Sounds like you're most of the way there...
You didn't say what you were using to power the amps - perhaps a recycled / reused power adapter from some now defunct gear.
Try it with a 12V gel battery: that should eliminate any possibility of hum. If it does, then you're amp is just fine: you'll just probably have to build a small, approximately 12.6 volt *regulated* power supply.
It's simple enough - a couple of 1000 uf capacitors on input, an LM317 and a few resistors (probably one of them will be a 2k or 5k trimpot so you can adjust to the right voltage you want out). You'll probably need a heatsink - as it'll draw about 400ma or so. And bypass the output with a .1uf capacitor (mylar, ceramic - doesn't matter). This should give you a CLEAN, regulated output. I built one and it makes the amp completely silent with the input audio muted.
Many of the wall-warts (A/C adaptors) are filtered, but unregulated, and will pass some form of 'ripple' which is probably the source of your 50hz Hum. Even some of the regulated laptop-type (switch-mode) supplies which ARE regulated will pass some line-frequency ripple or even some switching ripple which will show up at a higher frequency.
If it still picks up hum while ON a battery - then you might be picking up some strong, nearby A/C hum via induction. Some sources might be fluorescent lighting, a (HUGE!) transformer nearby, nearby power lines, or some other electrical source.
Mike Y
Dallas, Texas
J&B wrote:
Me and a friend built this amplifier and it worked first time, but there is a horrible hum. To me it sounds like a 50Hz hum (we live in Europe) but we're not feeding 50Hz into the amp. Anyway, the volume control controls the level of the music but the hum is constant. It's the same on both our amps and on both channels. Anybody's got any idea of what's wrong? I have tried two different 12V sources and it's the same with both, I haven't tried a battery so far though.