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From keen@eden.com Sat Jan 14 13:20:00 1995
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From: keen@eden.com (r g keen)
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Subject: Guitar Effects FAQ
Date: 12 Jan 1995 01:17:56 GMT
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 ===========================================================================
 GUITAR EFFECTS FAQ

 Version 2.02, 12 January 1995

 This FAQ is periodically posted to:
 rec.music.makers.builders
 rec.music.makers.guitar
 alt.guitar

>It is also available on Web Pages:
>http://www.wwu.edu/~n9343176
>http://www.eecs.umich/~tjs/guitar/effects.hmtl
>
>Changes from the last version are marked with a ">".
>
>This FAQ was created by R.G. Keen, keen@eden.com
>Last update 12 January 1995

 ===========================================================================
 INDEX:
         Introduction
         Contributors
         What kinds of effects are there?
            amplitude based effects
            waveform distortion effects
            filter/frequency response effects
            time delay effects
            other miscellaneous effects
            common combinations
         Answers to questions about terminology
            Difference between overdrive and distortion
            Difference between vibrato and tremolo
         Where can I find schematics for guitar effects?
            Books
            Magazines
            Kits
            The Net
         How practical is building your own effect?
         Skills and tools for building effects
         Effects Packaging
         Procuring Parts
         Recommended suppliers are:
         Answers to questions about fixing broken effects
            Completely dead pedal
            Sound, no delay in older delay pedals
            Scratchy Wah Pedal
         Appendix A - Effects Economics 101
         Appendix B - Making Circuit Boards
         Appendix C - My Personal Best Way to Package Effects
         Appendix D - A Musical Distortion Primer
 ===========================================================================
 INTRODUCTION

 This FAQ is a generic answer to the questions which begin:

 I have heard of an effect called a DoWhizzer.  What is this?

 "My Phaser/Blaster/Clinker pedal is acting funny/dead.  Can this be
 fixed/how can I fix this?

 "I have <or have a friend who has> some electronics abilities and I'd like
 to build my own guitar effects.  Where can I find schematics for this?"

 I have done this for a long time, so I thought I'd write down some facts
 and some of my opinions.  I have tried to weed out a lot of possible-but-
 difficult things in writing this.  Where I simply state that something
 ought to be done some way, this is usually an opinion based on having
 tinkered.  My opinion is subject to being wrong at times, so if you know a
 better way, or if I have left out something that is much simpler and
 easier, let me - and the rest of us - know.  I certainly won't be offended
 at having a mistake pointed out.

 If you have something to contribute along these lines, by all means, send
 it to me or post it.

 ===========================================================================
 CONTRIBUTORS:

 Many thanks to those of you who have sent additions and corrections
 including:
>Mark Hammer        no longer on the net that I can find
 David Mourning     <dam@dcs.gla.ac.uk>
 Murray Traue       <traue@otago.ac.nz>
 Jamie Heilman      <n9343176@gonzo.cc.wwu.edu>
                     KY_YIP@PAVO.CONCORDIA.CA
 Scott Lehman       <slehman@mit.edu>

 ===========================================================================
 What kinds of effects are there?

 Effects are usually based on some facet of the human ear's abilities to
 figure out from amplitude and frequency content variations what a sound
 source is doing.  This was critical when the sound source might be a saber
 toothed tiger.  Now it is a means for us to express ourselves musically by
 directly invoking emotion.

 I tried to count up all of the things I could think of that an effect could
 even possibly be based on.  The raw list is stated first, just as a
 reference, and afterwards I give a short definition of each effect.  Since
 distortion is perhaps the most musically interesting effect to a guitarist,
 I also include a primer on distortion and an exhausting
 ...er...  exhaustive (to me at least) list of ways to get musically useful
 distortion in Appendix D.



>AMPLITUDE BASED EFFECTS               TIME DELAY EFFECTS
>based on the instantaneous loudness   Reverb
>of the signal, how it changes, and    Echo
>how quickly it changes.               True vibrato
>                                      Flanging
>Volume control                        Chorus/ADT
>Tremolo                               Slapback
>Auto tremolo                          Reverse echo/reverb
>Panning/ping-pong                     Sampling?
>Gating/repeat percussion
>Compression                           OTHER MISCELLANEOUS EFFECTS
>Expansion                             Octave division
>Asymmetric comp/peak compr.           Harmony generation
>Noise gating                          Phase lock tracking
>Attack delay                          Noise addition
>ADSR                                  Talk box
>Limiting                              Voice tracking (vocoder)
>Auto swell                            Ring modulation (DSBSC generation)
>                                      SSBSC generation
>WAVEFORM DISTORTION EFFECTS
>All the generic ways to distort the   COMMON COMBINATIONS
>waveform that comes from the guitar.  Leslie (vibrato, tremolo, varying
>                                       filtering generated by rotating
>Symmetrical clipping                   speakers)
>Asymmetrical clipping
>Infinite limiting                     Aphex (filtering, selective frequency
>Half wave rectification                band distortion)
>Full wave rectification
>Arbitrary waveform generation         Enhancers (Split the signal into a
>                                       few bands, slightly distort some,
>FILTER/FREQUENCY RESPONSE EFFECTS      remix)
>EQ/tone controls
>Treble/mid/bass boost
>Cabinet simulation
>Resonator
>Wah
>Auto wah
>Tremolo-wah
>"vibrato"
>Phase shifting


 EFFECTS DESCRIPTIONS
 ====================

 AMPLITUDE BASED EFFECTS
 Volume control
 Manual level control.  Twist the knob, the sound gets louder or softer.

 Tremolo
 cyclical variation of volume by a low frequency oscillator of some sort;
 parameters are waveform of the LFO, LFO frequency, and depth of modulation;
 note that while the terms tremolo and vibrato are often used
 interchangeably, tremolo is actually variation in loudness, vibrato is
 variation in pitch or frequency.

 Auto tremolo
 tremolo where the modulation frequency is varied by some feature of the
 input signal, generally amplitude.

 Panning/ping-pong
 generalization of tremolo to more than one channel; as one channel goes
 down in level, another goes up.  With non-square LFO waveforms, gives the
 effect of the sound source moving from place to place in stereo or more
 channel setups.

 Gating/repeat percussion
 tremolo with 100% modulation of the signal by a square wave.  With
 exponentially decaying waveforms (guitar is a good one), gives the effect
 of striking the same note again at decreasing levels. Some Thomas Organ
 Vox amps have this as a built in effect.

 Compression
 makes soft inputs louder, and loud ones softer, giving a one-level kind of
 sound with lessened dymanics.  This is effectively volume control with the
 level determined by the negation of the averaged envelope of the input
 level.

 Expansion
 Makes loud sounds louder and soft ones softer.  Effectively volume control
 with the level determined by the averaged envelope of the input level.
 Compression and expansion can be complementary, as in
 com(pression/exp)anding for noise reduction.

 Asymmetric compression/peak compression
 Only the peaks of the input waveforms get compressed, not the overall level
 of the waveform envelope.  Effectively, there is no averaging of the
 envelope and the instantaneous waveform level is compressed.  This amounts
 to a much softer form of clipping, and is part of the tube sound, since
 tube with a soft B+ supply are prone to this.

 Noise gating
 modulates the output off when the input level is below a threshold.  The
 modulation may be a square wave, or a variation of expansion where the low
 level inputs are "expanded" down into silence, which gives a less abrupt
 transition.

 Attack delay
 A variation of noise gating where the transition to "on" from the "off" or
 no signal state is slowed.  This gives an output which perceptibly rises in
 level with each new note envelope, reminiscent of a tape recording played
 backwards.

