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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.
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