N-Power Amplifier

Posted on Feb 4, 2014

The amp has been designed to be an ideal upgrade for builders wishing to step up from Gainclones. This amp will kick out the power where Gainclones fall over, yet still maintain a sweet sound. The use of n-channel MOSFETs in the output makes the amp easy to source parts for as well as being very rugged. Provided that high power MOSFETs such as the

N-Power Amplifier
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specified IRFP250 are used and the appropriate fuses are used on the board, the amp should shrug off short circuits at the output. With the recommended supply voltage of 41 V per rail (30 V transformer secondaries), 160 W RMS into 4 ohms is achievable, again if the recommended IRFP250 devices are used. The use of less powerful MOSFETs will mean you need to use higher load impedances and/or reduce the rail voltage. A differential pair is used at the input, which features a constant-current source in the tail and resistor loading. There are tracks and pads on the PCB to implement a current mirror for future upgrades. The voltage amplifier stage (VAS) uses a bootstrap capacitor to act as a current source, primarily to allow the drive to the upper output device to swing above the positive supply rail to increase efficiency. The lower output device is run as a Sziklai pair so has quite good efficiency too. The bias generator (T6, R11, P1, R12) is configured so that if the potentiometer goes open circuit, bias current will fall to a small amount. The Zener diodes on the MOSFETs are to protect them from over-voltage which can `punch through` the gate destroying the device. The corresponding diodes in series with them prevent any back EMF from the load turning the MOSFET on. Do not apply any power to the amplifier until the bias transistor and output devices have been mounted to the heatsink. Failure to do this will most likely...

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