Is a Fuzz Face a common-emitter or common-collector circuit? - diyAudio
Go Back   Home > Forums > Live Sound > Instruments and Amps

Instruments and Amps Everything that makes music, Especially including instrument amps.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 8th December 2011, 11:02 PM   #1
diyAudio Member
 
Join Date: Dec 2011
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!
  Reply With Quote
Old 9th December 2011, 12:01 AM   #2
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
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.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 9th December 2011, 12:20 AM   #3
diyAudio Member
 
Join Date: Dec 2011
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 common-collector...it'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!
  Reply With Quote
Old 9th December 2011, 12:41 AM   #4
Enzo is offline Enzo  United States
diyAudio Member
 
Join Date: Sep 2004
Location: Lansing, Michigan
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.
  Reply With Quote
Old 9th December 2011, 12:50 AM   #5
diyAudio Member
 
Join Date: Dec 2011
Thanks for the clarification...I will keep hacking at it!

I really appreciate the help.
  Reply With Quote
Old 9th December 2011, 01:31 AM   #6
diyAudio Member
 
tubelab.com's Avatar
 
Join Date: Jul 2005
Location: West Virginia panhandle
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.
__________________
Too much power is almost enough! Turn it up till it explodes - then back up just a little.
  Reply With Quote
Old 9th December 2011, 02:38 AM   #7
diyAudio Member
 
Join Date: Dec 2011
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!
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
Common Source versus Common Drain output stages alaskanaudio Solid State 36 1st June 2014 03:35 AM
CE (common emitter) Amplifier help SCKentrol Solid State 3 8th May 2011 08:52 AM
Need help understanding current sourced common emitter/source amplifiers GregH2 Solid State 9 26th February 2011 07:14 AM
Calculation of input impedance (Common emitter amplifier) jazko Solid State 22 22nd January 2011 11:35 AM
Basic Common Emitter Amp Help Obe1 Analog Line Level 99 21st February 2010 06:08 PM


New To Site? Need Help?

All times are GMT. The time now is 09:13 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2