Let's see what kind of trafo and heatsink to I need for this amplifier.

• As stated before, the average output power is expected to be 80W at 8 Ohm load. During one cycle, power will be delivered from positive supply rail half the time, and from negative supply the other half, meaning that each rail will have to deliver at least 40W.
• This implies effective current of 3.16 A, or 4.5 A peak , if we pretend that signal is pure sine (which is not). Power supply at 42V delivering 4.5 A peak half the time, gives away on average 42*3.16/3.14 = 60.2 W.
• Transistors will, therefore dissipate 60.2 - 40 = 20.2 W each.
• We re talking about efficiency of 66%, what is reasonable for Class B/AB amplifier
• Knowing that maximum junction temperature is 150°C, the maximum allowable thermal resistance is (150-25)/20.2 = 6.18 K/W. This means, that Rth of junction-to-case, case-to-heatsink and heatsink-to-ambient combined must be less than 6.18 K/W.
• Transformer I need for this amplifier is then 2x60.2 = 121 VA minimum, with a +/- 45V secondary voltage and a center tap.