Adam's Gain Setting Tutorial
There are only three different gain setting methods (at least three correct ones) that I'm aware of, I'm sure there are more, but these are the most popular. Each person has their preference, as long as their method isn't the turn-it-up-until-it-sounds-loud method then it should be fine, but for those of you who don't know how to set the gain properly you should pick one of these methods and stick with it. The first two are the best ways in my honest opinion. They'll give you almost identical results, they're just different ways of going about it. The third method really shouldn't be used unless you feel really uncomfortable doing the first two, as it can yield results that are pretty far off depending on the dmm used, the amp used, and the frequency being played. Also listed are the pros and cons of each method, so you can decide which is the best for you. All of these will require one or more 0dB test tones, if you don't have any then I have a set I've made that you can download here.
Also, If you don't already know the basics of gains and their function, it would be best to read
https://www.bcae1.com page 63 (index on RHS) before starting.
One more thing before you get started, these procedures will make it so that your amp clips at the same time as your headunit, that's it. You still need to listen to your speakers to make sure they're not being stressed, because they can still blow even if your signal is not clipping.
Method 1 - Oscilloscope
Pros: Flawless results, extremely quick, extremely easy
Cons: Expensive equipment
Required Equipment: Portable oscilloscope
Anyone who owns an oscilloscope should know how to do this already, but I'll cover it anyway for the sake of completeness. This is my personal favorite because of the pros listed above, since I already own a scope there are no cons, so naturally it's my favorite.
1 - Turn off all of your headunit's processing (crossovers, EQ, etc) and if you have an Alpine, set the bass level to the full +15.
2 - Turn your amp's gain and your headunit's volume all the way down (counterclockwise).
3 - Unhook all speaker(s) from the amp and hook up the scope's leads to the + and - on one of the amp's channels.
4 - Put on a test tone near the center of your amp's bandwidth. I like to use 1khz for class ab amps and 50hz for class d amps personally.
5 - Slowly turn up the headunit's volume (remember to adjust the resolution on the scope accordingly so you can see the entire waveform on the display)
6 - When you see the waveform start to clip, turn the headunit's volume back down a notch or two until it becomes clean again. If you don't know what clipping looks like, Basic Car Audio and Electronics has a nice figure illustrating it for you here (it's the first figure on the page, the white curve is what it should look like, the yellow curve is what clipping looks like). If this maximum volume is at a weird number (32, 47, etc) then it's perfectly fine to turn down the volume a little more to get it to a nice round number, this will make it easier to remember later.
7 - When you find the maximum undistorted volume of your headunit, it's time to start turning up the amp's gain. You're looking for the same thing here, as soon as it starts to clip you want to back off of the gain slightly to make sure it's clean again. If your headunit has powerful preouts and your amp has a small input range, there's a chance that it might clip as soon as you start to turn up the gain. This is perfectly fine, just leave the gain at the minimum setting and you're good to go.
8 - If you're at this step then your gain is set, all you have to do is remember the headunit's volume at which you did the test and you're set. Remember also that you should not turn your volume higher than this setting unless what you're playing is recorded at a very low level, or else you risk clipping. Another thing to remember is that any EQing that you do will change the point of clipping, if you have anything boosted more than a dB or two then you should try to stay a good bit below this maximum volume found in step 6 or you risk clipping at those boosted frequencies.
Method 2 - Listening
Pros: Very accurate results, very easy on mids/highs
Cons: Using it for subs can be a pain
Required Equipment: Other than the tones above and your ears, nothing
This used to be my method, that is until I got my scope. When I got the scope I went back and checked all my gains, and surprisingly they were perfect. This method yields accurate gain settings and for mids/highs it's extremely easy. For subs it's a little bit of a hassle, but it can still be done. Before you start, you should know that at one point in this test you're going to be sending a full power test tone to your speakers. This means that if your speakers are being overpowered, or even if they're being properly powered, there's a chance they could blow. For this reason, it's a very good idea to go in and rewire your speakers to drop the power going to them so this is no longer a problem. Below I've listed several popular wiring configurations and how they should be temporarily changed to drop the power.
Mids/highs, one side per channel - wire both sides in series and put them on one channel of the amp. If the amp does 100x2 @ 4ohm you will be using 1 channel at 8ohm, so you will have ~50rms going to the pair of speakers, which is 25rms per speaker, the power is cut by a factor of 4.
Single DVC sub (or a pair of SVC subs) wired in parallel to a mono amp - re-wire the sub in series and attach it back to the amp, the impedance will quadruple and the power will be cut by a factor of 4.
Anything that is running on a bridged amp (pair of components on a bridged 4ch, sub on a bridged 2ch, etc) - Simply unbridge the amp and connect the speaker to a single channel instead. Again, the power being delivered to the speaker will be cut by a factor of 4.
After you've done all of your re-wiring you can dive into the procedure.
1 - Turn off all of your headunit's processing (crossovers, EQ, etc) and if you have an Alpine, set the bass level to the full +15.
2 - Turn your amp's gain and your headunit's volume all the way down (counterclockwise).
