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Let’s discuss the question, “what’s the best way to set my gains?”

There are 3 common techniques for setting gains, by ear, with a voltmeter and test tones, or with on oscilloscope. People swear by their technique, and one common belief is that since the oscilloscope technique is the most “scientific” it must be the most accurate method. Well, those people would be wrong (hint: neither method is the most accurate).

First of all, let’s talk about what setting gains does. The amplifier will amplify the signal that comes into it from the head unit, DSP, etc. but it does not need to amplify a strong input signal as much as it needs to amplify a weaker signal to produce a given output. The gain knob is used to set the amount of amplification the amp needs to do, based on how strong or weak the signal it is receiving is.

There is no single optimal gain setting. I’ll say this again. THERE IS NO SINGLE OPTIMAL GAIN SETTING!

When setting the gain you are trying to do two things, reach the SPL levels you want (within the equipment’s limits) and prevent distortion (primarily from clipping). This is accomplished by finding an acceptable range within the gain structure, the gain should be high enough to allow for the SPL you want, and low enough to prevent clipping from over driving the amplifier. Anywhere in that range is equally as optimal as any other point in that range (for most music). This is the part that people tend to be confused about.
You may be thinking, “no way, an o-scope will get you as close to perfect as possible.” An o-scope will show you at what point the signal clips. This can be useful, but it is not particularly useful for gain setting. The reason it is not particularly helpful is that the o-scope will only show the exact clipping point of whatever frequency you send it, at whatever recording level you send it. If you set your gains with -3dB tones and end up playing a louder recording, you could have clipping. If you play a quieter recording, you will leave power on the table. Keep in mind that 3dB represents half, or double the power, so just a couple of decibels will dramatically change what the o-scope shows by cutting power in half, or doubling it. So, a couple of dB an take your gain setting from perfect to “way off.”

The good thing is that the acceptable range between nasty distortion and not being loud enough is pretty wide, and clipping isn’t automatically going to be a problem. Heavy clipping will be, but light clipping at low frequencies won’t be a big issue. From 20-40hz humans don’t notice distortion until it reaches 100% of the original signal, so you can have some low frequency clipping and never even hear it. As long as it’s within the capabilities of that speaker you won’t cause any damage to the speaker either. The shape of the wave (clipped) does not hurt a speaker, the power it sees does.

Ok, so you’ve got your new amp installed, and you just spent $100 on your new o-scope. You hook it up, play some tones, and set your gain to the maximum point before it clips. You’re stocked that your gains are now perfect, and they are, at that specific frequency, and that recording level, but they are less than optimal everywhere else. “Everywhere else” is where music lives. Music has dynamic swings, some is recorded louder than other music, some has a lot more content in certain frequency ranges than others, etc. so your gains are now set to be perfect for a test tone that your stereo won’t be playing. This isn’t a big deal because you’re still in that safe range I mentioned earlier, but guess what… You can get within that range just as easily without spending a dime on an o-scope just by listening to music, and using your ears.

Using your ears isn’t going to be any better than using an o-scope (necessarily), but it will be every bit as good, it will be free, and it will be quick. People are afraid of the ear method because they don’t trust their ears, but it’s not as difficult as you think. I’ll outline this technique later if anyone is interested. There are a couple of simple things that make it a piece of cake.

Alright I’m done rambling, it’s your turn to chime in, maybe we can all learn something from each other before this site completely falls apart.