I've built an amp using TDA2003. If I use a common 9V battery, it works nice, with very low noise.
But, if I change to a power supply (that I borrowed from a cell phone charger) that delivers 8.4V (1.7A), the noise gets too high. I can hear the noise even when the amp is playing. I tried to change the power supply, but the problem was the same.
So, It seems that the problem is with my power lines. I live in Brazil, and as far as I know, we use 127V/60Hz.
Searching the internet, I found that, in order to reduce noise, I should use decoupling capacitors. But the circuit already has two decouple capacitors: a 100uF (electrolytic) and a 100nF (polyester).
I'm just a hobbyist. I don't have an oscilloscope. Assuming that the circuit is OK (since it works with a battery), how do I reduce noise that is coming from the power lines? I was thinking about put an small extra cap (maybe ceramic), but I'm afraid to blow something.
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