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sequential amplifier stages are used. Most modern
high performance amplifiers use as many as seven
cascaded stages. In addition the input signal does
not only have to pass through several serial amplifier
stages but most of the amplifiers use several
power transistors in parallel to achieve a higher
power output. In some “monster” amplifier designs
up to 20 power transistors are switched in parallel.
Serial and parallel signal processing have two
major disadvantages. First of all any transistor –
even highest-grade versions produce distortions.
The more stages are used, the higher the distortions
of the circuitry become. And theses “distortions”
do not only add to each other, but multiply together
as well. This is because every stage amplifies the
distortions of the previous stage and modulates its
own distortions on top of it!
The second problem refers to the speed in which
the feedback readjusts the problems at the output.
The more stages an amplifier has, the slower the
“forward” processing of the signal is, and the bigger
the lateral off-set between the input and the
output of the amplifier becomes. In practice this is
called “slow forward amplification”. In the laboratory,
distortion occurring during the amplification
of the signal can be eliminated with a high feedback,
but these corrections only become active due
to the slow forward amplification with a subsequent
time delay. This is to say that the corrections do
happen with inertia and thus probably at a moment
in which the original distortion has already disappeared.
The feedback thus generates an inevitable
new distortion, which is again amplified by the
forward processing and is again corrected by the
feedback etc. etc. The delay between the start and
the elimination of the problem permanently generates
new distortions. The amplifier gets into a stage of
permanent oscillation and has more work fixing
itself generated problems than with processing the
original signal. Fortunately these self-produced
distortions are mostly of linear nature. This is to say
that the amplifier circuit corrects the same distortion
all over again, regardless of the music signal. The
described time delay when correcting these self
produced linear distortions, in practical operation
is, subjectively not so critical.
Contrary to above, this problem is much more serious
with a speaker connected, e.g. a varying
complex load. Any speaker system is an unknown,
variable disturbance which is continously inducing
interferences into the amplifier. These electro-magnetic-forces
are induced by electrical currents of the
speaker drivers and these are fed back to the amplifier
as variable error voltages. The feedback detects
these error voltages and tries to eliminate
them. As these electro-magnetic-forces permanently
change with frequency and level of the music, the
correction occurs with a time delay, so the correction
will logically always be too late. For an experienced
listener it is relatively easy to distinguish between
the relatively harmless self-generated amplifier
distortions and feedback distortions induced by the
speaker. Mostly in operation this problem shows up
as a “slow”, sometimes “hard” midrange/treble
response and a reduced resolution of the system.
Furthermore the delay of the feedback will cause a
permanent shifting of the phase of the signal, which
shows up as harmonic distortions. This results in a
“specific” tonality of the amplifier with reduced
sound staging and naturalness of the sound.
THE AVANTGARDE PHILOSOPHY
Avantgarde Acoustic loudspeakers are known to
react very quickly to any variation of the music
signal and to transform these into audible sound.
“With our speakers we use the horn technology to
make the forward acceleration and transient response
as fast as possible. What goes in, should
come out,” describes Matthias Ruff. Different to
most manufacturers who optimise the decay time of
their speakers (waterfall diagram) the company
from Lautertal regard the fast forward signal processing
as the main advantage of their speaker
designs. “Who cares about the second distortion, if
the first distortion has not even been detected. Most
speakers are purposely made slow and inert, only
to make the waterfall graphs look good in reviews.
It is so obvious: the less I let pass through from the
front, the better it looks from behind. But this doesn’t
mean that you have a good speaker. It is like reducing
the maximum speed of car drastically only to
improve the breaking distance,” explains the
Avantgarde designer furiously. And with multi-stage
amplifiers having high forward gain and strong
feedback Matthias Ruff is seeing the same problems.
“In the HGHF-principle you purposely put
up with errors during the forward amplification,
because later at the end you can flatten everything
with a high feedback”. In other words these designers
do not even try to prevent distortion in the
early gain stages, but count on heavy reprocessing
of the signal at the end. “If I already falsify the signal
at the front end, I can later correct as much as I
want, the information simply gets irretrievably lost”.