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How To: Properly Set your Amplifier Gain w/o the use of an oscilloscope

13103 Views 24 Replies 12 Participants Last post by  old_skool_noma
I was browsing sound solutions audio today and found this and figured it would be very useful here as well


Most audio equipment dies for one simple reason. Most people push their equipment beyond its limits. Whether it is the amplifier, subwoofer or full range speakers, clipping is the number one cause of failure. To prevent clipping, use this tutorial.

To figure out what voltage you should set the gains to, multiply the RMS power of the amplifiers output by the impedance of the speaker, then find the square root of that number. If you are using an amplifier that has an RMS rating of more than your speaker(s) can handle/rated for, then use the RMS rating of the speaker (instead of the RMS of the amplifier) to determine the voltage to set your amp to. This is also referred to as gaining down.

Gain Setting Equation

Voltage of the output = sqrt(RMS Power X impedance of the speaker)
  • Example
    Say the amp provides 100WRMS into a 4 ohm speaker:

    Voltage = sqrt(100W X 4 ohms)
    Voltage = sqrt(400W*ohms)
    Voltage = 20V

    Again, that was only an example, use the ratings of your amp to figure that out.
Setting the Gain(s)
To set the gain(s), you need two things:
  • 1. A DMM (digital multi-meter) that is capable of measuring AC voltage (needs to be able to measure up to a range of 200V).
    2. A test tone CD to use to set the gains at the correct setting.
Now, to set the gain(s):
  • 1. Start the vehicle, and pop the test tone CD in the head unit.
    2. DO NOT hook up the sub(s) or speaker(s) to the amplifier while doing this, just leave the outputs unused at this time.
    3. Now, time to set up the head unit.
    • a. If the loudest you listen to your music at on a regular basis is 22/35 with bass @ +3 and treble @ 0 with MX (or any other sound processor) on, use those settings. NEVER turn the headunit above 3/4 of the maximum volume.
      b. Remember to have the car turned on.
      c. If you want to use bass boost on a sub amp, set it prior to setting the gains on the amp and use the center frequency of the bass boost (45 Hz for most amps) as your test tone.
      d. Please remember that if you have a subwoofer volume control on the headunit and/or a bass knob for the amplifier, set it to the maximum before you set the gains on the sub amp.
    4. Take the leads from the DMM and but them on the outputs from the amp.
    5. Set the gain so that the outputs of the amplifier equal the voltage you found above. This is a MUST.
Here is JL Audio tutorial on their site:

Here is where you can download some test tones for system testing/gain setting:

For test tones higher than 80Hz, download this program and you can create your own:
Adobe Audition Trial Version

It is best to use 50 Hz tone for a sub amp (unless you have bass boost, use the frequency that is boosted as the tone), and a 1kHz tone for a full-range amp.

This is a good way to set the gains, but if you have access to an oscilloscope, by all means use it. Then you can set the gains to their absolute maximum as you can see when the amplifier clips.

If you are wondering what exactly clipping is, and what it looks like, read this:

If you have any questions about this, post up, I�ll try my best to answer them.

Also, remember a sub can only handle what it can, if you set the amp to its RMS you have to remember that the sub can handle only so much. It is box dependant, but it is best if you are not experienced to follow the manufacturers recommendations.

Enjoy, and remember to thump responsibly!
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reread the title... my bad
I has a sillyscope :D

Great info old skool noma :clap:
I has a sillyscope :D

Great info old skool noma :clap:
i can haz sillyscope?
Just to throw in the thread:


^that's diyma's test tone site. Lots of good tones on that site, some fun ones too. Oh and they've got the bass testers, you know, to make sure youre subwoofer is working ;)
owning a fluke 123 makes my life easier

Having ears makes my life easier.:D
Can we get a sticky? I know a lot of people would benefit from this.
i will definately benefit from it when i put my stuff in. got too much money in it to half ass it and blow stuff up.
Can we get a sticky? I know a lot of people would benefit from this.
x2 very interesting and good to know
what im going to disagree with here on that chart is where it tells you to turn off your EQ, loudness and filters, all of this will affect your output voltage and could overdrive your amplifier(s) and speaker(s)
well since you are disagreeing, i might add i disagree with that chart (to an extent)

if that chart is right, then i am putting 1300 watts to my sub, but with a clamp, DMM, and thanks to ohms law, i found out i am putting ~800 to my sub. i am not saying that chart is mathematically wrong, just cant be used the way its intended.

so whats the deal? i dont understand why you would use that chart because the speaker impedance changes while a speaker plays so the amp doesn't see a load of 2 ohms all the time (technically it doesn't even see 2 ohm!). being that, why do you want to set your amp to "xx" voltage and when you hook up the sub sees a different load?

one example, based on that chart, if i follow the instructions 100% and lets say i set my amp to 40 volts since i am running @ 2 ohms so i can feed my sub 800 watts...when i hook up the sub after this procedure, will it see 800 watts? I think not...

