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Soft Machine

Build Your Own Guitar Soft Distortion Unit

It's easy, and you can build it yourself for under 20 quid!

This simple project by RA Penfold gives you the chance to construct a fully-functioning low-cost distortion processor for your guitar for under £20 — a little money, a lot of fun.

Distortion or 'fuzz' effect units must have been around for almost as long as electric guitars themselves. Distortion is a very simple effect to produce, but some distortion units are very much more musical than others. Many of the published designs for distortion units produce hard clipping, which gives a very harsh sound. Figure 1(a) helps to explain what is meant by hard clipping. Here a sinewave signal is clipped symmetrically (clipping simply means that the signal is not allowed to go above a certain voltage). No matter how high the input voltage becomes, the output voltage is not allowed to go above the clipping threshold level.

This is basically what happens if a modern amplifier is overloaded; a hard clipping 'fuzz' unit produces the same severe distortion. The distortion products generated are predominantly at high frequencies, giving a 'bright' effect, which does have its uses. This effect is considered to be too extreme by many guitarists, though, and it does have some practical limitations.

Figure 1: Hard clipping (a) produces a harsh effect, while soft clipping (b) gives a thicker sound.

Other Projects

This project is just one of those presented in Electronic Projects for Guitar by RA Penfold. Also detailed in the book are:

Guitar Preamplifier
Headphone Amplifier
Pedal Phaser
Dual Tracking Effects Unit
Treble Booster
Dynamic Treble Booster
Dynamic Tremolo
Direct Injection Box
Improved Distortion Unit
Thin Distortion Unit
Guitar Tuner

The worst of these is that it generates very strong intermodulation products. What this means to someone using a hard clipping unit is that they must not permit two strings to resonate simultaneously. The penalty for doing so is some extremely discordant sounds! This effect also tends to be rather uncontrollable; it is easy to have a massive amount of distortion or none at all, but anything in between is virtually impossible. This is simply because even a small amount of hard clipping tends to produce very strong distortion products.

There is an alternative to hard clipping, in the form of soft clipping. Figure 1 (b) shows the sort of waveform that is produced if a sinewave is subjected to soft clipping. The soft clipping effect is produced by not having a well-defined clipping level. In fact there is no true clipping level at all. Instead, as the input voltage increases, the gain of the circuit decreases. Rather than simply clipping off the tops of waveforms, the signals are rounded down.

As far as the sound of the effect is concerned, soft clipping is far less harsh. Not only are the distortion products less strong, they also have a much weaker high-frequency content. This gives a much 'thicker' effect, and one which most people find very much more musical than the hard clipping sound. Also, it gives much weaker intermodulation distortion. This gives acceptable results with polyphonic playing.

Figure 2: The distortion unit circuit diagram.

Circuit Operation


RESISTORS (All 0.25 watt 5% carbon film)
R1 220R
R2 100k
R3 100k
R4 27k (see text)

VR1 4k7 min hor preset

C1 47u 10V radial elect
C2 470n polyester
C3 10u 10V radial elect

IC1 LF351N
D1 OA90 or OA91
D2 OA90 or OA91

JK1 Standard jack socket
JK2 Standard jack socket
S1 s.p.d.t. heavy duty push button
S2 s.p.s.t. min toggle
B1 9 volt (PP3 size)

0.1 inch stripboard, 19 holes by 16 strips
Small metal or plastic case
8-pin d.i.l. IC holder
Battery connector
Wire, solder, etc.

Figure 2 shows the circuit diagram for the soft distortion unit. This is basically just an operational amplifier non-inverting mode circuit, having a voltage gain of over one hundred times. It differs from the standard non-inverting amplifier configuration in that D1 and D2 are included in the negative feedback circuit. One diode processes positive half cycles, and the other processes negative half cycles. On suitably strong positive-going half cycles D1 is brought into conduction, and it shunts R4. D2 has the same effect on negative-going output signals. When conductive, the diodes reduce the gain of the amplifier.

Silicon diodes have well-defined forward-conduction threshold voltages, and they start to conduct at a voltage of around 0.6 volts. A voltage only slightly higher than the conduction threshold voltage is sufficient to produce a large current flow. When used in a circuit such as this, the result is that the output signal is hard clipped at about plus and minus 0.6 volts. Any signal that tries to take the output outside these limits simply results in the diodes conducting hard on the signal peaks, and reducing the gain of the circuit to a level that keeps the output signal within the plus and minus 0.6 volt limits. If you require a unit which produces hard clipping, use silicon diodes such as 1N4148s for D1 and D2.

