I found some old speakers laying around without amplifier. I would like to buy a amplifier board but i do not know the wattage and ohms of the speaker. The wattage etc is not written on the back of the speakers.

It has 2 big speakers and 2 smaller speakers. The big speaker is wired to the smaller one and creating one connection. Per pair one connection.

What amplifier should i buy not knowing the wattage. There is no brand or number written on the casing of the speakers.


  • Do you have a multimeter?
    – Marc W
    Aug 26 '16 at 13:18
  • @MarcW Yes i have
    – joel
    Aug 26 '16 at 15:48
  • I suggest that if you have money to spend on an amplifier, you might as well buy something in anticipation of one day having money to spend on speakers too. Otherwise, you might find you've compromised on the amplifier only to discover that you're dissatisfied with the speakers anyway! If the speakers are "old and laying around" how likely is it that they're actually any good? If you buy a decent amplifier, it's highly likely it will happily drive these speakers too.
    – Paul
    Aug 26 '16 at 15:50
  • is there any label or writing on the coil/magnet of the speakers? (you have to open up the cabinet)... any label/writing on the inside of the cabinet? any electronics inside? if so: any writing on that? -- can you take a picture? can you take pictures from the inside / wiring ? Aug 31 '16 at 11:51
  • @EdwinvanMierlo No there isn't. But i figured out it is 50W 4ohms using Joseem's technique.
    – joel
    Aug 31 '16 at 12:34

Well, it's like a game of Blackjack: to know the answer for sure, you'd have to bust your speakers.

If you have any amplifier at home or that you can borrow for a while, connect the speakers, put some music on WITH VOLUME SET A ZERO, and VERY CAREFULLY, turn up the volume litlle by little to see (actually to hear) how the speakers behave. Unless the amp is of much lower power than the speakers can stand, there will be a point where the volume you hear will be appreciable and the sound will be clear and defined (depending, of course, on the quality of amp and speakers). You can try (at your own risk) to push the volume further up a little bit, untill you hear occasional distortion and buzz from the speakers. Then you'll know you've overcome the maximum advisable power.

Depending on the position of the volume knob, you'll have a sort of relationship with the power of the amp (as a simple approximation, use a proportional relationship).

An additional previous step is to measure the impedance of the speakers with a multimeter. Set the multimeter to resistance measuring and measure the resistance of each speaker. Do it for both speakers to compare results. If one or both of the speakers measure 0 or infinite resistance, then it's busted, and you don't need to loose any more time. Otherwise if all is well it should give a measurement of approximately either 4 or 8 ohm (as almost virtually any commercially available speaker has one of these two values of impedance).

That's an important characteristic (as important as rated power) and it's one you can actually measure, so it's worth spending 5$ on a simple multimeter if you don't have one.

Once you know the speakers impedance, do the power testing with an amp with the same output impedance (if you can) and buy one with the same output impedance.

  • 1
    Thanks for your answer. I measured the impedance and it's 4.6 ohms. My home stereo uses speakers with an impedance of 4 ohms and online it says its rated 67 Watts. So when I connect my speakers to the home stereo to half of the volume without distortion. It is most likely rated 33.5 Watts?
    – joel
    Aug 26 '16 at 15:46
  • 1
    Very roughly, yes. If that's the case a 20 to 30W amp will probably be adequate to your speakers. Take care allways comparing same power units (peak power or RMS power). Aug 26 '16 at 17:10

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