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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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  • \$\begingroup\$ 1. Polyester -> not good, use ceramic; the leads have to be very short;2. you may also try inductors between power supply lines and amp; \$\endgroup\$
    – ilkhd
    Commented May 24, 2015 at 4:00
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    \$\begingroup\$ What noise? Whistling? Hiss? Hum?. If it's 60Hz hum (or 120 or 180Hz) then 100uF is nowhere near enough decoupling. Try 2,000 to 10,000uF, or a linear voltage regulator. \$\endgroup\$
    – user16324
    Commented May 24, 2015 at 9:01

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The catch with low cost audio chips like this (or even the more expensive LM1875) is that guaranteed PSR is rather low, around 50dB, while the typical one can be much better (90dB promised in the case of LM1875, see below).

enter image description here

The TDA2030 is actually even less promising with only 40dB guaranteed and 50 typical:

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And TDA2003 is even worse with 30/36 dB min/typical:

enter image description here

That's definitely going to be audible if the power source is noisy. (Also beware that these numbers aren't quite directly comparable because ST tests at 100Hz noise and TI at 1kHz...) So... if a linear regulator supply is out of the question (which will totally kill any audible noise from that source in my experience [it gives you some guaranteed 90dB with a LM317/LM337 solution]), try a different TDA2030 chip, assuming you're not having fakes or the CXY knockoffs (or try a LM1875 as it's pretty much a drop-in replacement). Luck of the draw with these things.

Your unspecified "cell phone charger" is almost certainly an SMPS with atrocious noise characteristics (better ones exist, but usually not at this price point). See for example what cheap ones have inside: https://www.youtube.com/watch?v=wi-b9k-0KfE

Also, a linear regulator after an SMPS won't be anywhere near as effective as one after a "classic" (filtered) rectifier. The reason is that the typical linear regulator is very good at eliminating mains hum (actually 2x the mains frequency in a bridge circuit), but its performance gets a lot worse starting around 10kHz... and that's exactly where the SMPS noise usually starts. And the audio amp IC has similar curve, e.g. here's the one from the LM1875:

enter image description here

Summary: the guaranteed fix is better (preferably linear regulated) power supply, which you can build yourself. If that's not an option somehow... try the amp chip lottery. However no amp chip will will magically eliminate the horrible noise put out by a super-low cost SMPS.

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  • \$\begingroup\$ In case you want to filter well the output of a switching power supply, check this: www.linear.com/docs/11877 it explains very well and shows very good results with little effort. \$\endgroup\$
    – FarO
    Commented Jun 9, 2016 at 11:24
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If you do not have an oscilloscope, then how did you find out you have too much noise? If you can measure it with a multimeter it must be a relatively low frequency. It really depends on what kind of noise it is. Apart from the solutions already given, you could also try a ferrite bead(for noise currents) or a common mode choke(for noise currents in differential lines)

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    \$\begingroup\$ It's easy to notice/debug noise in such circuits using your ear. \$\endgroup\$ Commented Sep 22, 2015 at 22:14

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