Soft Distortion Pedal
An effects pedal with a difference.
A block diagram is shown in Figure 1 and from this it can be seen that the input signal is filtered into two channels. Distortion is applied to one channel then the signals are recombined to form the output. The amount of distortion is controlled by altering the filter characteristics to increase or decrease the bandwidth of the 'distort' channel.
The non-linear distortion network uses germanium diodes and this, along with the configuration employed makes the voltage/current characteristic smoother than most of the standard circuits.
Input to the unit is via JK1 a DPDT ¼" jack socket which is arranged to connect the battery supply when the jack plug is inserted. The gain of the preamp IC1b is determined by R2 and R3 and set to a level suitable for most guitars. If a very low output instrument is used the preamp gain should be increased by raising the value of R3.
IC2a and associated components form the 'pregain' circuit. This is a two stage, split gain control which has a superior 'law' and also performs some frequency tailoring.
The soft distortion generator comprises C8 to C10, R9 to R11 and germanium diodes D1 to D5. There is the equivalent of three diode drops — each approximately 0.3V — in each direction, so the maximum swing across the diodes is 1.8V. This gives a better noise performance than if a single pair of diodes were used which would require greater amplification at the 'postgain' stage.
The 'postgain' circuit determines the relative volume required between the normal and effect modes.
TR1 is switched on when the footswitch, S1 is operated, allowing the signal through to the output stage where it is combined with the normal signal from IC1b. The LED D6 lights when the circuit is in the 'distort' mode.
In the 'normal' mode with S1 open, the pedal remains active and thus retains the advantages of its buffered high input and low output impedances. (Many designs simply route the input straight to the output whereupon clicks can occur and the loading seen by both instrument and amplifier vary.) The normal signal from the preamp IC1b is not combined with a distorted signal but is passed to the output stage IC1a. The gain of the output stage is determined by R19 which is designed to give an overall gain of about 2 in the 'normal' mode. Consequently the pedal is a useful buffer/preamp which can be inserted prior to the effects devices.
Note the centre frequency of the filtering section which comes in at full pregain can be moved by altering the value of R8. The timbre can also be modified by adding a 1nF capacitor between R9/C9 and R4/R5. These two options do not affect the sound of lower pregain settings.
The pedal uses a PCB which is shaped specifically to fit inside the pedal footswitch case. This case has an integral push-on, push-off switch operated via the large area hinged pad.
Start by drilling the holes in the case for the pots and jack sockets according to Figure 3. The LED position has a plug which can be pushed out from the inside.
Clip off the top set of contacts on the footswitch so that they won't foul the PCB. Bend up a pair of the bottom set of contacts.
Trim down the ledges around the base of the case by a few mm where the PCB is to rest. Cut the pot spindles to length and bend the contacts back through 90°. Mount the pots and sockets in the case, both pots with their contacts facing the LED opening.
Before mounting the components on to the PCB and if you have a multimeter, test all the diodes for a reverse impedance of at least 2ML2. Ideally, to maintain symmetry, if one diode shows a marked difference from the rest, use it, as the 'central', D3 diode, if two diodes differ markedly from the rest, use these as the pair D2, D5.
The track layout and component overlay of the PCB are shown in Figure 4. Solder the components in place using sockets for the ICs and taking care with the orientation of the diodes, ICs and C11.
With the PCB resting component side upwards upon the rounded end of the case, wire up the pedal by following the wiring chart. Veropins are advisable for connections on to the PCB. The main circuit is not sensitive to interference since it operates at low impedance and relatively high signal levels. The input is, however, at high impedance so screened cable should be used from the input socket to the PCB. Note that the screen should be connected to earth at the socket end only, the socket itself is connected to earth by a separate lead.
Once complete the PCB can be hinged over and rests, components downwards, on the small ledges around the sides of the case. One battery sits on top of the pots and the other in the rounded end of the pedal.
Soft distortion effect is suitable for guitars, basses and all monophonic instruments, but is of limited value with polyphonic unless single notes are played. With polyphonic the intermodulation products caused by the distortion often sound out of place. This intermodulation effect can be reduced by replacing C8 with a direct link, but this will give a less rich sound when used with guitar.
At low pregain settings the effect of the distortion is subtle and does not sound like 'distortion' as such, but it gives a warmer, thicker quality to many sounds, as in the 'valve sound'. C7 and R5 allow a degree of bypass for any higher harmonics that would otherwise be missing due to the treble cut by R9/C10 in the distortion path.
With an increase of pregain more of the diodes conduct and the effect becomes rougher and more obvious but remains soft enough to enable full guitar chords to be played without intermodulation being audible.
At high pregain C5, C6 and R8 cause a pre-emphasis of the midband frequencies, i.e. IC2a becomes an active bandpass filter, which makes the distortion circuit produce a distinctive and sustained sound suitable for guitar solo, heavy rock etc.