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Fuzz Face Technology

9/23/1997 6:36 AM
R.G.	Fuzz Face Technology 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? To start with, 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. 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. The voltage swing in the other direction, when the input is trying to turn the first transistor off, is not as constrained as the swing towards saturation. Fuzz Faces naturally tend to bias with only about half a volt on the collector of the first transistor, so there is a lot of room upwards. The input stage will first hit mushy saturation on one polarity of signal and then if driven hard enough, hit cutoff on the other polarity. The Fuzz Face has asymetrical clipping designed into it! The second transistor serves a couple of purposes. The base is directly coupled to the collector of the first transistor; the bias current then is largely set by the value of the emitter resistor - in this case a 1K pot. 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, divided by the 1K resistance. That same current, essentially, flows through the collector resistance of the device, and the voltage across the two collector resistors is equal to the static current times the sum of the resistances. Because the first transistor biases up so close to ground, there is still plenty of room for the second transistor to have some collector-emitter voltage left over for a linear swing. The gain of the second stage is determined by the ratio of the AC load in the collector to the AC load on the emitter. The AC collector load is just the sum of the collector resistors, and the emitter load to AC is the portion of the 1K pot not “shorted” to ground through the 20uF capacitor. 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. 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, so when the control is turned full down, the feedback reduces the first stage’s gain most. As the control is turned up more, less AC is fed back to the input, so the effective gain goes up. As the second stage is driven harder, it can hard clip in cutoff on the same polarity that the first stage clipped softly on, so the clipping begins softly for smaller signals (and gains) and then transistions to hard clipping with harder playing - 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 amount of signal tapped off at the junction of the two resistors is deliberately small, 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. The output volume control is an entirely ordinary volume pot. The output capacitor blocks the DC level from saturating any device following the Fuzz Face, as well as determining the lowest frequency that gets out. Making this cap bigger will let more low frequencies out. The value of the output level control has been the subject of some debate. Eric Johnson supposedly favors 100K over the stock 500K. This could have some effect, as the 100K acts as a load on the collector resistor. More importantly, it cuts more lows out by it’s interaction with the output coupling cap, and is less subject to stray capacitive loading cutting highs than the 500K; a 100K should sound somewhat brighter. So - let’s get to the meat of it. What sounds good in there? We’ve just hand waved the transistors as being there; there is a whole mythology that has grown up around those transistors. The earliest FF’s were made with AC128’s, a common Euro type at the time the FF was designed. It seems likely that the AC 128 was picked for it’s cost and availability (in retrospect) rather than any super sound qualities. It’s hard to remember for people who were not into electronics 30 years ago, but transistors were not all that good or reliable. 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). Silicon is much better about both of these things, and modern processes turn out closely clustered gains and very high frequency responses routinely. The fact is that the AC128 is not a super high quality transistor, and wasn’t at the time. It is a moderate gain (90 typical) device with only modest frequency response. It was HIGHLY variable in gain. I have measured several hundred AC128’s for gain, and they vary from a gain of 16 to probably 180. I say probably because the measured gains actually turn in numbers up to 500 or so, which are clearly leaky devices, not high gain devices. The semiconductor industry was new back then, and had not developed the technologies that let us make cheap, reliable, identical-as-peas-in-a-pod devices like we have become accustomed to. The variation in gain is important because it is crucial to the tone of the FF. 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, right out of the box. It was common for guitarists to make deals to presort a batch to get a good one. This practice continues today, with Eric Johnson’s tech supporters tracking down and interviewing FF’s to find the few “magic” ones. We can understand this today. I’ve done a lot of circuit simulation on the FF, twiddling the values of the transistor gains, and looking at the clipping waveforms and resulting harmonic spectra. There is a definite sweet spot for musical sounding clipping for gains around 80-110. If you allow combinations of one high and one low gain device, the range widens out to 70 or so on the low end and perhaps 130 on the high end. Keeping in mind that preferences for distortion tone are definitely a matter of personal taste, the range of gains for unselected AC128’s in this circuit would produce some really clunky-sounding devices. This seems to be borne out in practice. Mike Fuller, maker of the Fulltone “69” pedal, posted his preferences for Fuzz Face transistor gain to the usenet news groups, and they fall right in this range. 