 ADSR
 term borrowed from the synthesizer folks; stands for Attack Decay Sustain
 Release, which is the most general way to describe a musical envelope.  It
 is possible to generate an artificial ADSR envelope for a musical note to
 help fool the ear as to which instrument generated the note.

 Limiting
 Like compression, but operates on signals over some threshold only.  Well
 suited to keep an input from going over some level, but un-processed below
 that level, as in getting signals on tape without overloading the tape.

 Auto swell
 generally, a rise in level from some starting level to a final level when
 keyed manually or electronically.  Can effectively add sustain to some
 notes and not others when keyed manually, or can add a "swell" in volume
 over a run of notes, or can help with presetting the level of a lead.

 WAVEFORM DISTORTION EFFECTS
 Symmetrical clipping
 For a given input waveform, say a sine wave, the tops and bottoms of the
 waveform are clipped equally, symmetrically.  Although the musical
 implications are more involved than this simplistic explanation, for a
 simple sine wave, symmetrical clipping generates only odd-order harmonics,
 giving a reedy, or raspy sound to the resultant waveform.

 The hardness or softness of the clipping matters.  Hard clipping results
 when the output wave equals the input up/down to a certain level, then
 stays at the clipping level until the input drops below the clipping level
 again, giving perfectly flat tops and bottoms to the clipped output.  Soft
 clipping has no abrupt clipping level, but gently rounds the top/bottom of
 the output wave so the waveform is "softly" rounded on top/bottom, not
 flat-topped.  Some solid state devices actually flat top, then invert,
 producing a hollow topped output waveform at hard clipping.  There is a
 continuum of clipping hardness, depending on the circuitry used to clip.
 Soft clipping emphasizes the lower- order harmonics, the third and fifth,
 etc.  Hard clipping has a mix slewed to the higher order seventh and up
 harmonics, which are harsher sounding.

 Intermodulation distortion, the production of sum and difference
 frequencies from frequencies in the input waveform, varies with the amount
 and hardness of clipping.  Intermodulation sounds harsh and ugly.  The
 amount of intermodulation is a characteristic of the circuit that produces
 the distortion.

 Asymmetrical clipping
 The top(or bottom) of the waveform is clipped more than the bottom (top)
 half.  This causes the generation of both even and odd harmonics, in
 contrast to symmetrical clipping's odd-order only.  The even harmonics are
 smoother and more musical sounding, not as harsh as the odd ones.  The
 hardness of the clipping and the degree of asymmetry affect the sound.  The
 more asymmetrical, the more pronounced the even-order harmonics; the
 harsher the clipping, the more the harmonics are slewed toward higher
 order.  See Half Wave Rectification.

 Tubes in general produce asymmetrical distortion unless the circuitry is
 set up to remove them, as happens in push-pull.

 The comments on intermodulation apply here.

 Infinite limiting
 In essence, the waveform is amplified "infinitely" and hard and
 symmetrically clipped, producing a rectangular output waveform which shares
 only the zero crossings with the input waveform.  Sounds buzzy and
 synthesizer-ish.

 Half wave rectification
 Half wave rectification represents the logical conclusion of asymmetrical
 clipping.  One half of the waveform is flat, the other half is unchanged.
 This produces a prominent second harmonic, heard as an octave.  There are
 analog devices which produce an octave effect injust this way; I have heard
 that the "Octavia" effect is based on this.

 Full wave rectification
 In full wave rectification, one half of the input waveform is "folded" to
 the opposite polarity, producing an output with a net DC component, none of
 the original fundamental frequency of the input waveform, and only the
 second and higher harmonics of the original input frequency.  Produces very
 strong octave of the input waveform, as well as a slew of even-, odd-, and
 intermod- distortion products when more than a single frequency is the
 input ( as is the case for all musical instruments).

 Arbitrary waveform generation
 This effect generates a completely new waveform of arbitrary shape which
 shares the same frequency as the input waveform.  Guitar synthesizers do a
 version of this.


 FILTER/FREQUENCY RESPONSE EFFECTS
 EQ/tone controls
 Allow you to cut or boost the highs, lows, mids etc.  Tend to be
 broad-brush kinds of controls - all the "high's" get raised or cut.  Range
 is typically +/- 12 to 20 db boost/cut.

 Treble/mid/bass boost
 like an additional eq control, but tends to be narrower in frequency range,
 and perhaps more boost range, no cut.

 Cabinet simulation
 A filter network designed to mimic the two- or four-pole low frequency
 rolloff of a guitar speaker cabinet, usually to get that "miked cabinet"
 sound into a PA without really miking a cabinet.

 Resonator
 a filter with a boost in frequency at a narrow range of frequencies.  This
 sounds like a wah pedal when the pedal is not being moved.

 Wah
 A resonator that can have its center frequency moved up or down in
 frequency by moving a pedal.  The "wah" name comes from the way it mimics
 the moving resonance of the human vocal tract in speech as the sound "wah"
 is made.

 Auto wah or "Envelope Follower"
 A wah filter where the center frequency is determined by the loudness of
 the input signal, making a moving resonance on every note

 Tremolo-wah
 Wah where the center frequency is moved back and forth cyclically, as
 though the pedal was connected to a motor or some such.  This can generate
 effects similar to a rotating speaker or phasing.

 "vibrato"
 a cyclical variation in the basic frequency of the input signal, similar to
 the effect of moving the whammy bar on a suitably equipped guitar.  True
 vibrato as an add on effect requires some kind of time delay, and was hard
 to do until analog (and now digital) delays came to be.

 Phase shifting
 This effect is a filter response generated by using long phase delays and
 mixing with the original signal to cause a number of deep notches and/or
 peaks in the overal filter response.  This mimics the larger number of
 notches and peaks caused by true time delayed flanging.  Most simple phase
 shifters or phasers do this by generating two notches, although some pedals
 make four notches.  Flangers may make many notches.  Phasers may also
 incorporate feedback to sharpen up the effect of the notches.

 TIME DELAY EFFECTS
 Echo
 Reverb
 True vibrato
 Flanging
 Chorus/ADT
 Slapback
 Reverse echo/reverb
 Sampling?

 OTHER MISCELLANEOUS EFFECTS
 Octave division
 Takes the fundamental frequency of an input signal, divides it by two, and
 creates an octave-lower, sometimes a two-octave lower signal, which are
 usually mixed back with the original signal.  This is most often done with
 digital logic flipflops to divide the signal by two/four after squaring up
 the input to drive the flipflops.  This provides outputs that are
 substantially square waves, sounds like fuzz bass.  Some kind of filtering
 is usually provided to tame the sharp buzz of the square waves.  The simple
 dividers like this get very confused when fed more than one tone at once,
 so single note runs are all that is really practical - unless you like
 confused effects.

 Harmony generation
 Generation of other notes at musically-interesting intervals along with
 your notes.  The classic device to do this is the Eventide Harmonizer.  It
 is very difficult to do this electronically so that the effect produces
 musically-useful sounds consistently, hence Eventide's high price.


 Phase lock tracking
 An electronic circuit called a "phase locked loop" can produce an output
 signal that is exactly an integer multiple or small-numbers fractions of a
 reference signal in frequency.  You can generate:  a signal that follows
 your notes, perhaps lagging a little with a glide onto the note an octave
 or two above a third/fifth/seventh, etc.  above or below your notes.

 Sounds kind of like a computer playing harmony with you.

 The outputs are usually square wave or filtered square wave, and sound kind
 of synthesizer-y.  Modern all-digital MIDI-fied effects do something like
 this in their computer processors, and may not be as limited in output
 waveform.

