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I don't understand the meaning/relevance of output impedance.

Discussion in 'Audio' started by j4cobgarby, Oct 7, 2018.

  1. j4cobgarby

    j4cobgarby

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    Sep 18, 2018
    So let's say my amplifier says it has an output impedance of 4 ohms. I know, then, that I should connect this amp up to a 4 ohm speaker. However, will speakers with higher impedance also work? How about lower impedance?

    What's an intuitive way to think of the output impedance? Why does an amplifier even _have_ an output impedance?
     
  2. Externet

    Externet

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    Aug 24, 2009
    The output impedance rating is the one that will deliver more power transfer.
    You can connect 600 Ω speakers and it will work, but delivering much smaller power.
    Anyone who talks about the impedance of a device, MUST express "at such frequency".
    It does not happen. Marketing is who usually decides publications about a product and they do not care about proper technical terms.

    The impedance varies with the frequency, by its nature. The formula "impedance = function of frequency"
    Change the frequency, and the impedance changes.

    A plain speaker with impedance rated at 8 Ω; should had said "8 Ω at 1KHz".
    But nobody cares about doing things right and learning/understanding becomes difficult due to those omissions from careless people.

    About matching :

    Form a cotton ball; throw it as far as you can; it will land about 4 feet away.
    Make a paper ball; throw it as far as you can; it will land about 25 feet away.
    Throw a tennis ball as far as you can; it will land about 100 feet away.
    Throw a golf ball as far as you can; it will land about 150 feet away.
    Throw a baseball as far as you can; it will land about 200 feet away.
    Throw a football as far as you can; it will land about 120 feet away
    Throw a basketball as far as you can; it will land about 80 feet away.
    Throw a bowlingball as far as you can, it will land about 15 feet away.
    Throw a lead demolition ball as far as you can, watch your feet.

    Now, have the same test done with a seven year old kid:
    Distances reached will diminish accordingly to the strenght of the kid.-

    What is going on ?

    It is not the lightest nor the heaviest the one that got the farthest away.
    It is not the lightest nor the heaviest the one that used up the full strenght
    CAPABILITY of the arm muscles.

    It is the one that MATCHES the arm strenght. There is one object
    that flew the farthest for each person... you ; the kid.

    When a rf or audio amplifier drive an antenna or a speaker or a
    subsequent load, there will be an optimum FORCE -TO- LOAD ratio
    that will deliver the most POWER. That load will not be a tiny nor a huge one.

    A rated power output is the capability of a certain device AT a given
    impedance load.

    Unmatched driver-to-driven does NOT mean that there will be no action.
    Your 400 watt stereo will burn yours ears off at full volume knob through
    headphones but will be delivering only a couple of watts power, not
    the 400 rated watts... because your headphones impedance is 600 ohms
    instead of the at-rated 4 ohms... it is like the grown up throwing the paper ball.

    As mismatch extremes, it is possible to push a ship by swimming,
    or use a bulldozer to tow a bicycle.
     
  3. j4cobgarby

    j4cobgarby

    38
    1
    Sep 18, 2018
    Thank you for explaining very well the relevance of matching impedance of the amplifier and speaker! That was a very useful read. Now all I don't understand is how I would calculate the impedance?
     
  4. Externet

    Externet

    632
    131
    Aug 24, 2009
    ----> https://en.wikipedia.org/wiki/Electrical_impedance

    ----> https://www.wikihow.com/Calculate-Impedance

    Somehow, the impedance unit was decided to be named also Ohm. It could had been whatever else, like 'acres' or 'blurb' , but we are stuck to Ohms, as in the Ohm's law.
    Do not let that confuse you.

    Just apply the left-top replacing "R" by "Z" for impedance

    Screenshot from 2018-10-07 12-23-29.png

    Look at typical output impedance plots of audio amplifiers. It is not glued to 4Ω nor 8Ω; it varies with frequency :
    ---->https://duckduckgo.com/?q=impedance+plot+audio+amplifier&t=canonical&iax=images&ia=images
     
    Last edited: Oct 7, 2018
    j4cobgarby likes this.
  5. BobK

    BobK

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    Jan 5, 2010
    Everyone gets this wrong. You do not try to match the output impedance of an amplifier to the impedance of the speaker. There are multiple reasons for this:

    1. If the output impedance of the amplifier is matched to the speaker, only half of the available power will go to the speaker, the rest will go into heat in the amplifier. That is not ideal.

    2. If the output impedance of the amplifier is matched to the speaker, the amplifier cannot control the movement of the speaker well. You want the output impedance to be much lower than the impedance of the speaker because it can then maintain the correct voltage on the speaker coil even though the speaker is producing a back EMF. This causes the speaker to move in concert with the driving signal instead of, for example, going crazy at it's resonant frequency.

    3. If you actually did match the output impedance of the amplifier to that of the speaker, the amplifier in all likelihood could not supply that much current and would burn up, or go into protection mode.

