Hi, I'm doing audio amplifiers (Rapid electronics project kit) with Y9 and housing in a vac formed case. Everything is going fine, except... half of the circuits have a really loud hum/buzz that completely drowns out the music. I've checked all the usual stuff - polarity, joints, bridges, visual inspection of speakers. I've gone at them with a multimetre but can't find any vaults. As far as my eye and my multimeter are concerned, the circuits are perfect. There's a little copper wire behind the joints of the speakers and I thought the problem might have been that some of them were damaged, but even when I'm sure they're perfect the buzz persists. Any suggestions which components might be at fault here? I've checked the pins of the chip and they aren't bent or pulled out. We're using the correct value - or rather, the value given in the project pack. That seems to work for those that are fine. I'm sure I'm missing something simple but I just can't find the problem, and I really want all of the students to have a high quality amplifier to take home. Any suggestions? Thanks!