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