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Case
The prototype was constructed in a plastic case, approximately 170 x 70 x 190mm, however a somewhat smaller case could be used if required. Since the unit contains dangerous (mains) voltages the case must be held together with screws, a clip together case is not suitable. If you intend to include a power amplifier circuit (see later), use a metal case and make sure it is large enough to house all the components comfortably.

Since the PCB is so small and light, it will be perfectly secure mounted by the potentiometers, unless the unit is to receive some particularly rough treatment! Two holes are required in the front panel for the pots, 2 inches apart and the right diameter for the bushes on the pots being used. If the small PCB mounting pots from Maplin are being used, the holes need to be 7mm (9/32") in diameter. If the pots have any small lugs to locate into the panel, these can be carefully cut or broken off.

A third hole will also be required on the front panel, to suit the power switch being used. The switch used on the prototype was supplied by Maplin as a Miniature Rocker Switch (YX65V), and required one 18mm diameter fixing hole, and a small notch to prevent rotation. It may be easier to use this type of switch, rather than one which requires a rectangular cutout.

The front panel can then be marked with rub-down transfers and sprayed with lacquer if desired, before the components are fitted. When tightening the pot nuts, support the body of the pot from behind, and do not over-tighten. The washer should be placed against the front panel, behind the nut. The suggested switch has a plastic nut which should only be tightened a little more than finger tight. The knobs can now be fitted, ensuring they do not scrape on the panel.

The rear panel can now be prepared, with a hole for the mains cable grommet or clamp, and appropriate holes for the audio connectors being used. The prototype has 5 pin DIN sockets, however phono sockets would probably be more suitable in most setups. You may prefer to use two sockets each for Left Input and Right Input, connected in parallel, to save using Y splitters or making custom cables (see the Installation Diagram, Fig 4). The same could apply for the Output if using a stereo amplifier, so the signal can be taken to both channels.

The transformer should be mounted in the base of the case. Mount it away from the circuit board and audio connectors, but where the leads will still comfortably reach the board and switch. Fit a solder tag under one of the mounting nuts for the Earth connection, having scraped away any lacquer from around the hole in the transformer first. The prototype used a 100mA wire ended transformer, which was more than adequate for the job.

Wiring Up
The Interwiring Diagram is shown in Fig 3. Start with the audio connections, using suitable screened cable. If you wish to sleeve the screens, use a piece of the outer insulation removed from the wire. If two phono sockets are being used for the Inputs and/or Output, as suggested earlier, they can be linked together with lengths of tinned copper wire, providing the sockets are no more than about 40mm apart.

Connect the "Left Input" socket to P4 (core) and P5 (screen) on the PCB, the "Right Input" socket to P6 (core) and P7 (screen), and the "Output" socket to P8 (core) and P9 (screen).

The mains wiring should now be carried out, with great care! Mistakes here can be costly and dangerous.

The mains cable is standard 3 core mains flex, rated at 3 Amps or greater. A grommet or cable clamp must be fitted in the hole where the mains cable enters. The cable must be secured in some way. As a last resort you could tie a knot in the cable, but try to do something better.

Remove sufficient outer insulation for the earth (Green/Yellow) wire to reach from the switch to the tag on the transformer mounting. Trim the Live (Brown) and Neutral (Blue) wires to about 30mm, strip the ends, and solder to the switch as shown in the Interwiring Diagram, ensuring that there are no loose strands. These joints should be sleeved for safety.

The thick Brown and Blue wires from the transformer primary should now be soldered to the switch, as shown in the Interwiring Diagram, and sleeved as before. To save possible damage to the transformer, leave these wires the full length as supplied.

The Earth (Green/Yellow) wire in the mains flex should now be soldered to the tag under the transformer mounting nut, together with a length of wire sufficient to reach the PCB. If the case has a metal front panel, a wire should be connected from this to the earth tag (a metal rear panel will probably be earthed by the connectors mounted on it, if not connect it to the tag also). It may be easier to remove the tag whilst soldering. Recheck the tightness of the transformer mounting screws after soldering these joints.

The wires from the secondary of the transformer will be two of one colour (possibly Green), and one of another colour (probably Black). Connect the single wire, together with the wire from the transformer earth tag, to terminal P3 on the PCB. Connect the two same colour wires to terminals P1 and P2 on the PCB, either way round. As before, it is best to leave the transformer wires full length.

Connect a 13 Amp plug, fitted with a 3 Amp fuse, to the end of the mains flex.

Now thoroughly recheck the wiring, particularly the mains connections. Remember that any mistakes in the mains wiring are potentially lethal.