At present, MOSFETs are 'the business' in the world of the power amp. They eliminate a large portion of the problems inherent with previous bi-polar designs, and offer an excellent frequency response, fast slew rates (time taken to reproduce transient peaks), and are exceptionally reliable.

To be more specific, FETs (the species from which MOSFETs derive) are voltage amplifiers, as opposed to bi-polar amps which are current amplifiers; and although the MOSFETs are FETs, they are not true voltage amplifiers, and are built along lines that are quite similar to valves.

Thus, MOSFETs are less liable to bi-polar distortion, and are characteristically brighter and crisper sounding than bi-polar amps. They are also self-correcting — they conduct less as they get hotter, and as long as the power amp is designed to compensate for the slight power fluctuation, this then results in the virtual elimination of secondary breakdown (blowing themselves up).

Studiomaster have a new range of MOSFET powered amplifiers called the 'Stellar' range, and from this range I was able to have a look at the 100W per channel Stellar FET-1.

This is a small and attractive looking power amp which can either be free standing or 19" rackmounting. Its actual dimensions are a mere 10"x8"x3.5", and for rackmounting purposes a 2U 19" plate can be fixed onto its front panel.

On the front panel itself there are comprehensive LED indicators for different functions. In addition to the power on/off indicator, there are very useful clipping and 10% output level indicators for each channel, a thermal overload warning indicator, a mono and a bridged mode indicator. On its rear panel is the switching for Stereo/Mono or Bridged/Normal operation, and there are a choice of bare wire, ¼" jack or 4mm Banana plug output connections, whilst input connection is via two ¼" jacks.

In Use

Power amplifiers are arguably the most subjective pieces of equipment in the audio chain, as specifications vary tremendously from manufacturer to manufacturer. An amp's RMS power rating is usually a decent guide to the sound pressure levels you should expect to experience. However, the sensitivity of the speakers themselves do have quite a substantial effect on the SPL.

It is always a good indication, however, to see how wide the parameters that manufacturers allow themselves for measuring an amp's specs are. For example, an amp with a very wide frequency response within ±3dB is less likely to be as good as an amp with perhaps a slightly smaller frequency response within the limits of ±1/2dB. This is simply because a manufacturer who quotes specs within the tighter limits of ±1/2dB would probably be taking more care over the small details while, of course, having a very flat response throughout the frequency spectrum.

In the case of the Stellar range, their specifications are measured within a tight +0-1 dB limit, and have a very good frequency response from 5Hz to 280kHz!

First impressions of the FET-1 in comparison to a Quad 405 and an HH 500 MOSFET power amp were, initially, that it tended towards having a slightly 'rounded' tone whilst maintaining the clarity which is inherent in MOSFET powered amps. It is quite capable of reproducing at high levels the dynamic ranges one gets in a studio environment without any trouble, though at first it was noted that the clipping indicators flashed perhaps too readily. This did not prove to be a problem, however, as the occasional illumination was ignored since they only light up prior to the onset of clipping.

The FET-1 will give a solid 100W into either four or eight ohms in the normal mode. However when the channels are bridged, it will give 200W into either 16 or eight ohms (8 ohms being the bridge mode's minimum load).

At full output, hum and noise levels are better than 95dB; and total harmonic distortion is a mere 0.005%. It is arguable, however, whether THD figures of less than 0.05% have any audible relevance, even though it does indicate that the manufacturers are on their proverbial toes...

The Stellar FET-1 also has very thorough circuit protection with Open/Short circuit, thermal overload, DC detection on outputs as well as AC/DC fuses.

Conclusion

Much thought has gone into the design of this power amp. It is stylish, good sounding, and suitable for a wide variety of applications — from domestic PA to studio monitoring.

The other power amps in the range are the 200W FET-2 (£508.30), and the 400W FET-4 (£704.95) — which, if the FET-1 is anything to go by, might be well worth checking out if you are after higher wattage.

The Stellar FET-1 has some useful features which will certainly make it quite a popular amp — comprehensive switching and circuit protection, a selection of output connectors, and not least its small size. Although it will be up against some stiff competition in the 100W/£400 bracket, the FET-1 should be able to make its mark.

For: Small size, sound quality, LED indicators.
Against: Price

STUDIOMASTER STELLAR FET-1 MOSFET POWER AMP - RRP: £382.95