What does impedance have to do with sound quality?
Mar 11, 2012 at 2:59 PM Thread Starter Post #1 of 35

abraham_tek

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DISCLAIMER:
I've looked at several threads here on the forum about what impedance is, and if I get a high impedance (100ohm+ or whatever) pair of headphones then I should get an amp, etc.
 
But I still don't get the point of looking at that.
 
What does it have to do with sound quality? I have the very famous ATH-M50 and I just looked at the box and they are 38 ohms. And I agree, they are pretty loud, and the sound is pretty sweet too.
 
Do I still need an amp for them to sound better? And if so, what does the amp do? (thinking about the Fiio E7 to plug to my laptop).
 
Another thing: if I bought let's say the DT770 250ohm, then I should buy an amp. But why not then get the 80ohm version and not bother with the amp?
 
I think I have 2 questions mixed in one here:
1) What does impedance have to do with sound quality?
2) when I buy a high end pair of headphones, I hear that the amp improves the quality. But, what does it exactly do?
 
 
I hope I'm making sense here, since I'm a little confused.
 
 
Thanks in advance for all your help!
 
PS. if possible, please avoid any super advanced technical stuff as I'm not interested in that, UNLESS it helps on explaining about what impedance has to do with sound quality or what not, and about the amps. Thanks =)
 
Mar 11, 2012 at 3:38 PM Post #2 of 35

stv014

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It is not universally true that you need an amplifier for a high impedance headphone, and not for a low impedance one. There can be many reasons why a source is not driving a headphone perfectly (in the sense of not sounding the same as an "ideal" source), and many of those actually affect low impedance headphones more.
 
 
Mar 11, 2012 at 3:47 PM Post #3 of 35

stv014

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Quote:
Originally Posted by abrahamleay /img/forum/go_quote.gif
 
And if so, what does the amp do?

 
An ideal amplifier applies a voltage gain to the input signal, while having infinite input impedance, zero output impedance, and no distortion (be it linear or non-linear) or noise. In practice, the headphone output stage of a low quality "unamplified" source may fail to get close enough to meeting these requirements by having:
- insufficient maximum voltage or current output, so you cannot get sufficiently loud sound without clipping distortion
- high output impedance that reduces the electrical damping of dynamic headphones (this usually most noticeably affects the bass response)
- capacitor coupled outputs that roll off the bass with low impedance headphones
- high distortion when driving low impedance headphone loads
- stability issues when driving reactive loads
- too low signal to noise ratio at low volume
The above problems can be fixed or at least improved by the use of an amplifier, since the input of the amplifier is an easy "load" to drive, and the source can be used at its optimal output voltage level. Interestingly, the "not enough voltage" issue is the only one that specifically affects high impedance headphones more.
 
 
Mar 11, 2012 at 4:31 PM Post #4 of 35

xnor

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Every device with a headphone jack has an amplifier in it. The idea of using a separate amp is to reduce distortion, noise, low frequency roll-off etc. and increase volume.
 
If you compare a nice amp to, lets say, the one integrated into a cheap notebook you'll get improvements regardless of the headphones you use but you're most likely to be able to hear the differences with better headphones. This is why people tell you to get an amp with expensive headphones. However there's no guarantee that you'll get an actual, audible improvement.
 
Impedance per se doesn't tell you anything about sound quality.
 
Mar 11, 2012 at 10:18 PM Post #5 of 35

Mauricio

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The issue of impedance and sound quality is as follows.
 
1.  All amps have an output stage, that is, the stage that interfaces with the outside world, in this case the headphone.  The output stage of an amplifier is connected in parallel to the input of the headphone.  Therefore, from the amp's perpective looking out, the load that it must drive is its own output stage's impedance in parallel with the impedance of the headphones.
Conclusion 1:  the amplifier drives its own output stage in parallel with the headphone
 
2.  Basic electric circuit theory says that a voltage across two resistors in parallel will be divided between the resistors in a manner proportional to their relative resistance.  If one resistor has 10x the resistance of the other, it will also receive 10x (in relation to the other resistor) the voltage from the amplifier.  Taking stock of this fundamental fact, you will want the headphones input impedance to be significantly larger than the output impedance of the amplifier.  Why?  Because you want the bulk of the amp's voltage applied to the headphone, not to the amp's output impedance.  Conclusion 2:  The ouput impedance of an amplifier should be small relative to the input impedance of the headphone.
 
3.  Now, the input impedance of a headphone depends on the frequency of the input voltage.  If the voltage signal is at 50Hz, the headphone's input impedance may be very different from its input impedance at 5kHz.  If the headphone's input impedance becomes comparable to the amp's output impedance, then the voltage applied to the headphone at that frequency becomes attenuated, leading to a decreased frequency response at that frequency point.  This has an impact on the ability of the headphone to reproduce the signal accurately (aka sound quality).
 
Your AT-M50's nominal impedance is 38ohms (usually specified at a single frequency).  In real practice, its impedance at 50Hz, 500Hz, 5kHz and 15kHz may be vastly different.  Dynamically its impedance swings.  In all cases, the amp's output impedance should remain low relative to the headphones.  In many cases, it does not, leading to the attenuation of certain frequencies, thereby affecting the sound quality.
 