 Noise addition
 Noise (hiss, rumble, etc) is deliberately added to the input signal.  If
 this is done with restraint and matching the input signal envelope, it can
 add a breathing effect like the hiss of air in a flute.

 Talk box
 This effect is produced by using a small amp to produce sound that is
 conducted into your mouth by a tube, so you can mouth the words to a song,
 using your vocal tract resonances to shape the instrument sound, which is
 then picked up by a microphone.  This is the archetypical "talking guitar".

 Voice tracking (vocoder)
 Ring modulation (Double Side Band Suppressed Carrier generation) Single
 Side Band Suppressed Carrier generation

 COMMON COMBINATIONS
 Leslie (vibrato, tremolo, varying filtering generated by rotating speaker)

 ===========================================================================
 Answers to questions about terminology

 Q. What is the difference between "distortion" and "overdrive"?
 A. Effectively none. "Overdrive" started as what you got when you put too
    large a signal into the input of an amp, causing the signal to be
    distorted at the speaker.  You were "overdriving" the inputs.
    "Distortion" is the more generic term, and started when folks noticed
    that you could get a distorted sound from a little solid state amp that
    was VERY nonlinear.  The terms have been used so interchangeably that
    there is no real difference, although some people will swear that only
    tubes being overdriven sound good, etc.  Let them insist.  It won't hurt
    much either way.

 Q. What is the difference between "vibrato" and "tremolo"?
 A. In strict musical terms, vibrato is a cyclic change of the frequency
    of the note, while "tremolo" is a cyclic change in the amplitude of the
    notes.  The amp and effects makers have used the terms interchangeably a
    lot, so for amps and effects, you cannot tell what you will be getting
    from the term.

 ===========================================================================
 Where can I find schematics for guitar effects?

 Books
 "Electronic Projects for Musicians" by Craig Anderton
 "Electronic Projects for Guitar" by Robert Penfold

 which outline not only the schematics, but how to read them and how to find
 parts, how to make boards and packages and boxes, etc.  If you have no
 experience with effects, get one of these books first.  In fact, get them
 anyway.  They are very good references for people interested in effects.

 Magazines
 Other schematics are available commercially in back issues of Guitar Player
 magazine, where Anderton wrote an irregular series of articles starting in
 the seventies in which he would outline an effect and how to build it.
 Some of these are significantly different from or more flexible than
 commercial effects.

 Other electronics-related buildit magazines have published articles on the
 odd effect or two over the years.  These include Popular electronics,
 Radio-Electronics, Electronics Technology International, Electronics Today,
 a little-circulated rag called Poly phony, and another called Electronotes
 (although the Electronotes articles are primarily oriented towards analog
 synthesizers).

 Kits
 Effects kits are available from PAIA Electronics, 3200 Teakwood Lane,
 Edmond OK, 73031, 405-340-6300.  It is useful to write these folks and
 request a catalog if you're interested in effects. John Simonton is
 the president at PAIA, and is on line at PAIA.aol.com, or was for a
 while.

 The Net
 There are a few schematics available as postscript files at
 ee.bode.ualberta.ca.

 There is a WWW page maintained by Jamie Heileman that is now beginning to
 have a good collection of schematics in postscript form.  Jamie has been
 embellishing this collection at a fair clip.  It is likely that this will
 grow to seventy or eighty schematics over time, although it has only twenty
 or so now.  This can be reached at

 http://www.wwu.edu/~n9343176

 There are some schematics on the Analogue Heaven Web page or its
 connections at:

 http://www.cs.washington.edu/homes/map/analog/

 Informal Sources
 The ...ah...  less structured...  ways to find schematics are to know
 someone who has them somehow, like a musical instrument repairman, or to
 trace out the schematic from a commercial unit.  There are a number of
 people who have collected libraries of schematics for effects, if you can
 find them.  These folks usually want to trade schematics to build up the
 library.

 ===========================================================================
 How practical is building your own effect?

 It is NOT cheaper to build your own than to buy, at least in most cases.
 If you want to tinker and play around with building effects because you
 like it, go ahead.  Also, if you're after a vintage effect, the prices may
 be so outrageous for re-creations or originals that you can duplicate it
 for a song. (sorry, I had to...)

 If you want a few effects and think it is cheaper to build your own, think
 long and hard about it.  The economies of scale being what they are, the
 commercial companies can produce a finished effect and sell it at retail
 for less than your cost of parts.  Common semiconductors are from three to
 ten times cheaper in thousand unit lots than in ones and twos.  Finished,
 painted,lettered boxes to put this stuff in are ten to fifty times cheaper
 for a manufacturer to make than for you to do.  See Appendix A - Effects
 Economics 101 below.

 Most people who start down this path never build an effect.  Effects are
 hard to build -* mechanically *- not electronically, so the interest in
 electronics is immediately subverted when you try to package one.  Making
 one reliable under typical music conditions is even harder.  If you are
 not already involved to a signifcant degree in electronic tinkering, it
 will be expensive to acquire the tools and parts to build effects.  Faced
 with these problems, most folks give up.

 If, on the other hand, you just love tinkering with guitar effects, have
 some electronics know how, and have some money to put into the hobby, forge
 ahead.  It is my personal choice of a good time.  Has been for a couple of
 decades.

 You have to be really good with digital logic and programming as well as
 prototyping to make a sophisticated Digital Signal Processing kind of
 integrated effects box like the rack units.  With some experience, you can
 make effects which are not commercially available, or have your own
 personal likes written into the wires and parts.

 It is also in general NOT possible to build a good wah pedal or other
 rocker kind of pedal, as the mechanical construction of a reliable
 rocker-pedal mechanism is impractically difficult for the average Joe.
 However, you can often find a dead-or-dying Cry baby or other wah pedal to
 cannibalize for the case and pedal; I've seen dead ones for as little as
 $15.

 Oh, yeah...  what about what you thought this section was about.  Yes, you
 can build effects that perform as well as commercial units, or surpass them
 in terms of noise, performance, etc.  It's not hard, for the analog kind of
 stomp box.

 ===========================================================================
 Skills and tools for building effects

 There are some minimal skills and tools you'll need to be able to
 understand an effects schematic and have some hope of building something
 similar.

 Reading Schematics
 You'll have to be able to understand what the schematics are telling you.
 A schematic is a form of shorthand which just tells what parts are hooked
 together and what the values of the components are.  There are many ways to
 physically wire up the same schematic.  Unfortunately, there is no super
 easy way to understand schematics without some basic understanding of the
 electronic parts for which the symbols stand.  I recommend you pick a book
 or two from the bibliography and put in some study time if you have no
 electronics background.

 Basic Construction Tools

 For circuit boards:
 Pencil soldering iron (15-25 Watt)
 Needle nose pliers
 Small diagonal cutters
 Rosin core solder (acid core plumbing solder will eat up your circuit
   boards)

 For Boxes:
 Straight and Phillips screwdrivers
 Electric drill and bits
 Hack saw
 File
 Nibbling tool

 Basic Construction Skills
 You'll have to be able to get the electronics to work to complete an
 effect.  This generally means soldering parts to a circuit board.  Although
 it is possible to solder the parts all together, space-frame style and then
 pot them up in epoxy or something, o r to use wiring lugs, I don't
 recommend these methods (and I've done both of them!)

