A foot operated volume control, or swell pedal, is one of simplest effects pedals there is - it is also one of the most useful. The most common application is for reducing an instrument's volume during accompaniment playing, allowing it to be increased for a solo. Whilst it is easy to set a pedal at either end, i.e. minimum or maximum, a half-way setting can be difficult to duplicate accurately. The auto swell enables the player to set a consistent accompaniment level, and increase the volume (at a preset rate) by pressing a pedal. When the pedal is released, the volume reverts immediately to the lower level. Noise and wear problems associated with the pedal operated pot type of mechanism are also eliminated.

The unit can be put to a variety of uses and some of these will be suggested in the section on applications.

The circuit is essentially a voltage controlled amplifier, the gain of which is controlled by a variable rate ramp which is initiated by a foot switch. The advantages of this method over such devices as a conventional swell pedal or compressor unit will only really become apparent when you start to experiment with the possible uses it can be put to on guitar or keyboards.

The Circuit

The input is passed to IC1a inverting input via C1 and R1; this first op-amp increases the signal by a factor of four; its output passes the signal to the circuit comprising R3, C2, RV1 and TR3 (TR3 being the FET). This circuit is in effect a variable potential divider, the top half being R3, the bottom half being the series combination of RV1 and TR3 whose resistive value can be altered by adjustment of RV1 or control of the gate voltage on the FET. C2 is for DC blocking only. The output of this potential divider is passed on to the second op-amp IC1b via R4 and C3 and finally via C4 to the output. So what we have is an amplifier whose gain is variable and can be controlled by a DC voltage on the gate of the FET. R9 and R10 provide a mid-point voltage for op-amp biasing, decoupled by C6.

The circuit around TR1 and TR2 gives us our control voltage, by providing a variable speed ramp voltage; this is achieved in the following way: With S1 (a single contact push-to-make momentary action switch) in its normally open position TR2 will be forward biased via R12 and will charge up C7 to positive supply voltage. When S1 is closed TR2 will be cut off via R14 and no longer supply a charging current to C7, however the circuit containing TR1 will be completed giving a linear discharge path to the voltage on C7, the rate of which can be controlled by RV2. This discharge ramp is passed on to the gate of TR3 via RV3 which is set for the gate voltage of the particular FET used. The circuit is powered by a PP3 9-volt battery that is switched on when the input jack is inserted.

Construction

Assemble the PCB in the usual way, following Figure 2. Also fit wires of the appropriate length to connect to the controls and jack sockets. The board is designed to fit in a Cliff CFS-1 pedal box which comes with a switch similar to Maplin's two pole latch switch; this needs to be modified as follows. First, bend back the four tabs holding the switch in the frame, remove the switch and disconnect the wire that comes with the unit. Now is a good time to drill the pedal box to take the pots and jacks, whilst the fragile parts are out of the box. Next, the switch needs to be converted to non-latching operation: push the spring back and carefully remove the action lever (see Figure 3). Now solder the switch on to the board, and mount the whole assembly in the pedal. Note that the switch is now upside down in the bracket; because of the PCB you will only be able to bend two of the tabs over, but this should be sufficient to hold everything securely. Finally, wire up the sockets and controls: don't forget the battery negative lead, which connects to the centre tag on the stereo jack socket.

Setting Up

After the unit has been constructed RV3 will have to be set to suit the gate voltage of the FET. First of all plug the unit into an amplifier and check that with an input present, the output can be varied in level by the range control. If this is the case then set RV3 to its mid-point, set the rate control to fast and the range control to x2, adjust RV3 such that by pressing the switch you just get a doubling in gain at the output. The best way to do this is to adjust RV3 to the end that gives no alteration in gain at all when operating the switch, and gradually advance it until a doubling in level is achieved on operation of the switch, leaving it set at this point. This should ensure that the FET is working over its correct range and the range control is correctly calibrated. The unit is then ready for use.

Applications

The idea of this design was to produce a simple effects pedal that would give a gradual increase in gain to a guitar or keyboard signal at the touch of a foot operated control, the rate of the increase to be variable from almost instantaneous to around 3 seconds and the amount of increase variable from times two to times 10. The original requirement being to give guitarists the facility to apply sustain to individual notes within a solo by having a slow increase in gain that could be applied as and when required to counteract the natural decay in volume of the string. At this point I hear you all shout, "Why not use a compressor"? Well, the reason is that it cannot be applied to just odd notes here and there within a solo piece, and you also have to alter your technique to overcome the clipping effect to notes where you don't want it.

Although this was the main reason for designing the unit, it was soon found that it could be used to provide a vast number of other effects, and not only on guitar. I will mention a few of these and leave it to the constructor to discover the full potential of the unit. On guitar or keyboard it can be used with a fairly fast rate to produce violining or bowing effects by applying it at the start of each note played, or with a slightly slower rate the sound of a steel guitar can be imitated (very effective on chords). It can be used as a straight swell pedal with the range set to mid-way, or with the range set to x10 and the rate on slow a very intense crescendo effect can be obtained by playing a chord, applying the effect and letting it build in volume. With a range of x2 and instantaneous rate it can be used as a straight boost for guitars and with the range midway and the rate fairly fast you can make it sound as if you're playing backwards by applying it on each note and cutting it off sharply before playing the next note. (Like the guitar part on the Beatles' "Tomorrow never knows.")

Because the unit is in circuit all the time your instrument and amplifier settings will have to be adjusted to suit the way in which the effect is to be used, but as a guide you will find with the range control set to its mid-position you have a gain of unity, i.e. input signal equals output signal (that is with the switch in its normally open position).