I have an PC audio amplifier using TDA8947. One 30W subwoofer and two satellites. It produces a lot of background noise and I would like to cut it down a bit. The noise is all in the subwoofer. It was always there since I purchased the system.
I supposed the source of the noise is:
1. poor filtered power supply (it already has a 4700uF/35V capacitor)
2. noise in the input cable (which in my case is pretty long)
To test 1) I removed the power supply and hooked the amplifier to batteries. Yes people, you are reading this right! I put together nine 1.2V rechargeable cells and one 9V battery and I got close to 21V, while the DC voltage from the transformer was 24V. Results: the noise didn't changed a bit! So it is not a power supply filter problem.
To test point 2) I shorcuted the input, right on PCB. Result: the noise didn't changed a bit! So, it is not a noise on the input cable problem.
The upper part of the circuit board is shielded by ICs heat sink which is really thick. The lower part (where the connections are) is not shielded. I will try some aluminum foil.
What else should I try?