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Tuning your car stereo system can be rewarding. One lynchpin of tuning a car audio system involves setting amplifier gains. This guide will help you tune your car audio system for the greatest performance. These instructions can also protect your speakers from clipping distortion.
Setting gain matches the amplifier's input voltage with the source unit's output voltage. We can determine this by reading the output voltage. We can read the amplifier's output voltage with a digital multimeter, or DMM, for short.
A Couple of Notes
Understand that this is not the "best" way to set up your amplifier. This is what we did in the 1990s to make sure we weren't frying equipment. It's still useful if you want to set the gains with a tool you already own.
Using this method on a high-quality amplifier may leave some performance on the table. Using this method on a low-quality amplifier may result in equipment damage.
For more information, check out this article detailing how to set your amplifier gains with a Piezo Tweeter.
Some Definitions
What is the "Source Unit"?
For our purposes, a source unit is the last component before the signal reaches the amplifier.
What is clipping?
An amplifier has electronic limits to how much power it can produce. Attempting to surpass those limits will result in equipment damage. The reason for the damage is heat. The power you attempted to produce gets turned into heat inside the amplifier. The subwoofer voice coils also get sent high power signals with sharp bends. They're expecting a smooth curve.
A word about Class D amplifiers
Class D amplifiers are wonderful. They produce large amounts of clean power at a minimal cost. They're able to match the performance of Class AB amplifiers with fewer components. Class D amplifiers work by turning the input signal into a pulse width modulated signal. They send that signal to some transistors that act as high voltage switches. Engineers then filter out the switching "noise" with ordinary low pass loudspeaker filters. This yields an amplified signal that's as smooth as the input signal. This allows manufacturers to produce a high-quality amplifier at an affordable cost.\
But the low cost and high power output of a Class D amplifier will come at a price. Class D amplifiers misbehave at clipping. Even worse than traditional Class AB amplifiers.
When a Class A or Class AB amplifier reaches clipping, the result is a simple clipped signal. It looks like a sine wave with the tops and bottoms clipped off at the voltage limit. Oftentimes, the amplifier will also introduce "harmonic frequencies" into the signal at clipping. You may not notice these if you're using the amplifier's built-in low pass filter with a subwoofer.
A Class D will introduce extra spikes and dips and "fuzz" at the peaks and troughs of the clipped sine wave. This is on top of the harmonics and clipping distortion.
Some of the "old guys" have painted Class D amplifiers in a bad light because of how they behave when over-driven. But if you don't over-drive them, they're fantastic.
Tools Needed to Tune Amplifier Gains with a Digital Multimeter
You will need the following:
- a digital multi-meter,
- a source to send a tone signal through your system,
- a tool to turn the dial. Sometimes you can use your fingers. Other times, you might need a small screwdriver.
- A tool to disconnect and reconnect the speaker to and from the amplifier. These should never be "finger tight". Use the tool. They're usually an Allen key or a screwdriver.
- You'll also need some knowledge. We'll use Ohm's Law.
Volts = SquareRoot(Watts * Ohms)
The amplifier manual and the subwoofer spec sheet will provide us with the watts and Ohms.
An Exercise in Tuning Amplifier Gains
Let's go through the paces of setting an amplifier's gains. Our amplifier is a Rockford Fosgate R500X1D Prime 1-Channel Class D Amplifier. Our subwoofer is a Rockford Fosgate P3D4-12 Punch subwoofer driver.
This is a good match: the amplifier produces 500 watts into 2 ohms and the subwoofer has a 2 ohm voice coil. It can handle 600 watts.
Wait, I heard underpowering subwoofers was bad!
I go full Mr. Spock when this discussion comes up on Facebook, in a forum, or on Reddit. Turning down the volume is also under-powering the speakers. The flawed same logic means that you'll destroy your speakers listening to NPR. That same flawed logic means the only option to keep our speakers alive is to blast some hair metal at full volume. Skip the vocal interludes!
You and I know that's not true.
What kills speakers is abusing equipment. Don't turn up the gain because you think you'll get more power out of the amplifier. You'll get voice-coil killing distortion.
Determine Power
In our exercise, we're using an amplifier rated at 500w. We're wiring this to a subwoofer rated at 600w. We will aim to produce 500w at the amplifier output terminals.
Determine Impedance
An amplifier will not last very long if there's no resistance at the terminals. You'll pop the fuse if you short the amplifier at the speaker terminals. By that same notion, we need to make sure the subwoofer produces enough resistance. If it doesn't produce enough resistance, the amplifier could short and you'll pop the fuses. Consult your manual to determine how much resistance your speakers need.
For our lesson, we have a 2ohm loudspeaker connecting to an amplifier rated to handle a 2ohm load. We're good.
Determine output Voltage With Ohm's Law
To reiterate ohm's law: Volts = SquareRoot(Watts*Ohms)
We know the amplifier can produce 500w into 2 ohms.
So, to take a lesson from high school algebra, we'll show our work, one step at a time.
target voltage = SquareRoot(500 watts *2 ohms)
target voltage = SquareRoot(1000)
target voltage = 31.6
When we connect our digital multimeter to our amplifier, we want to target 31.6 volts.
When you set the gains on your amplifier for your speakers, change the numbers in the above equation. And make your math teacher happy: show your work!
Set Your Tone and Volume
Disconnect your speakers. Set your source unit to play a 40hz tone at 0db. 0db here does not mean absolute silence. 0db means it's "reference", you can also find tracks at -5db or even -20db.
Probe the amplifier outputs and Turn the gain!
Turn your amplifier gain all the way down. Play your track at 90% volume. If your deck volume range is from 0-80, set the volume to 72, for example. Hook up the digital multimeter to the speaker outputs on the amplifier. Check the voltage on the meter. If the voltage is below 31.6 volts, turn up the gain. When the DMM reads 31.6v, we have set our gain on our 500w amplifier at 2 ohms. We can now reconnect the subwoofer and boom away with less fear of clipping distortion!
Some Final Thoughts about Tuning Amplifier Gain with a Digital Multimeter
As I mentioned at the top of this tutorial, we aren't going to be perfect with this method. This knowledge is a tool we can add to our arsenal. We can use it to get our amplifiers "close enough for government work" in most cases. When it comes to edge cases, we may leave some amplification on the table. We may also over-drive an amplifier that is over-rated.
