High Watts Amplifier

The main changes made in this design was the addition of a clipping detector circuit and bias circuit modifications for the use of International Rectifier HEXFET MOSFET’s. Later modifications where made to the error amp and VAS stages which improved the overall sound of the amplifier. The power supply components for this amplifier are as follows and are expressed for Two Channels. 1 x Toroidal Transformer with a Core rating of 625VA. 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.

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The secondary windings are as follows.

2 x 50 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.

A suggested power supply schematic is shown below with the schematic of

The amplifier.

When using this type of MOSFET in the AV800 amplifier is strongly recommended that the output stage devices be matched. As it has been found that if this is not done then there is no guarantee that they will share the current under load. The Source resistors provide only a bit of local feedback and don’t in any way force the devices to current share. The best method I have found to work very well utilises just a 150 Ohm 1 watt resistor and a +15 volt DC power supply. If you look at the schematic below it shows how to connect and measure the N-channel devices and the P-channel devices. With the devices connected, as shown measure across R1 with a multimeter set to DC volts and measurement of between 3.8 volts and 4.2 volts will be shown. Simply match the device in-groups to a tolerance of +-100mv. Please note that you only have to match the n-channel to the n-channel devices and the pchannel to the p-channel devices, not the N-channel devices to the P-channel devices.

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