I've just spent a couple of days building an Amplifier for MP3 Playback,I used the Vellerman's circuits for the pre-amp & two mono 7W Amps.Pre-amp uses a TL072 IC,& the Amps TDA 2003 ICs.All the components are new & I thought I'd made a good job of building it (it looks verry pretty with the rainbow LED)The problem is,it doesn't WORK;All I get from the speakers is a HUM.The connections between the Volume control,pre-amp & amps are all screened wires.The gain adjustment & volume control make no difference,so I suspect the pre amp? because whatever the problem is,it affects both channels.Is there any way to test the pre-amp,without ripping the whole thing apart? I might not have music,but I've got a pretty light to watch while I'm listening to the HUM;