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Thread: Gain Setting with a DMM

  1. #1
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    Gain Setting with a DMM

    I hade made a post in the amplifier tuning guide stickied at top, but since that is supposed to be a very basic guide I have upon request removed my post in there to prevent any confusion. Some of the terms used in this article are not commonly known and the method may seem a little confusing. If you just want a simple no think way of setting your gains follow the method set forth by Jason stickied at the top. It is a good method for setting the gains on your amp(s), but using a digital multimeter is probably the safest and easiest (once you know what youre doing) way of setting your gains, short of using an o-scope at least.

    Gain Setting tutorial ...
    http://www.jlaudio.com/tutorials/Inp...nsitivity.html

    Test Tones ...
    http://www.teamtreo.com/view.php?showme=Free_Test_Tones (50 hz for sub)
    http://www.realmofexcursion.com/downloads.htm (1 khz for comps)

    Here's the formula you'll need ...
    Sqrt(Power*Resistance) = Voltage


    (i.e. If you bridge a 4 ohm subwoofer to your amplifier that is rated to produce 100 RMS @ 4 ohms in bridged mode, you would want Sqrt(4*100) = 20 volts) Make sense?





    The simplest way to do this is to just take the rms rating on the amp @ whatever ohm load you wish to use and just plug it into the equation. If the rest of this writeup is a jumble of words then just remember that using the rms power rating of your amp will work most of the time.



    The power you use can be either the rms power of the amp if you know you cans supply the amp enough voltage to do the rms power or any power value below rms. You can just check the voltage at the amp when it is playing to see what kind of voltage you can supply your amp. If you can only keep it around 12v then you may want to set your gains for a little below the rms power since most manufacturers do rms power ratings at around 14v although some do measure them @ 12v or less. However, keep in mind, test tones play louder than almost any music. i.e. they use up more power. So if you wanted to get 100w @ 4 ohm then its gonna be done at 20v at full tilt or on test tones. Music probably wont draw as much power so you could probably set your gains a volt or 2 higher and stil only get 100w out of the map @ 4ohms. Everybody with me?

    The rms power on your amp is the most power your amp can play continuously without clipping. In other words RMS power is clean continuous power. Clipping is when you try and force your amp to create more power than it is capable of and as a result it generates a harmful dc current. A clipped wave is nearly twice as powerful as a clean sine wave and in addition its abnormal shape does not allow for proper cooling. In case you didnt guess clipping ussually leads to more heat and burnt voicecoils i.e. blown subs, which is something none of us want. If you use this method to set your gains properly you will not have to worry much about clipping your amp or destroying your equipment. Its kind of confusing, but is really helpful once you understand it.

    Heres one more example for you:
    Amp- eD Nine.1
    Desired Power- 900w (This is the rms rating of this amp and iknow it is done at 12v which I know I can supply to this amp no problem)
    resistance - 2 ohms
    so formula is sq rt (900*2) = 42.4 V

    Realistically, if you dont know at what voltage the rms rating of your amp is at dont worry about. If you always set your gains according to the rms setting of your amp you will be safe at least 90% of the time. I hope you guys understood this and if not just ask and I'll do what i can to help.


    Before you begin, DISCONNECT YOUR SPEAKERS!!!!, unless of course you like the smell of burnt voice coils
    Last edited by eric77; 05-18-2005 at 09:07 PM.

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    Quote Originally Posted by Benny Z
    eric, will you come set my gains for me?

    haha, any idiot can set their gains

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  5. #5
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    Quote Originally Posted by Benny Z
    *your first link is broken...
    fixed

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    so i think i've got it...

    for my ppi pcx-4125... 125 x 2 @ 4 ohms and 12v for the front two channels.

    sqrt(125*4) = 22.360679774997898

    so where do i measure the voltage? at the speaker wire ends where they connect to the passive crossovers?

  7. #7
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    Quote Originally Posted by Benny Z
    so i think i've got it...

    for my ppi pcx-4125... 125 x 2 @ 4 ohms and 12v for the front two channels.

    sqrt(125*4) = 22.360679774997898

    so where do i measure the voltage? at the speaker wire ends where they connect to the passive crossovers?
    measure the voltage at the speaker outs on the amp.

