How to set amplifier gain - Home Theater Forum and Systems -

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post #1 of 10 Old 01-04-10, 02:02 PM Thread Starter
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How to set amplifier gain

Contrary to popular belief, car audio amplifier gain controls are not volume controls. They are input sensitivity controls and are intended to allow the user to adjust the sensitivity of an amplifier to match the output voltage from the head unit. This is needed in car audio because there is no standard for head unit output voltage.

The purpose of properly setting the amplifier gain is three fold. First and most importantly, it is to allow the amplifier to reach max output when the head unit volume is at max without causing the amplifier to clip. Secondly, gain can be limited to prevent over-powering your speakers if the amp can put out more than the speakers can handle. Third, gains can be used to help balance a system, meaning adjusting the relative output levels between amplifiers so that they are operating at the correct levels for any given head unit volume setting. This tutorial is primarily aimed at addressing the first: achieving max output without clipping the amplifier.

Here are a few things you'll need:

Hearing protection
You need ear plugs or muffs while setting gain as the system will be operating at high output levels. Hearing loss is very quick as sounds become louder, and you really need to protect your ears because building a good sounding sound system will be a waste of time if you can't hear it. Also since the output will be high while gain is being set you should perform this in a location and at a time when it won’t disturb other people.

A CD with proper test tones
You need a CD recorded at 0db reference level that contains various test tones. 50-60 Hz is good for setting subwoofer levels and 1000-1500 Hz is good for setting main stereo amp levels.

The basics:

Fader, tone controls, loudness/expansion, etc.
Fader and balance controls should be set to “normal” or zero or “in the middle.” Loudness and bass boost should be OFF and all EQ set to flat.

Dedicated sub volume controls
Many amps have outboard sub volume controls and head units have dedicated internal sub volume adjustments. These should be set to MAX.

Set all amp gains to their lowest point before starting. This is usually full counter-clockwise.

Input sensitivity switches
If your amp has a selector switch for different input sensitivities, start by setting it to the highest setting. These are typically expressed in voltages, for example .2-1v, 1-3v, 3-8v. Start with the higher numbers (ex. 3-8v which is the lowest sensitivity). If you can't get the amp to clip at those settings, try the next one down until you find the clipping point. You can disregard generally what the markings themselves say since there's no real standard for marking them. Never trust your system's well-being to those arbitrary numbers, they're just there for reference and are best ignored.

Work with one gain at a time.
For example, if you have both L&R gain for your front speakers, you'll set each side separately. If multiple amps, unplug (remove the signal from) all but the amp you're working with. If it’s a 4+ channel amp, typically you'll have only a single L & R gain, so treat it like a 2 channel. If it has multiple gains adjust each one separately.

Now, the procedure.

Play the appropriate test tone
Put on your hearing protection and start your car. Select a test tone that is in the pass band of the amplifier channel you are setting. For example, if it’s a subwoofer amp select something audible below the subwoofer LP crossover setting. I like to use 60Hz. Set your CD player on its lowest volume setting and play the track in repeat mode.

Find the head unit clipping point
Slowly turn up the head unit volume and you should begin hearing the tone. Keep turning it up until you can hear the tone change. It will become “sharp” sounding and is an obvious change. This indicates the head unit preamp is now clipping. Note the volume level and slowly turn the head unit back down until the tone changes back to its original sound. Note this volume level. If the head unit clips at below its max level, you should NEVER turn it up past the point where the clipping begins.

If the head unit never clips, this is good and indicates you have a quality preamp. Set the volume control to about 75% or 80% of whatever its max might be.

Now find the amplifier clipping level
Turn off the system and disconnect all the RCA cables except for the amplifier channel you are setting. Now turn the system on and place the test tone in repeat, and turn the head unit up to the max unclipped volume discovered in the previous step. Move to the first amplifier gain control and SLOWLY turn it up until you hear the tone change quality like before. This step may get loud, so be prepared. Once you find the gain setting where clipping begins, slowly back it off until the clipping stops. THIS is your proper max gain setting for this amplifier channel. Leave it there. Now turn off the head unit and repeat the above step for all the other amplifier channels in your system.

