audio amplifier ohms calculation

Discussion in 'General Electronics Chat' started by jatinparekh, Mar 7, 2012.

  1. jatinparekh

    Thread Starter New Member

    Jan 2, 2012
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    hello everyone
    how i can calculate or cross check that amplifier saying 6 ohms is actually giving me 6 ohms to my speakers ?
    is there any eqipments to calculate ohms?
     
  2. MrChips

    Moderator

    Oct 2, 2009
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    6 ohms is not a property of the amplifier. It is the impedance of the loudspeaker.
    Most loudspeakers are 8-ohm impedance.
     
  3. jatinparekh

    Thread Starter New Member

    Jan 2, 2012
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    yes true but any way to measure if the amplifier is actuallying giving 8 ohms
     
  4. vpoko

    Member

    Jan 5, 2012
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    I'm pretty new to all of this myself, but I don't think the question makes sense. Ohms is a measure of impedance, which is a property of devices through which current passes (the speakers). Amplifiers don't put out impedance, they have a certain expectation for what the speakers' impedance will be. Amplifier power is measured in watts.
     
  5. jatinparekh

    Thread Starter New Member

    Jan 2, 2012
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    actually i had the same thing on my mind but then why the amplifiers manufacturing company gives amp specification as say like 300 watts at 4 ohms
     
  6. kubeek

    AAC Fanatic!

    Sep 20, 2005
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    means it can output up to 300W when the speaker has 4 ohms impedance, and that the impedance should be 4ohm or higher.. If you use an 8ohm speaker, you will get lower maximal power. If you use 2 ohm speaker, you will burn the amplifier.
     
  7. wmodavis

    Well-Known Member

    Oct 23, 2010
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    The output impedance is of an audio amplifier is not the same as the spec'd load impedance. They are not designed for max power transfer rather for power output into a specified load resistance/impedance.

    More often than not the amplifier output resistance is much lower then the load impedance and some believed the lower the output impedance the better and that led to an amplifier spec I haven't seen mentioned for many years - "damping factor". See this article.
     
  8. crutschow

    Expert

    Mar 14, 2008
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    Most modern solid-state amplifiers have a very low effective output impedance (much less than an ohm) due to a large amount of negative feedback. It's so low that the "damping factor" (ratio of speaker impedance to amplifier output impedance) is no longer mentioned as a significant spec (often the resistance of the wire connecting to the speaker is higher than the amp output impedance). Damping factor was more of a concern with tube amps where the output impedance is typically higher, especially if there is an output transformer.
     
  9. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Most modern solid-state amplifiers have a very low effective output impedance (much less than an ohm) due to a large amount of negative feedback. Kind of what I said, isn't it? It's so low that the "damping factor" (ratio of speaker impedance to amplifier output impedance) is no longer mentioned as a significant spec (often the resistance of the wire connecting to the speaker is higher than the amp output impedance). Damping factor was more of a concern with tube amps where the output impedance is typically higher, especially if there is an output transformer. As an audio amplifier design engineer in the late 60s damping factor was a characteristic we measured and published for our solid state audio amplifiers with no output transformers. But you add good info on magnitude of output impedance and speaker wire resistance compared to output impedance of the amplifier.

    To OP: You can try to calculate it but it is not a trival exercise for the uninitiated. It can be measured using the appropriate equipment. We did not use specialized test equipment to do so rather a voltmeter, variable low resistence load and Ohm's law.
     
  10. #12

    Expert

    Nov 30, 2010
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    I suppose you could use an oscilloscope to see at what voltage clipping starts to happen when the rated amplitude is being driven into the correct impedance, but I don't expect the OP (jatin) to have an oscilloscope, a signal generator, and the right math skills. An easier way might be to buy a $30 sound level meter, calculate the SPL the speakers should produce with the wattage you have, and listen for clipping. Not deadly accurate, but pretty good if you can get a steady tone. Real music has peaks ten times as high as the average level so that's useless for measuring.

