This is the view from the component side, the copper tracks on the other side being shown in red. The small squares are a tenth of an inch, and the board size is 3.9 in. x 6 in. Small boards are often supplied in size either 6x4 inches, or 100 x 160 mm. The design shown will fit onto either size, with only a small gap at either a side or end. The exact layout may need to be changed depending on component sizes, for example a small 2.2uF polyester 'open frame' input capacitor may have a lead pitch as little as 6.5mm, but a 2.2uF polypropylene capacitor from the same supplier is specified as 27mm, and will not so easily fit on the board. Whatever the relative merits of the two types the smaller one will pick up less interference and make a smaller loop to pick up stray fields, which in practice may be more important than the low levels of low order harmonic distortion produced by capacitors.
The BF transistors are shown with a dark line to indicate the back, metal side (The base and emitter are reversed compared to the MJ types, but all have a centre collector.). The MJ transistors are all mounted on an aluminium bracket on the top of the pcb using suitable insulating kits. The bracket should be about 5mm or 3/16 inch thickness. Thicker metal will of course give better heat conduction, but the transistor leads may then not be long enough to reach through the board, and extensions or terminal pins will then need to be added. The bracket is bolted to a suitable heatsink.
The 'earth' is split into three sections to try to ensure that small, large and non-linear currents follow separate paths, and all three should be connected through their own heavy gauge wire to a single earth point. The earth end of the speaker should go to the same earth point, as should the negative side of the power supply capacitor. Two terminal pins are shown for the input at the bottom left, and two more are included near the top. These two can be used for an optional thermal switch of the bimetal normally open type bolted to the heatsink, operating around 70 deg.C. If this switch closes the amplifier will shut down until the temperature falls enough for the switch to open again. Originally a solid state detector circuit was included in the empty space at the bottom right, but this seemed over complex for such a simple function and is now omitted.
As always this is not a fully tested construction project for the inexperienced, and at the very least an ability to find and rectify faults is essential.HOME.