Specifications

With a push-pull amplier, the input signal is split into
two halves with opposite polarities with a stage called
a “phase-splitter”. Then each output tube amplies
one half of the signal, and these two “half-signals” are
recombined by the output transformer.
When solid-state came out, they were able (eventually)
to make two devices that were opposite and
(approximately) equal. So a designer could pull the
same trick of splitting the signal into two parts, one
for each output transistor, and then the output of each
output transistor was combined back into a single
output containing the entire signal without the need
for an output transformer (which is big, heavy, and
very expensive). This type of design is referred to as a
“complementary output stage”.
One can also do the same trick with transistors of the
same polarity by adding an extra transistor to invert
the drive signal to one of the output transistors, and
this is called a “quasi-complementary output stage”,
but it doesn’t change anything fundamental about the
situation.
Specically, the input signal on ANY modern power
amplier is split into two parts, sent through an
output stage that has a separate device for each half
of the signal, and then the two half-output signals are
recombined to make one nal output signal. This is true
whether it is tube or transistor, uses transformers or is or
not, is truly complementary or quasi-complementary.
A conventional output stage always has *something* in
between the two inputs of the two half-output stages
before they are recombined. Typically it is a bias circuit
of some sort that sets the idling current in the output
devices, but there is *always* something there.
The diamond circuit is the only commonly output stage
in use that feeds the two half-output-stages from the
same exact point. In contrary, here is a typical two-
transistor NON-diamond output stage. (Two transistors
for each half of the signal, for a total of four transistors)
- see diagram 4.
As you can see, there is *something* between the two half
signals in the traditional output stage. Invariably it is a bias
circuit that sets the amount of output current in the output
transistors. And so the two halves of the signal going
through the two halves of the output stage are slightly
dierent somehow and when the recombine to recreate
the entire signal out the output of the output stage, it
simply doesn’t sound as good as the Diamond circuit.
In a way, this entire argument makes no sense, because
if you look at the simplied Diamond circuit in the
second gure (near the top) again, there is no connection
whatsoever between to two halves of the signal at the
half-way point through the output stage. That is, at the
input, the signals are identical as they are hard-wired
together, and at the output the signals are identical as
they are hard-wired together. But *in-between* the two
transistors in series, there is absolutely no connection
between the top half of the circuit and the bottom half of
the circuit.
“ But that is the beauty, the mystery, and the
art of audio circuit design. If we knew all of
the answers to everything, then everything
would sound identical and everything would
sound perfect. There is still clearly more
awaiting discovery here. “
The only other power amplier that I have ever seen
that uses the Diamond buer for its output stage is
the original DartZeel amplier. But it works in a very
dierent way than does that Ayre AX-5. Remember how
we needed to have *three*stages in the output stage
to properly isolate the loudspeaker from the front-end
circuit, whereas the Diamond circuit has only two?
In the DartZeel, they solve this problem by giving some
extra “heft” to the front end of the amplier circuit by
using three transistors with a feedback loop around
them. This makes them “strong” enough to handle the
load presented by only using two transistors in series in
each half of the output stage.
t: 01727 865488 e: [e-mail] w: www.symmetry-systems.co.uk
Symmetry, Suite 5, 17 Holywell Hill, St Albans, Hertfordshire AL1 1DT
The AX-5 Story
from Ayre Acoustics March 2014
Page 9