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1 Construction Notes for the AV 800 watt MOSFET Power Amplifier Introduction The 800 Watt AV amplifier is based on My 1kw Amplifier and shares the same topology and basic PCB layout. The only real difference is the number of Output devices that the unit uses. The 1kw design has 20 O/P devices, while the AV amplifier has 14 O/P devices. This amplifier can be used for practically any application that requires High power, low noise, distortion and excellent sound. Examples would be Sub-woofer amp, FOH stage amplifier, One channel of a very high-powered surround sound amplifier etc. The AV amplifier has four main stages of amplification. We will begin by looking at each stage in reasonable detail. The Error Amp Stage The first stage is what I call an asymmetrical balance input error amplifier. It is a design, which allows only one single differential stage and yet has the ability to accept a balanced I/P source. An unbalanced source can be used if either the inverting or noninverting I/P is tied to signal ground. Now I will explain how each device in this stage works together. Q20, Q21, R51- R54, form the main differential error amplifier, which then has its collectors connected to a cascode load. Q18, Q19, R49 and ZD2 form the cascode stage which provides a constant 14.4 volts on the collectors of Q20, 21. Q17, R48, R50, ZD1 and C12 form a constant current source, which supplies 1.5milliamps to the first differential stage. These modules form the first stage of the amplifier and basically set up how the whole amplifier is biased from front to back. The Voltage Amplification Stage This next stage provides most of the voltage amplification that the next stage needs to drive the o/p stage to full power. Q22, 23, Q15, Q16, R34, R35, R36, R42, C7, C26, C27 form the second differential voltage amplification stage. Q15 and Q16 form what is known as a current mirror load for the second differential stage and basically force this stage to share the current supplied from R36, which is about 8milliamps. The remaining components, namely the caps provide local frequency compensation for this stage. The Bias and Buffer stage As the name suggests Q24, 25, 26, R15, 32, 33, 37, 43, 47, C6, ZD5, ZD6, form the Bias and buffer stages. Its main purpose is to provide the MOSFET Gates with a stable and compensated supply voltage and Buffer the Voltage amp stage from the high Gate Source capacitance. Which would without this stage cause the frequency response and slew rate to be very poor indeed. The down side of this is the extra stage does introduce an extra dominant pole in to the amplifiers feedback loop. 2 The Output Stage or Current Amplification Stage Once again as the name suggests this stage converts the voltage developed in the VAS and provides all the amperes needed to drive 8 or 4 Ohm loads. 2-Ohm loads are possible for several minutes at a time. In fact I have tested the 1kw amplifier to over 1600 watts RMS into 2 Ohms. But this would not be recommended as