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What is/are output impedance for?

Discussion in 'Sound Science' started by billsonchang007, Dec 15, 2012.
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  1. BillsonChang007
    Hello! :)
    I been reading ampflier reviews and they always measure the output impedance however, I have had no idea what it means! All I know so far about "output impedance" is that for headphone with low impedance, it is most recommended that the source output impedance is/are lower than 1 Ohm...but any spacific reason and what it mean by output impedance I have no idea.
    I hope you guys can help me here :)
  2. PurpleAngel Contributor
    Impedance is resistance, measured in Ohms.
    You want the impedance of the headphones to be at least 8 times (or more) then the impedance of whatever headphone jack you plug your headphones into.
    So, if your using 80-Ohm headphones, you would prefer your headphone amplifier, to have a 10-Ohm (or less) impedance.
    If the spread is less then 8 to 1, you get poor audio quality, like less then preferred bass quality.
    Nut I have used 50-Ohm headphones with a 10-Ohm amplifier and the sound quality was fine.
    Erfan Elahi likes this.
  3. BillsonChang007
    Thanks for the explaination!
  4. Chris J
    To make a long (and pedantic) story real short and real simple:
    Impedance = algebraic sum of resistance plus capacitive and inductive reactance.
    The output impedance of an audio power amplifier typically includes the resistance in parallel with the inductance of the output compensation network plus the output impedance of the output devices. 
    Night Crawler likes this.
  5. BillsonChang007
    Another question is what causes headphone to degrade in sound? Is it output impedance or what? xD [sorry if it's a stupid question]
  6. PurpleAngel Contributor
    My brain will never remember that well enough to able to repeat it to someone else.
    Erfan Elahi and Night Crawler like this.
  7. BillsonChang007
    Sorry but I prefer Purple Angle explain better. But very good explanation from both of you :)
  8. ClieOS Contributor
    To my own understanding, output impedance affects in two ways:
    First, dampening. Transducer is a mechanical device, so it has inertia. When you push the diaphragm out with a signal, ideally you want it to go back to its original position immediately after the signal ends before the next signal comes. But in the real world, it will vibrate just a tiny bit before it fully stops. The vibration becomes the noise since it isn't there in the electric signal. There is already mechanical dampening in most transducer design (i.e. the rubber ring that connects the diaphragm to the metal housing), but electronic dampening is better. As the vibration itself creates a tiny bit of voltage and will get fed back to the amp section, the amp section will then compensate / resist the voltage change which forces the vibration to stop quicker. However, this kind of dampening usually works best if there is a big enough difference between the output impedance and the load impedance. 8x is the minimum but some will say 10x.
    Second is impedance interaction. The impedance of a headphone changes with frequency. Most of what you read in spec is measured in 1kHz tone. A 32ohm @ 1kHz headphone can have a 16ohm or 64ohm impedance @ 500Hz, or 150ohm @ 15kHz. This is referred as a impedance curve. Sometime you can also have headphone with very linear / flat impedance curve and maintain 32ohm from sub-bass to treble (though it most case, impedance is usually higher at upper treble). The problem is when headphone has a curvy impedance curve - for an example, if a headphone is 4ohm @ 100Hz, 16ohm @ 1kHz and 120ohm @10kHz. When you drive it with an 10ohm output impedance, the headphone will see much less power at 100Hz (as the 10ohm output impedance is big in ratio compare to the 4ohm impedance of the headphone so most of the power will be waste), a little better at 1kHz but fine in 10kHz. This effect will change the final frequency curve of the headphone, thus adding coloration. This is particularly true for multi-way IEM since they tends to have very curvy impedance curve due to the use of crossover circuit. Again, a 8X difference is suggested in order to make sure the output impedance of the amp is far enough away for the impedance curve of the headphone so interaction will be minimum. In the case of IEM, less than 1 ohm is usually preferred since it will make sure the ratio is almost always greater than 8x.
