Level matching starts with the head : Audio Car

Level matching starts with the signal created by the head unit and is there- fore irrevocably tied to the preamp signed strength of the head. That’s why “ y I it’s best to start with a high-voltage signal and why a high-voltage head unit output is desirable. With a high-voltage head unit, you start with a high signal-to-noise ratio, and with a low-voltage head, you start with a low signal- to-noise ratio. Your system is limited to that baseline level no matter how much you turn up the input sensitivity or gain on subsequent components.

So setting the gains in an entire system is simply adjusting the level of the preamp or pre-amplified signal as it moves from one component to the next in a car audio system. If the gains are set too low, you won’t have the maximum amount of signal that the components in your system can collectively produce, but you’ll likely have less noise. If the gains are set too high, you’ll get the maximum amount of signal but you’ll likely get more noise as well. What you’re trying to do when setting the levels for an entire system is find a happy medium where you get the most signal with the least amount of noise. 

From the head to the amp

To keep things simple, look at how gain relates to the signal between a head unit’s output and an amplifier’s input. The gain control of the amplifier allows you to adjust the input sensitivity, which determines how much input signal from the head unit it takes to produce a given amount of power by the ampli¬fier before the amp begins to clip the signal. With high voltage coming from the head unit, the amplifier’s input sensitivity can be set lower to achieve a specific power output before it starts to clip, and the potential for noise is lower (therefore, a higher signal-to-noise ratio). But with a low voltage coming from the head unit, the gain has to be set higher to get the same output before it starts clipping, and the potential for noise is higher (a lower signal-to-noise ratio).

The gain setting on an amplifier also has an effect on the volume swing of the head unit, or the minimum to maximum volume level. If you have a head unit with a high-volt output, volume can get very loud, very fast if the input sensitivity of the amp is set high. If your high-volt head’s volume control ranges from, say, 0 to 30 and the amp’s input sensitivity is cranked up, the system might reach full power at, say, a setting of only 10. But with the same high- volt head, you can lower the input sensitivity on the amplifier so that the head unit has a wider volume range, all the way up to 30. With a head unit that doesn’t have a high voltage output, the opposite is the case. You could turn the volume all the way up to 30 and crank the input sensitivity on the amp, and still it may not play loud enough, and you’ll likely get more noise.

Setting gains and crossover points by ear

Some installers use a multimeter or test equipment such as an oscilloscope or a real-time analyzer to set gains, but it’s also possible to do it by ear. It takes practice and you won’t get it right the first time, but it’s a great hands- on learning experience on how your system works. As with everything else, the more you do it, the better you’ll get at it.

The same thing goes for setting your crossover points, the frequency at which the signal going to each speaker begins to be attenuated or turned down. If your subwoofers are crossed over at 100 Hz and down, signals from 100 Hz and up are slowly attenuated so that the sub doesn’t play them. If your midranges are crossed over at 100 Hz and up, signals from 100 Hz and down are slowly attenuated so that the speakers don’t play them.

Because crossovers are usually built into amplifiers, it’s convenient to set your crossover points at the same time that you set your gains. Setting proper crossover points is an equally important aspect of tuning your system for the best sound. 

To illustrate how to set gains and crossovers by ear, I use the example of a system with a high-volt head unit, a five-channel amplifier (with two front channels, two rear channels and a single mono subwoofer channel) with a built-in crossover, a pair of component speakers in the front doors, a pair of coaxials in the rear deck, and a single subwoofer (see Figure 20-1). In this sample system, you would only be setting the gain on the amplifier. But with a more elaborate system that includes multiple amplifiers and processors, the procedure would be much the same. You would just have more gains to adjust.

Head unit

Figure 20-1:

A system consisting of a high- volt head unit, an amplifier, front and rear speakers, and a single subwoofer.

Rear

speaker

Rear

speaker

Subwoofer

Courtesy of Audio Profiles

To set the gains and crossover points by ear in this system:

1. Turn the gains on your amplifier all the way down or to a minimal setting. The gain control will usually be marked as Input Sensitivity or Gain.
2. Set the crossover points at the settings recommended by the speaker manufacturer, or what you feel is best suited for each speaker set and the type of sound you’re seeking. This is when having crossovers built into the amp comes in handy. Regardless of whether you’re using crossovers built into the amplifier or separate electronic crossovers, the crossover controls are typically labeled Frequency or Filter. Frequency adjustments are either set (say, at 50, 80, and 125 Hz for a subwoofer crossover) or variable (you can choose any frequency between 50 and 125 Hz). In our sample system, the amplifier’s crossovers could be set at 100 Hz and down for the subwoofer and 100 Hz and up for the components and coaxials. (Passive crossovers, which are usually included with components and coaxials, separate the frequencies going to the individual speakers that are part of the components and coaxials.)
3. Turn the level of the bass and treble to 0 on your head unit, set any equalization to 0 or flat, and turn off any signal processing, such as a loudness button.
4. Play very dynamic music that has full-frequency range content — meaning plenty of bass, plenty of midrange, and plenty of treble — and turn the volume up about three-quarters of the way.
5. At each input of the amplifier — the front channels for the front speakers, rear channels for the rear speakers, and the subwoofer channel for the subwoofer — slowly turn up the gain until you start to hear the speaker distort or break up, and then back it off a bit from there. If you use music you’re familiar with, you’ll know when the sound starts to degrade. Start with the front speakers, then do the rear speakers, and continue on to the subwoofer. You should set the gains and crossover points for the subwoofer last, getting the rest of the system dialed in first.
6. Repeat Step 5 for your crossover points, setting them higher or lower until each speaker starts to break up or you get the type of sound you’re looking for, and then back it off a little.

At which point the sound begins to break up or distort also depends on the type of music you’re using and most often listen to. With rap music or anything that has low bass, you want to be careful not to crossover your front speakers too low. Otherwise, low bass could potentially damage them.

Take your time tuning your system, and listen to it carefully. If you start to get frustrated or your ears fatigue and everything starts sounding the same, walk away and come back to it in a few hours or the next day. In fact, it may take you several tweaking sessions to get the sound right. You’ll also want to go back in a week or two and tweak the system again.

You can also ask a friend or acquaintance who has experience and a good ear to help you. If you can find someone with the proper test equipment, they can tune your system much more quickly and with greater accuracy. As part of their service, many car audio shops tune your system for you if you have it installed there. But, as with a guitar, tuning a system by ear yourself is good practice and a great skill for a car audio enthusiast to develop.