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How to set amplifier gain using test tones

When the hum turns into a buzz, it's clipping

Buck in the Crutchfield Labs

Buck in the Crutchfield Labs

In an amplified car system, you need to set your amplifier’s gain correctly in order to enjoy your music’s full range of dynamics and frequency response — hearing all the notes clearly, whether loud or soft. You’ll feel your music’s impact better and hear exciting details that otherwise would get lost in your car.

There're quite a few ways to set gain, but I think listening to test tones is the simplest method giving the best results. Continue reading the article if you want to see how I came to this conclusion by my researching the issue in the Crutchfield Labs.

Instructions for setting amp gain using test tones
  1. Download 0 dB sine wave test tones of 40, 100, 400, 800, and 1000 Hz to your preferred medium (disc, chip, file, phone).
  2. Set your receiver’s EQ presets and the amp’s bass boost to the way you normally listen to your music.
  3. With the amp gain at minimum, play the 40 Hz tone and turn up the receiver’s volume until you hear it buzz. Back off the volume until the hum returns, and write down or mark the volume setting.
  4. Repeat Step 2, noting the top clean volume settings, using the 100, 400, 800, and 1K Hz tones.
  5. Pick the tone with the lowest clean volume setting and play it again at that setting.
  6. Raise the amp’s gain until you hear it buzz, back off until it hums, and you’re done.

Setting the gain by playing music

The quick and easy way to set the gain is by ear while playing music.

Most manufacturers recommend playing familiar music with the amp gain low, raising the receiver's volume until the music distorts, then backing it off until the music sounds clean again. Next, you turn up the amp's gain until you hear the distortion again, then back it off slightly, and you're done.

For a more detailed explanation, see my article about Tuning your subwoofers.

Setting the gain using test tones

The other methods of setting gain involve using test tones. A test tone is a single note played at a specific frequency, and is typically found on a level-setting disc, but can also be found online for downloading. In the Crutchfield Labs, I ran a set of tests and determined that "doing it by ear and music" works, but not quite as accurately or scientifically as using test tones.

1. Test tones and oscilloscope

Each tone creates a reference-level (0 dB) sine wave that you can observe on an oscilloscope screen. Instead of listening for distortion in music. As you adjust the volume and gain, you can see exactly at what point the signal of each frequency distorts and where it plays clean.

Waveform image

An example of a a clean waveform (left) and a distorted, clipped wave (right)

2. Test tones and speakers

But seeing as most people don’t have oscilloscopes, I thought that maybe by listening to the tones through speakers, one could also set an amplifier’s gain correctly. A non-distorted sine wave test tone sounds like a pure hum. When it distorts, you can clearly hear it buzz. By using test tones played through a speaker, I wondered how accurately I could set an amp’s gain as compared to setting it by the other methods.

A deep dive into my Crutchfield Labs project

I went into The Crutchfield Labs and set up an amplifier, wired to a power supply, a car receiver, and a pair of speakers. I also attached two sets of probes to a pair of speaker wires, one going to a voltage meter and the other to an oscilloscope. This way, we could see what the sounds look like and read the resulting power level the amp produced.

Pioneer DEH-3400UB CD receiver

Pioneer DEH-3400UB CD receiver

First set the tone controls to the way you listen

The receiver's and amp’s EQ and crossovers needed to be set to where they normally would be when playing music. This is so the gain would be set under real-world conditions. Adding boost, at any frequency, after setting the gain, can make the amp clip, distorting the sound and endangering speakers and subs.

I let the receiver (a Pioneer DEH-3400UB) stay in its factory preset "Dynamic" EQ setting, which boosts the bass and treble for a fuller sound. That meant certain frequencies would play louder than others. I needed to find out which tone clipped the receiver first, at the lowest volume setting. Then, I needed to use that receiver volume setting at that tone’s frequency to set the amp’s gain.

Part 1: Setting the gain with the oscilloscope

How loud can the receiver play and still play clean?

