The E&MM Guitar Buddy has been designed as a low cost kit suitable for both beginners (to the art of music or electronics), as well as providing a handy musicians' practice amp for almost any location. Its levity and compactness makes it very portable; in addition, it can be powered by an internal PP9 battery, an AC mains adapter or from the electrical system of your car via a cigarette lighter plug. Although designed primarily for use with electric guitar, the two input sockets will accept signals from other sources such as high impedance microphones and electronic keyboards.

An important feature of the Guitar Buddy is the provision of a headphone socket, a must to avoid disturbing the family, or (when out on the road) hotel clientele, or other musicians when tuning up in the galvanic atmosphere of the dressing room!

Circuit

Most of the amplifier circuitry is contained in the TDA1011 integrated circuit, but a few external components are required for the IC to operate.

The input signal is passed via paralleled sockets JK1 and JK2 to the passive tone network consisting of RV1, C1 and R1. Volume control RV2 permits attenuation of the signal before it passes to the preamp stage of IC1, via coupling capacitor C2. At this point potential radio frequency interference is attenuated by C4. The output of the preamp stage is then coupled via C5 and R3 to the power stage of IC1. C6 is an additional high frequency filter.

Bootstrap capacitor C9 allows the output to swing close to the supply rail, to squeeze as much undistorted output from the amplifier as is possible for any given supply voltage. As the output terminal rests at half rail volts, C11 is necessary to block DC voltage from the speaker. A Zobel network consisting of R4 and C10 compensates for variations in speaker impedance with frequency, as seen by the amp, and ensures high frequency stability. When headphones are connected via the socket JK3, the internal speaker is disconnected and R5 attenuates the output signal to avoid damage to cans and ears alike! Note that the contacts on JK4 switch off the internal battery when external power is connected.

Easy-build Construction

Since all of the components, including the control pots and jack sockets are mounted on the circuit board, the construction is very simple. The only external connections required are those of the battery and the speaker.

Using the component overlay shown in Figure 3 and with reference to the photograph of the completed board, the components can be inserted and soldered. Start with the resistors, then the capacitors, remembering to observe the polarity of the electrolytic types. There is only one link on the board which can be made with a short length of tinned wire or resistor lead.

The integrated circuit should be mounted to its heatsink before being soldered to the board. The orientation of the IC can be determined from the pin-out given in Figure 2. Take care not to bridge any of the IC pads when soldering.

The jack sockets and pots can now be soldered in place.

When all the components are on the board the battery connector and speaker leads can be connected. Take care with these connections as a mistake at this stage can be costly!

Insert the board through the holes in the case and the printed facia panel, then secure with the three chrome nuts for the input and headphone sockets. After connecting the speaker, glue foam strips to the inner sides and bottom of the front half of the case. This will help to reduce cabinet resonances. The large foam pad should be formed loosely over the speaker, making sure that the battery connector protrudes. This completes the internal assembly.

The battery connector goes through a slot in the battery compartment in the rear of the moulding. The two halves of the case can now be joined using the four fixing screws. The small foam pad is pushed into the bottom of the battery compartment to cushion the PP9 battery. All that is required now is to push on the control knobs and connect the battery.

Playing

The amplifier is switched on whenever a jack plug is inserted into either of the input sockets. A point about volume controls, often overlooked by musicians, is that their setting bears no fixed relationship to the power output; with a high output pickup, full power may be obtained long before the volume control is turned fully clockwise.

Most headphones on the market being stereo, the amplifier is kitted out with a stereo phones socket, wired to drive both earpieces in mono, and at the same time disconnecting the internal speaker. If an external power source is used, ensure that conventional polarity is observed, viz, the tip of the plug is positive, and the body negative. The battery is disconnected whenever an external power supply is in use. As with all battery powered gear, don't forget to remove the battery when the amplifier is out of use for long periods. Now you can begin those long hours of dedicated practice!

A complete kit of parts for the Guitar Buddy as listed is available from Electronics and Music Maker, (Contact Details), at a price of £19.75 including p&p and VAT.