Expression for guitar or keyboards
An expressive wah-wah effect with built-in equalizer. Design by Mark Stuart.
Pedal effects units can be generally divided into two types. One changes the amplitude characteristics of the signal, by waveform clipping or by compressing and expanding the signal envelope. Fuzz, sustain, companders and similar effects fall into this group. The second type of effects units are those which change the signal frequency or phase characteristics. Treble and bass boosters, parametric equalisers, phasers and wah-wah pedals all modify these aspects of the music signal. The Wah tone pedal combines three individual frequency modifying effects which may be used singly or in any combination. Treble boost and cut, bass boost and cut, and a pedal operated wah-wah effect are provided.
The block diagram of Fig 1 shows the Wah Tone system layout. The input signal passes to the input buffer amplifier stage. This stage has a high input impedance and a low output impedance to drive the wah-wah stage; a high input impedance is essential for some types of guitar pick up, but low impedance sources can also be used with a high impedance input because the 'mismatch' is the right way round — low to high.
A wah effect is produced by a circuit with a peak in its frequency response. This peak can be swept up and down the audio band. Dual potentiometer VR1 is linked to a rocking foot-pedal and sets the frequency of the response peak; by moving the pedal the response peak is swept through the audio frequency band, producing the familiar wah-wah effect. At the limit of travel of the foot-pedal an alternate action footswitch can be operated to completely bypass the wah-wah effect.
The signal from the wah-wah stage enters a passive tone control network with individual bass and treble controls. The bass control gives 10dB of lift and cut at 100Hz. The treble control gives 10dB of lift and cut at 3kHz. The maximum amount of boost and cut is 20dB. Because it is a passive network the tone control stage reduces the level of the signal passing through it by approximately 20dB (10 to 1). The output buffer amplifier has a high input impedance to minimise loading of the tone control network, and a gain of 10 to restore the signal to the original level. From the output buffer amplifier the signal passes to the output socket SK2. The unit is switched on by inserting the input jack plug.
A low noise operation amplifier, IC1b connected as a non-inverting amplifier stage is used as the input buffer amplifier. The gain of the stage is set to 2 by the feedback resistors R4 and R5. The input impedance of the stage is set by R3 shunted by the input impedance of IC1b. The input impedance of IC1b when used as a non-inverting amplifier stage is very high, therefore the shunting effect of IC1b on R3 is negligible, and the input impedance is equal to the value of R3 alone, ie, 100K. Input coupling capacitor C2 protects IC1b from stray DC input voltages and provides a low frequency roll off.
The wah-wah stage is a variable frequency tunable bandpass filter formed around IC2a. The frequency of the bandpass filter is set by the feedback network C3, C4, R7, R8, VR1a and VR1b. This network is called a Wien bridge. At very high frequencies, feedback signals from IC2a output are shunted away via C3 and so have little effect. At very low frequencies the reactance of C4 becomes very high and prevents significant feedback. Between the extremes of frequency there is a point called the centre frequency where the amount of feedback is maximum. At this frequency, set by the values of resistance and capacitance in the bridge, one third of the output signal from IC2a is fed back to the noninverting input. The centre frequency can be varied without changing any other characteristics of the circuit, by simultaneously varying VR1a and VR1b. By using a dual ganged potentiometer for VR1 a and b the centre frequency can be swept through the audio band as required. Resistors R7 and R8 set the centre frequency upper limit. The lower limit is set by the maximum rotation of VR1 that can be achieved with the pedal mechanism. With the type of pedal used the 100K pot can be turned one third of a revolution giving a maximum resistance of 33K.
If the only feedback around IC2a were the Wien bridge network the circuit would oscillate, because the gain of IC2a vastly exceeds the losses of the positive feedback network. A second feedback network consisting of R9, R10, and VR2 provides a controlled amount of negative feedback, which controls the circuit stability. Using VR2 the amount of negative feedback can be adjusted.
The combined effect of the two feedback networks is to produce a gain peak at the Wien bridge centre frequency. The sharpness of the peak is increased as the negative feedback is reduced, until at a critical setting of VR2 the circuit oscillates at the centre frequency. When VR2 is set correctly the centre frequency peak is neither too sharp nor too flat. An ideal wah-wah effect is produced when the centre frequency is varied by VR1.
A passive tone control network follows the wah-wah circuit. R11, C5, C6, R12, and VR3 control the bass frequency response. C7, C8, R14, R15 and VR4 control the treble response. Each set of components forms a potential divider chain. The input signal is applied to R11 and C7 at the top of the divider chains. The lower ends of the chains, R12 and C8, are connected to a signal ground point. The sliders of the two controls tap off a proportion of the signal from the divider chains and feed it to IC2b, the output stage. The component values in the divider chains determine the frequency bands affected.
As the tone control network is passive it produces a considerable loss of signal level. This loss is restored by IC2b which is a standard non-inverting stage with a gain set to 10 by feedback resistors R16 and R17. IC2b also provides low impedance output which is coupled via C9 to SK2, the output socket.
The use of a ready-built case complete with a rocking footpedal and linkage for VR1 minimises the amount of mechanical construction work. The case specified has holes already punched for SK1, SK2, SW1 and for the PCB mounting screws. Two additional holes must be drilled, for VR3 and VR4, on the opposite side of the case to SK1 and SK2. A printed circuit board holds the other components.
Begin assembly by building the printed circuit board. Refer to the component overlay drawing in Fig 3. Mount the low profile components first, then the larger components. IC sockets are recommended for both ICs. The wire leads should be fitted to the printed circuit board last of all. They can be soldered directly to the board or connected via terminal pins.
Drill the holes for VR3 and 4, then mount all the components inside the case. Fig 3 gives a guide to the positioning of the large components. VR1 requires special care; it is fitted to a plastic bracket supplied with the case. A slotted lever is also supplied to link the shaft of VR1 to the rocking pedal. The lever must be fitted so that when the pedal is pushed fully forward VR1 is fully clockwise. Bypass switch SW1 must be mounted using two nuts so that its height can be adjusted; the ideal position allows SW1 to be operated with slight overtravel of the pedal.
Mount the PCB to the case using three self tapping screws fitted with insulating pillars to ensure that the PCB tracks do not contact the case. Finally complete the wiring. Note that a wire must be soldered to the body of VR4 to link the cast to the negative supply voltage at SK1. The centre tags of VR1 a and b are bent over and soldered toqether.
Connect a suitable input signal source to SK1. Inserting the input jack plug automatically switches on the circuit power. Connect the output socket to an amplifier with its volume control set well down. Set VR3 and VR4 to mid travel. The wah-wah gain preset VR2 should be set fully clockwise. Operate SW1 and the wah-wah pedal whilst injecting a signal. As SW1 is an alternate action switch it will be necessary to establish by ear whether or not the wah-wah effect is connected. With the wah-wah effect switched on gradually turn RV2 anticlockwise. If RV2 is turned too far the circuit will burst into oscillation. The ideal setting is a matter of personal taste and should be established by trial and error to suit the instrument used.
The tone controls operate completely independently from the wah-wah circuit. When set to the centre position the frequency response is flat. The total current consumption of the circuit is approximately 6mA so battery life should be adequate with a standard PP3.
As the on-off switch is linked to the input socket it is obviously important to avoid leaving the input lead connected when not in use.
Feature by Mark Stuart
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