Burnt chinese amps

Hello guys, i have a 4ch chinese cheap amps... Yeah its burnt on half side of the amps.

My amps brand is 'AudioPyle', and the power output i assume 4 x 75 watts which i get from the pcb marks.

My installation is I use 4 ch amps for front and sub. Each channel 1 & 2 for my front speakers and ch 3 & 4 were bridged to push the subwoofers.

And my story started with my mistake when i disconnect the subwoofer cable. That cable touch my car body (ground), and my sub make sound like clipping sound. My amps went okay when I put that cable back to my sub, sounds okay without any noise when playing music.

After that, when i try to adjust gain, the problem aprroach. Just a little turn for the gain knob, my sub sound like clipping again and a few secs then my amps smoked. A all FET & the gate resistor burnt, which supply power to the ch 3&4.

Okay then i try to fix it. I pull all resistor and fet on half side which supply power to ch3&4. I just hook up to a battery and test the half fet, the result is ch1&2 working fine while the ch3&4 have no fet. Okay then I put a new fet and gate resistor which supply the ch 3&4. All when okay when fired up, music played clean.

After replacing fet, i try to adjust the gain then i turn the gain knob just a little and my amps make clipping sound again. I turn it off immediately before it burnt my fet again. But when i try to turn it on again it still make clipping sound and the protect LED were blinking like a disco. Okay then i turn that off again before my fet burnt again.

But the condition is now when i disconnect the speaker from the amps, the LED protects lamp were turned off and the amps were on like theres no problem. Ch 1&2 were working fine when turned on and ch3&4 not connected to speakers.

Thats my problem, anyone can figure which part is still broken?
My amps main PCB is on the attachment and theres still exist a small pcb beneath the main PCB which includes gain knob, Lowpass/highpass switch and knob.
 

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Confirm that the shield grounds on the head unit are intact:
RCA Shield Repair

Check all shields on the head unit.

If those are OK, confirm that the RCA input shields on the amp are directly connected (0 ohms) to the non-bridging speaker terminals for the corresponding channels.

Thanks for your response :)

Ah I forgot to mention, I use a smartphone to play music when I test the amp.

YES, for the RCA shield of the amp were directly connected to non bridge terminal for the corresponding channel.
 
After a few days not used, i turn on the amps and the channel 3&4 were normal.

I triend channel 3 only with speakers and music, channel 4 only with speaker and music, and also try bridge 3+4 with speakers and music. its normal, the music sound good.

I turn off the amps, and turn the gain knob a little bit higher, and turn on the amps again. The music went well, sounded good.

Turn off again, and turn the gain knob all the way down. And turn on with speakers and music, and the amp protected.

Turn of again, turn the gain little bit more higher. And then the amps went back normal, music and speaker played well.

Turn off again turn the gain more higher with speaker and music, turn it on and the amp protected.

Turn off again and slightly reduce the gain with speaker and music, turn it on and the amps back normal.

I'm confused with these...
Do the power supply fet could be burnt just because of the defective gain knob?
 
In some amps, a defective component like the gain pot can cause the amp to oscillate at very high frequencies which can cause excessive current draw in some amps.

It's best not to adjust the pot when you have a problem like this because you are changing two things (output level and gain position). This makes troubleshooting difficult. Set the gain low and drive the amp harder to see if it goes into protect.