50W pure class A ground push-pull amplifier 01

The circuit of the voltage amplifier stage is shown in Figure 7-1. The input stage adopts a complementary balanced differential amplifier circuit. The input tubes are Toshiba
's latest low-noise twin-pair tube K389/J 109 and crystal twin-pair tube A13 49/C 3. 381 constitutes a source-base differential amplifier
stage, which is symmetrically powered by a constant current source. The total analog system ratio is particularly high. This differential input circuit is quite mature. The Japanese audio publication
"Wireless and Experimental" has recommended its use many times. The works that won the third place in Japan and the first place in the fourth transistor amplifier competition
used this differential input form. Enthusiasts who are used to circuits know that there are two places that have the greatest impact on sound quality in the voltage amplification stage
: one is the constant current source circuit of the input stage differential amplification circuit; the other is the second-stage amplification circuit. In this machine,
effective measures have been taken for these two places. In order to ensure that the current of the constant current source is not affected by changes in the power supply voltage
and minimize distortion, a high-quality current Miller circuit is used. The biggest advantage of this circuit is that the current flowing through the constant current diode
is not affected by changes in the power supply, so the distortion is very small. Figure 7-2 is a more popular
constant current source circuit nowadays. This constant current source generates a reference current by dividing the voltage through a resistor between the power supply and the ground. Its
biggest disadvantage is that the current will vary with the power supply voltage. It changes with changes, which has a negative impact on the dynamics and positioning of the music. The current Miller circuit of this unit
has a significant improvement effect on the sound quality.
    The second-stage amplifier circuit plays a decisive role in the frequency characteristics and accuracy of the entire amplifier. This pole uses Hitachi low-
noise crystal transistor A1191/C2856 to form a common-base push-pull Schermann circuit. This pair of fever tubes has excellent linear amplification characteristics when the collector current is not greater than 10mA.
The Mann circuit not only has excellent impedance transformation, but also has
a high cut-off frequency and very reliable stability within the operating frequency band. It is the most distinctive and essential part of this machine.
Different from the traditional common-emitter amplification circuit, it no longer amplifies the power supply voltage , but only converts the signal voltage, so it will not amplify
the interference noise sneaking from the power supply. The clever thing is that the base grounding has an effect on the power supply. The interference noise signal plays a good isolation role,
and its noise suppression is greatly improved compared to the traditional common-emitter amplifier circuit, and its own linear characteristics are also greatly improved. This circuit is used in
the LUX B-lo power amplifier circuit that won the only gold medal for Japan's 25th Advanced Audio Products and the famous American Xavier Kardon PSZZOO circuit.