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LM3886 - Noisy signal unless inputs shorted

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#81
Bill_P

Thanks. Actually this was one of the wrong boards for the amp I wanted to build. Same board, but resistors result in higher gain. For the boards that I already finished with the lower gain, I need to test those. That leads to my next question, which I'll post elsewhere.

BTW, I don't understand something-why is it that the connection I made on the POT got rid of the hum. I mean, I didn't actually connect anything to ground, did I?

Thanks
 
#82
Often hum is the result of a connection to earth/ground causing a loop with other wires/equipment also connected to ground. It isn't the ground itself that causes the hum, rather it is this loop that makes a very good aerial that picks up electromagnetic interference. This may help you understand and is very useful info for making your amplifier hum free. http://hifisonix.com/wordpress/wp-c...to-wire-up-a-Power-Amplifier-October-2017.pdf
 
#83
Hogwild said:
Bill_P

Thanks. Actually this was one of the wrong boards for the amp I wanted to build. Same board, but resistors result in higher gain. For the boards that I already finished with the lower gain, I need to test those. That leads to my next question, which I'll post elsewhere.

BTW, I don't understand something-why is it that the connection I made on the POT got rid of the hum. I mean, I didn't actually connect anything to ground, did I?

Thanks
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The pot put a load across the input to the amp via the wiper and the resistance of the pot to ground. The resistance varies by turning the pot and never goes higher than the properties of the pot. (10k/20k/50k Ect.) The amp will always have an input load to ground through the resistance of the pot. Without a path to ground, the input of the amp is unbalanced and will pick up any electrical generated noise around it, as from your fluorescent lamp as you stated earlier. When things are open, the wiring between the amp and the pot is basically an antenna..


The grounds of the input of the amp and the source should always be connected together UNLESS both units are being powered with the same power supply. A "Ground Loop" issue could happen but that's another subject..


The positive signal input of the amp is connected to the wiper. The positive from the source is connected to the opposite end of the resistor in the pot. The resistance is fixed to the source by the properties of the pot. (10k/20k/ect..) The wiper is collecting signal from the fixed resistance load connected to the input source. The resistance varies and is seen by the amp. When the wiper gets closer to the source input of the pot, more signal gets to the amp. As the wiper goes farther away along the resistor load of the input, less signal gets to the amp. When the wiper gets to the grounded end of the resistor, the signal is gone.. (But the input of the amp still has a load on it)


Bottom line.. The input to your amp was unbalanced.
 
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