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.
Performance of Prototype
Output Power (RMS):... 140W into 8 ohms; 200W into 4 ohms
Frequency Response:. 20Hz - 80kHz at -1dB points (see Fig.1)
Input Sensitivity: ............ 830mV for 200W into 4 ohms
Harmonic Distortion:... <0.1% (20Hz - 20kHz) (see Fig.2)
Signal-to-Noise Ratio:. >102dB unweighted; 105dB A-weighted
with respect to 200W into 4 ohms
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
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
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.