Comments on this Amplifier

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I like to build a kit amplifier from Ebay, it can o/p a max of 120W into 8 ohm with +/- 50V supply. This amplifier is to drive the twin woofers with a total of 4 ohms. This amp should be able to drive the twin woofers giving a total of 240W into 4 ohms which is sufficient.

I welcome any comments on the design of this amp., whether it is any good. I think it is alright. See attachment for the photo and circuit diagram.

Thanks in advance.


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It is a split supply amp, which can toast your speakers if it overheats and the output transistors avalanche. Shorting the output with a 1/4 phone plug pulled 1/2 way out can also toast the output transistors. Buy a a diac-triac protection circuit to put on the output, which detects that the output voltage is too close to either +-Vcc, and shorts out, blowing the fuse from the power supply to VCC if you install two, or the line fuse from the house power, or the power transformer if you don't install any fuses. Peavey has inexpensive diac-triac PCB's somebody says, and they operate in Aust., but which one has the right voltage triac for your Vcc is up to you. You could look up peavy schematics CS400, CS600, CS800 and see which one has your Vcc voltages, then get the PN of the diac-triac PCB from the parts list. Okay edit, forget CS800s, has +- 75v rail, too much for +-50.
There are several protection circuit threads on here, read them. CS800x or CS800s has about 4 levels of output protection, all of which are very cool and why it costs $999 new. PV4, PV8,PV1.3k don't have the protection, part of why they are cheap. I got a CS800s with repairable issues and good protection for $200, best way to buy all the protection circuits. Some Crown models and some QSC models also have protection circuits.
Commercial amps also have a thermistor controlled fan circuit, which keeps the fan from making noise when you are running quiet.
I don't see any pots for adjusting the bias current on the output transistors, useful for tuning out the crossover notch noise.
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