Using test tones to set amplifier gain
When the hum turns into a buzz, it's clipping
In the 1950's, I'd take the family television's vacuum tubes down to Willow Grove Radio and TV Repair, check them with the giant tester machine, buy new replacement tubes, and reassemble the repaired television, so my mom and dad could enjoy their precious, respectively, Dean Martin and Red Skelton shows. In the 1960's, I studied radio and electronics at the Franklin Institute in Philadelphia. After college, in the early 70's, I joined a rock 'n roll band as the soundman, learning how to operate the electronics that make music sound good. Then, I worked in a music store in Austin manufacturing, installing, repairing, and operating sound systems and components for recording studios, nightclubs, and touring bands. I moved back to Charlottesville permanently in 1984 and opened a little demo recording studio. I also attempted to put to practical use the creative writing degree I had picked up along the way. In 2006, I finally came to my senses and got this job at Crutchfield where they actually pay me to ramble on, rant, and explain the things I love about music, electronics, and getting good sound.
More from Buck Pomerantz
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.
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
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 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.
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