In the May & June 2007 issues, we published the circuit and assembly details for our new high-performance 20W Class-A Stereo Amplifier modules, along with a suitable Power Supply module. Then in the July issue, we described a Speaker Protection & Muting module and followed that up in August 2007 with a Low-Noise Preamplifier & Remote Volume Control.

This month, we show you how to assemble everything into a custom-made steel chassis that’s been designed by Altronics. This precision laser-cut chassis is supplied with all the holes drilled and with pre-punched front and rear panels with screened lettering.

This case is similar to their "2U" deluxe rack cases (but is much deeper) and features a bevelled front panel. The completed amplifier looks very professional, although at 420 x 425 x 88mm (W X D x H) it’s really quite a large unit. This size is necessary to accommodate the large finned heatsinks used for the power amplifiers and to allow the various modules to be logically placed (and separated) inside the chassis.

The large chassis size is also important to aid ventilation, as the main heatsinks run quite hot in operation (about 30° above ambient). In addition, the bottom of the chassis and the lid have large ventilation slots which line up with the heatsink fins, to allow the air to circulate through them.

That’s one of the drawbacks of a class-A amplifier – they generate lots of heat that has to be dissipated.

OK, let’s assume that you’ve completed all the modules and that you’re ready to mount them in the chassis and install the wiring. Here’s how to go about it.

Preparing the case

As supplied, the case is finished in a tough powder-coating that’s also a good insulator. However, you must ensure that all sections of the case, including the side panels and the front & rear panels, are correctly earthed and that means ensuring they make good electrical contact with each other.

The first job is to ensure that the two side panels, the front & rear panels and the lid are all earthed to the bottom section of the chassis. This is done by using an oversize drill to remove the powder coating from the countersunk screw holes. Use a drill that’s slightly smaller in diameter than the screw heads and be sure to remove the powder coating right back to the bare metal.

Don’t just do this for one or two holes – do it for all the holes in each panel. Provided you use a drill that’s not too big, the bare metal will later be covered by the screw heads.

Fig.1: follow this diagram and the photos to install the parts in the chassis and complete the wiring. Note that the supply leads to the modules, transformer and mains switch are twisted together - see photos & text.