Construction Notes for the AV 800 watt MOSFET Power Amplifier
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 anyapplication 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 inputerror 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 itscollectors 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 sharethe current
supplied from R36, which is about 8milliamps.
The remaining components, namely the caps provide local frequency compensation for
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 theVoltage 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.
The Output Stage or Current Amplification Stage
Once again as the name suggests this stage converts the voltagedeveloped 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 a long-term load at all.
As it does exceed the SOA figures of the output stage.
Power supply requirements for the 800 AV Amplifier
The power supplycomponents for this amplifier are as follows and are expressed for
One Channel or One power module only.
1 x Toroidal Transformer with a Core rating of 1KVA. Primary windings are made to suit
your local mains supply. eg: for Australia One single primary winding with a 240VAC
rating. For USA, 110VAC, 115VAC and I believe there is a 220-Volt AC mains supply in
some areas of the United States.For the UK it would be 220 VAC to 240 VAC.
The secondary windings are as follows.
2 x 65 volts AC at full load.
One 400 Volt 35 Ampere, Bridge rectifier.
2 x 4.7K 5 Watt ceramic resistors
Minimum filter capacitor requirements would be 2 x 10,000uf 100 volt electrolytic.
Ideal capacity would be 40,000uf per voltage rail.
For stereo or dual mono operation the following power supply will be...