Believe it or not, the design can be pushed even further. You will need to add more power transistors and upgrade the power supply though. Supply voltages of up to ±150V can be used without changing anything other than increasing the number of output devices. Although the MJ15024/5 are rated at 250V, they will take more because the base is tied to the emitter with a very low resistance. 1200W into 8 ohms is quite possible, with around 2kW into 4 ohms (or 4kW / 8 ohms in BTL configuration). I suggest that the number of output devices be increased by at least 25% - a total of 13 devices for each supply rail (26 in total, including drivers
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