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. (Mike also prefers transistors with only certain colored epoxy sealant, which I can’t see making any difference except coincidentally, but then, who knows?) The frequency response of the transistors matters, too. The AC128 originals were not particularly stellar; this also turns out to be a happy combination, as we now know that clipping with razor edges sounds -- well, like listening to razor blades. Most guitarists (there’s that personal preference thing again) seem to want the rough edges smoothed off. At highest gain, any amplifier exhibits frequency response limited by the device characteristics. The AC128 has parasitic capacitances typical for then-current germanium devices, meaning that it was working hard to do the full audio range. This almost certainly has something to do with the softened edges of the sound. In simulation, I could play with the parasitic capacitances of the simulated devices, and found that adding high junction capacitances from collector to base made for softer distortion. 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. This dodge was used in later silicon versions of the FF. Silicon versions of the FF??? Yes. The FF was built with several transistor types, including silicon. The first ones seem to have been all AC128’s, a PNP germanium device, followed by a runs with the Newmarket NKT275, which is very similar. I recently obtained a batch of NKT275’s and they seem to be much more consistent than the AC128, but in other respects very similar. There were several versions with the BC108C and/or BC109/BC109C. These are NPN silicon devices, and the BC108C in particular is a HIGH gain device, typically 500. The reputation of the NPN silicon devices is spotty, although some owners (there’s that preferences thing again...) do like them very much. At one point in the past, Dunlop bought the trademarks and began manufacturing the FF as a reissue. The initial ones were made with high gain silicon transistors and sounded dreadful (IMHO). Dunlop would later make the JH2, which used the MPSA18, with a typical gain of 900! These all seemed to have a “sticky” or “constricted” quality about them. Dunlop has in recent years arranged for new manufactured germanium devices to be made for them, and is shipping these in its latest reissues. The JH2, by the way, is a great value for the DIY’er. You get that neato round case for cheap because they sound horrible. You can then rebuild the real FF circuit onto the circuit board in less than an hour, and have your own great sounding FF. This all makes good sense - in any device that has so few parts, the qualities of the parts will show through. I’ve also done some experimentation with germanium devices other than the AC128 and NKT275. I found a number of germanium types, measured their gains, and then listened to them. The results are pretty consistent - get the right gain and frequency response and the numbers printed on the cases don’t matter. I like the 2N527 from GE (very consistent!) and the 2N508, also GE. The 508 is a little hotter, for more of a metal tone. Reports of results on the ECG158 are mixed. Some people are very happy with it, some not. I suspect that this can be put down to the variablility of the devices. Measure what you get!! OK, what about things other than the transistors? Roger Mayer is reputed to have tweaked Jimi’s gear heavily, so maybe there’s some tone to be wrung from the stones there. There is a set of changes collectively referred to as the “Hendrix” mods or the “Roger Mayer” mods. These are Replace the 470 ohm output resistor with 1K Replace 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. This mod primarily seems to increase the output level and gain of the second transistor. Changing the input and output capacitors changes the bass response of the unit. To allow more bass response, you can double each of these capacitors. In the case of the output 0.1uF capacitor, raising it to 0.47uF may be useful. This will prevent the bass signal from being lost by the impedance of the coupling capacitor. Mike Fuller posted some recommendations on the Fuzz Face to the net. Among those are to add a 1K linear pot in series with the 470 ohm resistor in the collector circuit of the second transistor, still taking the output from the top end of the 8.2 K resistor. The pot is set up so it is acting as a variable resistor, wiper shorted to one end lug. Another is to connect a 50K pot, again as a series resistor, in series with the input of the circuit, before the 2.2uF input capacitor. If that last one seems a little odd, think back to what I said about the low input impedance. With a low input impedance, the input loads a guitar significantly; the base can only move a few tens of millivolts before cutting off or saturating the first transistor. If you put a resistor in series with the guitar pickup, it raises the apparent source impedance of the pickup, making it look more like a current source (albeit a tiny one) and less like a voltage source. A signal from a current source lets the input of the effect seek it’s own voltage level, and merely supplies a varying current. This can be much more linear than a voltage source drive. As a result, the variable resistor allow you to radically clean up the distortion that the FF produces, producing subtle shades of softer distortion. In effect, the output mod is a “more” control, and the input mod is a “less” control. As you would expect, the two added controls can interact to produce many shades of distortion. If you like softer distortion, you might also like to add some high end taming capacitors. 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. So - the Fuzz Face is a fortuitous combination of circuits that combine initial soft clipping with asymetrical clipping that changes toward symetrical clipping under drive. It’s easy to build, easy to modify, and has a very wide range of tones possible. This thing offers what I consider the most bang for the buck for the enterprising effects hacker. Dig in.... see what magic you can wring out of it.

