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Overview The circuit is all about a power amplifier that can provide a power output of 60 W while operating in Class A mode. Terminology - Class A – refers to an output stage where the bias current is greater than the maximum output current thus, all output transistors are always conducting current due to its linearity and low distortion
- MPSA56 – a PNP general purpose transistor enclosed in a TO-92 package and used for amplification and switching with features of low current with 500 mA maximum and low voltage with 80 V maximum
- MJ802 – a silicon NPN power transistor with TO-3 package and excellent safe operating area for use as an output device in complementary audio amplifiers to 100 Watts music power per channel
- 2N6515 – a high voltage NPN transistor in a through hole package with 150ºC junction temperature
Circuit Explanation The Class A mode of operation is one amplifier configuration that is accepted universally as the ideal use for audio. Although the most common problem of Class A is the heat dissipation and power supply design, they are still preferred because during biasing, the Class A is always responding instantaneously to an input signal since the transistors are always turned ON. Other classes like Class B and AB will require a microsecond or more to turn ON. Under the high current slewing conditions that happens when transient audio signal are provided with heavy loads, the Class A operation will allow a smooth process. In the circuit, each sub-amplifier consists of two voltage gain stages. The first stage is made up of 2-stage common emitter Q1-7 with a gain of 2.3 while the second stage serves as a mirror stage Q13-14 that drives the voltage across a 0V load resistor with a gain of 200. A 15V supply provides power to the input amplifiers with a 0.36mA current being constant in the first stage by the floating regulator stage Q2-3 while giving compensated temperature. At this stage, the gain is set by emitter resistor R6 which produces local negative feedback while the second stage has its base attached to ground. The output stage is using the Darlington emitter follower arrangement but with 3 parallel pairs of driver and output transistors. The trimmer TR2 sets the value of constant current via the heatsink mounted Q15. TR1 sets the output DC offset voltage to zero in the input stage due to the consequent mismatching and tolerances of components. The current in the first stage of one sub-amplifier is made higher or lower by the TR1. The bandwidth of the signal below that of the open loop amplifier is reduced by the RC network R2 & C2 from the low pass filter. This eliminates the generation of intermodulation distortion. The outputs stage is using a total of six 250W power transistors to provide high performance while the short term current capability is in excess of the current capability of the wiring by 90A. The transistors should be securely attached to the heatsink to ensure good thermal contact in all temperatures. The differences in thermal coefficients can result in some slightly loosing of the screws particularly in the heatsink and top & bottom covers. Application The 60 W power amplifiers are widely designed for commercial installations which can be used for multi-zone applications like the low impedance line speaker systems. It can provide amplification as well as advanced mix functionality for distributed background music and be used with home theatre systems. They are more suitable in providing power to public address systems, recording studios, and on-stage monitoring. Source:users.otenet.gr/~athsam/Power_amplifier_60w_Class_A_Eng.htm
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