The specified diodes are germanium devices, and these have much less well-defined forward conduction threshold voltages. They will actually conduct at quite low forward voltages, but they will have quite a high resistance. As the forward voltage is increased, their resistance steadily reduces. This gives the required soft clipping effect, with the gain of the amplifier steadily decreasing as the output voltage rises.

S1 enables the effect to be switched out when it is not required. The output level from IC1 is likely to be higher than the direct output from the guitar. Therefore, VR1 has been included so that the output of the circuit can be reduced to a level that is comparable to that from the guitar pick-up. Adjustment of VR1 has to be a subjective matter, since the output signal is at an almost constant level during the course of each note. By contrast, the output from the guitar pick-up will start at a high level, and substantially decay during each note. It is therefore a matter of giving VR1 a setting that gives no obvious change in volume as the effect is switched in and out.

R4 has been given a value that is suitable for low output guitar pick-ups. If the unit is fed from high output pick-ups it would be better to use a much lower value. About 2k7 should be suitable. You can vary the strength of the effect by altering the value of R4. High values give a stronger effect — lower values give a weaker effect.

The current consumption of the circuit is only about 2 milliamps. A PP3 size 9 volt battery is adequate as the power source, and each battery should give over one hundred hours of operation.


Details of the stripboard panel are provided in Figures 3 and 4, while Figure 5 shows the point-to-point wiring. The board has 19 holes by 16 copper strips.

Construction of the unit is very straightforward, and offers little out of the ordinary. Remember that D1 and D2 are germanium diodes, and that they are more vulnerable to heat damage than are ordinary silicon diodes. Ideally, S1 should be a heavy duty push-button switch so that it can be operated by foot. An s.p.s.t. switch of this type might be difficult to obtain, but a d.p.d.t. type is suitable. Simply use one set of three tags, and just ignore the other set.

In use, it should be borne in mind that the unit adds some extra gain to the system. This means that extra care is needed in order to avoid problems with feedback, hum pick-up, and so on.

Figure 3: The component layout for the soft distortion unit.

Figure 4: The underside or the soft distortion board.

Figure 5: The soft distortion unit hard wiring.


Like the other projects in the book Electronic Projects For Guitar, this one is built on 0.1 inch, copper strip matrix board, also sold under the proprietary name of Veroboard. It can be obtained from Maplin Electronics (their catalogue is on sale in WH Smith) or from electronics shops such as Tandy. The board may easily be cut using a hacksaw, but any burrs on the copper strip should be filed away to prevent short circuits. Where breaks in the copper strip are indicated on the drawing, (large black circle with small white centre hole) a drill bit (approx 5mm) twirled between the fingers will remove the copper quickly and safely.

Again, ensure that the copper track is cut right through and that there are no burrs shorting out the strips. Use a low wattage soldering iron (between 15 and 40 watts) with multicore solder. On completion, check that there are no solder splashes bridging the tracks and that there are no dry or faulty joints. Also check that the orientation of the IC, the electrolytic capacitors and the diodes is correct according to the layout diagram.


Though anyone who is reasonably adept at electronic construction should be able to complete this project, we don't have sufficient space to explain all the terms and techniques involved for the complete beginner. However, the book from which the project is taken (see details at end of article) includes a comprehensive section which should enable beginners to make a start. Note that we will be running an article dealing with soldering techniques in the near future, for anyone who would like some help with this important construction and studio maintenance skill.

The Soft Distortion project is reprinted with the kind permission of PC Publishing from Electronic Projects For Guitar by RA Penfold. Electronic Projects For Guitar is available from The RM Bookshop (reference number B179) at £8.95 plus £1.25 postage and packing. Telephone (Contact Details) to order by credit card. Alternatively, send a cheque made out to SOS Publications Ltd to: RM Bookshop, (Contact Details).

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AKG K500 Reference Headphones

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Live Sound

Recording Musician - Copyright: SOS Publications Ltd.
The contents of this magazine are re-published here with the kind permission of SOS Publications Ltd.


Recording Musician - Dec 1992

Feature by Robert Penfold

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