I have a few speakers that I salvaged from broken electronics and I plan on buying an amplifier board to drive the speakers. I'm currently looking at an amplifier such as this one and have sketched a diagram on how I believe it should be wired and powered as to not blow the speakers. I'm not sure if this is correct or if my calculations are entirely off. I appreciate any help and corrections you could provide.

Also, is there an ideal way to supply the 12V and 11V? I have an old pc psu and some adapters but I'm not sure if those are the best approach.



  • \$\begingroup\$ 12VDC is 6V peak output voltage, assuming your amplifier can get right up and down to the rails, which it can't, so at best 5.3V peak, which is 3.7VRMS, which gives you 5.8W across 8 ohms, or 11.6W across 4 ohms. Sorry to disappoint. \$\endgroup\$
    – user207421
    Jun 6, 2016 at 9:14

3 Answers 3


Supply 12V to both amplifiers, just on one don't turn the volume up as so far, or put a resisitive divider on the amplifier signal input.

You would still need this divider even if you were to use an 11V supply as you'd have to scale the input down to match the lower peak output

if you are connecting the aplifiers to diffrerent sources the divider is not needed, just adjuste the volume control to suit.

These amplifier modules are class D and so are no less efficient operating at reduced volume

to get full loudness it's going to want 50W or so of 12V. So a 3A or more "power brick" is indicated, but music power if often spiky so you may get acceptable performance with a smaller power supply. if you want to used the two amplifiers for different purposes using two powersupplies (one each) may be more convenient.


It is not clear what your goal is here? With such random, mis-matched speakers, you will not achieve uniform audio volume from each speaker. But we don't know if that is what you are trying to do?

Reducing the power input to the amplifier is not a valid method. Adjusting the audio level (volume control) is the method of keeping the power output within the maximum rating of the amplifier.

Simply operate the amplifier (whichever one you choose) of your "nominal" 12V supply (which probably isn't exactly 12 volts, but it doesn't really matter which is why the working voltage is shown as "10-26V".)

Using mixed 4 ohm and 8 ohm speakers presents an awkward combination, but then the speakers are different anyway. It is what it is. There is little benefit to over-analyzing connecting some random, mis-matched speakers to a commodity ultra-cheap digital amp board.


Two things.

First, your power calculations seem to be off by a factor of 2. The actual calculation for an amplifier is far more complicated but using the equivalent DC will give worst case scenario. V^2/R = W for the first case gives 144/4 per channel giving 36 watts. Then divided by two gives 18 watts per speaker absolutely worst case. In reality it's less than that.

Same thing for the second calculation 121/7.2 gives 16.8 watts per channel.

Second, the link you provided shows that the amplifies are not 4ohm stable when using both channels in stereo.

Working voltage: 10~26V, Max output: 2 x 50W (left and right channels)/8ohm, single channel BTL power 100W/4ohm

These specs show that when using the chennels individually they are 8ohm stable. If you bridge the amplifier the it will sustain a single output that is 4ohm stable. (Bridging is only available on certain amplifiers, it is when the positive of one channel is connected to the negative of the other channel across a speaker. The amplifier is designed so that one channel drives the speaker normally through the positive terminal. The second channel drives the speaker with an inverted waveform but through the negative terminal.)

There are several problems with calculating amplifier power.

First, using DC for the calculation gives a wide ballpark. This is because the calculation is like connecting the input source directly across the speaker. In reality the speaker is driven by AC and will only see maximum input power at the greatest volume and for only the peak of a sine wave.

Second, sound is not a pure sine wave. It contains different frequencies and harmonics between 20Hz and 20kHz with varying amplitudes. This means that using calculations for RMS (pure sinewaves) is also not practical.

Third, speakers are an inductive load. This further complicates the power calculation because the speaker induces reactive power.

In reality they best bet is to match the RMS power of the amplifier as close as possible to the RMS power of the speaker. Then adjust the gain until you hear the speakers distort and turn the gain down a bit. If you can't match the amplifier and speakers closely a more powerful amplifier and weaker speaker is better.

In your comment you wonder if the amplifier would regulate the power down. They won't. I've just scratched at the list of what needs accounted for when preforming the calculation. There are many more that easily account for the dependency.

  • \$\begingroup\$ Oh, I was thinking that at 12V the whole board would get 36W and I would have to divide it by the number of channels. So now I've got another question: say I were to supply the linked board with 26V over 8ohms then that would give 84.5W per channel; does that mean the board would regulate the input to output no more than 50W? \$\endgroup\$
    – Pierino
    Jun 6, 2016 at 1:06
  • \$\begingroup\$ Additionally I just found another amplifier which appears to be more proper. link. Would this one work for both circuits? \$\endgroup\$
    – Pierino
    Jun 6, 2016 at 1:14
  • \$\begingroup\$ @Pierino I updated my answer. \$\endgroup\$
    – vini_i
    Jun 6, 2016 at 2:00
  • \$\begingroup\$ you've got bridge stability backwards, dual 8 ohm bridged is 16 \$\endgroup\$
    – Jasen
    Jun 6, 2016 at 5:58
  • \$\begingroup\$ There is no such thing as 'RMS power'. \$\endgroup\$
    – user207421
    Jun 6, 2016 at 13:17

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