We have had quite a few enquiries from readers who want to build a Mosfet amplifier with a rating of about 200W.

We had not designed such a module and as has been mentioned previously in SILICON CHIP magazine, our preference has been to design high performance amplifier circuits around bipolar transistors rather than Mosfets.

However, many people prefer Mosfets because of their legendary ruggedness. Altronics had a Mosfet amplifier module which produced 200W into a 4Ω load and so we decided to take a look it.

It turned out to be based on the "Pro Series One" as mentioned above, although this version by Altronics has been derated and adapted to different Mosfets. It has a rated power output of 140W into 8W and 200W into 4Ω. Frequency response is within 1dB from 20Hz to 80kHz (Fig.1). Total harmonic distortion is rated at less than 0.1% up to full power (Fig.2) and signal-to-noise ratio with respect to 200W is better than 100dB unweighted.

Originally, the "Pro Series One" was based on Hitachi Mosfets with TO-3 metal cases. These are no longer available and their plastic equivalents are quite difficult to obtain as well. Altronics looked at this situation and have used essentially the same circuit designed around some equivalent plastic Mosfets made by Exicon of the UK. This has necessitated a re-design of the PC board so that all the Mosfets and the driver stage transistors all line up along one edge, allowing them to be mounted vertically on the heat-sink.

Apart from using plastic power transistors which greatly simplify mounting compared to metal TO-3 power transistors, Altronics have employed spring clips to mount adjacent transistor pairs, to make things simpler again. The spring clips apply just the right amount of tension to the transistors and there is no danger of damaging a transistor due to over-tightening a mounting screw.

The heatsink is a black-anodised aluminium extrusion with fins on one side. It measures 300mm long and is fitted with a cover which allows it to be cooled by an 80mm 24V DC fan. The fan runs continuously and this means that the heatsink is always cool (or at least, at little above ambient temperature).

Fig.1: as measured in the SILICON CHIP laboratory, frequency response is a very wide 20Hz-80kHz (-1dB).

A really attractive feature of the module is that it comes with its own power supply, consisting of a 300VA toroidal power transformer and accompanying power supply board. The bridge rectifier is mounted on the same vertical heatsink as the amplifier and so it also gets the benefit of continuous fan cooling.

The whole assembly is mounted on a sheet of black enamelled steel measuring 300 x 214mm. This could be built into a larger chassis for a PA system, stereo system, active sub-woofer or whatever.