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From keen@eden.com Sat Jan 14 13:20:00 1995 Path: zip.eecs.umich.edu!newsxfer.itd.umich.edu!gatech!newsfeed.pitt.edu!uunet!news.eden.com!matrix.eden.com!keen From: keen@eden.com (r g keen) Newsgroups: alt.guitar,rec.music.makers.guitar,rec.music.makers.builders Subject: Guitar Effects FAQ Date: 12 Jan 1995 01:17:56 GMT Organization: Adhesive Media, Inc. Lines: 1446 Message-ID: <3f2004$7rl@boris.eden.com> NNTP-Posting-Host: matrix.eden.com X-Newsreader: TIN [version 1.2 PL2] Xref: zip.eecs.umich.edu alt.guitar:43343 rec.music.makers.guitar:38152 rec.music.makers.builders:2952 =========================================================================== 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.