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Maybe stupid question… but when checking output voltage of aftermarket amplifier, shouldn’t all of the outputs match the labeled polarity?

I was trying to troubleshoot something and when I checked voltage with multimeter, the front left & rear left (ch 1 & 3) have positive voltage but front right, rear right, & sub outputs (ch2, 4, & 5) have negative voltage. And even swapping polarity on inputs doesn’t change it positive. First time installing full audio system so bit confused.

It's not a stupid question at all! When checking the output voltage of an aftermarket amplifier, you would expect all of the outputs to match the labeled polarity. However, it's important to note that the polarity of the output voltage doesn't necessarily correspond to the polarity of the input signal.

If you're seeing negative voltage on some of the outputs, it could be an indication that the amplifier is producing an inverted signal on those channels. This is not uncommon and can be intentional or a result of the design of the amplifier.

If you're not sure if the negative voltage is normal for your particular amplifier, you should consult the manual or contact the manufacturer to confirm.

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Also while I’m here, question about mounting my shallow 10” sub… It seems easiest way to ensure minimal vibration is to drill through floor to bolt brackets, is that typically what people do? Or would simply setting on top of carpet and maybe fasten a simple strap or 2 will be good enough?

Mounting a shallow 10" subwoofer can be done in several ways, and the choice of mounting method depends on personal preference, the type of vehicle, and the specific subwoofer and enclosure being used.

Bolting the subwoofer bracket to the floor of the vehicle can be an effective way to minimize vibration and ensure the subwoofer remains securely mounted. However, it requires drilling into the floor of the vehicle, which may not be desirable for some people. If you decide to go this route, make sure to choose a location that won't interfere with any important components of the vehicle and that the bolts are properly secured to prevent any rattling or movement.

Another option is to use a non-slip mat or padding between the subwoofer and the vehicle floor to help reduce vibrations. This can be effective but may not be as secure as bolting the subwoofer in place.

Using a simple strap or two to hold the subwoofer in place can also work, but it may not provide as much stability as other methods. Make sure the straps are properly secured and not too tight to avoid damaging the subwoofer or enclosure.