The design is very similar to that of a balanced opamp. Neither input nor output is ground referenced. This takes the influence of ground currents and loops out of the equation. The only thing that is referenced to ground is the common mode voltage of both outputs, this is done to insure that both outputs have the same dynamic range and the output remains symmetrical. Since the load is attached between the two outputs, it is not interested in the common mode voltage. The entire amplifier is DC-coupled, which is intentional as to guarantee that any coupling capacitors wouldn’t be adding their sonic signature to the amplifier.
Fig 2. The ExtremA class-A amplifier schematic.
The schematic of the amplifier shows the details discussed above. Without describing every component in detail it is clear that T1 and T2 are the input transistors with the second stage formed by a folded cascode around T7 and T8. Transistors T17/T18 and T20/T21 are used as a complementary feedback pair and buffer the output of the folded cascode. The two output stages employ an auto-bias scheme for which the bias current is set by the emitter resistors of the 2SC2922/2SA1216 output transistors. Subsequently overall gain is set to approximately to 20x (26dB) by feedback resistor networks R1/R3, R2/R4. Transistors T31/T32, T41/T42 in combination with the diodes connected between their emitters effectively form an overcurrent protection, whereas T49/T50 comprise a DC protection circuit. The DC protection does not simply engage a relay that disconnects the load, but more elegantly disconnects the amplifier from its power supply.
Fig 3. The regulated powersupply schematic.
The power supply itself has two separate sections. The high-current supply is straightforward with two bridge rectifiers and a few large buffer capacitors for the two output stages. The low-current supply puts out a higher voltage to insure that the input stage has enough voltage to fully modulate the output stage, hence upping efficiency. Although the PSRR (power supply rejection ratio) of the input stage is quite good, regulation does not hurt and we have opted to use a discrete regulated power supply which fares much better than conventional voltage regulators, such as the popular L78xx/L79xx. This power supply design effectively operates both the input- and output stages under optimum conditions, which will result in the excellent performance figures this amplifier is capable of.
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