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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.