Circlotron - History

History

Circlotron name emerged as a trademark of Electro-Voice. A U.S. patent for a circlotron High Fidelity Audio Amplifier was filed by Alpha Wiggins of Electro-Voice on March 1, 1954 and granted March 28, 1958. However, other inventors filed the same concept earlier:

  • C. T. Hall filed Parallel Opposed Power Amplifiers on June 7, 1951; U.S. Patent 2,705,265 granted on March 29, 1955.
  • Tapio Köykkä filed on September 2, 1952; Finnish Patent 27332 granted on November 10, 1954; improved version was filed on September 30, 1955, Finnish Patent 29642 granted on April 10, 1958.

All these patents called for a transformer-coupled, fully balanced design; commercial transformerless amplifiers were not feasible at this time due to high costs of power supply capacitors required for an OTL design (at least thousands of microfarads at 200 volts or better). In 1950s and 1960s circlotrons were produced by Electro-Voice (8 models, 15 to 100 watts per channel), Finnish Voima and Philips. These inspired other local manufacturers, such as Calrad. All these models employed output transformers and beam tetrode or pentode tubes (for increased efficiency). Similar amateur designs were published in the USSR.

The concept was resurrected in its transformerless form in early 1980s by Ralph Karsten, founder of Atma-Sphere which remains the principal contemporary manufacturer of circlotrons. Other OTL circlotrons are made by Tenor Audio and Korean manufacturers.

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