Hello, a long time ago I build 2 P3a's. They work great still no failiers. 
But I have noticed that when I try to get too much bass out of one of them, it starts to make cracking noise. I have a sine generator, and at any frequency below 70Hz it starts to crack with the same frequency except the cracking noise is coming out of the tweeter. I don't have an oscioloscope to check, but I suspect that I possibly damaged some of the transistors when I accidently soldered q9 the wrong way and all of the bc546 transistors blew up, then I checked all of the other transistors and they measured the right values, I just measured them with Ohm meter. So I got some new bc546's and I powered the amp without a heat sink, so when i touched the output transistors they burned my fingers, and I powered down immediately. I put the heat sink on and powered back again, and everyting worked fine until I noticed that cracking noise. Both amps are using exactly the same transformers and caps and everything so I am suspecting those transistors, I have 3 of each so I am going to try to replace them, even tho they measure the same resistance as the ones on the working amp, I didn't try to exchange them because it's just too much trouble. The amps are running at +/-24V DC so I am in the SAO. I am just looking for an advice and any information is greatly appreciated. Thank you very much.
Petar
But I have noticed that when I try to get too much bass out of one of them, it starts to make cracking noise. I have a sine generator, and at any frequency below 70Hz it starts to crack with the same frequency except the cracking noise is coming out of the tweeter. I don't have an oscioloscope to check, but I suspect that I possibly damaged some of the transistors when I accidently soldered q9 the wrong way and all of the bc546 transistors blew up, then I checked all of the other transistors and they measured the right values, I just measured them with Ohm meter. So I got some new bc546's and I powered the amp without a heat sink, so when i touched the output transistors they burned my fingers, and I powered down immediately. I put the heat sink on and powered back again, and everyting worked fine until I noticed that cracking noise. Both amps are using exactly the same transformers and caps and everything so I am suspecting those transistors, I have 3 of each so I am going to try to replace them, even tho they measure the same resistance as the ones on the working amp, I didn't try to exchange them because it's just too much trouble. The amps are running at +/-24V DC so I am in the SAO. I am just looking for an advice and any information is greatly appreciated. Thank you very much.
Petar