Kleinleistungsverstärker der absoluten Spitzenklasse mit 2 x 50 Watt Ausgangsleistung – auch mit FB
Ende der 80 er ist es uns gelungen, einen elekronischen Schaltkreis zu entwickeln ( ABR ) der den Einsatz selektierter Röhren in Endstufen völlig überflüssig macht. Dieser sog. ABR ist in der Lage den Strom beliebiger Endröhren zu korrigieren und auf einen exakt vorbestimmten Wert von z.B. 40 mA zu stabilisieren und dies, bis zum Ende der max. Lebensdauererwartung der Endröhren. Dieser Schaltkreis ist ein Meilenstein in der Entwicklung unserer Verstärker – einmalig in der Welt bis heute – und eigentlich unverzichtbar wenn man höchste Betiebssicherheit anstrebt und das mit Standard – Röhren. Durch rote LED´S wird die Brauchbarkeit der Endröhre bei korrekter Arbeitspunktfindung signalisiert. Im Fehlerfall leuchtet dann am entsprechenden Platz eine LED als Hinweis darauf die entsprechende Röhre zu ersetzen. Auch die Drift des Arbeitspunktes durch Alterung wird korrigiert und somit über lange Zeit die klangliche Eigenschaften der Röhre aufrechterhalten – eigentlich bis zum Ende der zu erwartenden Lebensdauer der Röhre.
Wir verwenden dieses innovative Konzept in all unseren Endstufen
Zahlreiche Versuche fremder Anbieter, diese Technologie zu entwickeln, scheiterten immer daran, das zwar der Ruhestrom statisch stabilisiert werden konnte, daß aber im Lastfall bei zunehmender Ausgangsleistung die Endröhren zugesteuerte und so die Endstufe vorzeitig ins Klippen geriet je höher die Ausgangsleistung wurde. Im Gegensatz dazu speichert der AUDIO VALVE ABR den statischen Ruhestrom und hält dessen Größe fest, auch wenn dynamische Passagen mit hoher Ausgangsleitung abverlangt werden. Dieser bislang weltweit einmalige und geschütze Schaltkreis, entwickelt von AUDIO VALVE katapultiert den Röhrenverstärker in ein völlig neues technologisches Zeitalter und hat kaum noch etwas mit konventionellen Verstärkerbau zu tun. Diese hochinnovativen Produkte von AUDIOVALVE überzeugen gerade deshalb durch ihre fantastischen Leistungsmerkmale hinsichtlich Klang und Power. Die ASSISTENTEN Vollverstärker harmonieren mit allen gängigen Lautsprechermodellen mit gutem Wirkungsgrad (ab 92 dB) ausgezeichnet.
(English) Audio Valve was born in 1984. This German firm first concerned itself with amplifiers for headphones and afterwards since 1990 has entered the market for valve amplifiers.
Helmut Becker, a German engineer, had spent his leisure hours listening to music over electrodynamic headphones. That the quality of music derived from an amplifiers headphone output terminals was only mediocre, he became very quickly aware of. He knew as well that on power amplifiers the headphone output signal was achieved through the utilisation of an attenuating resistance in the output circuit and therefore determinable for the quality a factor resulting in a compromised sound. The residual noise of the amplifier, almost inaudible over the majority of acoustic enclosures, became, over headphones unfortunately quite predominant, the latter requiring for their functioning less than fractions of one Watt. Further, he realised that in the CD players which have been on the market since 1983, the general practice has been to include a little integrated headphone amplifier. In some top range units a circuit with components discreetly constructed for quality was used but these were extremely rare. With these findings as a basis, this engineer decided to construct a headphone amplifier of very high quality for personal use. It soon became recognised, making an impression in its branch owing to its sheer quality. Shortly this product reached the market as a kit and was described in revues in Elrad and Steroplay. In 1984 Helmut Becker founded the Company Audio Valve. In 1990 the range was extended and embodied a new appearance an aesthetic achievement based on a striking symbiosis of glass and black anodised aluminium. The annual demonstrations in a Frankfurt salon earned the product great acclaim and very quickly opened the way for a rapid increase in exports to Great Britain and Asia. In 1994 a completely new factory was built. Its very modern equipment enabled a further modification of the products, bringing them into line with the CE requirements for the European market.
One of the original features of the Audio Valve products is to be found in the utilisation of the valves in such a way that they operate as closely as possible to their ideal reference values each according to its type. One recalls, for instance, the output pentode-triode PCL 805 and most particularly the small beam tetrode with a common cathode QQE 3/12. This valve with a noval (nine pin) base was manufactured formerly by Philips and Telefunken. (the latter was used in our test amplifier) It has a per plate power dissipation of 7,5 W and may be used with plate voltages ranging from 200 to 300 V. It is ideal in class A and AB amplifiers (whereas used in class C the grid bias voltage may reach +30 V!). This valve, according to Helmut Becker, is ideally adapted to be integrated in a circuit which includes another very innovative concept, an automatically operating grid bias control, refered to as „ABR“ and used to great advantage in the integrated amplifier Assistent 20s.
The Assistent 20s is one of the top products of the firm Audio Valve. In the near future we shall investigate the other products among which one finds the RKV, the amplifier for electrodynamic headphones, which was conceived at the time of the founding of the company.
