Mosfet Amplifier Design
The circuit diagram shows a power amplifier that I designed back in 1979. The idea for the circuit came from two different circuits in the book Op-amp cookbook by Walter Jung. One used buffer transistors to increase the output capabilities of an opamp while the other used two transistors to reduce the voltage supply to the opamp allowing it to be used with circuits of any voltage. I have seen both these techniques used in various circuit designs but never together. The characteristics of Mosfet power transistors meant that a simple high power amplifier could be built. The only difference between a 100Wamplifer and a 600W amplifier was the number of output transistors and the size of power supply.
Amplifier circuit using 1 output transistor
Amp rack circa 1982 total power 1200W
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The picture on the left shows a prototype amplifier and is a good example of how not to build a high quality amplifier. Considering it was knocked up on Vero board this amplifier is still in perfect working condition despite being pressed into service at many a gig. The board above the transformer provides DC offset protection and eliminates switch on surges reaching the loudspeaker. The black object at the bottom right is the cooling fan and the audio circuits are the ones at the right of the case. The reason for this prototype was to test the forced cooling and tunnel type heat sink and the output protection circuit. |
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This picture shows a bit more clearly the "non production" lash up nature of this particular amplifier. I did develop a proper PCB, but most of the bits here were borrowed from the "here's one I discarded earlier" box. The three boards at the top of the picture are from left to right, the two main amplifier boards, one for each channel, showing how few components are used in the design, and on the right is an input board that converts a balanced input into an unbalanced signal to drive the amplifier. The potentiometer that controls the input level, seen above the fan, is wired between the input board and the amplifier boards. Three of the twelve output transistors can be seen on the inside of the heat sink tunnel. Again the output protection circuit can be seen below the heat sink and to the left of the main transformer |