 Skills Bibliography
 Reading Schematics
  "The Design and Drafting of Printed Circuits"  by Darryl Lindsay,
  Published by Bishop Graphics
   ISBN 0-9601748-0-X

 Electronics Construction:
 "Electronic Projects for Musicians" by Craig Anderton
 "Electronic Projects for Guitar" by Robert Penfold

 Soldering
 (under construction)
 ===========================================================================
 Effects Packaging
 This is HARD.  An effects box needs to be sturdy, and either metal or lined
 with metal or a conductive paint to keep the circuit from picking up hum or
 radio interference.  It needs to be big enough to hold the circuit, but
 compact enough to be usable.  I have bent up boxes from sheet metal, with
 varying results, but to be sturdy enough, you need very heavy (hard to cut
 and bend) sheet metal.  Worse yet, good effects boxes are hard to find in
 commercial lines as well.  There are a few bent sheet metal boxes th at are
 about the right size and shape, but might not be durable in heavy use.
 Open up some commercial effects cases to start getting an idea about what
 is common in effects.

 You can package several effects together in a rack enclosure or in some of
 the schemes espoused by Anderton.  I don't personally like this, but it is
 a reasonable way to package your effects.  Penfold doesn't say much about
 boxes, for some reason.

 A good box for an effect should be about 3 inches wide, 5 inches long, and
 1 1/2 to 2 inches deep.  A sloping front for the bypass switch is nice, but
 not essential.  A very good starting point is the line of Hammond die cast
 aluminum boses - tough, durable , easy to work, almost ideal.  Try the
 1590B or the 1590BB, about $10 from DigiKey or Mouser.

 The 1590BB is a cast aluminum box with a fitted base/cover.  It is very
 durable, and inexpensive in quantities of one.  It is reasonably easy to
 find, and about the right size for an effects box.  The early MXR effects
 like the Phase 90 and Distortion plus were packaged in a box the size of
 the 1590B, just slightly smaller than the 1590BB, and were very tightly
 packed indeed.  The 1590BB is big enough for a non-manufacturer to get a
 whole effect shoehorned in.

 Other boxes that are about right are:
 Hammond 1590B -similar to the 1590BB, but smaller, the size of old MXR's
 Hammond 1590C -similar to the 1590BB, but bigger, about 2 1/4" high.
 LMB #138      -folded sheet metal, not too sturdy, but cheap
 LMB #139      -folded sheet metal, not too sturdy, but cheap
 LMB MDC 642   -folded sheet metal, sturdier, moderate price
 LMB MDC 532   -folded sheet metal, sturdier, moderate price
 LMB UCS 1 3/4-5-5 -folded sheet metal, much sturdier, also pricier


 Putting it all in the box
 Figure out in advance where things will go inside the box and how the board
 will mount, where the controls go, and where the battery mounts.  This
 amounts to a three dimensional layout of the box.  Once you are sure of
 your layout, mark and drill the box for mounting holes for the jacks and
 controls.  Do this before painting or finishing the box.

 A big part of making it come out right is the right selection of controls
 and their placement on the box.  Think about commercial effects you may
 have used, and how the controls are placed, how close together they are,
 etc.  Make several drawings, or better yet, mount your controls in a
 cardboard or foam-board mockup of your effects box before you drill and
 possibly ruin your box.  It is easy to drill holes and hard to grow them
 closed again.

 Making It Look Good
 There are lots of ways to do this.  Anderton outlines several in his book.
 My preference is to paint it with a good primer, bake it in my oven at 200
 degrees for an hour, cool, and paint with an auto touch up paint, then bake
 again.  When this is cool, the paint should be reasonably durable.  If you
 use a light color, you can use your laser printer or copier to print labels
 on clear laser-printer labels from an office supplies store, and letter the
 controls easily and neatly.  If you use a dark color, the direct transfer
 lettering from electronics shops will work, but this will need another coat
 of clear spray to be durable.

 Best of all is to get the box powder coated and baked, then have it screen
 printed.  This is really only affordable if you or a friend already have
 the equipment and skill to screen print, but it produces a
 commercial-quality finish.  I have seen this done, but never done it
 myself.

 ===========================================================================
 Procuring Parts
 I used to try to ferret out the used- and surplus-electronics stores in a
 town to see what I could find cheaply.  Two suppliers have changed my
 habits.  These are DigiKey and Mouser.  They stock a broad line of
 electronics parts, including parts to make packaging easy, and although
 their prices are not the best possible deals or surplus, they are
 reasonable for first quality commercial goods.  They ship fast, and stock
 what is in their catalogs.  Mouser, in particular ships the same day as
 your order, and sinc e they have a warehouse in Dallas, I get my parts the
 next day.

 Both Anderton and Penfold have good discussions of which parts are good,
 and which are not.  I will add to that only where I think I'm really
 adding.

 I list some recommended suppliers in the next section.

   * Integrated circuits
     Anderton's circuits used some strange op-amps and some opto-isolators
     that are not easy to find, although they DO exist.  Most commercial
     effects except recent Japanese ones pretty much stick with either
     single or dual 741-style op amps or CMOS logic IC's which are easy to
     find.

   * Transistors and diodes
     Some of the more interesting effects used germanium transistors and
     diodes.  You can occasionally find germaniums in surplus places and you
     can find germanium diodes in Radio Shack, of all places.

     Ordinary silicon diodes are in Mouser and DigiKey.  I usually don't buy
     from Radio Shack except as a last resort, like if you need a connector
     RIGHT NOW.  They are more expensive.

   * Resistors and Capacitors
     Mouser and DigiKey.  Read their catalogs and select parts BEFORE you do
     your layout, then the parts you actually get will fit in place on the
     board.

   * Jacks, switches, knobs, etc.
     This is the kind of stuff you usually think of last.  Big mistake.

     The classic stomp box has this big metal switch you stomp on to switch
     it in and out.  This switch is the Carling 317PP.  It costs $15 (yes,
     apiece!)  in unit quantities, and only drops to $8 in hundreds.  I have
     not found a good replacement that is the same function.  Just the
     economics of this switch has driven me to adapt some form of electronic
     switching in my effects, as I can make an Anderton-style CMOS
     equivalent for less than $3 in parts and one square inch of circuit
     board.  It is really important to know this before you hack up your
     box.

     (I understand that Maplin Electronics in the UK has Arrow DPDT stomp
     switches for the equivalent of $6 US, so this might help if you can buy
     from them)

     The control pots are another problem.  Effects units in small stomp
     boxes need small controls to fit in the box.  It is usually hard to
     locate a set of small, physically similar pots in different values for
     an effect.  Manufacturers can do this because they can buy large
     numbers and get essentially anything they want.  A bright spot is
     Mouser's selection of miniature pots, for about $1 apiece, in many
     values.

 ===========================================================================
 Some recommended suppliers are:
 In the USA:
 Mouser Electronics   Mouser Electronics       Mouser Electronics
 11433 Woodside Ave.  2401 Highway 287 North   12 Emery Ave.
 Santee CA 92071      Mansfield TX 76063       Randolf NJ 07869

 Mouser Electronics
 370 Tomkins Court
 Gilroy CA 95020
 Catalog Subscriptions: (800) 992-9943 (Continental US only)
 Sales & Service: (800) 34-MOUSER (800-346-6873) (US, Puerto R., Canada)
 Very complete catalog of brand-new components.  Usually quick service, $20
 "minimum" ($5 charge under $20).  When ordering, you may want to be sure to
 ask about availability and shipping locations; they have several
 warehouses, and frequently orders will get sent from several warehouses
 (which drives up the shipping costs).  Export orders have a $100 minimum,
 except for Canada and Mexico.

