Power Supply
| WARNING: Mains wiring must be performed by a qualified electrician - Do not attempt the power supply unless suitably qualified. Faulty or incorrect mains wiring may result in death or serious injury. |
The basic power supply is shown in Figure 2. It is completely
conventional in all respects. Use a 40-0-40 V transformer, rated at 300VA for normal
use. For maximum continuous power, a 500VA or bigger transformer will be
needed. This will give a continuous power of about 350W, and peak power of close to
400W is possible with a good transformer. Remember my warnings about using the amp
in this way, and the need for the additional output transistors.
Figure 2 - Basic Power Supply Circuit
C1 must be rated for 240V AC (or 120V AC) operation - do not use standard 250V DC caps under any circumstance, as they will fail, and R1 will explode! This is not intended as humour - this is fact! C1 and R1 may be omitted in most cases, and if you cannot get a mains rated capacitor I suggest that you don't install these components.
The supply voltage can be expected to be higher than that quoted at no load, and less at full load. This is entirely normal, and is due to the regulation of the transformer. In some cases, it will not be possible to obtain the rated power if the transformer is not adequately rated.
Bridge rectifiers should be 35A types, and filter capacitors must be rated at a minimum of 63V. Wiring needs to be heavy gauge, and the DC must be taken from the capacitors - not from the bridge rectifier.
Although shown with 4,700uF filter capacitors, larger ones may be used. Anything beyond 10,000uF is too expensive, and will not improve performance to any worthwhile degree. Probably the best is to use two 4,700uF caps per side (four in all). This will actually work better than a single 10,000uF device, and will be cheaper as well.
NOTE: It is essential that fuses are used for the power supply. While they will not stop the amp from failing (no fuse ever does), they will prevent catastrophic damage that would result from not protecting the circuit from over-current conditions. Fuses can be mounted in fuseholders or can be inline types. The latter are preferred, as the supply leads can be kept as short as possible. Access from outside the chassis is not needed - if the fuses blow, the amplifier is almost certainly damaged.
What Does It Look Like?
I have included a photo of the prototype amp, fully mounted on its
heatsink. For normal use, some brackets would also be needed to mount the heatsink,
unless two assemblies were used as the side panels of a conventional (stereo) amplifier
chassis.
Figure 3 - Completed Amp Module
The driver and main heatsinks shown are adequate for up to 200W into 4 ohms with normal program material. The bits of wire you can see were used for hooking the amp up to a test supply for the measurements and listening tests. The power transistors are all mounted underneath the board, and the mounting screw heads can be seen on the top of the board.
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