the indicated value on your meter is "false leakage gain" and will have to be subtracted from the total reading that you do next. In fact.9888V . and from this tester http://www.4K resistor is then (4uA)*(100)*(2472) = 0.330V. and use a 9. and read 1. I'm being picky about the ohms because if you get exactly 2. We stick the device into the socket.geofex.4K .that's why were doing this little dance in the first place. Because of the resistor scaling.229V from 1. and read (93uA)*(2472) = .2M and 2.101V. that's from this being an imperfect world.4K resistor. the voltage across the 2. you can carbon film at 5%. You hook up a couple of resistors and a DMM to the device.472 ohms. It reads (100E-6)*(2472) = 247mV. which would be ideal. Hey! How come it's 110.330V and get 1. That is incredibly too much leakage. and more than 500uA I would say is bad. huh? But we know that germanium really does have leakage . you'll find that the voltage across the resistor will be numerically equal to the indicated gain! That's why the somewhat odd resistor values. This will set you back about US$0.0V battery. and not 110? Well. and has a gain of 110 .4K resistors and measure them. This is one way to sift the leakage from true gain. and slightly more or less than that from other sources.2M resistor and a 2.multiply the voltage by 100. So. so you can easily test a large number of devices. and read the voltage before we press the switch.49K metal film 1% resistors. and the sum of the leakage (which doesn't change with base current) and the amplified base current.a prime specimen. and the resistors set up conditions you can control to see what is what. so a milliamp of leakage will cause 2. we subtract 0. The true gain is just 100 times the reading. get a transistor socket to. Then we press the switch. this just makes a leaky device look like a higher gain device. To do the test. If you can. More than 300uA means the device is suspicious. if you're clever.. The JH1 and JH2 are a great value for the DIY'er. This all makes good sense . I've also done some experimentation with germanium devices other than the AC128 and NKT275. a PNP germanium device. These are NPN silicon devices. and much better than you actually need to make a fine sounding FF. The exact base current is 4. The JH2 used the MPSA18. Later. and the BC108C in particular is a HIGH gain device. Dunlop made FF clones..in any device that has so few parts. Dunlop bought the FF trademarks and began manufacturing the FF as a reissue. well. and that the resistors are 2.. Simple finger heat will make the gain rise rapidly. but in other respects very similar.0000V.5% accuracy is doggone fine for work with such blunt tools.046..uA. The first ones seem to have been all AC128's. What's the real gain? All of them are . others prefer having a lower gain of 70-100 for the first transistor and 90 to 130 for the second. much better than the ill-fated JH series.) do like them very much. The right *real* gains are from 70 to about 130. You get that neato round case for cheap because they sound horrible. At one point in the past. followed by runs with the Newmarket NKT275.geofex. including silicon. and have your own great sounding FF. typically 500. There were several versions with the BC108C and/or BC109/BC109C. I obtained a batch of NKT275's and they seem to be much more consistent than the AC128.. Within that range. and is shipping these in its latest reissues. although some owners (there's that preferences thing again.1V (reasonable with germanium at these currents) and that the battery is *exactly* 9.20000M. The JH1 and JH2 were both attempts to combine the Fuzz Face circuit with the purchased rights to Jimi's name (bought from the JH estate. The FF was built with several transistor types. Look for low leakage. The reputation of the NPN silicon devices is spotty. the qualities of the parts will show through. Both were made with high gain silicon transistors and sounded dreadful (IMHO).they'll change in a second anyway as the device heats or cools. Dunlop actually arranged for new manufactured "NKT275" germanium devices to be made for them.htm 5/7 . you'll flip the switch and watch the voltage while you put your finger on the transistor.com/article_folders/fuzzface/fftech.13/07/2015 The Technology of the Fuzz Face being built with some approximations. which is very similar. people report the best sounds. these seem to have a "sticky" or "constricted" quality about them. Reports are that these are OK sounding. and approximately the right gains. Besides . and.. you get the picture. 