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Yamaha B-1

A class B design (1974), with a relatively high quiescent current of 400mA, dissipating 68W per channel. Noted by some as resembling the once familiar, single-ended, push-pull power stage featuring two pentodes (EL86) feeding an 800R loudspeaker, and worth a look simply from an educational point of view.

The pre-amp stage is comprised of a dual FET to reduce offset errors and three differential amplifiers in cascade with long tail pairs consisting of constant current sources. Each half of the second differential stage consists of two cascode connected P-FETs operating in a common gate configuration virtually eliminating the Miller effect (capacitive feedback) and guaranteeing linearity over a considerable drain potential swing. This is advantageous in the third differential stage, similarly cascoded, which operates as a phase inverter for the output stage.

Additional circuitry ensures sufficient bias (on power up/down) to the output pair to prevent dangerous drain currents and to ensure the quiescent currents are practically independent of power supply fluctuations.

All signal handling stages are equipped with FETs, the output pair being N-channel V-FETs (or V-MOSFETs), driven in push-pull. Bootstrapping makes the drain impedance of TR512 appreciably higher than that of TR513. V-FETs TR514/8 are arranged as source followers, both being tied to the -200V rail by constant current sources. This not only stabilises the quiescent current for these drivers, but also applies the entire output current swing to the gates of the output pair.

The B-1 had 39 FETs, 113 transistors, 3 LEDs, 64 diodes and 7 zeners. Increasing the component count can sometimes be disadvantageous, since obviously, there will be fewer hardware failures and costs will fall if there is less of it, especially if high-voltage supplies (+85V through -200V rails in the B-1) are involved. At the same time, any failure in a system which acts upon another, will produce a fault condition which will, in all probability, result in failure of the primary system which is working perfectly. Similarly, if the failure is beyond the owner/user's ability to rectify, finding an engineer locally who is equipped, competent and willing enough to do so promptly is a virtual impossibility. Thus the B-1 being referred to in some circles as '(in)famous'.

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