JLH Class A amplifier

JLH Class A amplifier

The 1996 version used specified transistors, with a +/-22V power rail and 2A quiescent current, and approximate RMS power output under resistive loads of: 10W at 16 ohms, 20W at 8 ohms, 15W at 4 ohms and 10W at 2 ohms.

To minimize distortion, Tr1 and Tr2 should be a matched pair. If this is not possible, the higher-gain device should be used at the Tr1 position. Low-gain output devices such as the 2N3055 should only be used with high-gain drive transistors, such as the 2N1711 or 2N3019 (or suitable alternatives—perhaps the specially chosen BD139).

Ideally, the output transistor should have an fT of 4MHz or higher, although many amplifiers have successfully used 3MHz devices (or even lower). For reference, ON-Semi's 2N3055 datasheet cites an fT of 2.5MHz, and ST's is 3MHz (although other 2N3055 manufacturers cite 0.8MHz). Tip: Power devices are typically 3MHz.