Back in November 2006 we asked the question: Do people really want a high-performance valve amplifier? Significantly, we are still getting the occasional email from people expressing their interest or otherwise. And while the consensus of the letters seems to be that people are interested in valve amplifiers, we are of the strong impression that comparatively few people would actually build the then proposed 60W/channel stereo amplifier with negative feedback.

Interestingly, we expected a number of letters expressing dissatisfaction with the proposed concept but that was not the case. A number of people also suggested an updated version of the output transformerless amplifiers produced by Cyril Murray and other designers in the late 1960s. These were designed to drive special high-impedance loudspeakers and if we were to update them, we would actually need to use an output-coupling transformer to drive conventional low-impedance loudspeakers, so there would not necessarily be an advantage to that approach.

Regardless of what approach we actually took, our previous experience leads us to expect that few people would build the final design when they were confronted with the high price and the amount of the work required to assemble the amplifier. So we decided to take the alternative approach and upgrade our very popular 15W class-A amplifier which was featured back in 1998.

We approached this with some trepidation because we well remember the problems we had with the power supply and the lengths we had to go to obtain satisfactory low residual noise. The problems were based on the heavy fixed current drain from the power supply and the relatively large hum field produced by the toroidal transformer because of that high current.

However, I am delighted to report that our efforts have borne fruit. As a result of some very careful analysis of the 1998 design by Peter Smith and drawing on some circuit techniques advocated by noted audio designer Douglas Self, we have been able to get some very significant improvements, as well as a little more power, while keeping overall power dissipation to levels similar to the old design. This has been achieved by quite a number of minor improvements to the circuit, a new PC board design and a shielded toroidal power transformer which has allowed us to eliminate the earlier design’s regulated power supply which was housed in a separate box.

This month we present the first in a number of articles on the new amplifier module and a subsequent stereo class-A amplifier delivering 20 watts per channel. Ultimately, it will give far better sound quality than any valve amplifier and at a far lower price. Nor will its audio performance be degraded over a relatively short time by component ageing, as happens inevitably in any valve amplifier.