    As has been said here, this is not a trivial exercise, and not usually a concern. We whipped this problem so long ago that most of us don't even think about it.
     
  11. wmodavis

    Well-Known Member

    Oct 23, 2010
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    #12 - Not sure what clipping or a sound level meter has to do with determining amplifier output impedance.
     
  12. #12

    Expert

    Nov 30, 2010
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    Just a check to see if the impedance is too high to provide proper signal to the load. Nowhere near good enough to find the actual impedance of the amplifier. If this is a matter of finding a faulty amplifier, a clipping test would be a start. If it's just curiosity about how to prove the published specs are true, my contribution is worthless.
     
  13. Audioguru

    New Member

    Dec 20, 2007
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    I think Crown and a few other very high power amplifier manufacturers rate the damping factor at 1000 or more. Then the output impedance of the amplifier is 8/1000= 0.008 ohms or less. When the amplifier drives a 4 ohm speaker then the damping factor is "only" 500.

    Many vacuum tube amplifiers sound BOOMY (one note bass) because their damping factor is only 1 to 10.
     
  14. jatinparekh

    Thread Starter New Member

    Jan 2, 2012
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    Thank you all
    all the information help me to understand electronics better.
     
  15. twister007

    New Member

    Feb 29, 2012
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    Not sure if this would work, but it might. Put a sine wave into the amp. Change the speaker from a 4 ohm to 8 ohm and monitor the wave form on a scope. If the wave is twice as large, the amp impedence was 4 ohm. :)
     
  16. wmodavis

    Well-Known Member

    Oct 23, 2010
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    Nope that would not work because the amp impedance is likely way less than 1 ohm, more likely close to .01 Ohms, and speakers are resonant devices with non constant impedance vs frequency. But if you use a known resistor as load and accurately monitored output voltage with and without the load you could then calculate the output impedance using Ohms law.

    On the other hand, IF the amp output impedance was in fact close to 4 Ohms, your method would be valid.
     
    Last edited: Mar 10, 2012
  17. Audioguru

    New Member

    Dec 20, 2007
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    The high amount of negative feedback in a half-decent audio amplifier keeps the output level the same if it has a speaker or if it has no speaker. Then you can calculate the output impedance of the amplifier to be extremely small.
     
  18. MrChips

    Moderator

    Oct 2, 2009
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    I don't know how to describe you guys but a bunch of poster junkies.
    Go back and read the OP #1. The OP does not understand audio amplifier ratings.
    The specs might have said something like 12W into 8 ohms (not usually 6 ohms).
    Most of you understand this and have gotten it right, but why belabor the issue.

    8 ohms is the load of the loudspeaker and has nothing to do with the output impedance of the audio amplifier.

    I gave the answer in post #2 and from OP #3 he still didn't get it.
     
  19. MrChips

    Moderator

    Oct 2, 2009
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    And he still did not get it at post #5.

    POWER = \frac{V^2}{R}

    In order to specify a power rating you must specify the conditions under which the power is measured, that is, you must specify R.
    Just as important, you must specify what V means, is it peak, peak-to-peak, RMS or average music power (what ever that means).
     
  20. Audioguru

    New Member

    Dec 20, 2007
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    The output impedance of modern amplifiers is extremely low so has nothing to do with the load impedance. The minimum load impedance causes high output current and high heat.
    The OP wrongly asked if his amplifier "saying 6 ohms" can "give" 6 ohms to his speakers.
    Maybe he means asking if his amplifier can produce 6 Watts into his speakers or 6 Volts to his speakers.

    Maybe the amplifier is "saying" 6 ohms because it might have its output power rated when it has a 6 ohms load. Then its output power will be different into 8 ohms or 4 ohms.

    Some amplifiers produce the same output power into 8 ohms or 4 ohms. an LM1875 amplifier IC produces 20W into 4 ohms or 8 ohms when it has a 50v total supply. With a 4 ohms load then the extra power is simply thrown away as heat.
     
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