I started with the amp’s gain set to its minimum, and the speakers disconnected. I played the first tone, 40 Hz, a low bass note, only fit for subwoofers, and set the scope to view the sine wave. Then I turned up the receiver’s volume until I could see something bizarre happening to the wave’s shape. It didn’t "clip" at the top and bottom, it distorted in the middle. But I could see exactly at what volume level the distortion first appeared, and where it disappeared.

I took note of what the receiver’s volume reading was: 52. (The receiver’s top volume number was 62.) That meant the receiver played 40 Hz clean and at its loudest at its "52" volume.

Test Tone CD Track 7 40 Hz Maximum Clean Volume = 52
Test Tone CD Track 8 100 Hz Maximum Clean Volume = 51
Test Tone CD Track 9 400 Hz Maximum Clean Volume = 57
Test Tone CD Track 10 1K Hz Maximum Clean Volume = 59
Test Tone CD Track 11 4K Hz Maximum Clean Volume = 56
Test Tone CD Track 12 8K Hz Maximum Clean Volume = 55

This receiver plays loudest at volume 51, otherwise 100 Hz notes would clip

I measured the receiver’s distortion-free top volumes for the other test tones on the disc. The 100 Hz tone stood out as the strongest — I had to turn the volume to its lowest setting to get it to play clean. Because that volume represented the level that all the tones would play cleanly through the receiver, I used the 51 setting for the receiver’s volume for the next step. Because the 100 Hz tone was the strongest, and would clip the amp first, I used the 100 Hz test tone to set the amp's gain.

Sound Ordnance amplifier

Sound Ordnance M-4050 4-channel amplifier

The amplifier’s turn

I played the tone and looked at the sine wave while turning up the amplifier’s gain knob. Any waveform distortion I then saw came from the amp, not the receiver. Turning the gain back down until the distortion disappeared, I set the gain exactly where the amp and receiver were both at their maximum clean output levels: perfectly gain-matched.

Setting the gain right optimizes the amp’s output

I turned up the gain to the amp’s top clean-playing point and read the volt meter. The volt meter read AC (alternating current) voltage, and the amp I used (a Sound Ordnance M-4050 4-channel) showed a top clean output for 100 Hz of 17.6 VAC. That translated to about 77 watts. Not bad for an amp rated at 50 watts RMS per channel.

Bench testing results in higher power readings

What was going on was the power supply the receiver and amp used was 13.5 volts DC, about the same as a running car’s system usually provides, but the amp wasn't connected to the speakers and so wasn't loading down the power supply with the increased current demand of the speakers. That explained some of the "extra" power. But the amp definitely performed above its specified rating. If I had wanted to, I could have set the amp’s output to exactly 50 watts, by turning the gain down until the voltage read a targeted number, in this case 14.14 volts AC.

Math formulas — skip this paragraph

The wattage equals the voltage squared divided by the speaker’s impedance in ohms, 4 ohms in most cases. The voltage equals the square-root of the product of the wattage times the speaker’s impedance (also usually 4). 50 watts times 4 ohms equals 200; the square-root of which is 14.14 volts AC. 14.14 volts through 4 ohms of impedance creates 50 watts of power. These formulae are based on Ohm’s and Joule’s Laws and you can’t break them if you tried.

A note on multimeter accuracy

To accurately measure your amplifier’s output power with a multimeter, use a 60 Hz tone for a subwoofer amp, and a 100 Hz tone for a full-range amp with its high-pass filter turned off.

  • This is because most meters are made to measure AC voltage accurately at 50-60 Hz (the common frequency of all power systems around the world). Using a standard hand-held multimeter to measure the voltage of a higher-frequency signal results in readings that are much lower and leads to inaccurate power calculations.
  • For instance, the Amprobe 15XP-B multimeter I used in this Labs demonstration reads the voltage of a 0 dB 1 KHz signal about one-fifth the level that it reads at 40 Hz or 100 Hz. This would result in a calculated wattage about one-twentieth of the correct output power.
  • Different meters will have different degrees of deviation.

