Gains Tutorial
MacLeod
Posts: 14,358
We reference this all the time so I figured Id post it. So here it is, our very own Neomagus' world famous gain setting walkthru so if you blow something up its not Polk's fault! 
Gains Tutorial
This is the most widely used method, but that doesn't mean it's the best. Strictly speaking, the most effective way to adjust the gains on your amps is using an occilloscope and a track of pink noise. Now, since most people don't have easy access to an o'scope, and many have never heard of one (so don't be embarrassed if you have no idea what that o-word was - there are more important o-words to worry about), this is a great secondary method.
A definition: Xmax, as used in this walkthrough, means the manufacturer's reccommended excursion limit for the speaker. This is listed, usually on the website and in the owner's manual, as, appropriately enough, the thiele-small parameter called 'xmax'. DO NOT use a parameter referred to as 'xmech' or 'xmag' here, they're not necessarily the same thing. If you can't find xmax, the easiest way to find out is to call the appropriate company. (Yeah, I know, you'll have to use a phone, what a pity...)
1. Turn gains on all amps down.
2. Pick just one to tune, it doesn't matter. We'll call it the sub amp. Disconnect the rest of the amps, either power or speakers, doesn't matter. This is so you don't get confused. See Note 1.
3. Turn your HU up to 75%; mine goes to 40 and i turned it up to 34. This is because the VAST majority of headunits on the market begin to distort after this. If you are SURE you have a non-clipping headunit, you can go higher, but I strongly reccommend that you do not exceed 90% - even 'non-clipping' HU's can clip when they're being pushed this hard.
4. Find or make a test tone. For a sealed or 4th order bandpass sub box, use around 40 Hz. For a ported box, or sixth order bandpass box, it's not that simple; you'll be best off using a sweep that goes from the tuning frequency up to the lowpass frequency, because maximum excursion occurs at some point above tuning. You can find the point of maximum excursion and tune using that tone, if you desire. If those two sentences didn't make sense, go sealed or take the time to learn a bit more, you and your subs will be happier. For mains, use the highpass crossover frequency (the lowest frequency they're set to play at). See Note 4.
5. Play the tone and turn the gains up slowly until it begins to distort (if you have enough clean amp power, this will happen around xmax; if you're underpowering the speaker this will happen before you hit maximum excursion); with subs this is relatively easy to hear, as it should manifest as buzzing or a grittiness in the sound or some such thing - something other than the smooth hum you've been hearing. For speakers, it may be more difficult to hear. If in doubt, turn it down. Oh, and if for some reason your gain is almost all the way up... something's wrong. Your amp will distort when you've turned the gain up almost all the way. If you need it this high, there's a problem with your system somewhere that the amp can't fix for you. See Note 6.
6. Back it down a tad. See Note 5.
7. Rinse and repeat with all other amps, starting from Step #2 (including disconnecting the just-tuned amp), without changing the gain on the amps you've already set. See Note 3.
8. From here on in, NEVER turn the gains up beyond where you've set them unless a system component changes. If one amp is too loud, like your mids and highs overpower your subs, turn the gain on that amp down to match the rest.
9. Equalize/shape if needed; attentuate rather than boost if you can. Just remember that if your sub amp is at the gain you set it at (it's maximum gain in your current setup) any bass boost will necessitate you keeping the volume down below where it was during the tuning. for example, if the gain on your sub amp is set at 12 'o clock when your hu is at 30, and you then dial in +3 bass on the HU, you'll be limiting your max hu volume to 27-29. If you want to be safe. See Note 2.
Notes
1. If you don't have some set of speakers amped (say you're running your fronts off the HU), whenever I say to disconnect that amp, you'll have to disconnect one or both of the wires at the speaker itself. Sorry, but the only other way that can be done is to fade completely away from those speakers; i.e. if the fronts aren't amped, and you're tuning the subs, you can fade all the way to the rear, and the fronts should be quite quiet enough for you to tune the subs.
2. If you're more advanced or have more time you can do equalization beforehand, then tune for maximum volume once the sound is properly shaped. The difficulty with this is in shaping with low volume and with the amps not precisely volume-matched... but it can be done, and if you can do it, you should, because it will allow you to squeeze every last centibel out of your properly tuned system.
