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How to limit amplifier output power

Discussion in 'General Electronics Discussion' started by vinod chandran, Apr 26, 2013.

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  1. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi there,
    I have a 100 X 100 watts stereo amplifier. But unfortunately my speakers are 40 watts. So i need to limit this amp's output power. I googled for a circuit and find this-
    but i think this is too complex to build. I have to find either a limiting circuit or a 40w amplifier circuit which is suitable for my power supply. (40-0-40). Please help.

    Moderators note : updated link
    Last edited by a moderator: Feb 14, 2021
  2. BobK


    Jan 5, 2010
    Unless you plan on using ear-splitting volumes, don't worry about it, just don't turn it up too high.

  3. Solidus


    Jun 19, 2011
    You can get L-pad attenuators for 100W. Google "100w lpad" and the first few results should come up that are what you need.

    Problem with using L-pads is that they load the device at full load constantly, and thus reduce part life - but if you're using a solid state amplifier this shouldn't be too much of a worry.

    Simply get two (one for each channel) and put it a bit less than halfway, adjust to taste. That should kill about 60W from the output.
  4. john monks

    john monks

    Mar 9, 2012
    I am assuming that your amplifier is designed to put out 100 watts into 8 ohms. And I am assuming that the output impedance of your amplifier is about 0 ohms because of internal negative feedback that almost all amplifiers of this type are designed with.
    So the voltage coming out of your amplifier is 28.28 volts. The voltage required by your 8 ohm speaker is about 17.89 volts. So the current is going to be about 2.24 amps. So if you place a resistor in series with the speaker it comes out to be about 4.65 ohms and will dissipate about 23.2 watts.
    So there you have it. You need two resistors of about 4.65 or about 4.7 ohms and about 23 watts in series with each resistors. How simple can you get?
    If you get 10 ohms at 10 watts each for each channel you may be close enough.
    This is not perfect but unless you are a great conisour of music I don't believe you can hear the difference.
  5. davenn

    davenn Moderator

    Sep 5, 2009
    I'm using 100W speakers on a 240W / channel stereo amplifier

    I agree with BobK's comments.....

  6. vinod chandran

    vinod chandran

    Jun 21, 2011
    Hi All
    Thank you. ,
    Hi john monks- my speakers are 4 ohms. anyway i am going to try your idea. Is it possible to measure an amp's output voltage with a DMM
    Hi davenn- what if somebody turn the volume pot accidently ?.
    Last edited: Apr 27, 2013
  7. john monks

    john monks

    Mar 9, 2012
    That's why you want to limit the power going to the speakers.
    Seeing that your speakers are 4 ohms then you want to use 2.35 ohms 23.2 watt resistors in series with both speakers.
    As a practical matter you might as well use some 25 ohm resistors rated for 5 watts and just run lower power because unless you really like loud music you will never notice the difference.
  8. DJ AMA Solomon

    DJ AMA Solomon

    Feb 14, 2021
    There's no need to limit the power of the amplifier, I am using a 440W per channel receiver on 500W speaker each side and even with such a perfect match i still over power the speakers.


    Dec 19, 2019
    I used to use a self-resetting fuse.
    ratstar likes this.
  10. AnalogKid


    Jun 10, 2015
    A single 6 ohm resistor in series with a 4 ohm speaker will dissipate 60 W when the speaker is dissipating 40 W, adding up to 100 W and solving the problem. It also will increase greatly the effective output impedance seen by the speaker, so the speaker will ring more on transients.

    As above, the simple answer is don't turn up the volume. If you want to maintain fidelity, then attenuate the signal at the *input* to the amplifier rather than at the output. In a component stereo setup you can put an L pad between the tuner output and the amp input. In an integrated receiver it is more difficult.

    Note that the power the speaker receives is dependent on the signal. With a very quiet music passage through a 1000 W amplifier, the speaker will see only 10 W. The ***only*** way the speaker will see 100 W long enough to cause damage is with a test tone. Also, in a normal home listening environment, 40 W rms of voice or music into a speaker rated for 40 W will chase you out of the room and wake the neighbors.



    May 20, 2017
    If you use a series resistor to limit the power your speaker receives, assuming it has a crossover in it, it is going to sound fairly different. This is because the crossover was designed to operate from a source impedance close to 0Ω. Realistically your amp's output impedance is going to be in the region of 10 to 100mΩ depending on its design.
    The bass driver in particular will suffer as it relies on the ability of the amplifier to damp the cone's motion. If it cannot do so because a large value resistor has been put in series with it, the cone is free to flap about in an uncontrolled manner.
    Just use the volume control sensibly.
  12. ratstar


    Aug 20, 2018
    What if you use a bypass cap after the resistor to get rid of the load?
  13. AnalogKid


    Jun 10, 2015
    A bypass cap would not "get rid of" the load, it would increase it.

  14. Audioguru


    Sep 24, 2016
    An amplifier with a +/-40V supply has a total of 80V which will allow an amplifier output of about 80W into 8 ohms per channel before clipping or 100 Whats with severe distortion. The output will be about 140W into 4 ohms if the amplifier can drive 4 ohms.
    I blew up the woofers in speakers rated at 40W. The woofers were stamped 5W. Yeah, I played them too loud.
  15. davenn

    davenn Moderator

    Sep 5, 2009
    OK this was a 7 year old thread that @DJ AMA Solomon resurrected

    it's now closed
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