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I don't know if I'd trust it...

Building an amp with that much power output capability can be a challenge. It requires lots of iron, and lots of patience.

There are a couple of circuit comments I'd like to make, there is no protection for the outputstage or the speakers. If you short the output, you'll lose transistors. If the transistors just die, they'll take a speaker with them. Lots of little things look like they've been left out. For instance, there are no clamping diodes to prevent the inductance of a speaker from putting higher than rail voltages on the output.

I would advise a MOSFET amp for PA use. In general, Mosfets accept more abuse than transistors. Also, they're not subject to thermal runaway. The bad side is that they cost more and put out more noise and heat.

If you can do the PCB yourself, check out the 550WPC (4 ohm) amp in Randy Slone's book.

If you need the PCB, check out ''. The designs seem a little dated, but the specs look good. I've never heard one, but have seen good things about them.

Good luck. The circuit board for this amp is the deal of the century. Still available for only $6 from Old Colony! Who cares? It's only 40 watts. Not really. Jumper Q11 and change R9 to 27K 1/2W(Pass suggests this for the A40 if you are having trouble finding the fet, only use 10K for his design) Change Q1,2,3,6 to the MPS8099 or 2N5551. Change Q4 and Q5 to the 2N3440/2N5416 pair. Use a heatsink on these. Change R10 to 68 ohms. Delete R13 and D3. R11 will be selected for bias. C4 will be for high frequency response. R14,15 will be changed to one 220 ohm per ouput device. The outputs are to be two each BUZ900D/905D This will give 400 watts RMS with a 53V+53V 800VA transformer and only two pair of outputs! Don't forget to add 10V gate zeners and output flyback diodes.


I am interested in your comments regarding both amp and speaker protection. Do you know of any sites which give details of how to go about incorporating these features in an amp design. Or do you have the time to go into it in a bit more depth on the forum.


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