Enter the output power and input power into the calculator to determine the amplifier gain in decibels (dB). This calculator helps in understanding the amplification level of an amplifier.
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Amplifier Gain Formula
The calculator runs in two modes. Use the target voltage mode to set amp gain with a multimeter. Use the dB gain mode to express a gain ratio in decibels.
Target voltage mode:
V = sqrt(P * R)
dB gain mode:
Power gain (dB) = 10 * log10(Pout / Pin) Voltage gain (dB) = 20 * log10(Vout / Vin)
- V = target AC RMS voltage at the speaker terminals, in volts
- P = amplifier RMS power output, in watts
- R = final speaker load, in ohms
- Pout, Pin = output and input power, same unit
- Vout, Vin = output and input voltage, same unit
The target voltage mode multiplies the RMS power by the load impedance and takes the square root. That gives the AC voltage your multimeter should read on a sine test tone when the amp is producing rated power. The dB gain mode takes the ratio of output to input and converts it to decibels using the log10 factor that matches the quantity (10 for power, 20 for voltage).
Reference Tables
Common target voltages for setting gain with a 0 dB sine sweep tone:
| RMS Power | 1 Ω | 2 Ω | 4 Ω | 8 Ω |
|---|---|---|---|---|
| 100 W | 10.0 V | 14.1 V | 20.0 V | 28.3 V |
| 250 W | 15.8 V | 22.4 V | 31.6 V | 44.7 V |
| 500 W | 22.4 V | 31.6 V | 44.7 V | 63.2 V |
| 750 W | 27.4 V | 38.7 V | 54.8 V | 77.5 V |
| 1000 W | 31.6 V | 44.7 V | 63.2 V | 89.4 V |
| 1500 W | 38.7 V | 54.8 V | 77.5 V | 109.5 V |
| 2000 W | 44.7 V | 63.2 V | 89.4 V | 126.5 V |
Common dB ratios you will see when reading specs:
| dB | Voltage Ratio | Power Ratio |
|---|---|---|
| 3 dB | 1.41 : 1 | 2 : 1 |
| 6 dB | 2 : 1 | 4 : 1 |
| 10 dB | 3.16 : 1 | 10 : 1 |
| 20 dB | 10 : 1 | 100 : 1 |
| 30 dB | 31.6 : 1 | 1,000 : 1 |
| 40 dB | 100 : 1 | 10,000 : 1 |
Examples and FAQ
Example 1. A 600 W RMS amp is wired to a 2 ohm final load. V = sqrt(600 × 2) = sqrt(1200) = 34.64 V RMS. Disconnect the speakers, play a 0 dB sine test tone in the same frequency range as the amp, and turn the gain knob until your multimeter reads 34.64 V AC across the speaker outputs.
Example 2. An amp produces 50 W out from a 0.5 W input. Power gain = 10 × log10(50 / 0.5) = 10 × log10(100) = 20 dB.
Should I disconnect the speakers when setting gain? Most installers do. The amp produces full rated voltage with no load, and you avoid sending a sustained sine tone through your subs or drivers.
What test tone frequency should I use? Match the range the amp drives. 50 to 60 Hz works for subwoofer amps. 1 kHz is standard for full-range and mid amps. The tone must be recorded at 0 dB with no clipping.
Why does my measured voltage drop when I reconnect the speakers? Real speakers have rising impedance with frequency, and amplifiers sag slightly under load. Set gain unloaded to the calculated target, then verify with the load connected. A small drop is normal.
Is more gain the same as more power? No. The gain knob matches the amplifier's input sensitivity to your head unit's output voltage. Turning it past the point where the amp hits rated power only adds distortion, not output.