 Digi-Key
 701 Brooks Ave. South
 P.O.Box 677
 Thief River Falls, MN 56701-0677
 +1-800-DIGI-KEY (344-4539)
 +1-218-681-3380 (FAX)
 No minimum, $5 handling under $25, free and very complete catalog, very
 nice indeed.  Prices aren't always the best, but rarely excessive.

 Maplin Electronics is a BIG supplier to the home hobbyist of electronic
 parts and kits in both the UK and Europe.
  They can be contacted at:
  Maplin Electronics
  PO Box 777
  Rayleigh
  Essex SS6 8LU
  UK
  Fax: +702-553935
  Modem: +702-552941 (sorry, don't know paramaters,
                    try 2400,8bit,1 start,1 stop)
 Maplin is reported to sell an Arrow DPDT equivalent for 3.76 pounds (~$6
 USA) each in one off quantity.  (I'll have to try that one!!)

 Antique Electronic Supply lists the following germanium transistors:
 Part No     Description                                 Price ($US)
 U-TRB-1     PNP similar to 2N107, 2N218, CK722 etc      $1.49
 U-TRB-4     NPN similar to 2N170                        $1.69
 U-TRB-5     NPN similar to 2N193, 2N388, 2N1302         $1.29
 U-TRB-6     NPN similar to 2N170, 2N292                 $.95
 U-TRB-7     PNP similar to 2N111,2N139, 2N218           $.95
 U-TRB-8     PNP similar to 2N107, 2N218, CK722          $.95

 The address  is:
 Antique Electronic Supply
 P.O Box 27468
 Tempe, AZ 85285-7468
 USA
 Ph (602) 820-5411
 Fax (602) 820-4643

 Hosfelt Electronics has the CLM6000 opto-isolator that is ubiquitous in the
 projects in Craig Anderton's book.  They are about $3.50 US apiece.

 Hosfelt Electronics
 2700 Sunset Blvd.
 Steubenville OH 43952
 +1-800-524-6464
 +1-614-264-5414 (FAX)
 No minimum, $3.75 S&H.  Surplus electronics.

 The Electronic Goldmine has a good listing of surplus electronics, useful
 for run-of-the-mill construction.

 Electronic Goldmine
 P.O. Box 5408
 Scottsdale AZ 85261

 1-602-451-7454 voice
 1-602-451-9495 fax

 ===========================================================================
 Answers to questions about fixing broken effects

 Q. My Belchfire Megablaster pedal doesn't work at all. How can I fix it?
 A. Most completely dead pedals - this means that no sound comes out of the
    pedal at all either when the pedal is not switched to bypass or in
    either position of the bypass switch - are either wiring or battery
    power related. Open up the effect, taking care to note how it came apart
    so you can successfully put it back together. Carefully examine all of
    the wires inside the box. All wires should be firmly soldered at both
    ends. If there is a wire with one end loose, carefully examine all of
    the places that it might have connected to, perhaps with a jeweler's
    loupe, ($5 at most hardware stores, very handy) to find where the wire
    broke off as it left the soldered joint. Wires almost never break in the
    middle of the wire, as there are tiny nicks in the wire where the
    insulation was stripped away. The wire breaks at the nicks when it is
    flexed too much. Melt the solder at the joint an push a wooden toothpick
    into the hole to make room for the wire. Carefully strip a little
    insulation off the remaining wire and push it into the hole, then solder
    it.

    If you find no broken wires, suspect power problems. This will sound a
    little simplistic, but measure the battery voltage, both with the
    battery out of the circuit and with it clipped in and the effect
    working. The battery may be dead, or alternatively may not have enough
    oomph left to run the effect. The battery voltage should be at least 7
    volts when it is actually running the effect. Measure this by poking
    voltmeter leads on the battery terminal, poking it between the battery
    and the battery clip (easy for a 9V battery). If the voltage is low, the
    battery may be dead, try a fresh one. If a new battery does not help,
    there may be a short on the board.

    Sometimes the isolation of the circuit board from the case is messed up
    somehow, and the board shorts out to the case. If this is true, the
    effect will work when the case is open and the board pulled loose from
    the case. The short may be a signal short, in which case the battery
    testing will not find it, or a power short, which will be caught.

    There may be an open wire between the battery and circuit board.
    Connect one voltmeter lead to the case ground at the input or output
    jack, and measure the voltage at the place where the battery lead goes
    onto the circuit board.  Note that for effects which use either the
    input or output plug to switch power on, you'll have to stick a plug
    into the approprite jack to connect the battery up.  This is very
    common, so you may just want to plan to stick a plug in each jack.  If
    you don't read battery voltage going onto the board, something is open
    between the battery and the board.  The easiest way to find this is to
    stick one ohmmeter lead on a battery terminal, poking it between the
    battery and the battery clip and the other on the ground or + voltage
    respectively at the circuit board. There should be almost zero ohms
    from the battery terminal to the board. The path from battery to board
    may go through a jack (often the (-) terminal does this), or through a
    power jack for external power (frequently the (+) terminal does this).
    You can rapidly go down the wiring chain from terminal to terminal until
    you find where the connection stops; you will have just passed the
    break.

    Sometimes the battery clip has a broken wire inside the plastic holder
    that fits on top of the battery. This is why you should connect directly
    to the battery terminals.

    If one of these tricks doesn't catch it, there is probably a fault on
    the circuit board, and you'll need to be able to debug it from the
    schematic, or trace out the board to make a schematic to work from. This
    is much more complicated, and you should only try this if you already
    know how. If you don't already know how to do this, get someone who does
    to fix the pedal for you.

    If the effect is not totally dead, just sounds wrong somehow, it is
    probably a circuit board problem, and needs the circuit board tinkering
    to fix it.

 Q. My (MXR Micro Chorus/EH Electric Mistress/ EH Memory Man/others) delay
    or chorus pedal doesn't delay/chorus/etc anymore, although sound comes
    through it in both effect and bypass positions. How can this be fixed?
 A. This is one of those rare cases where you can have an effect that can't
    be fixed, at least not easily. A common cause of this malady is that the
    analog delay chip, a Reticon SAD512 in the Micro Chorus and a Reticon
    SAD1024 in the others, is not commercially available any more. The only
    options are cannabalizing another box that contains one, or hacking
    in one of the Panasonic MN3xxx series delay chips. This hack is almost
    certainly possible, but I have not seen one fixed this way yet.

 Q. My wah pedal sounds scratchy/noisy when I move the pedal. Can this be
    fixed?
 A. Yes. First step is to look at the pot that is rotated when the pedal is
    moved.  If this pot is an open style pot (there are holes that go
    through into the innards), get a can of non-residue contact cleaner at
    an electronics supply store.  Squirt some of this into the hole and work
    the pedal/pot back and forth a lot while it evaporates.  This MAY cure
    it.

    If the pot is sealed, no holes into the interior, you may have to
    replace it.  The manufacturer may have the actual part with the little
    gear, saving you a LOT of time and effort.  If not, it is possible to
    get a new, same value pot and cut the shaft to length, drill a little
    hole for the pin that keeps the gear from slipping on the shaft, and
    reassembling.  This requires some significant mechanical aptitude,
    though.

    A slam-dunk solution is to refer to Craig Anderton's article from Guitar
    Player entitled "Clean up your Volume Pedal" , only use the LED/LDR pair
    to replace the pot on the Wah.  I have done this for a friend who wanted
    to have a remote pedal, the actual Wah circuit back in his rack.  Worked
    Great.

