Using test tones to set amplifier gain

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


Buck Pomerantz

Buck Pomerantz was born and raised in Philadelphia. His parents bought their first television set when he was born. He figured out how to run it by the time he was two. Besides athletics, his formative interests included electronics, amateur radio, music, and stage crew work. He got his BA in writing from Brown University. Then he joined a rock 'n roll band as their soundman and moved to Charlottesville, Virginia. After that venture failed, he spent time in Boston, New Orleans, and Berkeley. He worked in a music store in Austin manufacturing, installing, repairing, and operating sound systems for recording studios, clubs, and bands. He moved back to Charlottesville, ran a little recording studio and finally joined Crutchfield as a copywriter. He has 2 grown children and 3 grandchildren, but after a good nap he can still rock out.

More from Buck Pomerantz

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.

Setting an amp’s gain right results in the highest signal and the lowest noise and distortion. You’ll feel your music’s impact better and hear exciting details that otherwise would get lost in your car.

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.

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. 

Welcome to 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" 4-way 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.

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! To make it easier, I've summarized the process below.

Instructions for gain-setting using test tones
  1. Set the receiver’s EQ presets and the amp’s bass boost to the way you normally listen to your music.
  2. With the amp gain at minimum, play a tone from the disc (we recommend 40 or 100 Hz) 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.
  3. Repeat Step 2, noting the top clean volume settings, using the 400, 800, and 1K Hz tones.
  4. Pick the tone with the lowest clean volume setting and play it again at that setting.
  5. Raise the amp’s gain until you hear it buzz, back off until it hums, and you’re done.
  • dominicmonastra from Clifton heights pa

    Posted on 7/1/2015 10:23:46 AM

    This really helped me in so many ways !!!

  • Chad Barkla from Australia

    Posted on 8/4/2015 7:30:07 AM

    Great detailed article. Now i realise why my system sounded like rubbish when i set amp gains with receiver full tilt and no test tone Cd. Cheers

  • Ken Lacy from Baxter

    Posted on 8/24/2015 5:39:58 PM

    Please help? I have installed a Pyle 5 channel amp 6800 watts into a chevy cruze.. Powering an alpine 12" sub rated for 1000 rms. Power from Batt measures 14.2 at the amp. ground is good from the amp to the frame. 17ft rca's from the stock head unit with a pac AAGM-44 adapter to give me the rca outputs. Everything is brand new. All settings on hu and amp set to minimum or 0 - volume at 75%. I play the 50hz sounds and my multimeter starts at 6 volts and i turn the gain all the way up and the highest voltage i get is 44. I have two other gain adjusters for the other 4 channels and the highest reading i get on either of them is 42. My 2 ohm sub calls for 48 watts. Everything I read from others doing it, they can get the readings up over 60 watts and gain not even turned up all the way. Any ideas why I am getting less than normal wattage readings?

  • Buck Pomerantz from Crutchfield

    Posted on 8/25/2015 10:46:40 AM

    Ken, You might have your voltage and wattage a bit confused. Wattage equals the voltage squared divided by the sub's impedance in ohms. If you read that your amplifier's putting out 44 volts AC RMS, it means that it's outputting 968 watts into that 2-ohm subwoofer.

  • Jack S.

    Posted on 10/19/2015 6:45:39 PM

    Thank you so much for all the great information. I have read that I'm okay with headroom on an amp's RMS output being higher than the speakers' RMS handling. But should I always set the amplifier's output to the proper voltage for my speakers using Ohm's/Joule's Laws?

  • Buck Pomerantz from Crutchfield

    Posted on 10/21/2015 1:31:21 PM

    Jack, Setting an amplifier's gain with test tones and a multimeter is one of the three methods delineated in this article, and not necessarily the most accurate. It does not take into account whether the source signal is clipped or not. Personally, I think using test tones and listening for the buzz is an easier and more accurate method for setting an amp's gain.

  • thomas from Garden Grove

    Posted on 11/20/2015 7:30:03 AM

    My head unit preout is 4V and my 4-ch amp gain setting is 4V at full counter-clockwise and 150mV at full clockwise. Does this mean I leave my amp gain at the lowest setting (4V)? Also, I have an EQ on the head unit, do I set this to my preference before I set my gain?

  • Buck Pomerantz from Crutchfield

    Posted on 11/20/2015 2:16:57 PM

    Thomas, Just because a receiver's spec states it can put out a 4-volt preamp signal doesn't mean the signal is clean at that level. That's why I recommend using your ears to set an amplifier's gain, so you can hear distortion when it occurs. And yes, like the article says: "Set the receiver's EQ presets and the amp's bass boost to the way you normally listen to your music" before setting the gain.

  • Hayden

    Posted on 11/27/2015 6:09:03 PM

    Can't find the test tone disc. Is it at 0db or -10db?

Find what fits your car