Two-channel power amplifier circuits of the parallel type

A new power amplifier is described which amplifies both stereo signals by means of a single push-pull-like pair of transistors or tubes. This circuit is derived from the parallel-type push-pull amplifier and combines to advantage, especially its output-transformerless operation, with the possibility of controlling the stereo effect or producing an unlimited number of phantom stereo signals. All basic alternatives of this circuit are derived and a complete two-channel amplifier is described, which in essence is a Lin quasi-complementary amplifier adapted for stereo. The amplifier includes, in one of its branches, a built-in polarity inverter so that it can be used with any conventional stereo signal source. In addition to the outputs for the two flanking speakers, it is provided with a sum-signal output for the center speaker, and means of contracting or expanding the stereophonic basis. Owing to the sliding operating point of its power stage, it exhibits properties similar to those of a class A-B amplifier, as far as transistor dissipation and supply-current economy are concerned. Finally, the new amplifier is compared with two single-acting stages and the resulting advantages are evaluated. An analogy is drawn between this amplifier and the modern stereo disk recording.