Canjak is a unique self contained headphone amplifier designed to plug directly into the guitar jack socket for personal practice. For those learning to play the electric guitar, a Canjak is a must to keep on good terms with the family and neighbours! The professional guitarist will find the Canjak invaluable for developing technique, practising numbers, and writing new ones. With a Canjak and a pair of cans, you can practise your electric guitar anywhere; in the middle of a park, on a beach, in a coach or even on a plane. The Canjak is by no means limited to guitars; any electric or electronic instrument with an adjustable output level can be used.

A switchable fuzz circuit is included for additional flexibility. For simplicity and ease of use, the battery is switched on by the insertion of the phones plug, rather than using an extra switch. The complete circuit, along with the battery, connectors and switch are enclosed in a tiny plastic case which can be slipped into a pocket when not in use. When plugged into the guitar, it is completely unobtrusive, the only cable in use being the phones lead. To ease construction, as many parts as possible have been mounted on a single PCB, including the switch and phones socket.

Circuit

The circuit, shown in Figure 1, is based on the LM386 monolithic amplifier, IC1. This device contains all the necessary active components for the Canjak. C1 couples the input signal from the input jack plug, JK1, to the non-inverting input of IC1. C7 couples the amplified output signal from IC1 to the phones jack socket, JK2, via R4 and R5. These resistors serve several purposes; they limit the output power, protect the output stage of IC1, allow the use of mono or stereo phones, and reduce the size of C7 needed for good bass response.

With SW1 in the fuzz position, the germanium diodes D1 and D2 apply nonlinear negative feedback via R3 to produce the familiar soft-limiting distortion. R1 increases the initial gain of the amplifier so that the fuzz diodes have more effect. When fuzz is not selected, R2 provides linear negative feedback to produce clean, distortion free operation. C4 and C5 are required to prevent upsetting the DC bias conditions. C2 and C3 prevent HF oscillation while C6 decouples the battery supply. A make switch contact in the sleeve connection of JK2 acts as a battery on-off switch when the phones jack plug is inserted and withdrawn.

Construction and Testing

To achieve the compactness of this project, naturally everything has to be packed in quite tightly, so follow these construction notes very carefully otherwise you may find assembly difficult. Don't be over-generous with the solder since it will be necessary for the soldered joints on the PCB to have a low profile.

Assemble the PCB according to the component overlay, Figure 2, starting with the diodes and IC1. There is no room for luxuries like IC sockets, but with a little care, no problems will be encountered in soldering IC1; don't be frightened of it, though. Crop all the leads as close as possible to the PCB. Now insert all the resistors, then solder and crop. Note that all except R2 are mounted vertically. For the wire-ended components, it will be found helpful to bend the leads out at 45 degrees to the PCB after insertion to hold the components in place while soldering. The capacitors can now be inserted, soldered and cropped. Now insert the socket, JK2 and push it firmly down onto the PCB while it is being soldered. Again crop the terminals close to the PCB. Insert SW1 and lightly tack the connections with solder so that the assembly can be tested; It will have to be removed again later. The battery clip wires can now be connected to the PCB, after trimming them to a length of 10 cm.

Prepare the plastic box as shown in Figure 3, remembering to cut and file the ribs flush as shown. Cut the screen tag of the jack plug so that it is the same length as the inner tag. Solder a pair of 6 cm long insulated wires onto the two tags. Now fit the plug into the end hole of the case and secure it by means of a nut. Terminate the two wires to the PCB. The wiring connections are shown in Figure 4.

Before installing the PCB it must be tested, since fault rectification will be much more difficult after installation. First though inspect the assembly, checking particularly component orientation. Check the soldered joints with an eyeglass, watching out for bridged tracks. Now connect a PP3 battery, plug in a pair of phones, and connect the jack plug into your instrument. The signal should now appear in your cans loud and clear, the level being adjusted by means of your instrument volume control. Check that the fuzz switch operates correctly. If all is well, disconnect the plugs, sockets and battery. Remove the fuzz switch from the PCB, making sure that the PCB holes are left unobstructed.

Fit the switch into the small hole of the case. Adjust the rear nut such that the bush will be flush with the front nut. Slacken the front nut for now, so that the switch can be moved. Offer the PCB assembly into the case with the battery leads folded back underneath into the battery compartment. Manoeuvre the PCB into position, guiding C7 under the jack plug terminals, and move SW1 as necessary to avoid other components, such that the jack socket can be located into the remaining case hole. It will now be possible to swing the PCB towards SW1 so that the switch terminals can be located into the PCB. After ensuring that the PCB is straight in the case, the jack socket can be secured with its nut; similarly for SW1. The switch terminals can now be soldered, taking care not to melt the case edge. Trim off the excess terminal length if necessary.

To complete the assembly, place a strip of insulating material over the track side of the PCB, and secure with adhesive. This will prevent the metal case of the battery from shorting together any of the joints. Finally, fit the battery and screw on the lid.

You can use your Canjak with any electric or electronic instrument, using the instrument's volume control to set the listening level. The phones you use should be of the common low to medium impedance type. If you wish to use your Canjak with bass instruments, make sure you use high quality phones with adequate bass extension. Although the Canjak has built in fuzz, other effects can be used by plugging the instrument into the external effect unit in the normal way. Your Canjak can then be plugged into the output of the effect unit.

Now plug in your cans and play; but don't forget to unplug them when the Canjak is not in use, otherwise you will soon have a flat battery!

A complete set of parts for the Canjak, including PCB and case, is available from E&MM, (Contact Details), at a cost of £9.95, including postage, packing and VAT. Please order as: Canjak kit.