What a month! Having just finished with the wild and wonderful E.A.R. V20 integrated, here I am savouring an equally radical new power amplifier from an Italian designer as revolutionary and iconoclastic as E.A.R.'s Tim de Paravicini. After all, GRAAF's Giovanni Mariani is one of but a handful of electronics wizards brave enough to carry on with output-transformerless circuitry - only he chose not to emulate the seemingly standard practice of making amps that are ugly, unreliable and built like something which looked like as if it had been fashioned in a Russian farm tool plant - circa 1946. Then again, Mariani
Aah, Modena! Home of Ferrari, Maserati, Lamborghini, balsamic vinegar and the world's finest cappuccino. It's important that you know of the locale if you're to appreciate why all GRAAF amplifiers transcend the hi-fi norm, and why the GM20 in particular transcends the GRAAF norm. Just as de Paravicini has excelled himself with the V20, so has Mariani entered a new phase of excellence. And, again like de Paravicini, he's managed to produce something totally out of the ordinary, yet completely in character with the amps of his which preceded it.
Yes, the GM20 is an OTL amplifier like the GM100 and GM200. Yes, it offers balanced operation. Yes, it's built to standards surpassed by no other valve amp maker on the planet, and matched only by the likes of Wavac or Nagra. Yes, it contains exclusively Italian parts wherever possible (and begs the need for a valve plant to open somewhere between the Alps and Sicily if GRAAF is to make it nearer to 100 percent). But this particular model uses a rather special tube, one which I don't believe has been used in OTLs before...
Mariani's first OTL/OCL(output transformerless/output capacitorless) amplifier using the circuit now adopted for the GM20 appeared over a decade ago. The benefits of this type of circuit are recognised by a specific breed of tube junkie and may be summarised as providing unrivalled transparency, speed and dynamics. That's because OTL/OCL designs are as close as it gets to direct injection of signal into speaker: the format avoids the need to couple the loudspeaker load by means of an output transformer. In the GM20, as with other OTLs, the output stages are directly coupled to their respective loads. And the output tube in this case is the Russian military workhorse, the 6C33C tube known for a bunch of 'nipples' on its top surface.
Like its predecessors, GM 20 uses a fully differential and balanced configuration (i.e. symmetrical) and is DC-coupled between each of its stages. This explains the low levels of hum by OTL standards, and the general immunity to noise, including noise generated by AC mains. To reassure those who have been (metaphorically speaking) singed by OTLs, great care has been taken to ensure the amplifier's stability throughout, Mariani's expressed goal being to make them 'practically immune to variations in the characteristics of the active components (thermal, static and dynamic) and maintaining their special character unaltered over time.' And if reassurance is needed, the GM200 I've been using for - what? five years? - still behaves as if it were hardly used.
GM20's power supply is made up of six separate sections, four for the output stages and two sections for the driver and gain stages. Two of the aforementioned 6C33C triodes are used per channels, configured to exploit the ability of these tubes to deliver high current with a low voltage power supply. Mariani believes that these Russki masterpieces represent an ideal choice for an OTL design precisely because of their military origins, which endow them with great ruggedness...in addition to superb quality.
Completing the valve complement is an input stage using two 6922 double triodes responsible for the voltage gain and operating as impedance 'adaptors'. The driver stages employ two triode-coupled EF184 pentodes, to take advantage from the differential circuit mentioned above and to work as phase splitters. This guarantees good driving ability for the output tube grids and helps to maintain perfect symmetry between the two signals.
Who knows? Maybe in his youth Mariani had an unpleasant experience with an OTL. Whatever the reason, he's gone out of his way to ensure that the user can treat the GM20 pretty much as if it were a 'normal' tube amp. The stabilising offset and bias circuits ensure completely trouble-free setting-up, and loudspeaker protection is guaranteed by a novel and sophisticated circuit which avoids the use of series relays or current limiters; the tubes themselves are naturally current-limiting. GRAAF has employed only a small amount of feedback - 6dB - to allow the GM20 to drive difficult loudspeaker loads without problems.Read more about the GM20 on Page 2.