GM-50 is the most recent GRAAF creation, an integrated amplifier born to complete our production of the highest class. GM-50 includes in a single frame the features and the level of performance that in the past were an exclusive of GRAAF preamplifiers and power amplifiers. Construction qualities, sound performance and the highest finishing level are joined with the lower cost and the ease of use of an integrated amplifier. The "GM" monogram on the front panel is a synonimous of the greatest sound quality, until now exclusive for top level electronics like GRAAF GM-200 power amplifier and GM- 13.5 B preamplifier. The goal of GM-50 is to establish new quality standards between top class integrated amplifiers.
As usual for GRAAF electronics, GM-50 aestethics are very accurate even in the slightest details. They rely on the coupling of a massive 15 mm aluminium panel with a transparent crystal that allows the vision of the internal circuit. Led indicators show through the crystal the source selected by the user, making the front panel even more attractive. Large rotary knobs give a comfortable access to listening level control and source selection. They look solid and sturdy; the pleasant feeling given by their use is quite what the most demanding audiophile can expect of a GRAAF electronic.
The careful manifacturing and the plentiful sizing of the most critical components are suggested by a frame depth of almost 50 centimetres. GM-50 has also a remote control for on/off switching, source selection, listening level control and mute switching. The amplifier can manage up to six line sources: two of them with balanced outputs, the others with single ended ones. Two tape outputs are provided as well.
High quality binding posts allow the connection of a couple of loudspeakers. Elegance and refinement, joined with versatility and ease of use are just a preview for the exclusive GM-50 internal features. They have been considered with the greatest care in the aim to reach new limits for sound and quality in top-class integrated amplifiers. To attain this object some of the solutions typical of the OTL Series electronics, conceived by Giovanni Mariani, are used. The fully balanced configuration from input to output is between them. Its realization concerns the complete doubling of the electronic circuits, the only acceptable solution for the GRAAF quality standards, until now used only for very few electronics of higher cost. At the power switching, GM-50 enters in mute condition, until tubes reach the operating temperature.
The project of GM-50 integrated amplifier is based on a lay out of great simpleness, an essential rule to obtain the best sound quality. The first stage, for gain and impedance adjustment, serves the level control, in order to reach the most correct operation. Audio signal then enters a stage made of two differential circuits in cascade, with the second of them that works as the actual driver for the output stage. At this point the signals are compared each other and subject to a further level gain. Output stages are made from a push pull of KT 88 tubes, biased for A-AB Class operation. Each tube has its own automatic bias control system, a choice that allows to obtain the best performance in any operating condition. Another exclusive feature of the output stage regards the use of transformers with ground connected central end in the secondary windings. Such a solution is to have a true balanced configuration for the output stage. Also, this allows to obtain a balanced configuration for the feedback circuit. Usually, in audio amplifiers, the feedback circuit brings back to the input only a part of the output signal, in a point defined as comparison knot. With this solution GRAAF GM-50 has two comparison knots for each channel, allowing to have a balanced feedback net, more linear, precise and stable in its correction purposes. GRAAF GM-50 has a feedback rate limited to 6 dB only.