My LM3875 paralleled X 4 .... Hum problem solution

[coffin]

Hi
Recently I just finished installing a dual mono LM3875 X 4 paralleled amp, but it suffered from hum and buzz. So I looked for grounding solution, then I found one, although it is for discrete amp, but It works perfectly here! All the hum and buzz disappear.

http://circulator.bravehost.com/myPi...pStereoENG.jpg

tip: WIRE A 10 OHM 5W RESISTOR FROM FILTER CAPS GND TO CHASSIS FOR EACH CHANNEL.

cheers

Coffin

[coffin]

Here are some photos.

But there's another issue, that when I let the input unplugged, it seems oscillate, because when I tested it in this way, I hear my cheap testing speaker unit cried out, and the coil of the unit burned.....

Should I have to add a buffer at input, or should I do compensations at input?
Did anyone have this kind of experiences?

cheers

Coffin

[tomchr]

You definitely want the center point of your supplies tied to the chassis as you've done. I believe for safety, you are required to have a rigid connection anyway. In most countries, the single resistor you have used is not legal as in the event of a short to the chassis, the resistor will burn up before the circuit breaker and you won't have any protection. I suggest adding a diode bridge across the resistor as shown on Rod Elliott's page (about 2/3 down).

Are your input RCA connectors insulated? I'd recommend that they be - otherwise, you now have a ground loop.

I see a couple of different reasons why adding the 10 ohm resistors helped. If your input connectors are insulated, the amplifier ground is floating with respect to the chassis. Thus, any noise on the mains safety ground could be coupled into the amplifier. As the mains safety ground is normally tied to the mains neutral wire, it'll have quite a bit of 50/60 Hz (and harmonics) on it. Connecting the amplifier ground to the chassis "locks" the ground reference for the amplifier and noise on the safety ground will no longer couple into the amp. Now, if your inputs are not insulated, you now have a ground loop. I.e. a current that circles from the input connector ground through the chassis to the ground point and from there via the supply ground back to the input connector via the ground connection between the board and the input connector. This current can induce electromagnetic fields that will couple into the amplifier, usually resulting in a poor noise floor and/or mains hum. The 10 ohm resistor helps limiting this circulating current so it won't interfere too much. But ideally, you should break the loop. The best way of doing this is to make sure the input connector is insulated from the chassis. There are connectors that are made with an insulating plastic base for exactly this purpose.

~Tom

[coffin]

Hi
The inputs are of course insulated from chassis. I was tied gnd directly to chassis, but it humed....I'll try add a cap and bridge.
BTW, in Taiwan, most AC ground is same as Japan -- empty.

Thanks

Coffin