Build A Bass Guitar Pre-Amp
This article describes how to construct simply and inexpensively a small battery-powered preamplifier to build into the body of a bass guitar, in order to drive a remote slave amplifier. We also discuss the merits of using such a system. We have not attempted to describe how the unit should be fitted into a guitar and would recommend that if you are uncertain you should contact your local guitar repair man before tackling the fitting job.
Most musicians would agree that a system whereby the guitarist can use only the built-in tone controls on his instrument to control the overall volume would be more useful than the present system of amp controls at the back of the stage.
One answer to the problem is to have an elaborate P.A. System into which the voices and instruments are fed and balanced by a sound engineer or qualified roadie!
Another answer, especially for bass guitarists, who tend to cause some inter-modulation distortion on the P.A. if they are fed into the Communal System is to have a standard 'slave' amp and Speaker (monitor) at the back of the stage and to fit a small pre-amplifier into the guitar itself. The only controls on a slave amp are the volume control knob and a mains on/off switch.
The circuit and construction design of the pre-amplifier we are describing is intended to make possible direct signal-injection into the normal slave amp of your choice. The pre-amplifier unit is small enough to fit inside most bass guitars, and the time and money spent on the construction of the unit (approx. £2) could easily be recouped if you buy only a power amplifier (slave unit) instead of an integrated guitar amplifier; slave units normally cost less than integrated units.
The tools required are very basic. The essential items are: side cutters, small pliers, soldering iron, solder and a drill of about 3/16 inch diameter or a 'vero spot face cutter'. A junior hacksaw will also be required to cut the vero board on which the circuit is constructed.
If you have no previous experience of soldering it is a good idea to practise on a scrap piece of vero board with some odd pieces of wire before you start to make the pre-amplifier. A small soldering iron with a bit of about 1/8 inch to 3/16 inch will be found ideal. Do not use too much solder. Big 'blobs' of solder will run across the strips of copper on the vero board and short them out. When the soldering iron is sufficiently hot, put only enough solder on the iron to 'wet' the surface of the bit; then the iron to heat the job and apply a small amount of solder to the joint or pin, not the iron.
The components are all readily available and inexpensive. The total cost should not exceed £2 including the battery clip and a switched jack socket which will switch the battery off when the jack plug is pulled out.
You will also need a small piece of 0.1" vero board. This is insulated board drilled with a 0.1" matrix of holes. On the bottom side are copper strips which join lines of holes. Components are mounted on the top side with their wires going through the holes and soldered underneath. The copper strips can be cut where required with a special spot face cutter or an ordinary 3/16 inch or 1/4 inch drill held in your fingers.
The other miscellaneous items are: battery clip for PP3 type battery; 18 inches of insulated connecting wire; 12 inches of 22, 24 or 26 S.W.G. tinned copper wire and two jack sockets; one of which must have a 'switch' normally open contact on the front contact and the other a switch normally closed contact on the back contact. If this is not available a separate on/off switch can be put in the battery negative wire.
The circuit is not sensitive to layout and any of the normal construction techniques can be used. For those without a lot of experience the vero-board layout given will be found most convenient. The electrolytic capacitors, C2, C4 and C5 have a right and a wrong way round. The + and - are usually marked on the components but if in doubt ask the shop when you buy them. The integrated circuit also has a right way round. On the 8-pin 'dual in line' pack there is a dot by pin one or between pins 8 and one, and the pins are counted counterclockwise looking at the top.
The 'top hat' type package is counted the same way round but the tab is above pin 8. Not pin 1. This is most confusing unless you know!
Before switching on check the wiring, check that the copper track has been cut in the correct places and, most important of all, that the battery has been connected the correct way round.
If the unit is built into the guitar an arrangement is possible so that if a plug is put into one socket the signal from the guitar comes out directly bypassing the preamplifier and if put into the other socket the signal comes out through the amplifier after first switching the amplifier on.
The battery current is about 0.7 mA without any signal and rises to about 1 mA at full signal without a load. This is sufficiently little for the battery to last for several months. The amplifier will continue to work until the battery volts have fallen to about half their original value.
Distortion on the prototype was 0.15% THD at 100 Hz and the background noise was -72 dBm; giving a signal to noise ratio of 72 dB when the output level is 0 dBm.
The gain is 11 times. This can be reduced by increasing to value of R3 or increased by reducing the value of R3.
High frequency roll off is provided to reduce the high frequency response above 730 Hz and yet rid of Hiss. Low frequency roll off is provided to cut out frequencies lower than the lowest note of the bass guitar. This cuts the bass response at frequencies below 33 Hz.
The output is not sufficient to drive low impedance headphones fully, but will make enough sound to be heard. The unit was not designed to drive headphones so this is just a useful extra.