The design is largely executed using commonly available parts such as the BC550C and the BC547C/BC557C. The reader might wonder why fancier Japanese transistors like the popular 2SA970/2SC2240 were not used. Basically that is because the areas where these transistors excel at arenít really of concern here. Their strongest points are a combination of low noise, high maximum Vce and constant Hfe, neither of which are critical in this circuit. The design of this amplifier dictates a virtual short input, making the feedback network's Johnson noise the dominant noise source. The output stage is bridged, halving the supply voltage required for rated power. 60V rated transistors are wholly sufficient here. As for the transistors having a constant Hfe, simply matching the Hfe of the input transistors will be all that is required here, in other parts of the design a constant Hfe will not give better mileage, or better performance.
Fig 1. The 2SC2922/2SA1216 transistors as used with this amplifier.
For driving the output stage we have used common BD139/BD140 transistors that have long since been given this task in numerous other amplifiers for the simple reason that they work very well in this respect. Due to the requirements for dissipation and speed the output transistors however are of a less common variety. Here Sanken 2SC2922/2SA1216 ring-emitter transistors are used, which have found their way in many amplifiers of fame and are known for their excellent performance and reliability. Two pairs of these transistors are used in a bridged configuration with a maximum dissipation of about 60W per transistor, for an output power of 100W/8-ohm. We should mention that only genuine Sanken transistors should be used, equivalents, or counterfeits made by other manufacturers unfortunately will not suffice.
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