    Though might not always being the case, many dynamic driver have more linear impedance curve so dampening is more important. You can have a 32ohm dynamic headphone sounding very linearly / no coloration with amp of 10ohm output impedance, but you might not get as tight a bass since dampening is minimum at best. On the other hand, since balanced armatured tends to be very well mechanically dampened, impedance interaction is more important factor to consider when dealing with multi-way BA.
    Hope these help to explain it a further bit.
  9. gnarlsagan

    Most clear and concise explanation I've read on this subject. Thanks Clieos.
  10. jaddie
    You may, but it's not correct.  Impedance is not just resistance.  Chris J's explanation is much more accurate.  
  11. PurpleAngel Contributor
    Chris J's maybe way more accurate then my way,
    But I think my way is something normal people can use as a gateway for understanding impedance without overloading their brain.
  12. jaddie
    Interesting way to put it.  
    The thing missing here is that lower impedance headphones tend to have the flattest impedance curves as a general rule, where higher impedance models have much wilder impedance curves.  What that means is, if you have an amp with low output Z across the entire audio band, it won't have trouble driving either.  
    The other missing item here is, it's possible to have a low output impedance amp that cannot actually drive a low impedance load because of it's limited power output capability.  Some of those darn CMOY designs, for example, have this problem.
    The 10:1 rule is interesting, but then the examples cited don't actually meet it.  In practice, finding a portable amp that can maintain a 10:1 impedance ratio to low impedance phones is pretty much not going to happen.  But that's ok, because their impedance curves are mostly flat anyway. 
    The damping argument is a bit overrated.  There's so little mass in a headphone diaphragm that the back EMF it would generate shouldn't be an issue with just about any common amp.  The voltage generated by the inertia of a diaphragm is a function of its mass, the strength of the magnetic field its in, and the available mechanical damping supplied mostly by the acoustic system around the diaphragm, like the compression or rarifaction of surrounding air as it moves (I'm thinking sealed or IEMs here).  It's mechanical suspension is actually not that big a factor. In headphones the diaphragm mass is very low, the magnetic field its in is low, so the back EMF resulting from inertia that would need to be damped us also very low.  Even with a speaker's tweeters, damping factor (or lack of it) is just not an issue.  Now, a subwoofer diaphragm...that's a very different problem.  Those things are nice little generators looking for a load to dump into. They have huge XMax (maximum diaphragm travel), are massive and are in an intense magnetic field.  Now that's a generator!  And it needs to be damped.
  13. jaddie
    You're right, your way won't overload their brain.  But it also doesn't explain impedance, and raises the question: if impedance is resistance, why the different terms?  See, now you have to overload their brain anyway. 
    It also doesn't explain impedance changing with frequency, which is actually key to why you'd need to know about it in the first place.  Perhaps you can come up with a way to explain that without overloading brains.  Then you'd have a winner.
  14. PurpleAngel Contributor
    If others on Head-fi ever really to the time took the time to explain impedance to newbies better then me, on regular bases, I might feel bad.
  15. stv014
    The damping and impedance interaction are actually related issues, in other words, damping the bass response of a dynamic transducer occurs through the impedance peak at its primary resonance frequency. From a voltage source with near zero output impedance, the current drawn at the resonant frequency will be lower because of the increased impedance, and that dampens the resonance compared to a current source ("infinite" output impedance) which would have no electrical damping at all. However, impedance variations with frequency can also be the result of other things than back EMF, like the inductance of the voice coil (which causes rising impedance at the highest audio frequencies), or the passive crossover network used in a multi-driver headphone or loudspeaker.
    It is not true that high impedance headphones inherently have greater impedance variations relative to the nominal impedance (as a ratio, not as an Ω difference). Also, with the same ratio of highest and lowest impedance, a lower impedance load will be affected more by the same output impedance.
    One additional issue with high output impedance that may be worth mentioning is that it also increases the distortion of the driver somewhat. Basically, electrical damping works as a kind of negative feedback that flattens driver resonances and improves linearity. The difference is relatively small, though, even a near zero output impedance may make an improvement of a few dB over a near infinite one, but it is better than nothing.
    ultrabike likes this.
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