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MODEL5 VOLLVERSTÄRKER

MODEL5 INTEGRATED AMPLIFIER
MAXIMUM PERFORMANCE FOR THE
MONEY
Avantgarde Acoustic, the highly respected manufacturer
of exclusive spherical horn systems have
ventured into the lion’s den and now offers for the
first time a high end integrated amplifier within
their product portfolio. A speaker specialist designing
and manufacturing their own electronics? A
closer look this new project shows a well thoughtout,
coherent and most of all customer friendly
concept.
Horn loudspeakers are not only characterised by
their extreme sensitivity and dynamics, but sometimes
behave with all the agility of a flowering
mimosa. It is true that due to their high impedance,
the horn systems from Lautertal in Germany are
very simple to drive. They simply represent an easy
load for any amplifier, but their incredible resolution
of fine detail quickly reveals the limits of many
highly praised amplification. It is like in real life: for
a good sound you have to combine what is a good
match. An amplifier, which sounds great on a power
demanding speaker system, will not necessarily be
a good match with a horn. Like a big Diesel truck
engine might be ideal to pull a 30-ton load, but in
a racing car, it would certainly be the wrong choice.
The classical virtues of any amplifier “all things to
all men” attach great importance to – output power,
Based on these considerations Avantgarde Acoustic
has invested in a significant research and development
program to realise a unique amplifier concept.
The MODEL5 is a special integrated amplifier,
which ideally matches their ultra dynamic speaker
systems. With surprisingly simple means, the wellknown
virtues of their horns – dynamics, resolution
of fine detail and coherency are emphasized and
brought to life. In this respect the horn specialist
from Lautertal have made a courageous and most
of all consequential step towards further perfecting
their sound systems. Courageous because no other
electronic manufacturer wanted to think about
what’s really needed with the sensitive and revealing
Avantgarde Hornspeakers. Consequential
because a new benchmark in performance – and
in affordability – is now available. Now the music
lover can buy a perfectly balanced amplifier for
their horn system – no “TRIAL & ERROR” when
searching for the perfect sound combination, but
simply “PLUG & PLAY”. “I admit it. We have purposely
played the trump card of our high impedance
and high sensitivity horns, because in combination
with the MODEL5 we managed to achieve a
sound quality at a price which is simply unbeatable
when compared to normal speakers and amplifiers”
states a very pleased Holger Fromme.
In the following pages the special technical features
of the Avantgarde Acoustic MODEL5 amplifier are
explained.
load stability, damping factor etc are of no circumstantial
significance for a horn speaker system. With
a horn, quality is simply more important than
quantity. “I have never excepted that horn customers
have to spend a lot of money for features, they
actually do not need. It was our clear goal to concentrate
on the essentials and design a simple but
perfect amplifier for high sensitivity speakers. No
unnecessary ballast like a big power supply or
oversized transformers and numerous transistors.
Maximum performance for the money!“ explains
Holger Fromme, managing director of Avantgarde
Acoustic.
CLASS A MODE
The MODEL5 is an integrated transistor push pull
amplifier with a power output of 2 x 0.4 watts at
8 ohm in pure CLASS A mode. At 20 ohms 1.1 watts
is available in CLASS A before the amplifier converts
to CLASS AB mode. Thus Avantgarde Acoustic
consequently continiuous its high impedance technology
with all its advantages first introduced in the
TRIO OMEGA model.
By increasing the bias current the CLASS A mode
eliminates distortions when the signal passes the
null point of the signal curve in push-pull ampli-

