[Sideband]

I had an old Akai amplifier that developed noise when switching on....like a rushing noise that died away after a few seconds. When I investigated, I found that the small driver and pre-amp transistors were actually sensitive to tapping.....one actually seemed microphonic......you could tap the top or gently scratch the top and hear it in the speakers.....all very odd. I fitted standard BC548's for the pre-amp types and BC637/638's for the drivers. No problems after that.