I started electronics recently and I'm trying to make the most basic mp3 amplifier. First I tried to make the amp myself with reused electronic parts but I didn't succeed... Amplify speaker with 9v battery and one transistor. Do you have any recommendations about it ?

So I decided to use a LM386 because it's easier. I found a module which integrates de LM386 on dx.com for 3$. This one http://www.hamgadgets.com/LM386-AMP-MOD. But I guess I can make it myself for less and probably better/stronger.

It worked pretty well with the 9v battery if I set the gain to 50%. I read it provides up to 0.3w of audio power so it uses 0.3w/9v = 0.033a max ?

On the module it says 5 to 12v and I read that a battery provide the amps depending on the voltage and the resistance. But then I decided to try connecting it to an outlet. I retrieved an old transformer : 9V - 300mA - 2.7w. The LM386 chip on the module fastly became very hot so I disconnected it. When I tried again with the 9V battery, the led still lighted up but the sound didn't pass through.

I know home current is AC and a battery is DC but I don't fully understand it. The AC current works on cycles (is it up only during short periods ?) and DC is a horizontal straight line (in a graph). Doesn't AC provides the amperage like a battery and the 300mA value is the maximum amperage ? Why did my circuit burn ?

  • \$\begingroup\$ Was the adaptor outputting AC or DC? (Should be on its info plate). Without knowing what the circuit was it's impossible to say what's wrong with it. \$\endgroup\$
    – pjc50
    Mar 11, 2015 at 16:49
  • \$\begingroup\$ It's not written but I found the same transformer on ebay : befr.ebay.be/itm/… \$\endgroup\$
    – Servietsky
    Mar 11, 2015 at 17:11
  • \$\begingroup\$ 9V AC - yes, that means half the time it's providing current in the wrong direction and destroying the IC. \$\endgroup\$
    – pjc50
    Mar 11, 2015 at 17:15
  • \$\begingroup\$ Ok thank you ! But will it work if I use a DC transformer ? And am I right about the max amperage of the transformer ? \$\endgroup\$
    – Servietsky
    Mar 11, 2015 at 18:32
  • \$\begingroup\$ DC transformer is the right thing to use and the amperage stated is a maximum, yes. \$\endgroup\$
    – pjc50
    Mar 11, 2015 at 22:36

1 Answer 1


A 9V output from a transformer will be alterating possibly up to a peak of 13 Volts and down to a negative peak of -13 volts. It will be doing this 50 or 60 times per second and I'd bet on the first negative cycle from the transformer you burned the chip and possibly damaged any electrolytic capacitor on the module.

I'd throw it in the bin and get a new one. Welcome to the learning process. Next time, add a bridge rectifier and smoothing capacitor and measure with a voltmenter to make sure polarity is correct and the dc voltage level.

  • 1
    \$\begingroup\$ Alright thank you! I learn from my mistakes, at least it was just a 3$ module. \$\endgroup\$
    – Servietsky
    Mar 11, 2015 at 19:46
  • \$\begingroup\$ $3 is a cheap price to pay for this "learn" !!! \$\endgroup\$
    – Andy aka
    Mar 11, 2015 at 19:58

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