3. If you're biamping, do the tweeter amplification last, and just set the gain at an appropriate level - you have to have an enormous amount of power running to the tweets to blow them, and your ears will probably go before they do, if it's clean power. Emphasis here is on clean power; clipping is what destroys tweeters, more than too much power. Just be aware.
4. If you like listening to techno, rap, organ music, heavy metal or anything else with ridiculously deep bass, you'll need to use a lower frequency when tuning your subs (this only applies to sealed and fourth-order bandpass). This is because music like this has basslines that can be nearly sine-waves in the 20-40 Hz range, which WILL over-drive your subs if you've tuned them with a 40 Hz tone. Just be aware; this won't be necessary if you listen to 'normal' music, because the music in the 20-40 Hz range is far less than a sine wave, and thus won't destroy your drivers. The advantage here in tuning with a 40 Hz tone is that you get more volume with no increased risk of damage (with 'normal' music ONLY).
5. When you're tuning your sub amp (not any of the others), once you hear the distortion, back the gain down about 15 degrees (a little more than you would think necessary). This is because, by the time you notice any distortion, you could easily be up around 10% THD. Unless you're one of the rare few that can hear low-level distortion in the subbass range, you will need to back the gains off this far to ensure the sub isn't getting a clipped signal, even though you wouldn't hear it.
6. If you're using passive crossovers when you're tuning the fronts, also be sure to listen to regular music at the maximum volume level once you've tuned that amp (but still without the subs). This is because, rarely, enough clipping can get past the crossover to mess up the tweets, and you will NOT hear it with normal tuning signals. So, pick something harsh and heavy with a lot of treble, and just check for high-frequency distortion. If you hear it, back off the gain a bit.
Gains Tutorial
This is the most widely used method, but that doesn't mean it's the best. Strictly speaking, the most effective way to adjust the gains on your amps is using an occilloscope and a track of pink noise. Now, since most people don't have easy access to an o'scope, and many have never heard of one (so don't be embarrassed if you have no idea what that o-word was - there are more important o-words to worry about), this is a great secondary method.
A definition: Xmax, as used in this walkthrough, means the manufacturer's reccommended excursion limit for the speaker. This is listed, usually on the website and in the owner's manual, as, appropriately enough, the thiele-small parameter called 'xmax'. DO NOT use a parameter referred to as 'xmech' or 'xmag' here, they're not necessarily the same thing. If you can't find xmax, the easiest way to find out is to call the appropriate company. (Yeah, I know, you'll have to use a phone, what a pity...)
1. Turn gains on all amps down.
2. Pick just one to tune, it doesn't matter. We'll call it the sub amp. Disconnect the rest of the amps, either power or speakers, doesn't matter. This is so you don't get confused. See Note 1.
3. Turn your HU up to 75%; mine goes to 40 and i turned it up to 34. This is because the VAST majority of headunits on the market begin to distort after this. If you are SURE you have a non-clipping headunit, you can go higher, but I strongly reccommend that you do not exceed 90% - even 'non-clipping' HU's can clip when they're being pushed this hard.
4. Find or make a test tone. For a sealed or 4th order bandpass sub box, use around 40 Hz. For a ported box, or sixth order bandpass box, it's not that simple; you'll be best off using a sweep that goes from the tuning frequency up to the lowpass frequency, because maximum excursion occurs at some point above tuning. You can find the point of maximum excursion and tune using that tone, if you desire. If those two sentences didn't make sense, go sealed or take the time to learn a bit more, you and your subs will be happier. For mains, use the highpass crossover frequency (the lowest frequency they're set to play at). See Note 4.
5. Play the tone and turn the gains up slowly until it begins to distort (if you have enough clean amp power, this will happen around xmax; if you're underpowering the speaker this will happen before you hit maximum excursion); with subs this is relatively easy to hear, as it should manifest as buzzing or a grittiness in the sound or some such thing - something other than the smooth hum you've been hearing. For speakers, it may be more difficult to hear. If in doubt, turn it down. Oh, and if for some reason your gain is almost all the way up... something's wrong. Your amp will distort when you've turned the gain up almost all the way. If you need it this high, there's a problem with your system somewhere that the amp can't fix for you. See Note 6.
