Gain & Distortion

I just installed a mobile audio system in my vehicle. It breaks down like this,

Source > Amplifier > Loudspeaker

There's something called "gain" on my amplifier. From my understanding, the gain control is there so you can match the voltage from source to source. In my case, the voltage from my source to my amplifier. The voltage rating on the RCA outputs of my source is 4V, so I assume all I need to do is set the gain to 4 on my amplifier?

Could someone please touch on why I need to do this in the first place? Why the voltages need to match up from source to source?

Distortion; when a sine wave begins to clip. Here is how I see it. Again, please correct me if my logic is wrong (it probably is). I see this as a ratio. The closer you get your gains tuned right, the less distortion you'll get. For instance, if I set the gains at half of what they should be, I'm going to get distortion. Now if I set the gains EXACTLY what they should be, I'm going to get "half" the distortion I did before. In other words, I'm increasing the potential for the loudspeakers to perform?

I know it's rough:/

THanks.

Gain is defined in a logarithmic scale. Eg: gain= 10lg(Pout/Pin). In plain english (which is not my mother tongue) if you have a sound source ( a small mp3 player for example) which has a Pout of lets say 0.1W and you want to amplify to 10W the gain is : 10lg(10/0.1) = 2 db. Note that Pout of the mp3 player is the signal applied to the input of the amplifier which makes it Pin in the formula.
Distorsion is the clipping of the signal. To understand this you should think like this: If you have an op amp with double power supply of +-5V which has a gain of 10 and at the input you apply a sine wave of 1 v aplitude the op amp cant amplify it over 5V so it will clipp the wave.

This has been a debated topic. In short, there are suddenly two places to adjust the overall gain -- the source volume and the amplifier gain setting. Some (many) people will turn the gain all the way up or to some arbitrary level and then turn the source unit volume way down. This may incorrectly be seen as a good thing -- that the system is very loud at a low volume would imply it would be much louder at a higher volume. In reality, it just gets more distorted.

However, if you normally keep the volume low/moderate, eg -10dbFS, then the signal to the amplifier may not be high enough to give desired output when the gains are set to avoid distortion in the absolute worst case. In this case you should not generate excess distortion due to clipping. so this setting should be "distortionless" an not "half distortion". Of course this only accounts for clipping -- the speakers, ears, and environment may have various stress levels or imperfections that introduce distortion.

You also want to be able to adjust different channel's relative acoustic levels if you have separate amplifiers for subwoofers and other speakers.

Finally, most speakers have no problems with small levels of clipping. It is worse for tweeters in some cases though. In the automotive audio field, many prefer louder bass even if there is a tiny to small amount of distortion. For woofers, there is a myth that any amount of clipping causes the speaker to seize up and overheat.

Voltages (again, in the automotive audio field) need to match the source because there was not a standard level for line-out-converters or source units. Some people will also buy 8V line drivers in a belief that the amplifier will have less work to do and thus run cooler. The voltage amplification portion of the amplifier is tiny and uses almost no power in comparison to the current gain stages of the amplifier. As a result, though, amplifiers need to accept a wide range of inputs.

A useful paper on Gain Structure within Audio systems is available from Rane:
**broken link removed**

Although this applies primarily to professional audio, the same principles apply.
Also try following the links in this article.

hope this assists
Mik

A practical answer could be: There's a wide range of gain settings with no perceptible impact on distortion or noise respectively no need to set the gain EXACTLY.

Car audio manufacturers usually lie about the output power. If it is advertised at 200 Watts then the output at clipping is only 15 Watts RMS per channel (for each of the 4 channels) when the speakers are 4 ohms, or about 8.5 watts per channel when the speakers are 8 ohms.
The distortion is low or very low up until the output of the amplifier clips then the distortion becomes very high when the input level is increased because the clipped off parts of the output waveform are missing.