First, about the
idea behind a load switch. Vacuum tube amplifiers
(those that utilize the output transformer) have
secondaries with taps to accommodate different
speaker loads, and this feature ensures the
optimal functioning of the amplifier with various
speakers. Solid-state amplifiers are a different
story (meaning solid-state amps with no output or
matching transformers). It is commonly thought
that solid-state amp can be loaded with loads of
various impedances; therefore, the lowest value
of load impedance that can be connected to an amp
is taken into account during the circuitry design
stage of that particular amp. This means ONLY
that the amp will work with this impedance
without malfunctioning. In general, when very low
or relatively low (about 2-3 Ohms or less) load
impedances are connected to an amp, the quality
of sound becomes questionable. There are reasons
for that, and the main ones are concerned with
the correct choice of a Q-point to ensure at
least some resemblance of constancy of the
harmonic structure of (1) signal versus
frequency, (2) signal versus output power, and
(3) signal versus load.
As a rule, modern
design philosophies are quite removed from the
correct fulfillment of these three variables.
From our point of view, one of the ideal
solutions for these problems when solid-state
amps are concerned are: PURE class A operation of
an amp (without any tricks), and PURE class A
operation not only at the MAXIMUM load, but also
at the LOWER load impedances. Standard situation,
even if an amp operates in pure class A at 8
Ohms, is that it doubles the power at 4 Ohms
resulting in 1/4 of the doubled power in class A
and the rest in class AB. We, at LAMM, chose to
do the following to handle this issue. We
tentatively divided an entire impedance range
into two: from 16 to 8/7 Ohms and from 7/6 to 1
Ohm and below. As a result, we implemented a
special load switch in our hybrid amplifiers.
When the switch position is changed, it changes
the supply voltage and idle current of the output
stage, which allows the amplifier to operate in
PURE class A mode at the maximum and lower load
impedances. The M1.1 is a 100W monoblock
operating in pure class A at 8 AND 4 Ohms (the
M1.2 Reference is a 110W monoblock operating in pure
class A at 8 AND 4 Ohms).
This separation
into two impedance ranges is not cut in stone. If
speaker has an impedance of 6 Ohms, the M1.1/M1.2
Reference can
have the load switch at either 8 or 4 Ohms
position. In this situation, listening and
experimenting are necessary to figure out which
combination sounds better. Also, the room acoustics have
to be taken into account. Our equipment is
designed with a wide "safety" margin,
so that if the amplifier is set for an 8-Ohm
operation, it can be loaded it with 8, 4, 2, and
even 1 Ohm. In this case, only the output power
will increase. However, we don't recommend to do
this as harmonic structure of the sound will be
changed enough to become audibly noticeable.
However, this kind of experiments is very
educational in revealing the differences in sound
between the M1.1/M1.2 Reference with properly selected load
switch and the M1.1/M1.2 Reference with improperly selected load
switch (which in such situations operates and
sounds similar to conventional amps). Similar
tests had been made by Ken Kessler and described
in his review (HI-FI NEWS & RECORD REVIEW,
January'96, pp. 38-39). He came to a singular
conclusion about which speakers sound the best
with which load switch position on the M1.1. Our
amplifiers were designed by the purist for the
purists, with attention to every little detail.
M1.2
Reference
