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Stolen from Critical mass technical info. I thought it was very good (especially for those of you with problems blowing subs, or wanting more output)
When using multiple amplifiers for BASS it's important to remember the following.
1. Amplifiers must be Identical, SAME EVERYTHING! Any differences translates into less bass do to cancellation.
In other words, if you are an SPL competitor and DBs are everything, a 1 db gain adjustment difference between 2 amplifiers could cost you 2 db of bass output depending on the scenario. Hence the common "when I only had one 12" it sounded better than now with 2 12" subs and 2 Amps." Counting that most Car Audio installers understand polarities (+/-) the likely scenario is that there are differences in the bandwidth (frequency output ex: from 20z-80hz etc..) and or the amplitude of the 2 or more amplifiers.
2. The Gains Must be exactly the same on all amplifiers. We recommend having the Gains all the way down.
This way you are guaranteed to have exactly the same gain on both amps. Gains all the way up are a bad idea. The gains all the way up have huge disadvantages, whenever possible have all gains all the way down. This improves your S?N ratio (Signal to Noise) and eliminates many unwanted problems such as engine noise and Voltage mismatches which cause huge problems to your subs well being.
3. Use a head unit and or X-over/line-driver/preamplifier with a matching Voltage to allow your amplifier to reach maximum output with the gain all the way down. All amplifiers have gain controls that are variable. When set to the lowest setting your amplifier is designed to reach full output with an audio signal between 2.5V-10V. Read the owners manual and determine which Voltage you need. Than make sure your pre amplifier signal reaches that level without clipping (AKA: DC, distortion, BAD)
4. If at all possible (budget allowing) A pre-amplifier with Clipping IN indicator and Clipping OUT indicator is preferable, This allows you to set the pre-amplifier signal to maximum output without ever clipping. This is great because you will never blow a Woofer again. In a perfect world you would have a Line-driver with High Voltage out, subsonic filter and Lo-pass and Bass boost such as the Critical MASS ULX-9V http://www.criticalmassaudio.com/preamps/ulx9v.html This way you can set all Amplifiers exactly the same way (Gains all the way down, BASS Boost All the way Down, Lo-Pass all the way up, Subsonic filters ON all the way down) and by having all your bass amps connected to the ULX-9V you would now be able to feed all your amps the exact same bandwidth at the same exact amplitude (The exact same signal) This translates to more bass, cleaner Bass, and no blown Subs.
When using multiple amplifiers for BASS it's important to remember the following.
1. Amplifiers must be Identical, SAME EVERYTHING! Any differences translates into less bass do to cancellation.
In other words, if you are an SPL competitor and DBs are everything, a 1 db gain adjustment difference between 2 amplifiers could cost you 2 db of bass output depending on the scenario. Hence the common "when I only had one 12" it sounded better than now with 2 12" subs and 2 Amps." Counting that most Car Audio installers understand polarities (+/-) the likely scenario is that there are differences in the bandwidth (frequency output ex: from 20z-80hz etc..) and or the amplitude of the 2 or more amplifiers.
2. The Gains Must be exactly the same on all amplifiers. We recommend having the Gains all the way down.
This way you are guaranteed to have exactly the same gain on both amps. Gains all the way up are a bad idea. The gains all the way up have huge disadvantages, whenever possible have all gains all the way down. This improves your S?N ratio (Signal to Noise) and eliminates many unwanted problems such as engine noise and Voltage mismatches which cause huge problems to your subs well being.
3. Use a head unit and or X-over/line-driver/preamplifier with a matching Voltage to allow your amplifier to reach maximum output with the gain all the way down. All amplifiers have gain controls that are variable. When set to the lowest setting your amplifier is designed to reach full output with an audio signal between 2.5V-10V. Read the owners manual and determine which Voltage you need. Than make sure your pre amplifier signal reaches that level without clipping (AKA: DC, distortion, BAD)
4. If at all possible (budget allowing) A pre-amplifier with Clipping IN indicator and Clipping OUT indicator is preferable, This allows you to set the pre-amplifier signal to maximum output without ever clipping. This is great because you will never blow a Woofer again. In a perfect world you would have a Line-driver with High Voltage out, subsonic filter and Lo-pass and Bass boost such as the Critical MASS ULX-9V http://www.criticalmassaudio.com/preamps/ulx9v.html This way you can set all Amplifiers exactly the same way (Gains all the way down, BASS Boost All the way Down, Lo-Pass all the way up, Subsonic filters ON all the way down) and by having all your bass amps connected to the ULX-9V you would now be able to feed all your amps the exact same bandwidth at the same exact amplitude (The exact same signal) This translates to more bass, cleaner Bass, and no blown Subs.