The foregoing analysis applies to headphones and to passive speakers.  It does not apply, per se, to active speakers.
 
 
Mar 11, 2012 at 11:22 PM Post #6 of 35

wuwhere

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Quote:
The issue of impedance and sound quality is as follows.
 
1.  All amps have an output stage, that is, the stage that interfaces with the outside world, in this case the headphone.  The output stage of an amplifier is connected in parallel to the input of the headphone.  Therefore, from the amp's perpective looking out, the load that it must drive is its own output stage's impedance in parallel with the impedance of the headphones.
Conclusion 1:  the amplifier drives its own output stage in parallel with the headphone
 
2.  Basic electric circuit theory says that a voltage across two resistors in parallel will be divided between the resistors in a manner proportional to their relative resistance.  If one resistor has 10x the resistance of the other, it will also receive 10x (in relation to the other resistor) the voltage from the amplifier.  Taking stock of this fundamental fact, you will want the headphones input impedance to be significantly larger than the output impedance of the amplifier.  Why?  Because you want the bulk of the amp's voltage applied to the headphone, not to the amp's output impedance.  Conclusion 2:  The ouput impedance of an amplifier should be small relative to the input impedance of the headphone.
 
3.  Now, the input impedance of a headphone depends on the frequency of the input voltage.  If the voltage signal is at 50Hz, the headphone's input impedance may be very different from its input impedance at 5kHz.  If the headphone's input impedance becomes comparable to the amp's output impedance, then the voltage applied to the headphone at that frequency becomes attenuated, leading to a decreased frequency response at that frequency point.  This has an impact on the ability of the headphone to reproduce the signal accurately (aka sound quality).
 
Your AT-M50's nominal impedance is 38ohms (usually specified at a single frequency).  In real practice, its impedance at 50Hz, 500Hz, 5kHz and 15kHz may be vastly different.  Dynamically its impedance swings.  In all cases, the amp's output impedance should remain low relative to the headphones.  In many cases, it does not, leading to the attenuation of certain frequencies, thereby affecting the sound quality.
 
The foregoing analysis applies to headphones and to passive speakers.  It does not apply, per se, to active speakers.
 


So the output impedance of an amplifier also varies with frequency?
 
 
Mar 12, 2012 at 1:43 AM Post #8 of 35

wuwhere

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3.  Now, the input impedance of a headphone depends on the frequency of the input voltage.  If the voltage signal is at 50Hz, the headphone's input impedance may be very different from its input impedance at 5kHz.  If the headphone's input impedance becomes comparable to the amp's output impedance, then the voltage applied to the headphone at that frequency becomes attenuated, leading to a decreased frequency response at that frequency point.  This has an impact on the ability of the headphone to reproduce the signal accurately (aka sound quality).
 
If I have an amplifier that can supply enough voltage and current at that frequency for both amplifier's output impedance and HP, will that frequency still be attenuated from my headphone?
 
Mar 12, 2012 at 2:03 AM Post #9 of 35

Mauricio

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Quote:
3.  Now, the input impedance of a headphone depends on the frequency of the input voltage.  If the voltage signal is at 50Hz, the headphone's input impedance may be very different from its input impedance at 5kHz.  If the headphone's input impedance becomes comparable to the amp's output impedance, then the voltage applied to the headphone at that frequency becomes attenuated, leading to a decreased frequency response at that frequency point.  This has an impact on the ability of the headphone to reproduce the signal accurately (aka sound quality).
 
If I have an amplifier that can supply enough voltage and current at that frequency for both amplifier's output impedance and HP, will that frequency still be attenuated from my headphone?



The issue of impedance matching is not so much a question of whether the amp has enough "juice", but rather where that juice ends up.  You may have a mongo amp capable of providing humongous power, but if its output impedance is comparable to the input impedance of the headphones/passive speaker, most of that power will be wasted on the amp's own output stage.
 
Impedance mismatch is only one mechanism among many that can act on a headphones/passive speaker frequency response.  A headphone/speaker driver are imperfect, physical components with resonances.  Their imperfections can show up as dips or bumps in their frequency response even in situations where impedance matching is not an issue.
 
Mar 12, 2012 at 2:22 AM Post #11 of 35

mikeaj

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Well you didn't answer the question but that's alright.


Quote:
If I have an amplifier that can supply enough voltage and current at that frequency for both amplifier's output impedance and HP, will that frequency still be attenuated from my headphone?


If there is nonzero output impedance, you will lose some voltage over that output impedance, so it is attenuated.  If the output impedance and headphone impedance are flat across frequency, you will lose the same amount at every frequency; everything is attenuated equally, so there's no shift in the balance.  If the headphone impedance is not flat across frequency, then certain frequencies will be attenuated more so than others, which is generally undesirable.  Enough voltage and current is a separate issue.
 
Mar 12, 2012 at 2:44 AM Post #15 of 35

mikeaj

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Quote:
How does one know that the frequency is attenuated?


I'm kind of struggling with interpreting this question.  Is it about why electricity works the way it does, the general idea behind how to tell for a particular system, or specific tests to run to be able to tell for that particular system?
 

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