 ===========================================================================
 Appendix A - Effects Economics 101
 An effect that you build will likely cost you

     box          $10
     jacks          2
     stomp switch  15 (for Carling; 3 for electronic)
     paint          4
     controls       2
     knobs          1
     wire           2
     PC board       3
     electronics    5 - 12

 for a total of $32 to $51 if you have to buy all this stuff new.  This does
 not count any tools or other supplies like solder, pliers, etchant, and so
 forth, nor does it count your time and effort.  Musician's Friend sells
 stomp boxes for $30 to $100, the top end being some things that are not
 really reproducable at home.  Notice that the electronics that do the work
 are only 1/3 to 1/10 of the cost of the finished article.

 ===========================================================================
 Appendix B - Making Circuit Boards

 A lot depends here on whether you are building something from a schematic
 that has no provided layout or you are using a magazine article that has an
 already laid out circuit pattern to follow.  If you are doing the latter,
 you don't need to worry about how to make up your own layout.  If you will
 be working from a schematic only, you will probably need to lay out your
 own board for the effect.  You really ought to make a semi-formal layout or
 some sketches even if you intend to make only a perfboard or stripboard
 version.

 There is a much more complete FAQ on making printed circuit boards over on
 sci.electronics. I have thrown in only some basics.

 Layout
 > Laying out the board.  Go to your local used book store and in the
   technical books section, find a book with a title something like "Printed
   Circuit Drafting and Layout".  Read it, ignoring the things about how to
   lay pads and tape on mylar film, but paying attention to the sections on
   how to orient parts and run traces to and between them.  You can, of
   course skip this if your effect is a copy of a magazine article which
   contains a printed circuit layout.

   Lay out your board, keeping the board type you will make in mind.  There
   are some tools which will make this easier.  For general layout, get
   grid-ruled paper and tracing paper to draw sketches of layouts on, and a
   couple of colors of pencil, red and blue being traditional.  In general,
   make your board from a set of postage-stamp circuit fragments, each of
   which is a tightly connected lump of stuff, usually about one IC and its
   immediately connected resistors and stuff.  Most circuits are a set of
   such clumps, with signal and power supply lines going between them.  make
   the fragements, then interconnect them.  Make all the wires come off one
   side of the board, not all around it where you will have to knit the
   board into the box, and certainly not coming out of the middle of the
   board.  Have the inputs on one side of the board, and let the signal
   progress through the board like on the schematic, in one direction to
   avoid having unwanted feedback paths.  Do not forget to leave some way to
   mount the board in the box.  Wrapping the whole board in plastic foam and
   leaving it loose in the box is evidence of such forgetfulness, although
   that is exactly what MXR did in all its early production.

 Perfboard
 > Perf board and Strip board methods.  Hack a chunk of circuit board big
   enough to implement your layout.  Read Penfold for the full description
   of the stripboard method.  For the perfboard method with pad-per-hole
   board, bend the component leads to insert them into the board, then use
   the excess lead length below the board to make "circuit traces" by
   bending them flush with the board and routing them from connection to
   connection.  Use wire wrap wire to connect things that the leads can't
   easily.  Solder the leads where they go through the board, and at
   connections.  Solder as you go along, placing a few parts and soldering
   them.  This method is remarkably easy to do, and is reasonably durable if
   your soldering technique is good.  It is about as fast as doing a board
   layout, so if you have some experience, you can lay out the circuit from
   the schematic as you go.  I would, however, never, ever make more than
   one of a kind this way.  If I think I might EVER make another I would lay
   out a printed circuit board.

 Stripboard
   (under construction; Penfold uses stripboard exclusively)

 Printed Circuit Boards

   It is NOT in general a good idea to do double sided boards at home, as
   this more than doubles your difficulty, and plated through holes are not
   generally possible at home.

   Photo methods
   You can lay out the circuit patterns in black-on-white and have a
   commercial graphic arts store make you either positive or negative
   transparencies from this to use the commercially available photo-resist
   methods of making boards. This method produces premium results if you
   have the patience and technical consistency to get it to work. It is
   tedious, but produces great results when you finally get it down pat.
   There are commercially available kits to do this in a small-at home way.


   Dry transfers
   There are rub down etch resistant transfers similar to Letraset.  Not
   as easy to do but doesn't require a massive capital outlay.  You clean
   the copper surface, then rub down dry-transfer material in the shape of
   pads, lines, etc., then etch.  This can produce very good results if you
   are careful.  I recommend that you clean the copper surface carefully,
   then use kitchen plastic film to cover all except the small area that you
   are working on at the moment.  This helps keep the surface from oxidizing
   quite as much, and lets you rest your hands on the board without leaving
   skin oils that would foul up your etched patterns.

   Toner transfer
   This is today's winner. There exist plastic film and plastic materials
   that will accept the black toner from a copier or laser printer and then
   release it after the pattern has been ironed on to a copper clad board
   blank. These are sold by the names of TEC-200, Dynart, and Press-N-Peel.
   You can get -*very*- good results from this technique, lines as small as
   0.01" wide, and one wire between IC pads.


   Drill-and-draw
   This method is for the artists among us.  Lay out your board.  It is
   really better if you can lay it out completely and print some kind of
   real sized pattern.  Using the pattern, either just mark or and drill all
   of the holes in the board.  You now have a copper board with only holes
   in it.  Now, using an etch resist pen or lacquer or some other fluid that
   dries impervious to ferric chloride, draw in the circuit patterns.  This
   will take some time, but if you are careful, you can do a good job this
   way.  I know a fellow who actually laid out and etched most of the
   circuitry for an Apple II computer this way, using wire wrap for the
   parts he could not etch.  I recommend using the kitchen plastic film
   again, being careful to let the resist dry in a given area before placing
   the film over it to avoid smears.


 ===========================================================================
 Appendix C - My Personal Best Way to Package Effects
 This method turns out packages comparable to commercial ones.  It is not
 for the faint of heart.  It is based on the Hammond 1590BB mentioned above,
 Easytrax software, iron-on toner transfer PC board manufacture, and a good
 supplier.

 The Hammond 1590BB is 3.3" by 4.3" by about 1.1" inside, allowing for the
 slight slope of the sides.  A circuit board 3.25" by 4.25" with sheared off
 corners fits neatly on the bottom cover inside the raised lips that hold
 the cover in place when the box is closed.  This is a large enough circuit
 area to hold the input/output jacks, the circuitry, and a battery on the
 circuit board itself without any hand wiring.  The height is such that you
 can use a second smaller circuit board to hold the controls and switches,
 etc.  on the top of the box, connected to the main board by several wires
 of all the same length at one edge of the main board.  It is much easier to
 make a batch of all-the-same wires than to hand wire the jacks, battery,
 controls, etc.  together, and probably more reliable.

 I use four rubber feet on the bottom, with a 4-40 screw going through the
 bottom cover, with a nut on the inside of the bottom cover.  The screw is
 long enough to also go through mating holes in the circuit board which fits
 on the inside of the bottom cover, the nuts serving as standoffs to keep
 the board from shorting out to the cover.  The board holds the in/out
 jacks, which fit inside a notch cut in the side of the top cover.

 The top cover holds the control circuit board, which is attached to it with
 three 0.375" (3/8") threaded standoffs.  Mouser sells a line of miniature
 PC board mount pots that need - surprise - only 3/8" of clearance, and a
 line of miniature PC board mount switches that also need only 3/8"
 clearance above the board.  The controls and switches protrude through
 holes drilled in the top cover.  The pots have 1/2" long 1/4" diameter
 shafts that are embossed with an indicator and knurling to act as a self
 knob, or can take any of the small 1/4" shaft knobs.