3 - Lets do the mids/highs first. Put on a test tone near the center of your amp and speaker's bandwidth. I like to use 1khz personally
4 - Slowly turn up the headunit's volume while listening for the signal to clip. When you find this volume, turn it down a few notches so it's clean and it's a volume setting that's easy for you to remember (my headunit clips at 33, but I use 30 for setting the gains, just for an example). In case you don't know already, here are some tones to help you get an idea of what clipping sounds like: 1khz clean 1khz clipped
5 - When you find the maximum undistorted volume of your headunit, it's time to start turning up the amp's gain. You're listening for the same thing here, as soon as it starts to clip you want to back off of the gain slightly to make sure it's clean again. If your headunit has powerful preouts and your amp has a small input range, there's a chance that it might clip as soon as you start to turn up the gain. This is perfectly fine, just leave the gain at the minimum setting.
6 - If you're at this step then your gain is set, all you have to do is remember the headunit's volume at which you did the test and you're set. Remember that you should not turn your volume higher than this setting unless what you're playing is recorded at a very low level, or else you risk clipping. Also remember that any EQing that you do will change the point of clipping, if you have anything boosted more than a dB or two then you should try to stay a good bit below this maximum volume found in step 4 or you risk clipping at those boosted frequencies. Also remember to re-wire your speakers back to normal.
7 - Time for the sub amp. Unfortunately clipping can be hard to hear at low frequencies, especially with the crappy subs out there these days that couldn't play an accurate note to save their life. The higher the frequency you play the easier it is to hear clipping, but if you're using a mono class-d amp then you'll most likely be limited on the upper end at ~300hz due to the amp's limited frequency response, or it might actually have an adjustable lowpass filter that can't be defeated which you'll have to watch out for. If your lowpass(es) can be turned off then do it, if they can't then just turn them to the highest frequency possible. Pick a test frequency that's at least an octave below your lowest lowpass filter, 100hz or above would be preferable, but sometimes it's not possible.
8 - At this point it's all the same as before, turn the headunit to the volume found in #4, and then go back to step #5
Method 3 - DMM
Pros: Easy and fast
Cons: Some people don't like math, it's not amazingly accurate, and as cheap and useful as they are, some people don't own a multimeter
Required Equipment: Digital multimeter capable of reading true rms AC voltages
Probably the two most basic equations in electrical analysis are Ohm's law (V=IR, V is voltage, I is current, R is resistance) and the power equation (P=IV, P is power). When you combine the two, you get P=V^2/R. Rearranging this gives us P*R=V^2, or V=sqrt(P*R). This means that if your amp does 100rms at 4ohm, we have that V=20. In order for this amp to actually do 100rms @ 4ohm, it would have to put out 20Vrms on the speaker terminals. Unlike methods 1 and 2 above, this method depends on the power rating by the manufacturer to be exactly right in order for your gain to be right. If the amp is overrated, then by following this method you might actually be clipping the amp at full volume, and if the amp is underrated, then by following this method you might not be able to reach the full potential of the amp. Another disadvantage is you have to rely on the accuracy of your dmm, and many can be pretty innacurate, especially with AC signals. However, it's easy and fast, and some people like that.
1 - Use the equation above to determine the proper output voltage for your amp's power capabilities. If the amp does 1000x1 @ 1ohm then you want 31.6V, if your amp does 50x4 @ 4ohm then you want 14.1V per channel, etc.
2 - Turn off all of your headunit's processing (crossovers, EQ, etc) and if you have an Alpine, set the bass level to the full +15.
3 - Turn your amp's gain and your headunit's volume all the way down (counterclockwise).
4 - Unhook all speaker(s) from the amp, set the multimeter to AC voltage and hook up the leads to the + and - on one of the amp's channels.
5 - Put on a test tone near 50hz for all amps. Use this frequency because dmms are generally most accurate around 50-60Hz, and as long as your speakers are all disconnected they won't be damaged.
6 - Set your headunit's volume somewhere around 3/4 of max to ensure it isn't clipping and remember this volume setting.
7 - Now it's time to start turning up the amp's gain. Simply turn up the amp's gain until the output voltage matches what you derived earlier.
8 - If you're at this step then your gain is set, all you have to do is remember the headunit's volume at which you did the test and you're set. Remember also that you should not turn your volume higher than this setting unless what you're playing is recorded at a very low level, or else you risk clipping. Another thing to remember is that any EQing that you do will change the point of clipping, if you have anything boosted more than a dB or two then you should try to stay a good bit below this maximum volume found in step 6 or you risk clipping at those boosted frequencies.
That's about it, follow one of these methods and you should be good to go. Like I said above I used to use #2, but after buying an oscilloscope I now use #1. After going back and checking my gains with the scope I found that they were already set perfectly, so #2 is definitely accurate if you choose to take the time to do it right. #3 is fast so if you're in a hurry then that's probably the best one for you, though if there's one thing I've learned about audio, it's that you absolutely have to take the time to do things right or you'll just end up doing it again. Good luck, and don't blame me if you screw up and blow something
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