Amplifier Gain and Crossover Setting Tutorial

Prophet · Nov 1, 2007

Found this over in ICIX. Would be useful for you guys trying to set your gains...

http://www.icixsound.com/vb/showthread.php?t=27819

This is a Tutorial on proper settings of an amplifiers gain structure.

First a little knowledge on the gain and what it does. A common misconception is that the gain is like a volume knob, or another one is that Ã‚Â½ way equals 50% power, and all the way equals 100%. These are not true, the gain is a level matching device, to allow you to match the signal being sent from your cd player, and create the most amount of power, without clipping which is the cause of distortion and excess heat to the voice coil. With most newer cd players being at least 4 volt outputs or more, there isnÃ¢â‚¬â„¢t a need to turn the gains up as high as with, a 2 or even 1 volt RCA preouts. We will show you what clipping sounds like, and what it visually looks like at the end of the presentation.

We will first start with the items needed:

A digital multi meter or DMM , they can be purchased at a Radio Shack, automotive parts store, or Walmart in the automotive department, for usually around $15-20 dollars. The multi meter is a basic trouble shooting device that all 12-volt mobile electronics enthusiasts should have.

A screwdriver, or other device for adjusting the settings on the amp.

A cd with test tones, a 50hz 0-db, for setting of the gain on a sub-woofer amp, and a 150 Hz for setting the gains the other speakers in the vehicle, and then a tone for setting your crossovers. This tone is dependent on where you decide you want your amplifier crossed over at. For example. If you want to low pass your sub-woofers at 80hz. You will need an 80hz 0 db tone. If you want to lowpass them at 65hz, then a 65hz 0 db tone is needed.

Next we will be using a basic formula to figure out where the gain needs to be set.
For this you need to know how much power your amplifier is rated at.

We will use a 2 channel nine.2. It is rated at 400 watts by 1 at 4 ohms when bridged.
The formula is sq root of Watts x resistance(ohm load)

400 watts by 4 ohm is 1600 then we find the sq root of 1600 which is equal to 40

We now know that 40 volts is equal to 400 watts.

This 40 volts will be found with the DMM.

Now we will begin.

The first step is to turn your cd player on. Then set any boost settings, or eq setting to 0 or flat, turn bass boost or loudness features off.

If your cd player has crossover settings, and you decide to use those over the amplifiers, I suggest you turn those on now. A sub-woofer low pass setting should be no higher than 120hz, we recommend 80hz or lower, depending on listening preference. The high pass on the rest of the speakers in the vehicle should be around the same point as the low pass. For explanation purposes we will use 80hz today.

If you have more than one amp in the vehicle, I recommend you unhook the amps not being set. This will prevent any damage to the other speakers in the vehicle.

The next step is to go to the amplifier and set the x over setting to full. If using a mono block amp such as a nine, turn the crossover all the way up so it does not affect the signal.

Then proceed to make sure the gain is turned all the way down.

You will then hook the DMM up to the speaker terminals with the speaker wire in them, one in the positive one in the negative. You will keep the speakers connected during the process.

The dmm will need to be turned on, and set to voltage ac, your DMM owners manual should explain to you where voltage AC is at. As for the number settings, you will want to have the decimal place at a hundredth, for example 00.00

Since we are testing the nine.2 bridged for a sub-woofer. I will be using the 50hz tone. If setting the gain for upper frequency drivers the 150hz tone will be used.

Insert the tone cd and then proceed to turn the cd player volume to 75%. If using stock speakers running off the cd player, then turn it up to where they start to distort, and then down a little bit.

Turn the 0 db 50hz tone onto repeat.

With the DMM running, slowly turn the gains up, you will notice the voltage increase, slowly turn the gain up until 40 volts is reached.

With the 40volts being reached the speaker is seeing 400 watts. You can then stop the tone and turn the cd player down.

Now if you are using the amplifiers crossovers, we will be setting these now. Since we have a sub-woofer hooked up, we will turn the crossover to Low pass filter, or LPF. The LPF dial will then be turned all the way up. Since we decided to cross the sub over at 80 Hz, we will throw the 80hz tone into the cd player, we will then turn the volume up a little bit, so the sub is noticeable, no need to be turned up all the way. Put on repeat.

The LPF dial will be slowly turned down until the sub-woofer starts to not play the tone with the intensity it did before, The sub-woofer will not stop completely playing the tone, but it will get quieter. You will then slightly turn the crossover back until the speaker is playing with all intensity again. You are then crossed over around 80hz.

The same goes for a other speakers playing your upper frequency range, only you will want to set the amp to high pass filter or HPF, and use the HPF dial.

Now that your gain and crossover is set properly the chance of damaging your speaker and amps is cut significantly, but you need to remember, a lot of music today is recorded with clipping, so you still need to be careful and watch for signs of driver stress.
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