For example... I pasted the chart i made of the actual output of my amp @ 2 ohms (1500 @ 1 ohm, 800 @ 2 ohm...need to recone the sub to dual 2 ohm sometime.)

Volume----Frequency-----AC voltage-----AC amperage----Ohms------Watts


how i got that chart and took the readings explained here from another forum.

This is what we uesd to do for our SPL vehicles.
The first thing we did was use an O-scope to set all of the gains.
Unforfunetly you can't do that unless you have an O-scope.
So just set your stuff up the best you can by ear.
Next take your DMM hook the positive up to the positive speaker wire at the amp so your positive speaker wire and positive wire from the DMM are connected to the positive speaker terminal on the amp.
Now do that with the negative also. Do the negative the same way you just did the positives.
and set the DMM to AC voltage.
Now take your clamp meter and clamp that only around the positive speaker wire comming out of the amp.
Set the clamp meter to AC amperage.
Now the install part is done.
Next you need a test tone disc.
Set the volume on the head unit to were you feel everything is as loud as you think it shoud be.
Or if you have an O-scope set your volume rite at clipping. Just a hair less so your not running a clipped signal and remember that volume number becuase you will need to do all your testing at those same settings to get an accurette Idea of whats going on.
Now with the DMM set to ac voltage and the clamp meter set to ac amperage take the test tone disc and find a frequency to start with, burp your system and write down the results from the DMM and clamp meter.
Both those numbers need to be writen down at the same time or you won't get propper results.
then go up 5 hertz burp again and write it down.
then go up another 5 hertz and do it again.

start at 30 hertz burp then 35 burp 40 burp and keep doing that until you have gotten to 80 hertz.
Now the physical work is done.
Alright now you need a calculator and a place to sit down and relax and you want to well im going to write it for you the way I've got it wrote down in my notes.

AC voltage from the speaker outputs divided by the amp draw on the positive wire from the amp.
That will give you your impedance at each frequency.

Heres some thing extra for you do you really want to know how much power your amp is really making do this.
Multiply the AC voltage by its self then divide that number by the ohm loads you just came up with.
thats a heck of alot more realistic than just use 4 ohms or 2 ohms becuase you just did the work to find out what your amp is really seeing. so the wattages you come up with are alot more realistic.
I'm going to post this then I'll have another post for you that shows you how I set up my paper.
After my next post if you don't get it PM me and I'll try to help but I would like to have any question you ask to be posted in this thread so it can help others.

Edit 02/09/2010:
To have fun with and to help, Ohm's Law/Watt's Law Calculator:

Hers an example of a kicker ZR360 running to 2 10 inch solo L7 tens with a nominal 2 ohm load.

Volume-----Frequency-----AC voltage----AC amperage-----ohms------watts
31 --------------20--------- ----41.4------------10.7 -----------3.87------- 443

Do you get the idea.
Do you see that even though I am running a nominal 2 ohm load I never hit 2 ohms
but my amp did put out rated power.
Its supposed to put out 400 watts at a nominal 2 ohm load and if you look at the numbers I averaged 400 watts.
Do you see the impedence rise at 40 hertz and 50 hertz and look at the wattage.
IMO i think a combination of this and your hearing will set your gains correctly since you know what EXACTLY your sub is seeing power wise. Ideally i would say set it like this and find out the power you are throwing at the sub and then use a O-scope (if you can get a hold of one) to check for clipping. setting the gain solely by DMM wont help much either since some subwoofers are at more of a risk of mechanical damage than thermal. in the end, doesn't box design play a large role on what your speaker can take? since everyone doesn't have the same box design who is to say this is right? yes a sub might be able to handle XXXX amount of watts but if the box is too large then you are at risk of mechanical damage correct? so again, you can use this method to check for thermal damage, and if your box is larger than the recommended size then use common sense and listen to what the sub is saying for any signs of mechanical stress.

oh and not trying to start shit, just want to start a good discussion/learn so please correct me if I'm wrong ;).
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oh, i know exactly what youre saying, the 2 ohm nominal impedance is just an average of all the output voltages according to the variance in impedance
what im going to disagree with here on that chart is where it tells you to turn off your EQ, loudness and filters, all of this will affect your output voltage and could overdrive your amplifier(s) and speaker(s)
i'll have to agree with you on that point also
I agree as well...... shallow and pedantic.
I agree as well...... shallow and pedantic.
Lois, i find your meatloaf shallow and pedantic
did someone say meatloaf?

oh, i know exactly what youre saying, the 2 ohm nominal impedance is just an average of all the output voltages according to the variance in impedance

yup, so basically that graph is useless for setting the gain of an amp.

I want to go by 1 hz increments then throw it all on a nice fancy graph. i am curious how it would look like.
did someone say meatloaf?

yup, so basically that graph is useless for setting the gain of an amp.

I want to go by 1 hz increments then throw it all on a nice fancy graph. i am curious how it would look like.
you can do this with win isd
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