Specifications

To address the rst problem means that the amplier
needs to be larger and heavier, with a more powerful
power supply. To address the second problem means
that one needs to use a more powerful driving source
than the typical amplier uses.
The large majority of transistor power ampliers use a
“front end” circuit that provides the voltage gain, and
then an “output stage” with no voltage gain, but that
provides the current gain required to drive the low
impedance of the loudspeakers. (One version of the
Wilson WATT/Puppy dipped to around 0.3 ohms in the
upper midrange. While this is unusual, it is important
that a high-quality power amp won’t be bothered by
loads below 2 ohms or even 1 ohm. Most electrostatic
loudspeakers drop below 1 ohm at frequencies above 20
kHz.)
The typical solid-state power amplier only uses two
transistors signal path of the output stage. (There may
be more in parallel to provide more current *handling*
capability, but this does NOT increase the current gain.)
A really good output transistor will have a current gain of
around 100x, and the same is true of a driver transistor.
This means that a two-transistor output stage will have
a current gain of 100 x 100 = 10,000x. So if you connect a
4 ohm loudspeaker, the load on the “front end” (voltage
gain stage) will be 40,000 ohms.
While this sounds like a lot, it really isn’t. It is a low
enough load that it puts a strain on the front end and
increases the distortion signicantly. The solution
that 99.9% of all designer use is to add loop negative
feedback. Works great on paper, but doesn’t sound so
great.
“ At Ayre, we have always used *three*
transistors in series in our output stage. “
So if the third transistor also has a current gain of
100x, now the total current gain is 100 x 100 x 100 =
1,000,000x. Now if you connect the 4 ohm speaker, it
only puts a load of 4 megohms on the front end and
the distortion drops dramatically without having to use
negative feedback. These extra transistors only add a
dollar or so to the Bill of Material (BOM) so unless we
are talking ultra-cheap junk audio equipment, this is
completely negligible.
" Back to the Diamond circuit. If it has these
disadvantages, why in the world would
anyone use it? The short answer is it sounds
better. "
The long answer is that I really don’t know why, which
doesn’t help much as that’s not much of a story to tell,
and writers like to tell stories and people like to listen to
stories. At this point the best that I can do is to speculate.
So here goes nothing....
Before transistors came along (in the vacuum tube
era) there were two types of power amplier circuits
in common use. One was the single-ended circuit and
the other was the push-pull circuit. In the single-ended
circuit, there is one tube (or possibly more, but they are
simply connected in parallel to act as one larger tube),
and this tube must be “on” all the time or the signal will
be horribly distorted with clipping on either the top or
the bottom (depending on the particulars of the design.
This limits the power to a few watts (“ea-power”) unless
the designer resorts to huge power tubes originally
designed for radio station transmitting tubes. Even then
it is hard to get more than 20 (or *maybe* 25) watts out
of the amp and it draws a ton of juice from the wall,
which is pumped out as heat in your listening room.
So when the push-pull output stage was invented, it
allowed for MUCH more powerful ampliers to be built
very easily. Instead of a single 6L6 tube putting out
perhaps 5 watts, a pair in push-pull could easily put
out 30 watts. And when they developed bigger tubes
like the KT-88 (6550 in the US), instead of a single tube
putting out maybe 8 watts, a pair could put out 60 watts!
This was just in time for stereo, when all of a sudden
one needed TWO speakers and the small “acoustic
suspension” designs took over the market place. They
needed all that power, as they were so inecient.
t: 01727 865488 e: [e-mail] w: www.symmetry-systems.co.uk
Symmetry, Suite 5, 17 Holywell Hill, St Albans, Hertfordshire AL1 1DT
Page 8
The AX-5 Story
from Ayre Acoustics March 2014