A useful feature of many small practice amps is the headphone socket that allows you to play at a self-satisfying level without alarming the civilian population. Sometimes it’s good not to have to carry even a small amp, so this month we’re going to tell you how to build a gadget that will boost a guitar or medium-impedance microphone to a more-than-healthy level in your cans. It fits in your pocket, accepts a standard guitar jack cable and feeds Walkman-type headphones from its 3.5mm stereo phone socket. Powered by two low-cost AA’s, standard or rechargeable, it’ll play for 100 hours or more.
As the Pocket Rocket isn’t a kit there’s no special printed circuit board, so we’re going to tell you how to build it with parts you can buy yourself for around £16. The parts list is based on the widely-available Maplin catalogue.
There’s not much space in the box, so finding a small enough volume control isn’t straightforward, either to source or to fit. Instead we’ve used a mini switch… one way for ordinary guitar level, the other for microphones or loud guitar levels (take care of those eardrums, people!). The tone stays clear with low guitar volume settings, so this is a practical solution to the problem of not having a conventional volume control.
Building the pocket rocket
Begin by using the drilling diagram (right) to place the drilling centres in one end of the plastic box. Use a 3.5mm bit to pilot out the holes, and drill as accurately as possible to the template. Widen out the jack hole to 6.5mm and then to 12mm with a tapered reamer or a round file. The switch and headphone jack are both drilled out to 6.5mm final diameter.
Now, let’s prepare the stripboard. We’ve specified a piece that can provide two boards in case of unrecoverable errors the first time! Refer to the picture with the strip and hole numbering at the bottom of page 104 and score both ends of strip 10, where you want it to break. Carefully bend along the strip until a crack appears at one end. Gently increased pressure should cause the crack to follow the strip in a straight line to the other scored end.
All being well, your final board size should be nine full strips across and 31 holes long. The 39-hole board is broken across the 32nd row to get the right length. The rough edges can be filed smooth to reduce the width for a comfortable fit. Using a pair of sharp-nosed pliers or side cutters, nibble out one corner of the board to clear the corner post of the case (according to the matrix, holes 1 to 4 in strips A to C should be removed). Before attempting this, we’d advise you to score the corner with a straight edge and a hobby knife to prevent accidentally overshooting with the nibbling tool.
Drill a 3.5mm hole in the board, using hole E 29 as the centre. Use the same drill as a spot-face cutter to create breaks in the tracks at the points listed in the Breaks Matrix shown in the box below. Now fit the resistors across the board, passing the wires through the holes specified in the Parts Matrix (note that one of the resistors stands up on end), trim the wires under the board, and use the off-cuts to make the links. Fit the links across the board as specified in the Links Matrix (again shown in the box below), bend them back along the strips and trim them to about 2mm, then solder them to the strips. Take care not to solder-bridge any strips: likewise with the resistors. Fit the IC, whose pins should straddle the row of four broken strips D – G, holes 17 – 20; likewise the mini-jack, whose terminals should appear through E3, G1, and J3. Fit the capacitors and the wiring from the jack, switch and battery clip, as per the Parts Matrix. Note that battery-clip negative (black) goes direct to the middle ‘ring’ terminal of the three-pole input jack, and that the right-handmost 220uF capacitor is oriented inversely to the three others.
Finally, push the nose of the mini-jack through the 6.5mm hole nearest the roof of the box, and mark the case through the hole you made in the board. Drill through, and pass a short M3/6BA screw through the case and the board, using a nut to fix the board down to the case. You may like to include a small washer or spacer under the board to avoid bending stress with the nut tightened. Now fit the 6.3mm jack socket, with one spacer inside the case, and the switch, oriented with its throw horizontal. Fit two AA cells to the battery carrier and connect the clip. The carrier should fit snugly enough to avoid any need for further fixing with the lid in place. Connect your favourite phones and guitar, and you should get good clear sound at a reasonable level on one switch setting and much louder on the other.
We had so much fun with this little box that we’re working on a more sophisticated version. This will have mixable mic and guitar channels and we’re looking at a bit of stereo echo too .