06-21-2006, 12:36 AM
Join Date: Jul 2005
Location: Northeastern CT, near the cows and corn
My Ride: Black '99 328i Sedan
Setting Your Gains
Disclaimer: all info based on research and personal experience. I am NOT responsible for what you do with your system.
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)
Resistance (nominal impedance of your speakers)
Power (desired wattage)
Voltage (we'll measure this)
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.
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:
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:
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.
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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