The following is based on the 6 transistor mosfet design, but it all applies equally to the later 7 transistor version. Details of the volume control and input earthing problems can be found here.
The photos illustrate one way to construct a complete amplifier, including volume and balance controls. A 120VA toroidal transformer was used for the power supply. A simple production method using an etch resist pen is shown for the printed circuit boards, this being an easy method for amateur constructors without more advanced equipment.
First, the boards are drilled, I started by sticking a layout diagram on the component side and drilling the holes, mostly 1mm dia, but 1.2mm for the mosfet leads, the preset pots, the 2200uF capacitors, the inductor and the fuse holders, and 3mm or 1/8 inch for the heatsink fixing holes. It is necessary to trim off raised copper edges from the holes if they are drilled from the component side, but in practice I find this easier than drilling from the copper side. The heatsink bracket is 5mm aluminium with length 3.5 inches and width 1.5in and height 2in. The other circuit board shown is for twelve 1000uF 63V capacitors which were used to make a 12000uF supply smoothing capacitor.
The copper side is then cleaned and the circuit tracks marked with an etch resist pen. The one I used is type 'DECON-DALO 33'. Having drilled the holes first it is easy to draw the layout copying from a layout diagram.
Etching with ferric chloride and cleaning off the etch resist leaves the final boards. It is possible to tin the boards using a tin-plating chemical solution, or coat with soldering flux and coat with solder using a soldering iron, but personally I rarely do either. Left uncoated the copper surface will of course oxidise eventually, but this does no serious harm, though the components must of course be soldered in place within a day or two before oxidation makes soldering difficult.
The 12V zener diodes should be added before the mosfets to minimise the risk of excessive gate-source voltage during soldering. These and a few other components are added here. The 7R5 resistors are parallel pairs of 2W 15R, but single 2W resistors should be adequate in practice. The presets should initially be at full anti-clockwise setting so that quiescent current is initially small and output capacitor dc voltage low prior to setting up the correct values. A low value T type fuse, e.g. 250mA should also be used during initial testing, being increased to 2A when correct operation has been checked.
The mosfets are mounted on their heatsink bracket and 3mm bolts used to clamp the mosfet, bracket and board together. An insulation kit is needed, including a TO3-P rectangular mica washer between mosfet and bracket, and an insulating bush on the copper side of the board to prevent the fixing bolt touching or shorting the tracks. The bushes are normally used for insulating bolts from the top of transistors with metal cases or mounting tabs, but the mosfets have a plastic case not needing these, but they are essential for insulating the bolts from the board. Heat conducting paste should be used on both sides of the mica washers. (And later on the mounting bracket and heatsink.)
Note how the mosfet leads have been curved. This is to improve their flexibility to reduce stress from thermal expansion effects.
The rest of the components can then be added:
The copper side if left untinned will slowly oxidise. This does little harm except to the appearance, but can be prevented by the use of a 'PCB-lacquer' spray, e.g. Servisol Plastic Seal, £4.99 from Maplin. It can be seen that the fixing bolts would touch the nearby tracks unless insulating bushes are used. The relatively soft plastic of the bushes also helps prevent damage to the mosfets by overtightening of the bolts.
Further views of the finished board:
The case was chosen for low cost (£12.99) and is described in my Maplin catalogue as having an aluminium top. Their order code is LH42V, type WB7, size width 305mm, depth 159mm, height 133mm. When I recieved one I found that the top and sides are steel, i.e. magnetic, which I wanted to avoid. My original plan was to mount the toroidal transformer on the side of the case, but past experience suggests that transformers are liable to cause vibration when bolted to a steel panel, so I changed the layout to fix the transformer to the aluminium base. Constructors with metal-working ability could no doubt make a more attractive case, but for those like myself without the necessary facilities this case is adequate. Better looking but far more expensive cases are of course available, but I wanted to make this a low cost project.
The 12000uF supply smoothing capacitor is made from twelve 1000uF 63V electrolytics for lower cost, and a small circuit board is used to connect these in parallel.The case is drilled for heatsink mountings, speaker sockets, led indicator light, mains power input lead with suitable cable clamp, fuse holder, signal input cable, transformer and capacitor mounting bolts, single-point earthing bolt, and on/off switch (one suitable for high surge currents, I used the Maplin code FH57M rotary switch, at £1.79, rated at 80A for 10msec.). I needed an amplifier with volume and balance control, so these are included, but if a pre-amp is to be used these of course can be omitted.The heatsink used was rescued from an old computer, and is the type supplied with early pentium processors. The fan was removed and the end tabs had to be sawn off, but there are flat sections at just the right height to bolt on the mounting brackets.
The single point earthing can be seen in the next photos, the three earth leads from each amplifier, the earth leads to the speaker sockets, and the lead to the negative side of the power supply smoothing capacitor are all take to solder tags bolted to the base of the case. Three 3amp rectifiers in series can be seen connecting the bridge rectifier to the capacitor board, this is because the transformer used is 22V-0-22V 120VA, used as a 44V single output, which when rectified and smoothed would give slightly over the 63V rating of the capacitors with no load. Rather than use far more expensive 80V capacitors I used the three rectifiers to drop about 2V. With two boards installed with 100mA quiescent current each the voltage may in fact now be low enough, but to allow for supply line voltage fluctuations the rectifiers have been left in. If a 40V or 42V transformer can be found this could avoid the problem.The second amplifier is fixed above the first so that the inputs are both as far away as possible from the transformer. The quiescent current and output capacitor dc level should be set up on the lower board before fitting the top board.Here the signal input lead can be seen, the screened cables are taken to the volume and balance controls, or direct to the amplifier inputs if these controls are not required.