Kenwood KFC-6984PS 6"x9" 4-way speakers

Kenwood KFC-6984PS 6"x9" speakers

Part 2: Setting the gain using speakers and my ears

The noisy part of the test

I then repeated the whole performance with one speaker connected — a Kenwood KFC-6984PS 6"x9" 4-way. I want to say, in advance, that this was not a pleasant experience. Two hours later, my ears were still painfully ringing from the very high 8K Hz tone. Jordan, also in the Labs area at the time, complained that the 4K Hz tone was still ringing in his. This method can produce high-pitched, annoying, ear-drilling sounds that could hurt your hearing if you expose yourself for too long, and definitely will bother everyone within listening distance.

For using tones and your ears to set an amp’s gain, I recommend sticking with only the 40, 100, 400, or 1K Hz tones. They don't hurt at all. The 100 Hz tone alone will do for both subwoofer and full-range speaker amps.

Speaker hum
Speaker buzz

When a hum starts to buzz

A sine wave sounds like a hum. When it distorts, you can clearly hear it buzz. Again, the 100 Hz tone was the first to buzz, and at the exact same 51 volume setting. With the receiver at that top distortion-free level, I played the tone again and turned up the amp’s gain until I could hear the tone buzz again. Then I backed it off until the hum alone remained. The place the gain knob was set and the voltage readings were exactly the same as it had been using the scope.

I did this test after working hours so no one else would be disturbed. But I proved to myself, at least, that the ear-and-tone method worked just as well and as accurately as using a scope. The 40 Hz tone couldn't really be reproduced by the speakers, so was useless. The 100 Hz tone rattled everything on the desk, so it was a little difficult to pick the buzz-point out of the crowd of reverberations. The 400 Hz tone was the best tone to detect clip-points, with a very clearly defined hum-to-buzz point.

I hear music

Finally, I tried music and my ears alone. I performed this test twice, days apart, and also afterhours. Not everyone wants to hear my songs played loudly over and over again. At first, I played a favorite R&B-type song full of percussion, bass, horns, and lots of production — but I couldn’t hear it distort, only get loud. So I switched to a clear-voiced female vocalist singing swing. I also played a male singer, to see if it would be any different — it wasn’t.

Your hearing gets more acute when you close your eyes

I closed my eyes when I did this test, so no numbers were used to set the receiver’s top volume. I turned it up until I heard something go wrong with the vocal — it seemed thinner, not as bell-like, and harsher. The male singer's voice suddenly developed a rasp. After turning the receiver down a little, restoring the fine quality of the singer’s voice, I turned up the amp gain until I heard the same thing.

The two times I did this test, I got two different results. The first time, the receiver's maximum volume setting ended up one notch below the tones and scope setting. The second time, it was one notch higher than the tones and scope setting. But both times, the amp gain setting was exactly the same as the other methods.

The differences can’t be heard

On the first day, setting it by ear and music alone, I ended up thinking I should never turn the receiver higher than 50, and the gain was set so that at that 50 volume, the amp put out 15.7 VAC at 100 Hz, or 62 watts. On the second day, it ended up that I could turn it up to 52, and get 18.8 VAC at 100 Hz, or 88 watts. That 100 Hz tone was indeed slightly distorted visually, but it wasn't audible in the music. Plus, I don't usually listen to music full-blast for very long periods of time, so in real use, I would likely never be able to hear the difference.

It’s all about the music

I think either I was a little more or less sensitive to the singers’ voices on different days, and noticed changes at different levels than I could see in the waveforms’ shapes, or the music CDs I used were recorded at a different reference levels. I certainly cranked some swing those evenings in the Crutchfield Lab.

Whatever the differences between the methods were, they all resulted in having the receiver and amp properly gain-matched, and loud, distortion-free music ensued. Using the test tones disc was easier than listening to music. even without the oscilloscope, the tones made it possible for me accurately set the gain. It was very easy to discern when the hum distorted into a buzz.

Download some test tone files or pick up a test tone disc and try if for yourself!