6. Back it down a tad. See Note 5.
7. Rinse and repeat with all other amps, starting from Step #2 (including disconnecting the just-tuned amp), without changing the gain on the amps you've already set. See Note 3.
8. From here on in, NEVER turn the gains up beyond where you've set them unless a system component changes. If one amp is too loud, like your mids and highs overpower your subs, turn the gain on that amp down to match the rest.
9. Equalize/shape if needed; attentuate rather than boost if you can. Just remember that if your sub amp is at the gain you set it at (it's maximum gain in your current setup) any bass boost will necessitate you keeping the volume down below where it was during the tuning. for example, if the gain on your sub amp is set at 12 'o clock when your hu is at 30, and you then dial in +3 bass on the HU, you'll be limiting your max hu volume to 27-29. If you want to be safe. See Note 2.
Notes
1. If you don't have some set of speakers amped (say you're running your fronts off the HU), whenever I say to disconnect that amp, you'll have to disconnect one or both of the wires at the speaker itself. Sorry, but the only other way that can be done is to fade completely away from those speakers; i.e. if the fronts aren't amped, and you're tuning the subs, you can fade all the way to the rear, and the fronts should be quite quiet enough for you to tune the subs.
2. If you're more advanced or have more time you can do equalization beforehand, then tune for maximum volume once the sound is properly shaped. The difficulty with this is in shaping with low volume and with the amps not precisely volume-matched... but it can be done, and if you can do it, you should, because it will allow you to squeeze every last centibel out of your properly tuned system.
3. If you're biamping, do the tweeter amplification last, and just set the gain at an appropriate level - you have to have an enormous amount of power running to the tweets to blow them, and your ears will probably go before they do, if it's clean power. Emphasis here is on clean power; clipping is what destroys tweeters, more than too much power. Just be aware.
4. If you like listening to techno, rap, organ music, heavy metal or anything else with ridiculously deep bass, you'll need to use a lower frequency when tuning your subs (this only applies to sealed and fourth-order bandpass). This is because music like this has basslines that can be nearly sine-waves in the 20-40 Hz range, which WILL over-drive your subs if you've tuned them with a 40 Hz tone. Just be aware; this won't be necessary if you listen to 'normal' music, because the music in the 20-40 Hz range is far less than a sine wave, and thus won't destroy your drivers. The advantage here in tuning with a 40 Hz tone is that you get more volume with no increased risk of damage (with 'normal' music ONLY).
5. When you're tuning your sub amp (not any of the others), once you hear the distortion, back the gain down about 15 degrees (a little more than you would think necessary). This is because, by the time you notice any distortion, you could easily be up around 10% THD. Unless you're one of the rare few that can hear low-level distortion in the subbass range, you will need to back the gains off this far to ensure the sub isn't getting a clipped signal, even though you wouldn't hear it.
6. If you're using passive crossovers when you're tuning the fronts, also be sure to listen to regular music at the maximum volume level once you've tuned that amp (but still without the subs). This is because, rarely, enough clipping can get past the crossover to mess up the tweets, and you will NOT hear it with normal tuning signals. So, pick something harsh and heavy with a lot of treble, and just check for high-frequency distortion. If you hear it, back off the gain a bit.
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MECA SQ Rookie of the Year 06 ~ MECA State Champ 06,07,08,11 ~ MECA World Finals 2nd place 06,07,08,09
08 Car Audio Nationals 1st ~ 07 N Georgia Nationals 1st ~ 06 Carl Casper Nationals 1st ~ USACi 05 Southeast AutumnFest 1st
polkaudio SR6500 --- polkaudio MM1040 x2 -- Pioneer P99 -- Rockford Fosgate P1000X5D
MECA SQ Rookie of the Year 06 ~ MECA State Champ 06,07,08,11 ~ MECA World Finals 2nd place 06,07,08,09
08 Car Audio Nationals 1st ~ 07 N Georgia Nationals 1st ~ 06 Carl Casper Nationals 1st ~ USACi 05 Southeast AutumnFest 1st
polkaudio SR6500 --- polkaudio MM1040 x2 -- Pioneer P99 -- Rockford Fosgate P1000X5D