| The A40 brought a Class A design to audio equipment enthusiasts in 1978. It featured a
direct-coupled front end with doubly regulated constant current sources, which drove a
high current pure Class A push/pull output stage self-biased by a current sensing circuit.
The amplifier sounded good and didn't break. The circuit was simple and required no
adjustment (Fig. 1) In the past 14
years I have heard from over 500 readers who have built the A40. They have called, sent
letters, photos and, in a couple of cases, even sent the amplifiers. My favorite was the
gentleman in Alaska who built it on a cake pan. You might laugh to think of it, but I
assure you it was beautifully done, and he sent it to me to diagnose what turned out to be
a shorted compensation capacitor. (Norm was another person who actually sent the
amplifier).
The letters continue to arrive even at this late date, and the most common request is
for more power. Certainly this project will deliver more wattage, but I have never seen
power as a significant criterion for quality. Indeed, it often seems that power is a
substitute for quality. People who are dissatisfied with the sound of their system will
first keep turning up the volume, and then ultimately shut off the system.
My research over the past 20 years has led me to conclude that quality is not to be
achieved through close attention to bench measurements. An almost inverse correlation
exists, in which tube amplifiers sound good but have terrible specs: high distortion, poor
bandwidth, poor damping factor, and no slew rate. Solid-state design can have every spec
nailed down to what you would consider totally neutral performance, and yet be completely
un-satisfying. That is not to say that you can't build poor-sounding tube amplifiers, or
good-sounding solid state products.
The right brain/left brain theory applies here. You use one side of your brain for
objective logical kinds of processing, and the other for subjective and emotional. When
you are enjoying music, the subjective half is in play, and its process does not relate to
the specs very well.
The same phenomenon explains why A/B comparisons, no matter how well staged, do not
resolve what audiophiles claim to hear. When you are sitting there trying to hear the
differences between products, the objective side of your brain begins working and your
subjective responses get locked out by the pressure to make an objective decision.
For most audiophiles, it is only in the comfort of their home system with a favorite
piece of music land maybe a beer} that they can tell the difference between amplifiers. In
this context, the quality of the equipment is judged only by the response it elicits from
the listener, and power, frequency response, THD, TIM, damping, and slew rate are only
unneeded baggage.
If an amplifier, or any other product, has really great performance specs, you can bet
it was designed and built with one eye on the meter, and the numbers were a primary
criteria of performance. This means decisions were made based on measurements, and
typically the designer proceeds to hear what he measures. I have seen it again and again.
This amplifier was not designed with one eye on the meter; it was designed with the
belief that the simplest topology operated in the most linear fashion would yield the best
sound. This amplifier was not designed to please the meters: it was designed to please
you, the listener. |