 The two circuit boards are interconnected by a few (6-10) wires to carry
 signal and ground/etc.  between them.  The wires are all about 2-3" long.
 If you lay out the boards right, the wires do not have to cross or weave
 over one another, making it a breeze to wire up.  Since the jacks are on
 the main board, NO OTHER WIRING IS NEEDED.

 I use the publicly available Easytrax package from Protel to lay out
 boards.  This lets me lay out and save the board outlines and mounting
 holes, battery and in/out jack patterns, etc.  separate from the effect
 circuitry itself, so I can just substitute effect pattern into the "blank"
 board layout and have myself a new effect layout.  This speeds things up a
 lot.

 I print from Easytrax to a postscript printer, check the layout, then print
 on toner-transfer paper to make circuit boards, and etch in ferric
 chloride.

 Once the effect is constructed and working, I remove everything from the
 box, clean it with acetone, spray it with primer, bake, spray a light
 colored automotive touchup paint, bake, then letter with an overlay
 postscript printed on clear Avery laser printer label stock.

 The critical parts to all of this are:

 Hammond 1590BB die cast aluminum box Mouser# 546-1590BB
 Re-An PCB mount 1/4" phone jacks Mouser# 550-25301
 Alpha PC mount pots, Mouser#  317-2090
 Eagle 9V battery holder Mouser# 12BH611
 MountainSwitch Submini toggle Switches Mouser# 10T6xx
 3/8" standoffs Mouser #534-1450B
 Rubber bumper Mouser # 534-720
 Toner transfer paper DigiKey# TTS-5-ND
 Avery clear laser printer label stock

 (Do you sense a favorite supplier here?)

 ===========================================================================
 Appendix D - A Musical Distortion Primer
 How do I distort thee? Let me count the ways.

 This is a collection of the basic means of distorting a signal in a
 musically useful way.  I use a sine wave to illustrate, although a guitar
 is much more like a sawtooth than a sine.  These are the clipped waveforms
 in their raw state.  Tone controls or filters after the clipping will
 radically alter the waveshapes.

 Sine wave.  Smooth, roundey all over, no sudden changes in direction.This
 is the purest tone possible, just a single frequency.  Some flute tones
 approach this.
      * *                     * *
    *     *                 *     *
   *       *               *       *
  *         *             *         *
 *-----------*-----------*-----------*-----------*
              *         *             *         *
               *       *               *       *
                *     *                 *     *
                  * *                     * *

 Mild clipping. Flat topped sine wave.

    *  *  *                 *  *  *
   *       *               *       *
  *         *             *         *
 *-----------*-----------*-----------*-----------*
              *         *             *         *
               *       *               *       *
                *  *  *                 *  *  *

 REALLLY clipped. High gain, hard edged metal. Almost a square wave. Lotsa
 metalpedals do this then round the corners with some filtering.

  * * * * * *             * * * * * *
 *           *           *           *
 *           *           *           *
 *-----------*-----------*-----------*-----------*
             *           *           *           *
             *           *           *           *
              * *  *  * *             * *  *  * *

 Square wave. Some early fuzzboxes did this. Sounds like a synthesizer.
 Sounds OK, even useful, unless you hit two notes at once, then makes an
 unpleasant harsh distortion from the two (intermodulation distortion).

 * * * * * * *           * * * * * * *
 *           *           *           *
 *           *           *           *
 *-----------*-----------*-----------*-----------*
             *           *           *           *
             *           *           *           *
             * * * * * * *           * * * * * * *

 "Tube-y" distortion. Squashed, rounded tops. Vacuum tubes are not
 symmetrical enough to do this. The CMOS gate based pedals like the
 ElectroHarmonix Hot Tubes do this. Tubes are similar, but the tops
 and bottoms are not mirror images. Soft, smooth onset, touch
 sensitive, "bluesy".

    * * * *                 * * * *
  *         *             *         *
 *           *           *           *
 *           *           *           *
 *-----------*-----------*-----------*-----------*
             *           *           *           *
             *           *           *           *
              *         *             *         *
                * * * *                 * * * *

>Full wave rectified sine wave. The lowest frequency in it is an octave over
>the original sine. This has been used to one degree or another in the
>Octavia, UniVox SuperFuzz, Foxx Fuzz-Wah, and Experience pedals. It is
>commonly followed by diode clipping and some filtering, as the raw waveform
>sounds very buzzy and strange.

      * *         * *         * *         * *
    *     *     *     *     *     *     *     *
   *       *   *       *   *       *   *       *
  *         * *         * *         * *         *
 *-----------*-----------*-----------*-----------*

>Partial full wave rectified sine wave. Noticeable octave sound. note that
>the extent to which this resembles the true full wave rectified sound
>depends on how well matched the circuit is on rectifying the signal, so
>there is a continuum of sounds between the two.

      * *                     * *
    *     *                 *     *
   *       *               *       *
  *         *      *      *         *      *
 *-----------*---*---*---*-----------*---*---*---*
               *       *               *       *


 Half wave rectified sine wave. Noticeable octave sound.

      * *                     * *
    *     *                 *     *
   *       *               *       *
  *         *             *         *
 *-----------*-*-*-*-*-*-*-----------*-*-*-*-*-*-*

 Sine wave with second harmonic distortion. Uneven squashing top and bottom.

      * *                     * *
    *     *                 *     *
   *       *               *       *
  *         *             *         *
 *-----------*-----------*-----------*-----------*
              *         *             *         *
               *       *               *       *
                  * *                     * *

 High gain, asymmetrical clipping, hard clip on top, soft compression on
 bottom. The fabled Fuzz Face did this.

      * *                     * *
     *   *                   *   *
     *   *                   *   *
     *   *                   *   *
     *   *                   *   *
     *   *                   *   *
     *   *                   *   *
 ----*---*-------------------*---*-------------------*
          *                 *     *                 *
           * * * * * * * * *       * * * * * * * * *

 And some comments on the circuits that actually produce the distortions:

 I tried to catalog all of the ways I could think of that have been or might
 be used for musically useful distortion, with some running critique.  Can
 you give good examples of the circuit's use in gear that I haven't
 mentioned?  I'm trying to formulate some guidelines in my search for the
 perfect distortion.

 Simple harmonic distortion, which generates distortion products that are
 related to the fundamental frequency by low-integer multiples, is widely
 considered to add character to instrument sound.  Intermodulation
 distortion, which produces non-musically related frequencies at multiples
 of the sum and differences of two frequencies, is reputed to sound harsh to
 the ear.  All harmonic distortion methods also produce intermodulation
 distortion when fed two or more different frequencies simultaneously, but
 the exact method of producing distortion may result in lower or higher
 amounts of harsh sounding intermodulation; there is no simple way to
 predict the harshness.  In general, the "softer" the clipping or
 distortion, the less intermodulation, but this is only a generality.

 I have noted in poring over distortion schematics that the filtering of the
 signal both before and after the distortion may have more to do with how
 the distortion sounds than the actual method of distortion.  A combo amp
 with a speaker or two and an open back implements a two pole lowpass filter
 all by itself.  This is at least one of the reasons that a miked amp is
 preferable to running a distortion signal directly into a PA mixer.  This
 smooths off those offending treble shrieks.  An electronic multipole
 lowpass filter is the essence of all those "cabinet simulators" you see.

 Filtering BEFORE the distortion is also interesting.  Hendrix is reputed to
 have used a Wah pedal before his Fuzz Face.  The distortion is more intense
 at the frequencies boosted by the Wah, so you get an interesting change in
 distortion depending on what notes you hit.  This cuts a lot of the harsh
 sounding intermodulation distortion, too.  Anderton used this principle in
 designing his "QuadraFuzz", which split the incoming signal into four
 frequency bands and distorted each band separately, then mixed them back
 together.  This method gives a much lower intermodulation distortion by
 avoiding having more than one frequency distorted together.

