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stage we managed to eliminate, we reduced the
distortions of the transistors and increased the linear
bandwidth of the circuitry,” explains Matthias
Ruff. The MODEL5 achieves bandwidth of more
than 220 kHz. This is practically four octaves above
the frequency response of a CD. Even the high
resolutions of modern recording technologies like
SACD and Super AUDIO DVD are handled with
ease. The MODEL5 is so fast that even highest frequencies
above the audio spectrum are amplified
linearly. “The faster the forward signal is processed,
the higher the frequencies of the feedback distortions
occur. With the MODEL5 these problems are practically
immeasurable and far beyond the audio
range,” describes Matthias Ruff.
LOCAL LINEARISATION
Another disadvantage of an OVERALL LOOP
FEEDBACK is that it is not the cause of a problem
but only the result of a problem when corrected. A
distortion generated in the second stage of the amplifier
is not being eliminated at the point of origin,
but only at the end as the sum of all stages. Not the
actual reason but only the symptoms are being fixed.
Matthias Ruff gives us a simple example which
explains this correlation more easily: “Just imagine
you put on some shoes which are too small. After a
while you get blisters on your feet. A lotion and
band-aid will now alleviate the symptoms but not
the cause! If you have a better and faster feedback
from your feet, you would have immediately put on
some bigger shoes. The problem would have been
solved and the blisters would not have developed.”
MINIMAL Single Stage FEEDBACK
Nevertheless Avantgarde Acoustic did not want to
completely abandon the effective OVERALL LOOP
FEEDBACK. Matthias Ruff has a clear point of view
in this matter: “People normally only see black or
white. Mostly the feedback applied is completely
‘over the top’ or left out totally as in ZERO FEED-
BACK designs. Both solutions have many disadvantages.
For me the happy medium is the best solution”.
The percentage of distortions generated by the
MODEL5 itself are inherently small in comparison
to normal amplifiers, as it only has two transistors
in the signal path, which are even linearised independently
by special circuits. For this reason the
OVERALL LOOP FEEDBACK can be applied very
carefully. According to the manufacturer only the
large distortions of the circuitry are equalized.
“I purposely do without strong feedback only to get
good specifications. If you take into consideration
that normal speakers produce higher distortions
(by approximately 10%) than amplifiers, you can
see that amplifier distortions are highly overrated”.
The designer emphasizes that he has chosen maximum
resolution in practical operations instead of
optimising for negligible technical specifications.
The main objective is to make the amplifier as fast
as possible so that even the finest details of music
are reproduced accurately.
This example makes the biggest disadvantage of a
high OVERALL LOOP FEEDBACK in multi stage
amplifier designs more obvious: a mistake occurs,
after some time it develops into a problem and only
than is cured. “The most important thing is to detect
the defect immediately at its point of origin. If you
than know what’s going wrong, you have to treat
the cause precisely and the treatment must solve the
problem quickly”. According to Matthias Ruff the
key to success is a local linearisation concept. For
this reason both amplification stages of the MODEL5
are linearised separately. The BI-POLAR driver stage
as well as the MOS-FET power transistors have
their own linearisation circuitry. Each deviation is
detected at the point of origin and immediately
linearised, independent of the adjacent stage. “We
are working with automatically self-smoothing subsystems
which autonomously do their work and
efficiently reduce distortions”.
But how does the MODEL5 with its relatively low
feedback behave on complex loads? “The more
amplification stages one is using, the slower the
circuit is and the more negative the EMF forces