fiers. The high sensitivity of the Avantgarde Acoustic
horn systems ensure even at this low power output,
that up to 103 dB with the DUO and up to
110 dB with the TRIO the amp works in this optimal
CLASS A mode. Above this power the MODEL5
amplifier operates in CLASS AB mode. The output
power now increases to 2 x 27 watts at 8 ohms,
and in combination with the UNO, DUO and TRIO
this is enough power to really get “rolling”. If the
DUO is connected to the MODEL5 this will translate
into an incredible 118 dB sound pressure level.
This clearly shows that the combination, even when
connected to a small power amplifier is playing in
another league.
Small power and CLASS A mode spontaneously
gives you the subjective impression that this small
amplifier should be a perfectly match with dynamic
horn speakers. Matthias Ruff, the spiritual father of
the MODEL5 amplifier quickly concludes: “Small
power output and a high bias voltage are on their
own no guaranty for a good sound. These are just
simple engineering tools to further optimise the
sound”. The resultant sound is the combination of
all components, and this always includes the speakers.
“How many amplifiers exist, which measure
perfectly when connected to a dummy loudspeaker
load in the laboratory, but when hooked to a real
speaker they simply sound bad.” According to the
chief designer of Avantgarde Acoustic, the reason
can be found in the forces which flow back from
the speakers voice coil into the amplifier. With every
movement of the voice coil, magnetic field currents
are induced which are fed back to the amplifier.
These are called “Electro-Magnetic-Forces” (EMF).
Ideally the amplifier should “suppress” these forces
and control the speaker in every situation. In common
amplifier designs one tries to control these
forces with a high damping factor. With high feedback
the output impedance is reduced and the
damping factor is maximized.
TYPICAL PUSH-PULL TRANSISTOR
AMPLIFIER
The control of these Electro-Magnetic-Forces is difficult
to achieve. To better comprehend the problems,
one needs to understand the operation of a typical
transistor PUSH-PULL amplifier. To achieve outstanding
technical specifications, the vast majority of all
PUSH-PULL amplifiers are usually designed according
to the “High Open Loop Gain” concept. In this
concept the open loop gain of the amplifier is set
very high and the overall signal gain is later reduced
to the desired level by applying very high levels of
feedback. The intention of this circuit design is to
sub sequentially reduce distortions and increase the
damping factor of the amplifier. This “HIGH GAIN
High Feedback” concept (HGHF) in principle is
very easy to implement and at first sight, very logical.
By using several amplification stages the input
signal is first amplified to a level higher than in
actual fact needed. This excessive signal level is
then sub sequentially reduced to the required value
by a high feedback loop. Feedback continuously
compares the amplified output signal of the amplifier
with the input signal, if it detects deviations, the
feedback corrects accordingly. Distortions are effectively
eliminated and at the same time a high
damping factor is achieved. Simply spoken HGHFamplifiers
use the headroom generated by the
excessive high open loop gain to later correct the
“errors” at the output (i.e. distortions etc.) with the
feedback. HGHF-amplifiers thus in general have
very good technical specifications. “Superb technical
measurements in the laboratory are no indication
for good sound,” responds Matthias Ruff angrily.
“Static dummy loads are not speaker systems and
fixed frequencies of an oscillator are not music!
People naively believe in the specs of the data
sheets and are totally surprised when at home it
does not work”. The HGHF-principle is plausible in
theory, but in practical operation generates new
distortions. To better understand one has to take a
closer look at the circuit design of an amplifier. An
amplifier in general has the function to amplify the
input signal (for example: from a CD-player)
through several stages successively up to a level,
which is sufficient to drive a speaker system. To
achieve a high signal gain in HFHF-amps several

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”.

In combination to the concept of their speaker systems
Avantgarde Acoustic clearly focuses as well on
their amplifier to optimise the forward signal processing.
The designers around Matthias Ruff have
come up with something completely new and very
different. With their DSSF-circuit concept Avantgarde
Acoustic has found a clever solution to the
above problem. DSSF stands for “DUAL STAGE
SINGLE Feedback.” Put simply “Forwards fast +
backwards fast and little” the DSSF-circuit concept
is clearly described. The MODEL5 is a PUSH-PULL
transistor amplifier, which operates with only two
amplification stages. For each channel you will
surprisingly only find two transistors in the signal
path. It is impossible to design a PUSH-PULL amplifier
any simpler! Short signal paths with fast forward
signal processing is guaranteed. One BI-POLAR
transistor in CLASS A mode is used for the first stage.
Here the input signal is preamplified to the optimal
operating point of the power transistor. This power
output stage is equipped with a modern MOS-FET
transistor which amplifies the signal up to 1.1watts
at 20 ohm in CLASS A mode and 38 watts at 4 ohm
in CLASS AB mode. An additional small regulator
transistor – which is not within the signal path! –
controls and stabilizes the BIAS settings.
It is important to mention that the manufacturer is only
using one power transistor per PUSH-PULL loop in the
MODEL5. Transistors switched in parallel always have
(even when applying the strictest selection criteria)
small variations between each other. These add and
overlay with the audio signal and inevitably conceal
detail of the music signal. All information resolved as
a result of the circuitry gets irretrievably lost. Matthias
Ruff has found a simple and effective solution to
this problem: “What is the best selection? If I simply
do not connect any power transistors in parallel“.
DUAL STAGE FORWARD AMPLIFICATION
The advantages of this DSSF-concept are obvious.
The signal only has to pass through two amplifier
stages and thus is extremely fast. “To achieve a fast
forward processing of the signal we had to reduce
the circuitry to the absolute minimum. There is no
other way. Less is here more! With every amplification

