I picked up a powered home subwoofer at a garage sale. It did not have its power adaptor. All it shows is 12V DC (by the symbol) 2.5A. But it does not show the polarity anywhere on the subwoofer. Is there any way a rookie can figure out what the polarity is?

The subwoofer is a Granger Bessel brand. It was probably part of a home theater setup, since it does not show a model number on the subwoofer.

Thanks for any suggestions you can offer. If I can't find out anything, I will just try + polarity. It was just $10, so no real loss if I smoke it.

Picture of inputs

  • 2
    \$\begingroup\$ Should be easy to figure out if you open it up and look where the jack is. \$\endgroup\$
    – Ron Beyer
    Mar 18, 2021 at 20:42
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    \$\begingroup\$ As Ron points out, it's best to just go look. You may see a protection diode that makes it more obvious to the eye. But usually it doesn't take much to work it out by inspection. If you want, include a photo of the inside that may help us help you with this. And yeah, there's no polarity indicator present there. Just the DC indicator. I've seen that modified where the solid line (which is usually taken to mean the center pin) was annotated with a + or -. But only once did I see that. Usually, if they bother, they will use a clear indication. \$\endgroup\$
    – jonk
    Mar 18, 2021 at 20:54
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    \$\begingroup\$ Given it's audio equipment, my guess would be centre-negative. But as others suggest, you're best of either taking it apart, or perhaps using a beep tester to see if there is any continuity between either pin and the shell of the L/R inputs. \$\endgroup\$ Mar 18, 2021 at 20:58
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    \$\begingroup\$ Granger Bessel is one of the brands used in the famous "White Van Speaker Scam". Be prepared to be disappointed even if you get it working. \$\endgroup\$ Mar 18, 2021 at 21:16
  • \$\begingroup\$ For sure it's gonna be underwhelming. 12V x 2.5A = 30 watts input. Output has to be something less than that. Not exactly wall shaking power here. \$\endgroup\$
    – Kyle B
    Mar 19, 2021 at 2:19

2 Answers 2


Is there any way a rookie can figure out what the polarity is?

Yes, however NO GUARANTEES.

In nearly all devices (there can be exceptions) the ground is shared between the power jack and the input. I see what looks like a Cinch input on the right of the photo. One like this:

enter image description here

That (here gold coloured) ring is often connected to ground.

Use a multimeter to measure between that ground and both connections of the DC power input. The connection to the ground of the power input jack should measure a low value close to 0 Ohms. That ground connection should connect to the - of the supply power.

Again: there are no guarantees. The design can be different but usually the procedure above will work.

When measuring, use the Ohms range and measure until the value is stable. Often there are large capacitors connected to the power input. When these capacitors charge, the ohms measurement changes over time while the capacitors charge. If this happens, you're measuring the + connection of the power jack.


Well, I tried examining the board, but I am still such a rookie on these things that I just couldn't tell. So I just took a chance and went with a positive charger, and it worked. The subwoofer was just $10, so I figured it was a 50/50 shot. Obviously it's not a powerful subwoofer by any stretch of the imagination, but it did put out a decent amount of base, which is all I wanted for the small room that it is located in -- to enhance a couple of bookshelf speakers.

This definitely convinced me that I have a LOT to learn in this new hobby. Thanks for your suggestions and help.

  • \$\begingroup\$ Thanks for getting back to us. For future reference if you've gone as far as opening something up and examining it, you should add pictures to the question, as it's likely there are people here who could have identified polarity at a glance, and also pointed out why for the benefit of future readers. Unless for some reason you consider it to be objectively wrong you should use the checkmark button to mark Bimpelrekkie's answer as correct since it gives a better than 50% chance of success without disassembly. \$\endgroup\$
    – K H
    Mar 27, 2021 at 4:12

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