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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
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