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On the day of the old valve, most commercial audio amplifiers suitable for compact integrated single discs or players used stereo amplifier topology of a single valve. The circuit is usually carried out through a multiple type valve, for example, a triode pentode ECL86.
Common features of the amplifiers are Class A operation, the output power in the 3 - 5W range, input sensitivity of about 600mV of total output power, THD of 3% @ 1KHz and 3W. Best types showed figures of 1.8% THD @ 3W and 0.8% @ 2W.
This solid-state push-pull single-ended Class A circuit is capable of providing a sound comparable to that of tube amplifiers, offering more output power (6.9W measured across a load of 8 ohm speaker cabinet), less THD, higher input sensitivity and better linearity.
Voltage and current for this circuit is 24 V and 700 mA, respectively, compared with 250 HT rail and 1A@6.3V heating filament of the valve works with amplifiers. The penalty only for the transistor circuit that works with the need for a larger heat sink for Q2 and Q3 (compared with the maximum power delivered). In any case, the amount of heat generated by this circuit can be comparable to that of a single valve amplifier. A choice of low-boost facility can be added through R5 and C5.
This circuit was built and compared with a circuit of a valve box phonograph in the early 1950 by Aren van Waard, a Dutch biochemist working in the field of medical imaging (PET) with a strong interest in audio amplifiers valve. A complete description of the circuits and test results of subjective comparisons made by the distinguished author appeared in the magazine AudioXpress: February, March and April 2005 issues.
