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?

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    \$\begingroup\$ If the amplifier isn't already in a sealed metal box, try boxing it up. This would help cut down electrical pickup. Use a diecast aluminum box with sockets grounded to the box, if possible. \$\endgroup\$ – Dave Kielpinski Jan 31 '15 at 13:13
  • \$\begingroup\$ Describe what sort of noise. Buzzing, humming, whistling, sounds like garbled voices are all possible as well as the steady hiss of white noise. \$\endgroup\$ – Brian Drummond Jan 31 '15 at 14:24
  • \$\begingroup\$ A link to the amp IC is always appreciated. Do you have a 'scope? (then you can look at it and find any frequencies that are strong... that will give a hint.) Besides a metal box, try turning off other electronics, florescent lamps, scopes, computers, monitors. Move the Amp around is there an angle dependence, does it get bigger somewhere, can you shield it with your body? \$\endgroup\$ – George Herold Jan 31 '15 at 16:08
  • \$\begingroup\$ Has it always produced a lot of noise? \$\endgroup\$ – Andy aka Jan 31 '15 at 17:10
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    \$\begingroup\$ Can you post some clear photos of the PCB/circuitry? Does it produce noise when there is no input connected? If not, is it only when you connect it to a PC, or an ipod input as well? \$\endgroup\$ – Ryan Mar 1 '15 at 15:43

I've met a noisy low frequency in a portable cassette player with subwoofer a long time ago. The noise wasn't humming or buzzing, but more like a soft crackling noise (static) and the level of the noise was unpredictable, it slowly varied over time and had annoying louder "rushes". The volume or EQ controls did not affect it, so I feel this is a similar case.

I found out that twisting or compressing the heatsink against the PCB reduced or eliminated the noise. I fixed mine by attaching a stiff copper wire hook pulling the heatsink against the PCB. At least you can try if applying force onto the heatsink makes a difference.

I wasn't able to get into the root cause, I can only make guesses:

1) Chip fault, assuming that the applied external mechanical force enhances the internal connections.

2) Electromagnetic loop/interference with the currents of the chip and the aluminium heatsink -> make sure the heatsink is properly grounded.

I recommend to resolder the heatsink and chip in any case. Heatsink soldering can break due to temperature changes. The fracture circle around the "leg" can be seen in proper light, if at all. They're often nearly invisible to the naked eye.


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