After setting all the gains you can enjoy your system. Some additional thoughts:

Final tweaking -
Have an EQ? Want to use the "loudness" button? Want to adjust the bass/mid/treble controls? If you're making minor tweaks (+/-10%) there's no real need to worry about changing the gains. If you're talking about bigger changes (+3db or more) you may want to run the tones again with the new EQ settings to be sure you're still set right.

Also now that the gains are properly set you can adjust them DOWN to balance your system as much as you need to, just be sure you never turn them UP past the max points you discovered. Need more front volume but don't have a fader? Turn down the rear gains. Sub sound underpowered? Turn down the mains amplifiers. The important thing is to never turn them UP from where they are, just down.

Can't I just use an O-scope or DMM to set gains?
Sure, IF you know the exact output (not the rated output) of your amp and you understand how to do this based on speaker impedance and amplifier voltage, go for it. Indeed, some amplifier manufacturers like JL Audio prefer this method and give you detailed instructions for this. Generally setting gain by ear is more accurate & far easier and faster.

What about the gains on the EQ/X-over/line-output converter?

Ooh, good question. The general idea here is to follow the same process but start by setting the gains that are the furthest UPSTREAM (I.E. closest to the head unit) first and then set all the others as described above.

What if my head unit says "9v output" and the amp only says "5v" after I set the gain?
As I mentioned before, that doesn't really matter. Again, there isn't really any set standard for marking these controls. Just set everything according to the above process and you’ll be safe.
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post #2 of 10 Old 01-04-10, 02:43 PM
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Re: How to set amplifier gain

I remember seeing a document from Rockford Fosgate or JL (I forget which) a few years back about setting gain control using a multi-meter and not having to drive your amp/speakers to clipping. I'll see if I can locate it, but is this a viable method (less chance of hearing/speaker loss)?
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post #3 of 10 Old 01-04-10, 02:44 PM
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Re: How to set amplifier gain

Warning: PDF Link
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post #4 of 10 Old 01-04-10, 04:45 PM Thread Starter
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Re: How to set amplifier gain

Thanks for adding that!
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post #5 of 10 Old 04-13-11, 01:33 PM
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Re: How to set amplifier gain

Hi i read all the instructions you wrote and its seems easy.. Thnks ive been searching for something like this.. I have a question.. Where do i find to down load the cd test tones you said before? I read the jl audio page you just mentioned and they talk about this tones too.. Where do i find them to down load them.. I get the rew 5 but i think i need something,, i dont know about this i used it with my mini la acer mic i think its wrong,, do i need anothe mic? Where do i find the correct one? Please help me with this.. Do you have your instructions in spanish? Because my english is not so good.. Thnks please help me..
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post #6 of 10 Old 04-13-11, 02:35 PM
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Re: How to set amplifier gain

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post #7 of 10 Old 04-13-11, 03:56 PM
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Re: How to set amplifier gain

BETO wrote: View Post
OH my...............
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post #8 of 10 Old 06-26-11, 07:51 PM
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Re: How to set amplifier gain

Yes, in theory this method will work for small system but in systems with active amplification, I mean where in the car installed one monoblock for subwoofer, and 3 different amplifiers for 3 way front speakers only the way to set gain is listen and adjust, and this could take up to a weeks to set balance between sub,midbass, mid and tweeters.
For setting system with Db meeter I was using CDs: Professional Audio Test Disc by The Alan Parsons Project.
and Alan Parsons - Sound Check 2: Audio Test and Demonstration CD (2003).
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post #9 of 10 Old 07-04-12, 08:23 PM
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Re: How to set amplifier gain

I use a DD 1 as well
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post #10 of 10 Old 12-30-13, 08:45 PM
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Re: How to set amplifier gain

well i use a multimeeter
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amplifier , gain

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