Preamplifier Controls for Class A Power Amps

Having built the Class A power amplifier, shown by J. L. Linsley Hood (1969, Wireless world), it occurred to me that it would be more appropriate if I used a Class A biased preamplifier to go with it, including vintage period transistors. Usually, with small signals, such as that in preamplifiers, the mode of class does not matter so much, but I am sure there are purists out there who expect their preamplifier transistors to be biased in Class A as well. Over the years, many commercial amplifiers have emerged boasting Class A design throughout, and claiming superior performance.

At minimum, the JLH 1969 power amplifier requires a volume control, and balance control, however, I am sure many audiophiles would also like to have a preamplifier that provides equalization for their record players, including bass and tone circuit. After looking at many circuits, I settled for this group of circuits designed by Mullard Engineers and utilising the BC148 and BC149 transistors. An advantage of these circuits is that the design is clean and simple and works extremely well. Even better was the fact that I managed to get some period lockfit transistors with gold leads. These transistors often turn up on eBay as old stock and they are relatively inexpensive. I like the idea of my preamplifier utilising transistors of the same era as the power transistors and therefore decided to go this route. Another advantage of these circuits is that they are designed to operate at 18 V ±15 % hence there is a wide choice of possible power supply designs.

Equivalent Transistors

Comparing the parameters of the BC107, BC108, and BC109, with BC147, BC148, and BC149 we can see that they are very similar, and the gain figures, and collector currents being identical between the two groups.

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