9/23/1997 6:36 AM

R.G.

Fuzz Face Technology

9/23/1997 7:48 AM
J Epstein	Nice post. OK, That's Backlogged Project #4. -j

9/23/1997 10:06 AM
CJ Landry	Thanks RG for this valuable and enlightening article. I think this would be a great article you could put in the book you are going to write. Hint - hint!! Christian

9/23/1997 11:08 AM
Mark Amundson	Thanks R.G. for the excellent summary of the fuzz face circuit. I hope you archive your posting along with the stock schematic. Mark Amundson,

9/23/1997 12:46 PM
Steve Morrison	A truely great post, R.G. I actually felt the urge to stand up and applaud after reading it! Thanks for making my day.

9/24/1997 8:53 AM
M Helin	More mods for FF: Add an emitter-follower stage before first stage (like in TS-9), and use a pot from emitter to ground, wiper to FF input cap. This way the impedance matching is perfect (or use a FET, even better). Add a similar stage at the output, and add a usual tone control there between output of FF and extra stage. Add a resistor between the emitter of the first stage tranny and ground, or maybe like in the second stage, 1 k pot and cap. This could be a preset to adjust the gain of the first stage for different kind of and gained transistors. If this is done, the first stage collector resistor may be needed to be changed to a bigger one. Frankly, I don't understand why there are both 470 (or 330) and 8k2 resistors in series in the second stage output. It does lower the output level, but doesn't seem to affect the tone at all. Another thing which is obvious from schematics, is that the 1 k pot at the second transistor's emitter adjusts both the gain of the second stage and negative feedback to first stage (and this way the gain of the first stage) at the same time. It would be nice (but impossible, even two pots doesn't help) to be able to adjust both independently.

9/24/1997 8:53 AM

M Helin

More mods for FF:

Add an emitter-follower stage before first
stage (like in TS-9), and use a pot from
emitter to ground, wiper to FF input cap. This
way the impedance matching is perfect
(or use a FET, even better). Add a similar stage
at the output, and add a usual tone control
there between output of FF and extra stage.

Add a resistor between the emitter of the
first stage tranny and ground, or maybe
like in the second stage, 1 k pot and cap.
This could be a preset to adjust the gain of
the first stage for different kind of and
gained transistors. If this is done,
the first stage collector resistor may be
needed to be changed to a bigger one.

Frankly, I don't understand why there are
both 470 (or 330) and 8k2 resistors in
series in the second stage output. It does
lower the output level, but doesn't seem
to affect the tone at all. Another thing
which is obvious from schematics, is
that the 1 k pot at the second transistor's
emitter adjusts both the gain of the
second stage and negative feedback to first
stage (and this way the gain of the first
stage) at the same time. It would be nice
(but impossible, even two pots doesn't
help) to be able to adjust both independently.