    With that amp you are going to be able to turn the gains up a little further than 22v seeing as how it is rated at only 12v. I think they say you get a 10% power increase for every additional volt but my memory could be off. But if that is the carrect formula you should be getting 135-150w out of that amp. Check your voltage at the end of the power wires and see what you actually get back there.

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    i'll check it out tonight. that power rating sounds about on par with reviews i had read before i bought the amp. ...one said they saw close to 200w per channel at 4 ohms.

    right now i've got the gain at about 1/3 of the way up for the front channels. i'll see what voltage it reads as is...i don't think i want to turn it up much more than where it's at. it gets really freakin loud as is (and still SUPER clean sound).

  9. #9
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    Quote Originally Posted by Benny Z
    i'll check it out tonight. that power rating sounds about on par with reviews i had read before i bought the amp. ...one said they saw close to 200w per channel at 4 ohms.

    right now i've got the gain at about 1/3 of the way up for the front channels. i'll see what voltage it reads as is...i don't think i want to turn it up much more than where it's at. it gets really freakin loud as is (and still SUPER clean sound).
    the most important thing you can do is just be smart. If it starts to sound distorted for any reason turn down the gains a little. The dmm is great for finding a reference spot, but from there you should play it by ear. If its too loud, then turn the gains down, etc.

    I was just playing around with my dmm too and was very surprised to discover I was getting 13.8v at the amps with music blaring . That is much more than I had expected. I'm sure the new battery helped somewhat.

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  10. #10
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    i just noticed the test tones link you posted only has tones up to 80hz.

    ...since i'm setting gains for the components and not a sub, should i just use the 80hz tone, or should i try to find some mid and high range tones?

  11. #11
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    Quote Originally Posted by Benny Z
    i just noticed the test tones link you posted only has tones up to 80hz.

    ...since i'm setting gains for the components and not a sub, should i just use the 80hz tone, or should i try to find some mid and high range tones?
    you'll want 1 khz for the comps. I'll see if I can find one.

    edit: see first post. New link is up.

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  12. #12
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    can someone code that university calculus into a javascript

    i just want to be able to watch my dmm and know when to stop turnin the gain up.

    and not have to figure out all this fucking math nonsense

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    Great post Eric Really like the downloadable test tones/bass tracks

    Just to make sure I've got this right:
    My nine.2 is rated @ 400w x 1 going into a 4ohm load so I should set the gains to 40V.

  14. #14
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    Quote Originally Posted by Bremsen
    Just to make sure I've got this right:
    My nine.2 is rated @ 400w x 1 going into a 4ohm load so I should set the gains to 40V.
    right on. One thing to note though, the nine.x series are all rated at around 12.5v meaning that if you get anything over 12.5v in your car, which most do, you will be restricting your nine.2 somewhat. If it was me, I'd use 40v as a nice reference then go up a little more from there because test tones send out a stronger signal than regular music and because your nine.2 will probably do over 400 in your car.

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    so i set my head unit to just under "full blast" while playing the test tone, right?

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    Quote Originally Posted by Benny Z
    so i set my head unit to just under "full blast" while playing the test tone, right?
    I think 3/4 volume is pretty good for most head units. You want to set everyting with the hu/u a few notches below where the head unit starts clipping. That way you have the ability to squeeze (or clip, lol) a little more out of your amps if you should ever want to.

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    OK...I'm having some troubles. Here's the part I've got so far...

    Voltage at amp: 12.82v with the ignition off and 14.19 volts at idle.

    That was the easy part.

    I'm having trouble getting a reading on the speaker output voltage. I just put the postitve to the positive out and negative to negative out for one channel at a time, right? I'm getting +1.3 on the left side and -.7 on the right side doing it this way. ...and that's with the gain turned up close to 3/4.

    What am I doing wrong???????

  18. #18
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    Quote Originally Posted by Benny Z
    OK...I'm having some troubles. Here's the part I've got so far...

    Voltage at amp: 12.82v with the ignition off and 14.19 volts at idle.

    That was the easy part.