  • Mike from Sheboygan

    Posted on 2/22/2022

    If using test tones via Bluetooth from your phone, what volume should your phone be set at? The volume output of my phone can be set separate from my head unit. I want to make sure the tone my phone is sending to my head unit is as clean as possible.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 2/22/2022

    Mike, You set the output gain of your phone the same way you set gain for anything - turn up the volume until it distorts, then turn it down until it doesn't.
  • Mike from Glendale,NY

    Posted on 2/5/2022

    Very informative read! I hank so much.

  • Side WalkCracka from Compton

    Posted on 9/5/2021

    IOr you can fork out hundreds of dollars for the Steve Meade Designs meter SMD

  • Nicholas from Margate

    Posted on 8/1/2021

    Just tried playing the 1khz 0db test tone. I started bringing the gain up and at no point did I hear the sound switch from buzz to hum. It was a very high pitched sound all the way through that literally hurt my ears at high volume. Next I smelled something AWFUL (kinda like burning). I dropped the gains and volume and now hoping I didnt ruin my speakers. :(

    Commenter image

    Buck Pomerantz from Crutchfield

    on 8/2/2021

    Nicholas, You seem to have it backwards: a buzzing means the amp or receiver is already clipping, sending out dangerously distorted signals. Humming is the pure, undistorted signal you want to hear.
  • James from Carthage

    Posted on 7/5/2021

    My question is: if im doing the square root math is that for the total amp rms power? Im only reading one channel? Would this be correct? Dont seem right to calculate total amp power then set one channels gain to it? Help

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/5/2021

    James, That formula is used to calculate the output voltage of a single amp channel at a particular wattage and impedance.
  • Bobby Y from Imus City

    Posted on 7/4/2021

    Hello Buck, I want to use 8 ohm bookshelf speakers at home to be connected to my 4 ohm car amps. Can I apply this same principle in this setup. Thanks and greetings from the Philippines.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/5/2021

    Bobby, Setting a car amplifier's gain will be the same no matter what speaker is used.
  • Bill Mack from Roxbury

    Posted on 7/4/2021

    My system consists of an Alpine Ilx-207 HU, Infinity Kappa 50.11cs front speakers,Kappa 60csx speakers in my sound bar, an Alpine Type R ported sub (sbr-102cr) in the rear of my jeep wrangler. I just removed two older Alpine V power amps that I replaced with a JL XD700/5v2. Do I turn off the HP filters on the amp being that the speakers have passive crossovers? Do I also turn off the LP filter for the sub? I'm a total novice when it comes to setting up a system and find it quite confusing. Being that most of my components were purchased thru Crutchfield, the local place won't tune the system for me. I did purchase an Oscilloscope and have downloaded test tones, but I'm not sure how I should set my base settings.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 7/5/2021

    Bill, Because you purchased your gear from Crutchfield, you are entitled to free lifetime tech support. Give us a call so someone can talk you through your install and tuning.
  • John from Cincinnati

    Posted on 6/16/2021

    So where can I get the 0db tones? I can't find set anywhere on the web. I can find individual tones, but they are different db values.

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/17/2021

    John, When I type "0 dB test tones" into an online search engine, over 4 million results are offered. I'm sure one of them will work for you.
  • Carlos Cabrera from New York

    Posted on 6/16/2021

    Hello Buck, When you use an oscilloscope to measure and amplifiers output for clipping, does it matter if you're going to use 4 ohm or 2 ohm speaker if your amp can handle both?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 6/17/2021

    Carlos, When setting gain, it doesn't matter if speakers are connected or not - gain setting adjusts the amp's input sensitivity to match the source's output. If you want to look at and measure an amp's output accurately, you'll want to do so with the speakers, or some other kind of resistive load connected. Use protective ear plugs or headphones when doing so.
  • Billy from Killeen

    Posted on 5/21/2021

    When using downloadable test tones, either to a CD, Flash drive, Bluetooth, pugged in android auto/ apple car play, etc. Which one have you found to be better suited for testing? or do they perform about the same?

    Commenter image

    Buck Pomerantz from Crutchfield

    on 5/21/2021

    Billy, I have not found any differences in quality between different "brand" test tones.

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