5 Watt Class A Audio Amplifier

In the old days of vacuum tubes, most commercial audio amplifiers for compact integrated mono or stereo phonographs employed a single-tube amplifier topology. The circuitry was typically implemented using various types of vacuum tubes, such as the triode-pentode ECL86. These amplifiers shared common characteristics: Class A operation, output power ranging from 3 to 5 watts, input sensitivity of approximately 600 mV at full output power, and total harmonic distortion (THD) of approximately 3% at 3 watts and 1 kHz. The optimal THD figures were 1.8% at 3 watts and 0.8% at 2 watts. This solid-state push-pull single-ended Class A circuitry offers sound quality comparable to those tube amplifiers, providing higher output power (6.9 watts measured at an 8-ohm speaker cabinet load), lower THD, higher input sensitivity, and better linearity.