If Q4 continues to burn even if it is new, you probably have problems with the gate resistor, it is practically impossible that only Q4 gives problems, Q4 is in perfect parallel (except for the gate via resistor) to the other mosfets of that side (high or low side).
I repeat: all mosfets must be the same, and preferably replaced together, not just broken ones.
Even if the broken mosfets are on one side, also replace those on the other side, this is not essential but preferable.
On this amplifier you can use IRF640 but also IRF640N can be fine.
Thanks a lot , help me pls with bass-filtering also, I would like to decrease a little bit middle-treble-cut , maybe 4 eyes are better than 2 ...
"BRIDGE OUT" is not an output usable for any other amplifier, it is used only for the connection of another amplifier identical to the one in your possession, to be able to use them in bridge (full bridge configuration) in order to use the quadruple ( theoretical) of the power at the same load impedance, divided by the 2 amplifiers.
For this reason it is normal that it is already filtered, because in the second amplifier, the input will be devoid of cross-over, so the main amplifier will have to do the filtering job for him too, also if you control with the oscilloscope, you will notice that "BRIDGE OUT" is 180 degrees out of phase with the signal reaching MODU pin 1.
It is not impossible to change the "BRIDGE OUT" frequency cut but it is rather useless, since you will not be able to use it for your purposes.
L1 normally works very hot, you can feel comfortable, because in all the amplifiers of this type that I have repaired, L1 was always very hot even when it was empty, even without a signal and even without connected speakers.
Probably the output filter has not been optimized in the best way or perhaps the material used for the core is not suitable and saturated very soon.
But ultimately, it's not a problem, these amplifiers work, have worked and will always work under these conditions.
Thank you for your answers,
I gave up re-filtering the treble-cut, somehow there is a protection that forbid this action, if C18,C19 are adjusted (decreased) - more than 30% , an error beep is generated after the first checkings are completed when the sistem is powered on.
Decreased from 330 nF to 220nF the gain in middle-treble spectrum is weak, therefore I restored to original values, also beep ceased and now seems to work everything normally.
Thanks to everybody, after hours and hours of study now I have a working amplifier ...
I gave up re-filtering the treble-cut, somehow there is a protection that forbid this action, if C18,C19 are adjusted (decreased) - more than 30% , an error beep is generated after the first checkings are completed when the sistem is powered on.
Decreased from 330 nF to 220nF the gain in middle-treble spectrum is weak, therefore I restored to original values, also beep ceased and now seems to work everything normally.
Thanks to everybody, after hours and hours of study now I have a working amplifier ...
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