bridge amplifier problem

[l_ugavante]

I build bridge amplifier with a supply voltage of 50v, and operate 250watts on 8 ohms speaker and 500 ohms on 4 ohms. I done all necessary procedure in setting up the amplifier and test it for 5 hrs at low level adjustment on input. but when i test it again for the next day upon applying a power I heard a loud hum and the power transistor runs very hot, i shut it down and check my connection theres nothing wrong with my connection, but two power transistor damage with 4 driver transistor also affected. what went wrong with my amplifier does any body experience this kind of problem please give advise. :

 

[john1]

Hi,

On these fairly hi-power transistor audio amps,
i have found them to be susceptible to open circuit output,
shorted output, supply spikes, and poorly lined up crossover
leading to excessive current at crossover.

It certainly sounds like some of your transistors have blown.

I have never seen very good ways of protecting against over-
current in output stages, most of them are just current trips.

Ive often wondered about a low level high frequency tone, that
could be picked out by a small tuned circuit, and if it dropped
in volume, that could be taken as an indication of nearing
saturation, and the circuit could reduce the drive to the output
stage, but i never tried it, or anything like it.

Maybe such arrangements to 'monitor' the saturation are common
now. I haven't seen them if they are.

You will have to replace some transistors, and probably re-align
the crossover points to suit the replacements.

Best of luck with it, John