0. assuming that the transistor's base conducts that much with a forward voltage of 0. with a typical gain of 900! To my ear. You can then rebuild the real FF circuit onto the circuit board in less than an hour. Don't get too hung up on the exact numbers . as noted on the bottom of the pedal). I found a http://www. Some people prefer equal gains.at the temperature and conditions of the moment. Silicon versions of the FF??? Yes. Measure what you get!! Mods and Magic OK. With a low input impedance. The results are pretty consistent . so maybe there's some tone to be wrung from the stones there. also GE. for more of a metal tone.13/07/2015 The Technology of the Fuzz Face number of germanium types.2 K resistor. the input loads a guitar significantly.geofex. raising it to 0. making it look more like a current source (albeit a tiny one) and less like a voltage source. Some people are very happy with it.get the right gain and frequency response and the numbers printed on the cases don't matter. some not. again as a series resistor. This will prevent the bass signal from being lost by the impedance of the coupling capacitor. There is a set of changes collectively referred to as the "Hendrix" mods or the "Roger Mayer" mods. In the case of the output 0. Changing the input and output capacitors changes the bass response of the unit. Among those are to add a 1K linear pot in series with the 470 ohm resistor in the collector circuit of the second transistor. you can double each of these capacitors.htm 6/7 . the http://www. I suspect that this can be put down to the variablility of the devices. Mike Fuller posted some recommendations on the Fuzz Face to the net.47uF may be useful. I like the 2N527 from GE (very consistent!) and the 2N508. the base can only move a few tens of millivolts before cutting off or saturating the first transistor. and then listened to them. If you put a resistor in series with the guitar pickup. measured their gains. If that last one seems a little odd. and merely supplies a varying current. This mod primarily seems to increase the output level and gain of the second transistor. Reports of results on the ECG158 are mixed. in series with the input of the circuit. To allow more bass response.com/article_folders/fuzzface/fftech. A signal from a current source lets the input of the effect seek it's own voltage level. Another is to connect a 50K pot. it raises the apparent source impedance of the pickup. As a result. The pot is set up so it is acting as a variable resistor. The 508 is a little hotter. wiper shorted to one end lug. before the 2. These are Replace the 470 ohm output resistor with 1K Replace the 8. what about things other than the transistors? Roger Mayer is reputed to have tweaked Jimi's gear heavily. think back to what I said about the low input impedance.2uF input capacitor. This can be much more linear than a voltage source drive.1uF capacitor. still taking the output from the top end of the 8.2K resistor at the collector of the second transistor with 18K Replace the 1K control in the emitter of the second transistor with 2K. ..13/07/2015 The Technology of the Fuzz Face variable resistor allow you to radically clean up the distortion that the FF produces. The Vox Tone Bender (not to be confused with the Colour Sound Tone Bender) was a copy with only minimal changes to the the component values. only a few component value changes. One person reverse engineered the Fulltone "69" pedal and has posted the 69 pedal schematic to his web page. any of the circuits shown in this article (with the possible exception of the Fulltone "69" pedal because of the trimmer pots) can be built on the toner transfer/ready to solder boards sold at GEO. all variants on the basic circuit. http://www. and adapted to NPN silicon transistors.. There is also a new Roger Mayer "Classic Fuzz" which employs germanium transistors and essentially the FF circuit. It's easy to build. see what magic you can wring out of it. As you would expect. Dig in. considering the ease and simplicity of making the FF circuit. and the input mod is a "less" control. I suspect that there are many other boutique clones. and has a very wide range of tones possible.htm 7/7 . There have been several versions of the Vox Tone Bender and the Vox Distortion Booster. Adding 10 to 100pF from collector to base on the second transistor or 100-680pF across the collector resistor of the first transistor will soften things up a bit. In effect. So . The Fuzz Face also attracted a number of clones even in the Golden Age. producing subtle shades of softer distortion. you can see Fuller's application of his earlier suggestions..com/article_folders/fuzzface/fftech.the Fuzz Face is a fortuitous combination of circuits that combine initial soft clipping with asymetrical clipping that changes toward symetrical clipping under drive. The "69" pedal does enjoy a good tone reputation. If you like softer distortion. In this schematic. you might also like to add some high end taming capacitors..geofex. easy to modify. In fact. In summary. the two added controls can interact to produce many shades of distortion. the output mod is a "more" control. This thing offers what I consider the most bang for the buck for the enterprising effects hacker.