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An Output-Transformerless Power Amplifier

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An amplifier circuit using miniature tubes and no output transformer is described. Large amounts of negative feedback are used to reduce distortion to low values. The output stage is single-ended, with the tubes connected in series and working as cathode followers. A new method of symmetrically driving the output tubes from a phase-splitter tube is shown. Positive feedback from the output load to the input of the phase splitter is used. Three models of such amplifiers built by the author are described. Two of these produce an output of 10 watts each into a 16-ohm voice coil, and the third amplifier delivers 20 watts of power. Less than 0.1% harmonic distortion is claimed.

JAES Volume 2 Issue 4 pp. 252-256; October 1954
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AES - Audio Engineering Society