6
X
s
Owner’s Manual
so at a lower current level. This improved the performance at low
power levels, but allowed the higher dynamics afforded by balanced
push-pull operation at higher power levels.
This approach was so successful that we naturally looked to a greatly
expanded version for the Xs amplifiers. Initially we created a large
two chassis format that allowed more hardware for a larger power
supply and greatly expanded output stages and heat sinks. The four
massive banks of output devices used in the X and XA series became
six massive banks, with the bias current provided by the constant
current sources increased by a factor of ten. This expanded the
single-ended power range by a factor of 100. Of course this created
a need for more and better power supply, so we created a supply
with larger power transformer, paralleled fast/soft recover rectifiers,
and double the storage capacitance. The separate power supply also
holds the huge constant current sources used to bias the output
stage of the amplifier. It dissipates approximately half the energy
and contributes further to the performance by isolating the radiated
power supply noise from the main amplifier. The extra heat sinking
is very convenient, as the amplifier dissipates about three times its
output power rating.
It is not actually that difficult to gather the massive hardware and
fifteen kilowatts of output transistors. What is difficult is carefully
shaping the relationships between six banks of parallel transistors
so as to create the exact transfer curve that delivers just the right
harmonic structure, giving a palpable sense of life and realism to
music.
The question is inevitably asked, “Is this
pure
Class A, and where
does it leave Class A”. I expect such questions insofar as I have
probably been the biggest contributor to that sort of discussion.
Let me first say that this consideration is only in service to the sound
– if it improves the experience of the sound, then it is a virtue,
otherwise it has little value.
To answer the question, first we note that the bias is achieved solely
by constant bias values and there are no tricks. The topology of
each half of the balanced output stage is that of a push-pull follower
which is biased into single-ended Class A by a constant current
source placed in parallel. The resulting efficiency is about 33%.
For a balanced version, we see that four out of the six output banks
remain in Class A to the output power rating into 8 ohms, and the
remaining two output banks are there to supply additional current
when necessary.