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CarAudioCalc

Tuning calculator

Amplifier Gain Setting Calculator

Quick answer

Target output voltage is the square root of rated power times load impedance. A 500 watt RMS channel into 2 ohms should read 31.6 volts AC on a test tone with all processing flat, which is the number gain is set to.

Your numbers

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Result

Target output voltage 31.62 V AC Published figure
Power at the voltage you measured 392 W Published figure
Rated power 500 W
Load 2 ohms

Set the head unit to about three quarters volume with everything flat, play the test tone, and raise the gain until the meter reads the target. Gain is an input sensitivity control and it is not a volume knob.

The formula

V = sqrt(P_rms x R)

Use a 50 or 60 Hz sine test tone for a subwoofer channel, played from the same source you actually use, with bass boost, loudness and every equalizer band flat. A true RMS meter reads a low frequency sine accurately; many cheap meters do not, and a meter that reads high will have you set the gain low, which is at least the safe direction.

The figures above are computed with these exact inputs: Rated RMS power per channel 500 W, Load impedance 2 ohms, Voltage you measured 28 V.

What this calculator assumes

Some of these are published physics or manufacturer figures and some are conventions. They are not the same kind of number, so each one is labeled.

AssumptionKindWhere it comes from
Ohm's law and the series and parallel resistance relationsPublished figureOhm's law and the series and parallel resistance relations
Setting amplifier gain by earConventionSetting amplifier gain by ear, raising it until distortion is audible and backing off. It is folklore, it depends on hearing that is already fatigued by the time you are close, and it is why so many voice coils are burned by clipping rather than by power

Gear that matches this result

Every card below is in the page source. The calculator re-ranks them against your numbers and marks anything that falls short of what you just calculated.

Frequently asked questions

How does the gain setting calculation work?
V = sqrt(P_rms x R). Use a 50 or 60 Hz sine test tone for a subwoofer channel, played from the same source you actually use, with bass boost, loudness and every equalizer band flat. A true RMS meter reads a low frequency sine accurately; many cheap meters do not, and a meter that reads high will have you set the gain low, which is at least the safe direction.
Are these figures published or are they rules of thumb?
Both, and the page labels which is which. The Helmholtz port relation, the Thiele/Small alignment equations, cone area geometry, Ohm’s law, conductor resistance and ampacity, and manufacturer specifications are published and are labeled as such. Amplifier efficiency, cabin gain, the farad per thousand watts rule, port area per cubic foot and coverage percentages are conventions with no measurement behind them, and they are labeled as conventions.
Why does this calculator sometimes refuse to give an answer?
Because there genuinely is not one. A port can be too large in area for the box and tuning asked of it, a vented alignment stops describing a useful box outside a Qts range, and a sealed box cannot produce a total Q below the driver’s own. In each case the calculator returns nothing and explains why, rather than extrapolating past the end of a published relation.
Do I still need an installer if I use this?
Many owners do their own 12V work, and these are the figures that work is sized from. What matters is that a car battery delivers hundreds of amps into a short without hesitating, so the main fuse goes within 18 inches of the battery positive terminal and the battery comes off before power wire goes in. If any of that is beyond what you want to take on, a professional installer is money well spent.

Researched guidance, not professional advice. These results are planning figures from published relations and manufacturer specifications. Fit the main fuse within 18 inches of the battery positive terminal, because that fuse protects the wire and the wire is what starts a fire. Disconnect the battery before running power wire, use a grommet through every piece of sheet metal, and remember that sustained high sound pressure causes permanent hearing damage.