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20W Class-A Amplifier Module

This new 20W class-A is a refinement of our very popular 15W class-A module published in July & August 1998. It features ultra-low distortion levels, very low noise levels and a greatly simplified power supply which improves overall efficiency. Since it runs in pure class-A mode, there is no crossover distortion at all.

Pt.1: By Leo Simpson & Peter Smith

his upgraded class-A amplifier has been a long time coming. Virtually since the original circuit was published in July 1998, readers have been hankering for more power. Until recently, we have resisted because we knew that increasing the power output would bring a proportional increase in overall power consumption which was already quite high.

This is the great drawback of any class-A design. While they are beautifully distortion-free, they dissipate the same high power whether they are delivering a milliwatt, one watt or full power. And the total power consumption, and therefore heat dissipation, of the previous 15W/Channel Class-A Stereo Amplifier was 100 watts. That’s quite a lot of power dissipation for not very much audio output.

So how could we increase the power output while staying within the original parameters – ie, the original large single-sided heatsinks and the 160VA toroidal power transformer? The answer was not simple but essentially involved analysing the weaknesses of the original design to see if we could make worthwhile improvements.

In the result, we have made quite a few minor improvements to the original amplifier module. Together, these added up to an overall major improvement which enabled us to dispense with the regulated power supply. This makes the overall circuit more efficient and means that the amplifier can now use some of the power previously wasted in the regulated supply. That also reduces component cost and actually helps reduce distortion in an already exceptional design.

Some of the changes in the design are based on ideas and circuits published by the noted audio designer Douglas Self and outlined in a number of his books (available from the SILICON CHIP Bookshop).

All in the same case

The 15W/Channel Stereo Class-A amplifier presented in August 1998 also featured a separate power supply box because hum radiation from the power transformer was quite high. This new design will feature a shielded toroidal transformer which means that there is no need for a separate box. We will talk more about this aspect in a future article.

Redesigned PC board

We have completely re-designed the PC board so that the two power output transistors are spread much further apart. Instead of concentrating the heat in the centre of the heatsink, it spreads the heat over a wider area and makes more efficient use of the available heatsink area. In fact, while the new amplifier module can deliver up to 25W (instead of the original 15W), the heatsink temperature remains about the same as the original design; ie, about 30°C above ambient.

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Fig.1: this graph plots the total harmonic distortion (THD) at 1kHz from 100mW to just over 25W.
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Fig.2: the distortion versus frequency at 10W & 20W into an 8-ohm load (measurement bandwidth 22Hz to 80kHz).
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Fig.3: distortion vs frequency at 10W from 20Hz to 20kHz (measurement bandwidth 22Hz to 22kHz).
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Fig.4: the frequency response is ruler flat over the audible frequency range, with -3dB points at 1.5Hz and 190kHz.

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