    I'm having trouble getting a reading on the speaker output voltage. I just put the postitve to the positive out and negative to negative out for one channel at a time, right? I'm getting +1.3 on the left side and -.7 on the right side doing it this way. ...and that's with the gain turned up close to 3/4.

    What am I doing wrong???????
    are you playing a 1khz test tone?

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  19. #19
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    Quote Originally Posted by Benny Z
    OK...I'm having some troubles. Here's the part I've got so far...

    Voltage at amp: 12.82v with the ignition off and 14.19 volts at idle.

    That was the easy part.

    I'm having trouble getting a reading on the speaker output voltage. I just put the postitve to the positive out and negative to negative out for one channel at a time, right? I'm getting +1.3 on the left side and -.7 on the right side doing it this way. ...and that's with the gain turned up close to 3/4.

    What am I doing wrong???????
    Nevermind...I was an idiot. I still had the DMM set to DC. Switched it to AC and set the gain at 23 volts with the Alpine deck set to volume level 29.

    Woohoo! Thanks!

  20. #20
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    Quote Originally Posted by Benny Z
    Nevermind...I was an idiot. I still had the DMM set to DC. Switched it to AC and set the gain at 23 volts with the Alpine deck set to volume level 29.

    Woohoo! Thanks!
    29 volume might be pushing it. I believe most alpines start to clip around 25.

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    Quote Originally Posted by eric77
    29 volume might be pushing it. I believe most alpines start to clip around 25.
    grrr...

    this is where the local car audio store told me to set it. he said that i would still have some head room to go if i need it.


  22. #22
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    Quote Originally Posted by Benny Z
    grrr...

    this is where the local car audio store told me to set it. he said that i would still have some head room to go if i need it.

    i could be wrong...

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  23. #23
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    Setting Gain

    Quote Originally Posted by eric77
    I hade made a post in the amplifier tuning guide stickied at top, but since that is supposed to be a very basic guide I have upon request removed my post in there to prevent any confusion.
    I sure hope you aren't suggesting that people do this adjustment with the speakers connected!
    Many subwoofers can handle continuous test tones for a modest amount of time (especially at the responant frequency where the impedance is highest), but most voice coils will melt quickly with test tones. Tweeters will fry in seconds, and fine midrange drivers will be damaged or equally destroyed.

    The power spectrum in actual music is much different from sinusoidal test tones. For example, 100 watts of typical music will have less than 10% of that power above 3 kHz. That is why you can have a power rating of 100 watts on a tweeter---it is expected that the actual "music" power will be less than 10 watts.

    But if you use sinusoidal test tones, you get the FULL power at any given frequency. That will spell disaster.

    It would be wise to disconnect the speakers when adjusting the overall gain settings. If you just want to balance the system gains, such as with a multi-way active system, use only a few volts.

    Believe me, I've melted more than my share of drivers during tests, and S**T happens all the time!

    Matthew
    00 528 Sport

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    if you click on the first link he posted, jl instructs you to be sure to disconnect the speakers before playing the test tone...

  25. #25
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    Quote Originally Posted by exmaxima1
    I sure hope you aren't suggesting that people do this adjustment with the speakers connected!
    Many subwoofers can handle continuous test tones for a modest amount of time (especially at the responant frequency where the impedance is highest), but most voice coils will melt quickly with test tones. Tweeters will fry in seconds, and fine midrange drivers will be damaged or equally destroyed.

    The power spectrum in actual music is much different from sinusoidal test tones. For example, 100 watts of typical music will have less than 10% of that power above 3 kHz. That is why you can have a power rating of 100 watts on a tweeter---it is expected that the actual "music" power will be less than 10 watts.

    But if you use sinusoidal test tones, you get the FULL power at any given frequency. That will spell disaster.

    It would be wise to disconnect the speakers when adjusting the overall gain settings. If you just want to balance the system gains, such as with a multi-way active system, use only a few volts.

    Believe me, I've melted more than my share of drivers during tests, and S**T happens all the time!

    Matthew
    00 528 Sport
    Sorry for being unclear. I left the explanation of the process up to the jl tutorial at top. In which, you are told to disconnect your speakers before setting your gains.

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