    The output impedance of modern transistor amplifier is a fraction of an Ohm. They are designed to drive, typically, 4 or 8 Ohm speakers. Which means that they can supply the current required by a speaker of that impedance without straining the amp. Using a lower impedance speaker will strain the amplifier and cause distortion and, if sufficiently lower, destroy the amplifier. Using a higher impedance speaker will likely not cause damage or distortion, but will limit the power you can get out of the amplifier.

    So, you should match the impedance to what is recommended by the maker of the amp, which is NOT the same thing as it's output impedance.

    Bob
     
    (*steve*) and duke37 like this.
  6. Audioguru

    Audioguru

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    Sep 24, 2016
    Modern very low output impedance amplifiers are solid state. Old vacuum tube amplifiers usually matched the output impedance of the amplifier to the speaker impedance.
     
  7. Ylli

    Ylli

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    Jun 19, 2018
    And one of the things to do to make a SS amp sound more like a tube amp is to put a resistor in series with the ouput - reducing the damping factor.
     
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    When the damping of a speaker's resonances is reduced by a higher than very low output impedance of a solid state amplifier then the speaker resonates badly like a bongo drum.
    I think old speakers had good damping built into them so they did not resonate badly when driven from the fairly high matched output impedance of an old vacuum tubes amplifier.
     
  9. BobK

    BobK

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    Jan 5, 2010
    Which made sense because the output impedance of the amp was much higher than the impedance of the speaker. In that case, the power transfer would be limited to a fraction of the available power.

    The ideal case is zero output impedance, which we are closer to today. With zero output impedance all of the power goes into the speaker.

    Bob
     
  10. Audioguru

    Audioguru

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    Sep 24, 2016
    A linear amplifier with an extremely low output impedance still heats with about 45% to 55% of the power supply power when playing at about 50% of the power of clipping.

    In a Class-D amplifier, most of the power supply power goes to the speaker with only a little heating.
     
  11. Ratch

    Ratch

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    Mar 10, 2013
    You are asking two questions here. I can answer what output impedance is, but I will leave the question of how to hook up speakers to a such and so amplifier to others more in the know.

    Output impedance is the Thevenin impedance of the amplifier from the viewpoint of the load. It is either in series with the amplifier source voltage or in parallel with the amplifier source current. Theoretically, you can determine the Thevenin impedance by measuring the open output voltage and shorted output current, Then divide according to the definition of impedance E / I . However, that cannot be done in practice.


    If the amplifier output could be shorted without burning up. What would llimit its curren? Answer: Its output impedance.

    Because resistors have resistance and capacitors and coils have reactance.

    Ratch
     
  12. Audioguru

    Audioguru

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    Sep 24, 2016
    Only an ancient old vacuum tubes amplifier has a 4 ohms output impedance. A modern solid state amplifier has an extremely low output impedance, and its damping factor is specified, not its output impedance.

    We do not know if its specifications say if it can safely drive a 4 ohms speaker or if it can safely drive a speaker with an impedance lower than 4 ohms.
     
  13. kellys_eye

    kellys_eye

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    Jun 25, 2010
    Look up maximum power transfer theory. Basically the source and load must MATCH to attain max power transfer.
     
  14. BobK

    BobK

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    Jan 5, 2010
    Yep, for exactly the same reason as a linear regulator wastes power and a switching regulator does not. In fact, an class AB audio amplifier is a linear regulator that follows an input voltage and a class D amp is a switching regulator that follows an input.

    Bob
     
  15. Audioguru

    Audioguru

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    Sep 24, 2016
    Radio frequency amplifiers match impedances but modern audio amplifiers never match. The speaker resonates like a bongo drum and an amplifier with an extremely low output impedance damps the resonance very well so that the speaker faithfully produces the audio like it should sound.

    Look up the definition of "Damping Factor" that is a specification for all modern high quality audio amplifiers.
    This Crown audio amplifier is spec'd with a damping factor of >1500 so the amplifier output impedance is less than 0.0053 ohm.
     

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  16. Ratch

    Ratch

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    Mar 10, 2013
    Yes, that is true if one can only change the load impedance. But, if one is able to lower the source impedance somehow, even more power will be transferred. Consider a 1 volt source with a source and load impedance of 1 ohm each. That will transfer 1/4 watt. But if the source impedance is lowered to 1/2 ohm, then the power transferred is 4/9 watts. I have has big arguments with some folks who did not believe that.

    Ratch
     
    BobK likes this.
  17. BobK

    BobK

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    Jan 5, 2010
    Lower it further to 0 Ohms and you can transfer 1 Watt.

    The maximum power transfer theorem tells you what load impedance will allow the transfer the max power for a given source impedance. For a given load impedance, it is much simpler: The source impedance to transfer maximum power is zero.

    Bob
     
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