The classical chassis has been set into a caisson (a case-like frame). The valves sit in their sockets mounted on a double side printed circuit board. Little columns support the glass cover preventing dangerous hand contact burns or electric shocks could ensue conforming to the CE requirements. The cooling is achieved through the sides. The controls mounted at the front consist of an input selector and a volume control the two channels being coupled in their operation. The height of the main board constituting the chassis enables the deeper positioning of the power supply and filter condensers as well as the output transformers. At the back are the cinch input sockets, two pairs of output terminals and a mains switch.
The amplifying circuit uses only valves, but the push-pull output stages are coupled to the ABR circuit. This exclusive Audio Valve feature, allows a comparison of the reference voltage value with that engendered by the quiescent current and which appears at the terminals of a small value resistance in the return cathode circuit. A small operation amplifier monitors, as comparator, the divergence and deliverers the compensation voltage to the grid of the valve. Each valve, young or old, reaching reference values or not is thus compensated in its Grid bias to operate at a value which corresponds to its optimal quiescent currant. A red LED diode, at the foot of each value lights up preventing the use of inadequate or defect valves. For each channel the input stage and driver stage are served by an almost unknown noval valve, the American triple triode 6EZ8, whose bandwidth reaches 55 kHz. (Jean Hiraga)
Test no. 1
Ella Fitzgerald, „Reach for Tomorrow“, Verve Classic Compact, VSCD-4043.
J.H. The voice captured in its most intimate details surrounds itself with a delicate halo of reverberation coming through quite above the ordinary. It is sublime.
R.L. This amplifier is remarkable for its naturalness and for the richness of minute musical detail. It stands apart from many of the amplifiers we listen to in the course of a long year.
Test no. 2
Mozart, Fantasy for piano in D minor, Ivo Pogorelich, track 1, DG 437 763-2
J.H. Full, copious, but adequately controlled with more than touch of esprit yet with more expansiveness than in the majority of other recordings.
R.L. The piano presents a natural richness in the best sense of the world. The tonal coherence is exemplary. The piano remains clearly placed one and indivisible.
Test no. 3
Test of applause and percussion instruments. NRDS no. 10, tracks 14, 17 and 21.
J.H. The three tracks attain a very high standard . It is sublime and these praises are perfectly deserved even if track 21 makes one feel the limit of the amplifier`s power.
R.L. What class! One feels so very close to the veritable sound thanks to a quality of tone far beyond the usual. On the other hand the sound stage to me to have more width than depth.
Test no. 4
Mark Curry, It?s only time, „All over me“, Virgin CDVUS 49.
J.H. The completely calm voice is splendid. Its coast „rocailleux“ (rocky) is really there, but at the same time purer, cleaner, more melodic.
R.L. Rarely has Mark Curry come so near to his listener. And imagine, for all of this, a very efficient speaker has no more than 20 watts at its disposal!
Test no. 5
Johann Strauss, Egyptian March op. 335, Das Mikrofon, Tacet 17.
J.H. Not a trace of harshness in the strings only a little natural acidity. All other details are present and balanced.
R.L. This amplifier reveals a multitude of minute information. Excellent!
· The Audio Valve Assistent 20s as its nickname Little Marvel indicates, is a great discovery and very much so in the field of analysis of the minuteness of detail sitting in every recording in the realm of music.It is striking how often, despite the advent of our modern day electronics, a sound is produced which almost always turns out to be less lyrical, less melodic as well as more monotonous and diffused.As its nominally rated power is limited, it is necessary to seek out loudspeaker enclosures to hook it up to which possess a pretty high efficiency and sensivity to ensure an accompaniment of complementary quality. As the types of valves used in the Assistent 20s are not very common, it is advisable to buy a replacement set if possible in advance even from the date of purchase of the amplifier. (Jean Hiraga)
· Listening to this valve amplifier from Audio Valve is a revelation for me. I have been completely taken aback (surprised) by the extraordinary profusion of information embedded in our every day (normal) discs. Of course other electronic devices reproduce the information, but in a manner more reserved and less natural. Here the minute details permeate the listening experience and it is real treat because this all remains so naturally integral in a certain way to the music. However, be advised on one matter: It is imperative to choose loudspeakers having a high efficiency, and, it goes without saying, of very high quality. Without a doubt the valve has not finished astonishing us! (Robert Lacrampe)
Subjective dynamics: 9/10
Stereophonic realisation: 10/10
Units Design coherence: 9,5/10
The maximum power is limited to 2 x 20 W. We recommend using speakers possessing the same capacity for definition and having an even and balanced sound spectrum.
Our measured results
Amplifier preamplifier Audio Valve Assistent 20s, no. 2343 UA Ich
Maximum power, load 8 Ohm, voltage 230 V: 19 W
at 40 Hz: 0,55 %
at 1 kHz: 0,13 %
at 10 kHz: 1 %
Signal to noise ratio, medium Level A: 91dB
Slew rate on square wave signal: 9m s
The operation of the grid potential assistance circuit (ABR) tends to vary slightly depending on the power that is being delivered. This phenomena presents itself particularly at the point of delivering maximum nominal power. The amount of harmonic distortion was measured as sufficiently high but nevertheless acceptable being typically encountered in equipment with low feedback. The square wave signal slew rate at 10 kHz is only 9 m s. Its demise requires 12m s.
Translation from the French original which appeared in august 98 in „la nouvelle REVUE DU SON“ no.221
nov.98; translate by J.E. Rostron from John Paul Hiraga (La Revue de Son) france