Recent articles in this series — February and March, for example — have mentioned the interaction that occurs between the controls of a conventional guitar. Various ways of reducing this effect have been discussed, either by different wiring or by switching the controls out of circuit altogether.

The only satisfactory method is to incorporate amplifier circuitry in the guitar itself, making it 'active' as opposed to 'passive'. The advantages are these:—

1) Each pickup can be matched into the correct impedance to obtain the optimum frequency response.

2) Proper mixing circuitry can be used to eliminate interaction between the controls.

3) Since the amplifier comes before the guitar lead, it can be given a low output impedance which will avoid the high frequency loss caused by long cables, minimise interference and hum pick-up, and enable the guitar to feed directly into a mixing console if desired.

4) Elaborate tone controls can be included, allowing cut and boost of any frequency range, limited only by the number of knobs which will fit on the guitar! Normal passive tone controls tend to be limited to treble cut on guitars.

5) Low output pickups can be compensated for simply by using higher amplification. Low impedance pickups therefore become a more attractive proposition: these have less wire in their coils, and can have better high frequency response since there is less inter-winding capacitance (to be technical about it).

There are disadvantages, of course; the possibility of higher noise, for instance — this is really not a problem if the circuitry is designed correctly. Many players seem worried by the possibility of their batteries running out in the middle of a performance; easily cured by a sensible approach to regular battery replacement. Some commercial guitars include a switch to bypass the circuitry in case of such a mishap, but it is difficult to make the passive performance of such a circuit acceptable without compromising the active side.

Whatever the pros and cons, I decided to find out for myself and when building my most recent guitar, I tried a simple active circuit.

Design Considerations

The final circuit is shown in Figure 1. The 'forest of knobs' approach has been avoided in favour of simplicity and (hopefully) reliability, important if you have to fit the circuitry into an existing guitar; it also makes operation easier in live situations. (Now let me see, which knob do I turn to get an extra 5dB at 440Hz? Was it the fourth from the left, or third from the top behind the bridge?)

Just one integrated circuit is used as the basis of the circuit, an LF 347 quad FET input op. amp. This has a high input impedance, low noise, and will give good life from a single 9v battery. Standard high impedance pickups were used since they are much more easily available, and an extreme treble response is not needed from a bass guitar anyway.

IC1a and IC1b are buffer amplifiers, which have no gain but present a high (1 megohm) impedance to the pickups for the best response. One pickup — it doesn't matter which — has a phase switch for those penetrating nasal whines. R3 keeps C2 charged up to avoid unpleasant clicks when operating the switch, and possibly nasal whines of a different sort if the unpleasant click blows your loudspeaker up.

Separate volume controls are provided, one for each pickup in true Jazz Bass style; however, thanks to a proper mixing amplifier (IC1c) there is no interaction or loss of tone at low volumes. This stage also provides a constant low impedance for the correct operation of the tone control circuitry around IC1d.

To my mind, elaborate tone controls are unnecessary on a guitar; most of the work will be done by the amplifier or mixing desk's equalisation, and all that is required is the facility for slight changes to be made whilst playing. To this end, a single knob is all that's provided, in conjunction with a switch which gives a choice of bass or middle control. RV3 allows boost or cut of the chosen frequency range, and is truly 'flat' in the middle of its travel. The tone switch is S2, shown in the bass position in Figure 1.

IC1d feeds the output direct, and should be capable of driving any reasonable length of cable. The output jack JK1 switches on the battery automatically when the instrument is plugged in, so you must remember to unplug it when you've finished. R12 and R13 generate a 'mid point' voltage so that a single battery may be used; normally, op. amp circuits require balanced positive and negative supplies.

Construction

The eventual size and shape of the circuit will, of course, depend on the cavity in your guitar, and therefore no hard and fast layout can be given. It should be stressed that you will need to be experienced in building small, neat circuitry if you intend to tackle this project.

Reliability is paramount in this application, so good soldering is a must; the IC should be soldered direct, not mounted in a socket where it could work loose. Also, use only known good components of a reasonably miniature size: surplus parts are always false economy unless you have the facilities to test them thoroughly.

The prototype was in fact built on the small piece of Veroboard given away with May '81s E&MM; together with a PP3 battery, these fitted so well in the body cavity that it could have been made for them!

Whether you are modifying an existing guitar or building one up from parts, your choice of pickups is likely to be limited to those which will fit the recesses provided, unless you're skilled in woodwork. A wide variety are available as replacement parts nowadays. If you're modifying, in many cases the original pickups will be given a new lease of life by the circuitry and need not be replaced. Special characteristics are certainly not required to enable the active circuit to work satisfactorily.

The Guitar

First there were replacement pickups, then there was brass hardware, and finally new necks and bodies. Nowadays it's perfectly feasible to build a respectable guitar completely from 'replacement' parts without having had a guitar to replace in the first place! You save a bit of money and you get the instrument you want, in terms of equipment and finish if not in terms of style. Original designs are being slow to appear in this market.

The neck of my guitar is in fact genuine Fender, fretless Precision species, but the body, bridge and pickups are Jazz Bass replacements from an outfit called Double Eagle. A black and white photograph can't do justice to the very nice piece of mahogany which forms the body. I've finished it in teak oil which really brings out the grain. This guitar is primarily a studio instrument (what, me play fretless bass in public? You must be kidding!) and so a protective finish is not a necessity; otherwise, several coats of polyurethane would be a better bet, and is a must on the neck. Light woods, such as ash and maple, should also be varnished. If your bit of wood is somewhat unprepossessing, I've found car spray gives a good solid colour finish provided you use a primer first.

A couple of tips: on this guitar I've used woodscrews to fix the bridge, strap buttons and machine heads, but other fixings which get more use are in fact bolts screwed into threaded metal inserts in the wood. The control plate will need to be removed occasionally to replace the battery, and the pickup screws serve to adjust their heights, so this system avoids the wearing out of the holes in the wood which would occur if woodscrews were continually being screwed and unscrewed.

Although threaded inserts are available commercially, I have used ordinary threaded spacers glued in with Araldite; this idea worked even on the neck fixings. These would not normally get much wear, but any damage in this area would be very troublesome, and the bolts make it possible to switch necks reasonably quickly. I started out with one body and two necks, one fretted and one not; two guitars for the price of one and a half!

As you can see from the photograph, the Jazz Bass body is quite strongly slanted at the base, which means it will fall over if you try to lean it against anything. What may or may not be evident from the picture is that the lower strap button is offset from the body's centre line just enough to let the guitar stand vertically on a flat surface. You should really use a proper stand, of course.

Modifications

The circuitry described was designed specifically for bass guitar, but there's no reason why it shouldn't be modified for lead guitar use. Here, the tone control would be more use if it were a treble/middle type: to do this proceed as follows. Miss out C7, and the pole of S2 which bypasses C9 - instead, connect C9 permanently between the wiper of RV3 and the junction of R9 and R10.

Whatever sort of guitar the circuit is used in, the characteristics of the tone control can be changed by altering the capacitors:— C7 for the bass function, C8 and C9 for the middle and C9 for treble. Using a smaller value will increase the frequency at which the control works.

If more output is required, say to compensate for low sensitivity pickups, R6 may be increased. Beware of noise, however, and possible distortion if the pickups were high output types to begin with. Conversely, the gain may be reduced by reducing R6.