Setting gain with a test tone and a multimeter
The gain control matches the amplifier to the voltage your head unit sends it. It is not a volume knob: set too high, the amp clips long before the head unit reaches full volume. The multimeter steps below follow JL Audio’s procedure. Kicker’s guide for its KXA amps sets the gain by a light in the gain knob instead of a meter, but agrees on the rest: tone controls flat, a phone’s volume all the way up, about three-quarters volume, speakers disconnected, and afterwards only ever turning gains down.
- Download the tone and play the file, not this page. The page opens on 50 Hz, the tone for subwoofer amps, set up as a one-minute WAV at 0 dBFS and 44.1 kHz, the CD sample rate. Press 1 kHz for a full-range, 4-channel or high-pass amp and download that too. Put the files on the source you will listen with (a flash drive, a CD or your phone) and play them from the head unit or the phone’s music player. This page’s Play button and tone buttons (which start playing when pressed) follow the Volume slider, about −18 dB at the default setting, not 0 dBFS.
- Flatten the source and the bass boost. Switch off bass boost, loudness, EQ and any other processing on the head unit, and the amplifier’s bass boost. From a phone, set its EQ flat and its volume all the way up. Leave the amplifier’s crossover filters on, at the settings they will keep: a low-pass for the subwoofer and a high-pass for full-range speakers (Kicker suggests 80 Hz for both as a starting point). Switched off, they would send deep bass to small speakers once those are reconnected.
- Disconnect the speakers from the amplifier and turn every gain control fully counterclockwise.
- Play the tone at about three-quarters of the head unit’s maximum volume, the setting both JL Audio and Kicker give. Loop it so it doesn’t stop mid-measurement.
- Measure AC volts across the amplifier’s speaker output and turn the gain up until the meter shows the target voltage from the table below.
- Turn the volume down, stop the tone, switch the system off, then reconnect the speakers. If you rebalance the system by ear later, only ever turn gains down.
Keep the WAV as it is. Kicker asks that any other test tones you use be “unclipped, not attenuated, and uncompressed”, which the file from this page is until something re-encodes or re-levels it.
Target voltages
The voltage for a given power follows from watts = volts² ÷ ohms. Look up your amplifier’s rated RMS power per channel at the load it actually drives (a bridged pair or a sub wired to 1 Ω changes it).
| Rated power (RMS) | 1 Ω | 2 Ω | 4 Ω |
|---|---|---|---|
| 50 W | 7.1 V | 10.0 V | 14.1 V |
| 100 W | 10.0 V | 14.1 V | 20.0 V |
| 250 W | 15.8 V | 22.4 V | 31.6 V |
| 500 W | 22.4 V | 31.6 V | 44.7 V |
| 750 W | 27.4 V | 38.7 V | 54.8 V |
| 1,000 W | 31.6 V | 44.7 V | 63.2 V |
| 1,500 W | 38.7 V | 54.8 V | 77.5 V |
| 2,000 W | 44.7 V | 63.2 V | 89.4 V |
If your speaker or subwoofer is rated for less RMS power than the amp delivers at that load, set the gain for the speaker’s rating instead. A 1,000 W amp on a 300 W sub at 2 Ω should be set to √(300 × 2) = 24.5 V, not the 44.7 V in the table. That limits the power only up to the volume you set it at: turned up further, the amp can still deliver more, which is why Crutchfield’s car amplifier FAQ advises never connecting an amp whose RMS output is higher than the sub’s RMS rating.
0 dB, −5 dB or −10 dB
A tone’s level in dBFS says how close its peaks come to the loudest value a digital file can hold. The page starts at 0 dBFS, the full-level tone JL Audio’s guide asks for. Kicker and MTX also offer −5 and −10 dBFS versions, which you can pick under Peak level in the download box. With a tone 5 dB quieter you end up setting the gain 5 dB higher, so music plays louder at a given volume setting, but its loudest peaks reach the amp’s clipping point sooner. Kicker’s own guide warns that the quieter tones leave you with less protection from overdriving your speakers and more potential for speaker damage, and describes its −10 dBFS tone as meant only for subwoofers.
Before you trust the reading
A multimeter’s AC range is only accurate across a stated frequency band. The Fluke 101, for example, is specified for AC volts from 40 Hz to 500 Hz, so it suits the 50 Hz tone but gives no guaranteed reading at 1 kHz. In a Crutchfield test, a handheld meter read a 1 kHz tone at about a fifth of the voltage it showed at 40 or 100 Hz, which would lead you to set the gain far too high. Check your meter’s manual. If 1 kHz is outside its range, use an oscilloscope or a meter rated for it; Crutchfield’s own workaround is a 100 Hz tone with the amp’s high-pass filter switched off, and if you use it, switch the filter back on before reconnecting any speaker.
Once the gains are set and the speakers are back, the speaker test checks left and right and catches a speaker wired out of phase. For a 1 kHz reference at studio levels such as −20 dBFS, see the 1 kHz test tone; any other frequency is on the tone generator.