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Old 06-21-2006, 12:36 AM   #1
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Setting Your Gains

Disclaimer: all info based on research and personal experience. I am NOT responsible for what you do with your system.

Setting Gains

So many people have their amp gains set improperly, and to be honest setting them by ear is quite a crapshoot. Doing it properly with a multimeter is actually quite simple.

Here's how you do it:

1) DISCONNECT ALL SPEAKERS

2) Calculate voltage

P = Power in watts
I = Current in amperes
R = Resistance in ohms (effectively the nominal impedance)
V = Potential in volts (Voltage)

Knowns:
Resistance (nominal impedance of your speakers)
Power (desired wattage)

Unknowns:
Voltage (we'll measure this)
Current

Formulas:
P = I*V (formula for power)
V = I*R (Ohm's law)

So after a little substitution to get Voltage in terms of simply power and resistance we get:

V = square_root(P*R)

So, for example, say you have a 4 Ohm load presented to a 150 watt amp.

V = square_root(150*4) = 24.5 volts AC

This means you should increase the gain until you read 24.5 volts AC on the speaker outputs of your amplifier.

or

Let's say you have a 1 Ohm load presented to a 1200 watt amp

V = square_root(1200*1) = 34.64 volts AC

As a source, use a sine wave recorded at 0db at a frequency within the range you intend to amplify. You can generate tones in cool edit or use a program such as NCH tone generator, or purchase a Test Cd which will have the proper tones as well.

Additionally, you could use a scope to actually check if the signal is clipping, but I'm not going to get into that here as I doubt very few people have access to that type of equipment.

However, if need be, you can use these pre-recorded sine waves:
http://www.ronelmm.com/tones/

3) Set your gains

To measure the output of your amplifier, you will need a multimeter that can measure AC voltage. The one I'd recommend (and have) is:
Link

Using the number you have calculated in the previous step, you can now set your gains appropriately. Connect the respective lead on the multimeter to the speaker terminals on the amplifier (positive to positive and negative to negative). Set the multimeter to read AC voltage. Ensure that the gain is set all the way down, and that you have your filters/crossovers set (on the amp itself). Set head unit volume to 3/4 of maximum. Turn off all eqs/presets in the head unit. Play a track that will be in the appropriate frequency range (i.e. 50 Hz for a sub amp). I usually just play it on a loop. Turn the gain up until the multimeter reads the voltage that you calculated in step 2. Reconnect the speakers, and enjoy.





Note:
It is generally a good idea to have a more powerful amp than what your speakers can handle, and set the gains low. For example, I have a 1500 W RMS amp, and two 600 W RMS subs. I have the subs wired to present a 1 Ohm load to the amplifier, and I calculated the voltage in step 2 with 1200 W RMS (what the subs will use) as compared to 1500 W RMS (what the amp can put out, but will strain the amp and be too much for the subs). Therefore, I purchased an amplifier more powerful than what the subs require, and tuned it using the 1200 W RMS figure. Therefore, the amplifier has a lower strain on it than what it would trying to put out 1500 W RMS, and the subwoofers are receiving their nominal power.
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Old 06-21-2006, 03:36 PM   #2
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I have never thought about approaching it that way. Pretty ingenious way of doing it without specialized equipment. Just wanted to make a couple comments to benefit people that don't have much experience with this.

Most multimeters are designed to measure voltage at 60Hz (house AC US-standard frequency), so a 60Hz, even a 50Hz tone should give you an accurate RMS reading (i.e. good for setting up an amp for subs), but if you try the same procedure for upper range (I normally use a 1,000 Hz tone), then, depending on the meter, your voltage reading might not be very accurate (especially with a cheap meter).

Another thing to point out is the fact that a no-load voltage reading, could be different than a voltage reading with speakers connected to the amp. Unfortunately, a sine wave can damage a speaker, so it is best to do it without a speaker connected to the amp, and you have to be very careful if you try to do this with a speaker. Sometimes I use an old subwoofer as a dummy load to setup gains for front/rear speakers.

Using common sense is also very important. For example, if the head unit is rated at 3-volts output on the RCAs, and the amplifier dial on the gain reads (for argument's sake) 1 volt at a minimum and 5 volts max, then you expect a properly adjusted gain to be at the middle (around the 3-volts mark). If it isn't then you can suspect something is not right.

As suggested above, it is very good practice to get a bigger amplifier than what you need. An amplifier that is not being pushed to its limits will perform better, will not heat up as much and last longer. Kind of like having a big engine in a car. You will not be going full-speed all the time, but it does take less effort to accelerate and it is not being stressed as much during normal driving.

There are other ways to setup amplifiers. The best way to do it is with test tones and an oscilloscope, but as stated above, most people don't own test equipment. Most shops don't own scopes either, they just set gains based on experience/by ear on their garden variety installs.

Another way of setting up gains is using your ears as a tool. I would highly recommend getting a copy of the Autosound 2000 amplifier setting CD. It is cheap and can be used with either test equipment or for ear-tuning. The disk will help "train" your ears to learn what distortion sounds like.

For more details on system setting/tweaking, check out an article I wrote a while ago in my website: http://home.comcast.net/~customcarstereo/tweak.html
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Old 06-21-2006, 06:51 PM   #3
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ive been using test tones and my fluke123. its kinda nice to see it with the o-scope it makes you look smart doing it this way..hahah
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Old 06-29-2006, 12:57 PM   #4
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sweet
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Old 06-29-2006, 03:41 PM   #5
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Originally Posted by low
ive been using test tones and my fluke123. its kinda nice to see it with the o-scope it makes you look smart doing it this way..hahah
Fluke scope

I tried to use my scope to setup an amplifier last weekend and it is fried (acid + water from cleaning an adjacent basement wall probably had something to do with it). Although I would love to get a fluke scope, there is no way I can justify spending over $1,000 for something I don't use very often. Oh well, back to tuning by ear.
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Old 10-07-2006, 04:17 PM   #6
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What frequency range would one use to setup the amp for their sub? I used 30-39hz and set it to a solid 28.7/28.8 (800w) volts (AC). I then used the 90-99hz range to set the LPF on the amp. The sub is a 12" FI SSD (800w RMS) and the amp is an Orion 1200D (1200w RMS). The system absolutely pounds, but I just want to make sure that I did it right.

Thanks
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Old 10-07-2006, 10:08 PM   #7
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Depends on the crossover point. If your xver is set at 90, then 40, 60 would be good mid-points.
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Old 07-09-2007, 10:45 AM   #8
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Question

What frequency ranges would be suggested for amp tuning with sine wave for a subwoofer, and also MB Quart 2-way?

The link has many different frequencies to choose from, and yes, I am very new at this.

Amp is a JL Audio e6450, if that matters.

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Old 07-09-2007, 10:56 AM   #9
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Depends on your crossover frequencies. The frequency you choose can't be close to the crossover freq.

For bass, I use 40 Hz (My crossover frequency is 60Hz), you can use 60Hz if yours is higher.

For mids, I normally use 1,000Hz.

Are you using a meter or your ears?
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Old 07-09-2007, 11:04 AM   #10
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I'm using a meter.

The crossover frequencies listed by MB Quart are 5,000hz for the front components and 5,200hz for the rear components.

Does this sound right?
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Old 07-09-2007, 11:06 AM   #11
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I have no idea, but if you have a single input going into the MBQ crossover, it doesn't matter, just do 1khz.
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Old 07-09-2007, 11:07 AM   #12
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Got it. Thanks!
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