Circlotron - Benefits and Drawbacks

Benefits and Drawbacks

Perceived fidelity (quality) of audio reproduction is a hardly quantifiable, subjective measure that should be left to each individual listener's taste. Apart from audio fidelity, circlotron OTL have certain benefits versus asymmetrical OTLs (i.e. Futterman amplifier) and conventional, transformer-coupled amplifiers:

• Scalability. The same basic design can be easily scaled up or down by adding (removing) output tubes.
• Loudspeaker safety. Even when either V1 or V2 bank fails completely, direct current through speaker is limited to non-destructive values.
• Reliability. Failure in one triode (or a double-triode tube) can be offset by re-biasing the amplifier; it will continue operating, although at reduced maximum output and with increased even-order distortion caused by output stage asymmetry.
• Linear frequency response and fast transient handling. Absence of output transformer and global negative feedback extends full power frequency response into hundreds of kilohertz. On the lower end of frequency scale, bass response is limited by power supply capacity and driver circuit, and can be easily extended below 1 hertz.

All these benefits can be negated by power consumption: minimal stereo power amplifier (Atma-Sphere S-30) continuously burns 400 watts, while practical low-impedance stereo setups approach 1 kilowatt. Apart from ethical considerations and costs of electricity, such heat sources bring discomfort to listening room and may require dedicated ventilation and air conditioning.

Circlotrons, unlike minimalistic single-ended amplifiers, need to employ quite complex balanced input and driver stages delivering at least 100V peak-to-peak voltage and able to drive relatively high capacitance loads (due to the number of paralleled power triodes and lengthy wiring). The first requirement is typically solved with a long-tailed cascode stage (2 or 3 double triodes), the second - with two cathode followers inserted between the driver stage and the power stage. Both need a bipolar power supply, independent of the power stage power supplies, and firing up well in advance of the power stage supply.