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

appear due to the time delay occurring when the
feedback signal has to pass through all amplification
stages. A really fast amplifier does not need a high
damping factor to control complex loads”. Fast forward
amplification is a mandatory requirement for
effective control of the EMF forces according to the
Avantgarde concept. By removing everything that
is not really essential the MODEL5 operates with
only two amplification stages. The OVERALL LOOP
FEEDBACK can thus be connected from the DRAIN
of the power transistor directly to the EMITTER of
the driver transistor. There are two separate feedback
loops for the positive and negative PUSH-PULL
side. Distortion-free summation of both feedback
loops is ensured as the driver stage operates in
pure CLASS A mode. Whether the distortions are
generated by the amplifier circuit itself or induced
into the outputs of the amplifier by the EMF forces,
the feedback signal only has to go back through
one single stage. The feedback signal is thus immediately
present at the input of the driver transistor
and passes ultra fast through only two stages to the
output. Fast error detection, feedback generation
and error elimination at the amplifier outputs are
guaranteed with this Single Stage FEEDBACK circuit.
“As I am forward fast, I am automatically backwards
fast and have to run the feedback through
only one single stage?” raves the designer. Due to
the speed of the SINGLE STAGE FEEDBACK combined
with the speed of the DUAL STAGE FORWARD
amplification, the EMF induced distortions and
phase shifts only occur in a frequency band far
beyond the audible spectrum. “We did not need to
dampen the EMF forces any more with a brutal
damping factor, but made the amplifier practically
immune to critical loads. The amplified signal is
faster than the induced error and the MODEL5 is
resistant against very complex loads and signals”.
its high impedance, Avantgarde Acoustic developed
a special test set-up to evaluate complex loads
during operation. During the listening tests, several
very low impedance speakers placed in an adjacent
building were remotely hooked up in parallel.
“With this test set up we could simulate very complex
loads under normal listening conditions and evaluate
the results while listening. The smallest impact on
the audible results when switching complex loads
on and off was for me one of the most important
criteria’s when designing the MODEL5. Actually
it is a very simple amplifier, but which achieves
optimal results under listening conditions.”
LISTENING, LISTENING, LISTENING...
During the design phase of the MODEL5 these
experiences have been exclusively verified in extensive
listening tests. As a monitor speaker Avantgarde
Acoustic has used their top of the range
model: the TRIO OMEGA. “A horn is the highest
resolving and most precise physical principle of all
speakers. It will disclose detail, which remain hidden
with normal box speakers. For this reason it was
clear that we use our best horn system – the TRIO
OMEGA – during the design of our MODEL5 amplifier.
With the TRIO you can hear immediately
even smallest modifications in the circuit layout”.
As the TRIO OMEGA is very easy to drive due to

The MODEL5 is not only an ideal amplifier when
connected to high impedance horns, but even
excels with critical speaker systems. Although it’s
damping factor is allegedly not very high, the DSSF
concept ensures optimum control and surprisingly
neutrality even at extremely low impedances.
buffered by capacitors. For the power transistors
40,000uF of capacitance ensure ample reserves
for the relatively small output of this amplifier.
OTHER FEATURES
The MODEL5 integrated amplifier has five RCA
POWER SUPPLY WITH 7 VOLTAGES
The power supply of the MODEL5 utilised a specially
manufactured toroidal transformer with a total of
7 secondary windings! This expensive power supply
configuration is a surprise if you consider that normal
amplifiers can operate with only one voltage.
But Matthias Ruff has his own philosophy in this
respect: “In the MODEL5 we work with a minimized
circuit design but ensure the extensive power
supply is absolutely stable in all operational modes”.
The special transformer provides (with its shielded
secondary windings) a total of 7 different voltages
for the independent electrical supply of the different
circuit modules, power transistors, displays, relays
etc. Interferences are minimized, as the supply of
power to the different components is completely
independent from each other. The toroidal transformer
has a static and magnetic shielding to avoid
electromagnetic radiation into the other components.
The power supply of the various circuit modules is
INPUTS. As an option INPUT 5 can be reconfigured
internally with a jumper into AV-mode. In AV-mode
the volume control of the MODEL5 is bridged for
the INPUT 5. If you want to integrate your high-end
stereo within your home theatre system, you simply
have to switch the input selector to INPUT 5. The
volume of the left and right front channels is then
centrally controlled by the AV processor.
The MODEL5 has one RCA RECORD-OUT, one
PRE-OUT with XLR connectors for use with active
speakers (ex. SOLO) and a SPEAKER-OUT fitted
with WBT connectors is used for the connection to
the main passive speaker systems.
The main power switch is located at the backside of
the unit. Here you can completely turn the MODEL5
off and detach it from the mains. In the STANDBYposition
of the input selector, a red LED in the display
indicates that the amplifier is ready for operation.

