![]() When engineers learnt in the 60s that there is a device that exists with low noise, no wear and thus no replacement necessary, and small size they were onto it. So an overall higher performance circuit could be realised in roughly the same space as a four-valve preamp. Four valves were typical in preamps of the time. To have added another valve to make the same SC22 circuit in all-valves would have made the unit bigger, more costly, and more liable to go noisy as the valves wear. The BC107 is at the input of the MM / microphone stage, right where the advantages of a small, low voltage transistor would work best. It is incorporated within the negative feedback loop with the first valve to make it a hybrid phono stage. In thinking about this circuit now, I wonder why Radford didn’t direct-couple the BC107 straight to the next stage. It would have made the circuit more stable in the subsonic bass region. But as it stands it represented good forward thinking and is still a good circuit. ![]() Although the output impedance of the phono stage is far too high to be able to properly drive the tape-out socket. A buffer, such as a cathode follower, would work wonders here. ![]()
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