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Amp Headphone Jack Curiosities
PhantmShado
post May 9 2012, 21:44
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Hello all, I'm looking for some technical understanding of how my headphones are playing with my stereo amp (Onkyo TX-8050) though admittedly my background is pretty limited.

First I attempted to play some new ATH-A1000X headphones through the headphone jack (headphone impedence 32ohm, sensitivity 101db) and found though generally the quality was good, the response in the bass was particularly messy, in a way that especially didn't make sense to me considering the apparent quality in the rest of the response. After reading some of nwavguy's blog, I got the impression that output impedence from the jack may be too high and a poor damping factor could be hurting the bass. My best guess at impedence factor on the output jack is 75 ohms as this is the only number in the provided literature that seems relevant though it does say for composite video output (en-54 in http://www.eu.onkyo.com/downloads/1/4/6/2/...TX-8050_En.pdf). So my first question is does this sound accurate?

Second I saw some dt880s (600ohm, 96db) at a decent price, so I got them thinking this would eliminate muddled bass on the account of damping, especially due to reviews saying these phones performed quite cleanly in that area. My experience though was that the phones were just generally unimpressive including the bass response. More surprising to me though and more relevant to this thread was that the phones seemed to play just as loud as the ATH-A1000X while all literature has lead me to believe that there is no way this should be the case with the higher resistance and lower sensitivity (I verified the phones jack said 600 on it). So what I'm wondering is if someone with a little more technical knowledge can point out a way this could be possible that perhaps, but not necessarily, also explains a bad response?

I will also note that I'm running the amp in sound direct mode and that in terms of noise I could not detect anything besides what my music caused to be produced. Also, my AKG 702s (62ohm, 105db) seemed to have a much more even response then either of the already mentioned pairs, though I recall them sounding cleaner from the last amp I had (though this is somewhat meaningless as no back and forth occurred).

Any input from a more technically abled person appreciated.
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saratoga
post May 9 2012, 22:57
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QUOTE (PhantmShado @ May 9 2012, 16:44) *
First I attempted to play some new ATH-A1000X headphones through the headphone jack (headphone impedence 32ohm, sensitivity 101db) and found though generally the quality was good, the response in the bass was particularly messy, in a way that especially didn't make sense to me considering the apparent quality in the rest of the response. After reading some of nwavguy's blog, I got the impression that output impedence from the jack may be too high and a poor damping factor could be hurting the bass. My best guess at impedence factor on the output jack is 75 ohms as this is the only number in the provided literature that seems relevant though it does say for composite video output (en-54 in http://www.eu.onkyo.com/downloads/1/4/6/2/...TX-8050_En.pdf). So my first question is does this sound accurate?


Do you mean the 75 ohm composite video connector? 75 ohms is just the impedance of the RCA connectors, which are those cables you use if your TV doesn't support HD.
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PhantmShado
post May 9 2012, 23:02
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QUOTE (saratoga @ May 9 2012, 14:57) *
Do you mean the 75 ohm composite video connector? 75 ohms is just the impedance of the RCA connectors, which are those cables you use if your TV doesn't support HD.

I was also wondering if a poor impedance match on the jack could make bass in particular suffer?
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saratoga
post May 10 2012, 00:01
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QUOTE (PhantmShado @ May 9 2012, 18:02) *
QUOTE (saratoga @ May 9 2012, 14:57) *
Do you mean the 75 ohm composite video connector? 75 ohms is just the impedance of the RCA connectors, which are those cables you use if your TV doesn't support HD.

I was also wondering if a poor impedance match on the jack could make bass in particular suffer?


Headphone jacks aren't supposed to be impedance matched (since that leads to problems), but yes, if the impedance is high (that is, similar to the headphones) you will tend to get worse bass on a lot of headphones. You may also get other unpredictable effects.
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Speedskater
post May 10 2012, 14:01
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We are co-mingling "Radio Frequency Characteristic Impedance" which applies to circuits that deal with radio signals or TV signals or video signals or digital audio signals and the "output impedance" of the small audio amplifier that powers the headphone output.
Some receivers don't have a separate headphone amplifier, they just tap off the main power amplifier. This tap might have a high output impedance which will effect the frequency response of some headphones.

This post has been edited by Speedskater: May 10 2012, 14:01


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Willakan
post May 21 2012, 07:00
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Such receivers are likely to have very high output impedances: >120ohms. This not only effects frequency response, but has a considerable effect on LF distortion.
http://www.benchmarkmedia.com/discuss/feed...e-headphone-amp

There are other places that compare distortion measurements, but this link goes through it rather well.
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stephan_g
post Jun 7 2012, 15:10
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JFTR, headphone output resistance on this model receiver is a standard (for Onkyo) 390 ohms - it's an ordinary power amp tap indeed.

That not only explains the messy bass on the A1000X (you'll probably find an impedance peak in this area), but also why the 600 ohm Beyers play just as loud - they actually receive more power than their 32 ohm colleagues but are somewhat less efficient than the ATs.
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