The display is illuminated in white as soon as an
INPUT (1 to 5) is selected. If the MUTE-button of the
remote control is pressed the illumination of the
VOLUME display is switched off. The brightness of
the various displays can be adjusted by the means
of three dimmers controls on the underside of the
unit. These are accessible from the outside. A specially
assigned dimmer to control the brigthness of
the AV-channel can be set separately. Thus it is possible
to automatically dim or switch off the illumination
of the MODEL5 in home theatre systems with
projection as soon as the AV-channel is selected.
The remote control, which is included with the
MODEL5, allows the setting of the volume and
MUTE function.

TechniCAL DatA MODEL5
INPUTS
5 x Audio, (asymmetrical) RCA connectors 5 x stereo-pair
Input impedance
10k Ohm
optional (internal jumper):
4 x Audio, (asymmetrical) + RCA connectors 4 x stereo-pair
1 x Audio AV-direct (asymmetrical) RCA connectors 1 x stereo-pair
(with bridged volume control) Input impedance 10k Ohm
OUTPUTS
1 x Audio record-out, (asymmetrical) RCA connectors 1 x stereo-pair
1 x Audio, (symmetrical) 3-Pol XLR connector 1 x stereo-pair
Output impedance
200 Ohm
1 x Speaker output WBT connectors 1 x stereo-pair
Output impedance
0.4 Ohm
SPECIFICATIONS
Power output CLASS A mode at 20 Ohm 2 x 1.1 watts
at 8 Ohm
2 x 0.4 watts
Power output CLASS AB mode at 4 Ohm 2 x 38 watts
at 8 Ohm
2 x 27 watts
Frequency bandwidth -3 dB, 3 watts at 8 Ohm 10-220.000 Hz
-3 dB, 10 watts at 8 Ohm 10-35.000 Hz
Damping factor 20 Hz 44
(at 8 Ohm, dynamic measurement) 200 Hz 21
2.000 Hz 19
20.000 Hz 5
Signal/noise ratio
86 dB
Distortions (0,5 Watt, 8 Ohm) 100 Hz 0.03 %
1.000 Hz 0.03 %
10.000 Hz 0.07 %
Distortions (38 Watt, 4 Ohm) 100 Hz 0.25 %
1.000 Hz 0.30 %
10.000 Hz 2.00 %
Max. Power consumption
130 watts
High precision power supply
toroidal transformer
with 7 secondary windings
primary
2 x 115 Volt (50/60 Hz)
secondary
4 x 20 Volt (1.2 ampere)
secondary
2 x 15 Volt (0.5 ampere)
secondary
1 x 12 Volt (1.5 ampere)
static shielding 1
magnetical shielding 1
safety standards UL/CSA + ENEC EN 61558
Mains power connection
IEC 60320-C14 power entry ON/OFF module
Fuses (internally) circuit board fuse 4 x 2.5 ampere (slow)
mains fuse
1 x 2.5 ampere (slow)
Battery remote control 2 x AA with 1.5V
FEATURES
Extremely stable chassis heat sink 0.393 in (10 mm)
(protection against vibrations) front plate 0.393 in (10 mm)
rear plate
0.118 in (3 mm)
chassis cover
0.078 in (2 mm)
Analogue display of
volume + input
Display illumination STANDBY-mode LED-RED*
(* individual variable brigthness) ON-mode (channel 1-4) LED-WHITE*
ON-mode (channel AV-direct)
LED-WHITE*
MUTE-mode
VOLUME LED-OFF
Remote control (included) volume + mute
Dimensions (WxDxH),
13.4 x 14.9 x 2.8 in (340 x 379 x 72 mm)
Weight
16.5 lbs (7.5 kg)