Power amplifier circuit using Sanken tube

Japan's "Sanken" power amplifier has excellent performance on tubes 2SC2922 and 2SA1216 (or 2SC3264 and 2SA1295)
. It has good linearity and strong overload capability. Use it as a power amplifier-level high-fidelity power amplifier, and its performance is very good. Figure 2-30 is
the schematic diagram of a high-performance power amplifier made with this pair of tubes
    . The circuit consists of a first-level differential input, a first-level differential voltage amplification and push, and a first-level fully complementary push-pull power amplifier output stage.
In addition, this circuit also has overload protection and speaker protection circuits.
    At the input end, the low-pass filter composed of RI, c2, and C3 can limit the upper limit frequency of the input circuit to reduce transient intermodulation
distortion.
    VTl and VT2 form a differential input stage. VT3 and VT4 form a differential voltage amplification stage. The collector output of VT4 is directly
connected to the power amplifier output tube VT70Vr5 and VT6 to form a mirror current source. The collector output of VT3 is connected through the collector-
base connection of VTs. Input to the base of VT6, and after amplification by VT6, it is output by its collector and input to the power amplifier output tube VT80. VT7,
VTg, VT8, and VTlo form a composite tube complementary push-pull power amplifier output stage. VT12, VI)4 and VT13, VIGa form the output
overload protection circuit.
    VT14 ~ VT16 form a speaker protection circuit, which allows the speaker to be connected 5 seconds after the power is turned on to avoid startup
shock. It can also automatically cut off the speaker when the DC voltage at the output terminal exceeds 1-5V.
    Since the feedback capacitor has been eliminated in the DC power amplifier circuit, the DC bias
resistance of the feedback input terminal of the input differential stage to the ground is I   RF &; RF, and generally RF is only about 1kfl. In order to balance the differential pair, the signal input end to
ground resistance J52 should also be reduced to the level of RF . This will greatly reduce the input resistance of the circuit, making it difficult to achieve
a good match with the pre-stage. For this reason, the general II: Power amplifier circuits mostly use voltage-controlled device field effect transistors as differential input stages.
The input stage of this circuit still uses bipolar transistors, but the base bias resistor and input resistor are set separately (the principle is as shown in the figure) 1-35 (a)), it is a good solution to
adjust the balance of the circuit by trimming the potentiometer, which does not reduce the input resistance, but also achieves balanced adjustment of the differential circuit .
Since the differential input terminal is biased, the input capacitor Cl cannot be
canceled at will
    . Another feature of this circuit is that it adopts a floating grounding method. The floating ground of this machine uses
two voltage regulator tubes and a resistor R13 in the ground part of the input end to isolate it from the negative power supply of the power amplifier stage. This prevents
thermal noise generated by large currents in the power supply circuit from flowing into the input circuit. On the other hand, it can also suppress the "interface hum modulation distortion" (referred to as IHM distortion) caused by the back electromotive force generated by the speaker when playing back the input circuit.