 Based on an article in Electrical Engineering Times, the actual distortion
 method may not be as important as the pre- and post-clipping filtering, and
 the amount of offset in the duty cycle of the resulting clipped waveform
 and the variation in the duty c ycle with input signal level.  This article
 (by Brian Murphy, interviewing John Murphy, who used to be chief engineer
 at Carvin; EET, October 3, 1994) outlines Murphy's ideas on the degree to
 which he can simulate tube distortion in solid state designs.  He holds
 that if you filter properly, soft clip, and arrange for the duty cycle of
 the clipped signal to vary with varying signal levels, you get something
 very close to tube distortion.  This is the basis of the "tube emulation"
 circuitry in Carvin's SX series of amps.


 Circuit Types:

 Common Cathode Triode
  - "preamp distortion", used in the revered Bassman, Marshalls, modern
    tube preamps; frequency response easily made lowpass (sometimes
    unintentionally) by grid circuit and stray capacitances.  Thought to be
    the object of our search.  Typically 2% distortion when "clean".  Clips
    asymmetrically, although as typically used, cutoff is mushy and more of
    progressive compression than clipping, and "saturation" is defined by
    the grid going more positive than the cathode, with a dramatic drop in
    grid impedance when this happens, perhaps "saturating" the previous
    triode stage.  Lots of even order distortion products until harder
    clipping reached, then third harmonic rises Almost no fifth or higher
    order harmonic production until massively over driven.

 Single ended Pentode
  - "output stage distortion", as in smaller, cheaper amps, like Fender
    Champ, several Gibsons, others.  Asymmetrical clipping, asymmetric
    distortion from the single ended transformer.  Distinct second and third
    harmonics.  Very noticeable compression effects from screen grid bias
    drop, power supply sag.

 Double ended Pentode
  - "output stage distortion" as in large amps, including the Bassman. Push
    pull operation cancels even harmonic distortion generated in the output
    stage, so it can contribute only odd harmonics, principally third.
    Noticeable bias shift when driven hard can cause compression.  Other
    compression effects include screen grid bias sag, and power supply sag.
    This may be the origin of the "tube rectifiers sound better" school of
    guitar amps, as silicon replacement power supply rectifiers give much
    less sag on signal peaks and higher power supply voltages, so sound
    cleaner.  Symmetric distortion.

 Voltage feedback/biased bipolar - Germanium
  - "Fuzz Face" distortion. Single high gain stage saturates with guitar
    signal, is biased in a way that can not hard saturate, but can swing a
    long way towards cutoff without distortion.  Low input impedance loads
    guitar and pre-filters out highs.  Asymmetrical clipping, with prominent
    second harmonic.  Prominent third, and noticeable fourth and fifth
    harmonics.  This is the basic circuit in several vintage fuzzes,
    including the Vox Tone Bender

 Voltage feedback/biased bipolar - Silicon
  - Some rebuilt fuzz face units have this. Widely reputed to sound a little
    harsher than germanium.

 Common emitter bipolar - Silicon
  - "transistor sound". Nominally well designed CE stages clip symmetrically
    and hard.  No noticeable even order harmonics, prominent fifth, seventh,
    and higher harmonics.  Widely held to be an example of what is wrong
    with transistors for musical instruments.  It can be a useful effect, as
    the Big Muff Pi from Electro-Harmonix used a variation of this circuit
    with limiting diodes from collector to emitter.

 Common emitter bipolar - Germanium
    Never seen this. Probably same as silicon.

 Common Source JFET
    Never seen this. Well designed JFET CS stages generate only second order
    harmonics in the linear range, but who knows about clipped? I have seen
    postings that say that a well-designed JFET common source stage produces
    clipping very similar to tubes, but I've never seen any schematics or
    heard this.

 Common Source MOSFET
    The common source MOSFET is the basic circuit in Anderton's Tube Sound
    Fuzz.  A similar circuit is used in Fender's Stage Lead, and at least
    one stomp box.  This circuit can produce very convincing tube-like
    distortion if it is carefully designed.  The commonest way to do this
    circuit is with the CD4049 CMOS logic (yes, logic) IC.  It can be
    misapplied by biasing it into its linear region and to function as an
    amplifier.  I don't know of any discrete transistor versions, although
    Fender used a CD 4007 IC in the Stage Lead amps to get tubey sound.  The
    4007 is essentially a CMOS transistor array.

 Differential amplifier
  - Diff amps have a 25 Millivolt input range over which the output is
    linear.  Clipping is symmetrical.  Probably same defects as Common
    Emitter bipolar.  This is the basic stage in most op amps.  This basic
    circuit can be implemented as silicon bipolar, germanium bipolar, JFET,
    MOSFET and vacuum tube.  I have never seen this used as a clipper in a
    musical effect.

 One diode
> - One diode as a half wave rectifier or clamp generates prominent second
>   harmonic distortion;

 Back to back diodes
     Silicon, Zener, Germanium, LED
  - Many examples of this. This and the back to back feedback op amp are
    the basis for a lot of effects, such as the Ibanez Sonic Distortion 9,
    Proco Ratt, MXR Distortion +, and the DOD Overdrive.  Also many that
    need to remain unmentioned.  A lot seems to depend on how the diodes are
    driven and what filtering is done before and after the diodes.

    Silicon is reputed to clip abruptly, causing a harsh sound.  Germanium
    clips at about half the voltage of a silicon diode, but is reputed to
    turn on more slowly in its smaller range.  LEDs turn on at about two
    silicon diode drops, and are also reputed to be slower turn on, with it
    and germanium giving more "tube like" sounds.  Some units with "tube" in
    the title use LEDs or germanium for clipping.

    Oddly enough, the Marshall JCM800 uses a set of two pairs of silicon
    diodes back to back as clippers.  This seems strange in a Marshall, but
    it is there.  When I looked at the schematic, I first thought it was a
    protective clamp for a tube grid, but the circuit doesn't work that way.
    They are there for the distortion.

    I've never seen a schematic with zener clipping for music, but they are
    used in the signal processing world.

 Back to back diode feedback Op Amp
     Silicon
     Germanium
     Zener
     LED
    In well designed units, this is very much the same as the back to back
    diode units.  If the opamp gain is low, the diodes produce about the
    same end waveform as if they were driven by an external circuit.  In
    poorly designed circuits, the opamp gain is high, and the diode
    characteristics are lost, giving just a lower-voltage clipping op amp,
    which suffers from all of the problems of the bipolar common emitter
    stage, and perhaps a few of its own.

    The most famous example I can think of of the
    diodes-in-the-feedback-loop pedal is the Ibanez TS-9 Tube Screamer.
    Almost every commercial pedal manufacturer has made one of these.

 Op amp or comparator
  - Craig Anderton published a design for a "triggered fuzz" that is pure,
    open loop, op amp clipping.  This is an odd effect, akin to a
    synthesizer rather than a guitar in sound.

 Full wave rectifier
  - Anderton again. An op amp circuit for emulating ideal diodes produces a
    full wave rectification of the input waveform.  Gives a very prominent
    octave up by producing a second harmonic and removing the fundamental.
    Works best with single notes.  Multiple notes simultaneously give a
    metallic or clanging effect something like a ring modulator.  An odd
    effect.

    The Octavia pedal, as well as the new "Experience" pedal use a form of
    full wave rectification followed by diode clippers and filtering. The
    clipping and filtering seem to tame the odd buzziness of the raw
    rectification.