ABSTRACT
The author presents the design of a Trimodal power amplifier. Bare PCBs, kits, and complete tested versions of an updated Trimodal amplifier are available from The Signal Transfer Company. A stereo pair of Signal Transfer Trimodal PCBs were put in a box, together with power supply, protection system, and ultra-low noise balanced input card, all by Signal Transfer. The author provides an updated version of the Class-A circuit, in response to requests for a PCB for the Class-A amplifier designed with methodology. Naturally, the class-A circuit bears a very close resemblance to a 'blameless' Class-B amplifier. In the Class-B mode, the amplifier has the usual negligible quiescent dissipation, but in Class-A the thermal efflux is naturally considerable. The author decided to use a complementary-feedback-pair (CFP), output stage for best possible linearity, and some incremental improvements have been made to noise, slew rate and maximum d.c. offset. In a power amplifier, noise performance is not an irrelevance.