9/24/1997 10:48 AM
R.G.	Good post. The best way to get sounds you like is to dig in and tinker, and the best thing about building your own stuff is that it positively invites you to mod it to get the sounds that you personally like, and that may be unique. There is a possible problem with putting an emitter- or source-follower in front of the Fuzz Face. The low input impedance and the interaction with that impedance is something that guitarists have come to depend on when working with the fuzz face - or its many clones. The low impedance means that if you back off on the volume control, you don't get just a lower signal, you also get more of the volume pot impedance inserted and the signal source looks more like a current source, so the distortion cleans more, and in a slightly different way, than just the decrease in signal voltage. A follower does give you complete independence from the guitar impedance, which may be good or bad, depending on the sound you are trying to achieve, and your personal playing style. Note that this is not bad, necessarily, unless you WANT the clean-up effect of not having the buffers. Many players do. Usually, putting any other effect in front of the FF does this same thing, as does an on-board preamp. Matter of taste. On the tone controls - yeah, add whatever you like. I have this affinity for the scooped-mid tone of the Notch setting in the Univox SuperFuzz, and I've cobbled that setting into a selectable "afterburner" tone control stage that I can use after several fuzzes. It's also useful to put something like either a wah or a graphic EQ pedal in front of the FF and maybe even another graphic EQ after it for full control. The emitter resistor in the first transistor is useful in some circumstances. Putting too big a resistor in there takes away some of the mushiness of the saturation, though. As I understand it the 470/330 is there purely to lower the signal level. Mike Fuller makes a big deal about the adjustable 1K in series with this resistor affecting the midrange tone, and on his '69 pedal calls this control "Mid Contour". It is possible that this control does most of its effect on a supposed tube amp input following the FF. You can adjust the gain of the first stage independently of the second stage. If you split the 100K feedback resistor into two 51K resistors, then connect a capacitor to a series-resistor style pot to ground, you can selectively short out the feedback AC voltage produced at the Q2 emitter and eliminate the AC feedback to the first transistor. You could use 1uF and 1M for the cap and pot to start with (I haven't done the math on that, so some experimentation would undoubtedly be needed) and twiddle until you got values that you liked. It might be that a switch on the pot could disconnect it at full CCW rotation for stock operation. Very fertile grounds here for experimentation. In this circuit, everything matters!! Tinker!!!

9/24/1997 10:48 AM

R.G.

Good post. The best way to get sounds you like is to dig in and tinker, and the best thing about building your own stuff is that it positively invites you to mod it to get the sounds that you personally like, and that may be unique.

There is a possible problem with putting an emitter- or source-follower in front of the Fuzz Face. The low input impedance and the interaction with that impedance is something that guitarists have come to depend on when working with the fuzz face - or its many clones. The low impedance means that if you back off on the volume control, you don't get just a lower signal, you also get more of the volume pot impedance inserted and the signal source looks more like a current source, so the distortion cleans more, and in a slightly different way, than just the decrease in signal voltage. A follower does give you complete independence from the guitar impedance, which may be good or bad, depending on the sound you are trying to achieve, and your personal playing style. Note that this is not bad, necessarily, unless you WANT the clean-up effect of not having the buffers. Many players do. Usually, putting any other effect in front of the FF does this same thing, as does an on-board preamp. Matter of taste.

On the tone controls - yeah, add whatever you like. I have this affinity for the scooped-mid tone of the Notch setting in the Univox SuperFuzz, and I've cobbled that setting into a selectable "afterburner" tone control stage that I can use after several fuzzes. It's also useful to put something like either a wah or a graphic EQ pedal in front of the FF and maybe even another graphic EQ after it for full control.

The emitter resistor in the first transistor is useful in some circumstances. Putting too big a resistor in there takes away some of the mushiness of the saturation, though.

As I understand it the 470/330 is there purely to lower the signal level. Mike Fuller makes a big deal about the adjustable 1K in series with this resistor affecting the midrange tone, and on his '69 pedal calls this control "Mid Contour". It is possible that this control does most of its effect on a supposed tube amp input following the FF.

You can adjust the gain of the first stage independently of the second stage. If you split the 100K feedback resistor into two 51K resistors, then connect a capacitor to a series-resistor style pot to ground, you can selectively short out the feedback AC voltage produced at the Q2 emitter and eliminate the AC feedback to the first transistor. You could use 1uF and 1M for the cap and pot to start with (I haven't done the math on that, so some experimentation would undoubtedly be needed) and twiddle until you got values that you liked. It might be that a switch on the pot could disconnect it at full CCW rotation for stock operation.

Very fertile grounds here for experimentation. In this circuit, everything matters!! Tinker!!!

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