Is a Fuzz Face a common-emitter or common-collector circuit? - diyAudio
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Old 8th December 2011, 10:02 PM   #1
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Default Is a Fuzz Face a common-emitter or common-collector circuit?

I've been trying to understand the functionality of the Fuzz Face circuit (Fuzz Central -- Arbiter Fuzz Face) and I just don't get what's going on.

R. G. Keen says that the PNP Q2 transistor is setup as an emitter-follower (or common-collector) in his amazing circuit analysis here: Technology of the Fuzz Face - Frame Definition

But I keep looking at it and looks like a PNP common-emitter circuit: Designing the Common-Emitter Circuit with TransistorAmp 1.1 Software. Isn't the emitter (ground) what is common to both input and output?

I'm sure R. G. Keen's analysis is bulletproof, I just don't understand how it works...can anyone explain?

Thanks so much!
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Old 8th December 2011, 11:01 PM   #2
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Both stages are common emitter amps. If the fuzz control is turned up all the way the emitter of Q1 is grounded and the emitter of Q2 is bypassed to ground. The input of each stage is on the base and the output is on the collector. This IS the definition of a common emitter amp. The 100K resistor takes some DC negative feedback from the emitter of Q2 to the base of Q1 to help stabilize the operating point of the notoriously temp sensitive germanium transistors.

As you rotate the fuzz control away from maximum, the bypass on Q2's emitter becomes a partial bypass so that some AC negative feedback is applied from Q2 to Q1 along with the DC feedback. I suppose that it could be said that Q2 then functions somewhat like a phase splitter since signals of opposite phases are taken from the collector and the emitter. The desired output is taken from the collector, so it still looks like a common emitter circuit to me.

The 470 ohm and the 8.2K ohm just form a voltage divider on the output so that the output isn't strong enough to possibly damage an amp if it's turned all the way up.

Note that there is another possible negative feedback path caused by the internal resistance of the battery allowing some of the output signal to flow through the 33K resistor back into Q1's collector. This is why the old Fuzz Faces and Tone Benders were picky about what kind of batteries were used and how fresh they were. Add an electrolytic cap across the battery to eliminate this dependency. The value is not particularly critical, try a 10uF. If you like the sound of your Fuzz Face with a nearly dead battery, add a 1K pot between the battery (with bypass cap) and the rest of the circuit to adjust this effect.

I made dozens of these things about 40 years ago and I added extra pots, caps and switches to most of them. I just poked around in one while it was live and if I found something I liked, I wired it to a switch and added it. I had switches to select different values for all of the caps. Each one makes a different contribution to the tone.

All of my notes from back then are long lost, but my original unit with the transistor sockets is around here somewhere. It will turn up someday when I open up a "mystery box". When it does I will trace it out. I have been systematically going through stuff, documenting it and tossing it out. I have found stuff I built as far back as high school.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 8th December 2011, 11:20 PM   #3
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Thank you so much for your response!

I wonder why that Technology of the Fuzz Face writeup by R. G. Keen still says that the circuit is a's been up since 1998 and lots of forums link to it. Thanks for setting me straight!

I've seen some mods for this circuit which call for switching the voltage divider from 470 and 8.2k to 1k and 18k - the ratio is pretty close to the original resistance ratings. Does the increase make any other difference?

Btw, the tubelab website is great...nice tube amps you've built!
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Old 8th December 2011, 11:41 PM   #4
Enzo is offline Enzo  United States
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Go back to RG's place and read the paragraph again. It says,

"For DC purposes, the second transistor acts as an emitter follower. " He didn't say "this is an emitter follower."

SO for the moment, ignore the signal part, and look at what DC voltages wil be doing.
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Old 8th December 2011, 11:50 PM   #5
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Thanks for the clarification...I will keep hacking at it!

I really appreciate the help.
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Old 9th December 2011, 12:31 AM   #6
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Back in circuits analysis class they taught us to analyze the DC paths and AC paths seperately. They are seldom the same unless the circuit contains only resistors.

For the DC analysis simply remove all capacitors and analyze the circuit. The DC analysis is just used to determine the operating conditions. In the DC case the second stage can be considered an emitter follower with degeneration at the collector.

For the AC analysis it is assumed that the DC conditions satisfy proper operation. All capacitors are simply shorted, or replaced with resistors equal to their reactance at the frequency of operation. The AC signal path can be seen more clearly with this assumption. In this case both transistors operate as common emitter amplifiers.

Of course I learned all this in the 1960's before compter circuit simulations were possible for anyone other than large corporations. There are some extreme simplifications in the above assumptions, but they are still useful to see the circuit functions.

In many circuits including this one the presence of the AC signal affects the DC operating point, but does not change the circuit topology.
Tubelab, it's 5 year mission. To explore strange new tubes, to seek out new circuits and topologies, to boldly go where no tube has gone before......
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Old 9th December 2011, 01:38 AM   #7
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That is great information...I will do more research on how to do circuit analysis. I have been studying the Navy Electrical Engineering courses for a month now and doing my best to work through all of the problems and circuits.

Without being able to ask specific questions it is a challenge to get it all to make sense so I really appreciate the knowledge you are sharing! A book lays it all out for you, but it's talking to people like you that make it digestible.

I was reading your website and you mentioned that you wanted to setup a shopping cart (back in 07!)...have you looked into Wordpress? There are some great templates out there that would allow you to keep all of your content but also add shopping carts, Facebook integration and lots of other bells and whistles. And it's super easy to update and modify.

I'd be glad to help you set it up or talk to you about adding those kinds of components if you are still interested.

Thanks again!
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