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.

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.

Circuit Operation

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.

Construction

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.

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).