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Old 13th September 2014, 03:51 PM   #1
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Default importance of output impedance for headphones?

I've recently been digging into headphone amplification, particularly amp output impedance.

In the loudspeaker world, people are intent to have low output impedance - say, below 0.5 Ohms - and hence good damping of cone drivers.

But does damping improve headphone sound? That my question.

Sometimes in reading reviews, I think some headphone users glory on tubby undamped bass. And most headphone amps (often IC driven) are happy to have 35 Ohm or bigger resistors sitting in the output leg and rarely tell consumers what the output impedance of their amps is.

With my Audio-Technica ATH-A900 headphones (50 Ohms??), I think I hear a hint of damping when driven by a speaker-like output circuit as compared to a garden-variety headphone amp circuit.

BTW, can anybody tell me the output impedance of the headphone amp in a Behringer Xenyx 1204 mixer? Or where I can get a schematic?

Thanks.
Ben
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Old 13th September 2014, 04:35 PM   #2
RNMarsh is offline RNMarsh  United States
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I dont know about that model, but recievers often use 100 Ohms in series from speaker to headphone jack. Mostly, it was just a simple and cheap way to reduce the output level for headphone use. The affect on sound wasnt much of a consideration and headphone makers standardized on 100 Ohm for their designs. But now we have new headphone models and dedicated headphone amps so it is harder to tell what Zo is expected without consulting the mfr.


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Last edited by RNMarsh; 13th September 2014 at 04:38 PM.
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Old 13th September 2014, 04:42 PM   #3
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Quote:
Originally Posted by bentoronto View Post

But does damping improve headphone sound? That my question.
In the opinion of several cell phone manufacturers, yes (to my surprise BTW). Picking the right damping resistor is of great concern.
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Old 13th September 2014, 05:31 PM   #4
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Quote:
Originally Posted by RNMarsh View Post
I dont know about that model, but recievers often use 100 Ohms in series from speaker to headphone jack. Mostly, it was just a simple and cheap way to reduce the output level for headphone use. The affect on sound wasnt much of a consideration and headphone makers standardized on 100 Ohm for their designs. But now we have new headphone models and dedicated headphone amps so it is harder to tell what Zo is expected without consulting the mfr.


THx-RNMarsh
Loudspeaker amps had those resistors to keep from blowing up headphones and to keep the noise-floor less hearable on the phones.

Worth noting that loudspeaker amps generally have the feedback loop at the tail end, driving down output impedance. But headphone amps I've seen have the protective resistor last in the line and thus a passive resistance.

Yet most boutique headphone amps have IC output transistors and protective resistors in the 25-100 Ohm?? range whether or not that IC really needs protection.

Ben
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Last edited by bentoronto; 13th September 2014 at 05:36 PM.
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Old 13th September 2014, 05:34 PM   #5
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Quote:
Originally Posted by scott wurcer View Post
In the opinion of several cell phone manufacturers, yes (to my surprise BTW). Picking the right damping resistor is of great concern.
Any details to share?

You are implying something less than max damping might make some manufacturers happy??

Am I wrong to take for granted that headphone motors - like quality loudspeakers - have the moxie to benefit from damping? Or is the physics of closed or open headphones different and hence the need for damping (AKA degenerative feedback) of less importance?

Ben
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Old 13th September 2014, 05:42 PM   #6
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Quote:
Originally Posted by bentoronto View Post
Any details to share?
Either way I don't think there is a consistent story to report. Some of the next generation phones will support a separate "audiophile" signal chain. I think someone mentioned the HTC Harmon Kardon phone here already. Obviously bass is an issue with earbuds so some folks are fiddling with using the resistor to tune it and supplying custom designed earbuds to take advantage.
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Old 13th September 2014, 05:59 PM   #7
jcx is offline jcx  United States
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Headroom plots the impedance - lack of axis auto scaling is annoying though

Click the image to open in full size.

the biggest effect is usually the bas mass-spring resonance - visible in some impedance plots - V drive gives the typically plotted frequency response, a series R causes some peaking where the headphone Z rises

Tyll Hertsens has done more measuring of headphones at inner Fidelity: Headphone Data Sheet Downloads | InnerFidelity

some high end iem use multiple drivers, can have multiple impedance bumps over audio

since bass sensitivity is low, bass bumps may not be very audible

Quote:
Originally Posted by jcx View Post
...
practically there is little point in extreme accuracy at 10 Hz - several dB, 10-20 % mismatch won't reach audible thresholds that low


we just don't hear, resolve small frequency response differences that low - even if they are easily measured

Click the image to open in full size.

Clark, David L., "High-Resolution Subjective Testing Using a Double-Blind Comparator", Journal of the Audio Engineering Society, Vol. 30 No. 5, May 1982, pp. 330-338

ABX Amplitude vs. Frequency Matching Criteria
...


headphones are way more linear than most loudspeakers - but there could also be some distortion changes - as you increase output Z some dynamic driver nonlinearities are reduced - really unlikely to be audible if you believe Geddes

Last edited by jcx; 13th September 2014 at 06:09 PM.
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Old 13th September 2014, 06:03 PM   #8
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Quote:
Originally Posted by jcx View Post

since bass sensitivity is low, bass bumps may not be very audible
I'm afraid this falls into the customer is always right category for me.
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Old 13th September 2014, 07:12 PM   #9
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Quote:
Originally Posted by jcx View Post
the biggest effect is usually the bas mass-spring resonance - visible in some impedance plots - V drive gives the typically plotted frequency response, a series R causes some peaking where the headphone Z rises

Tyll Hertsens has done more measuring of headphones at inner Fidelity: Headphone Data Sheet Downloads | InnerFidelity

some high end iem use multiple drivers, can have multiple impedance bumps over audio

since bass sensitivity is low, bass bumps may not be very audible


headphones are way more linear than most loudspeakers - but there could also be some distortion changes - as you increase output Z some dynamic driver nonlinearities are reduced - really unlikely to be audible if you believe Geddes
Headphone acoustics seems much more benign than for loudspeakers (which is pretty close to unsolvable with current devices).

If a user could see an impedance curve (for example, for my A-T A900's) then it should be clear enough if the moving parts are under-damped. If no big resonance peak, no tight coupling and/or no floppy diaphragm and no tubby bass. So no problem having high output impedance amp (like most are).*

Ben
*in preparing this thread, I noticed a sponsor/vendor thread who was giving away pc boards... they had 100 Ohm protective resistors.
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Last edited by bentoronto; 13th September 2014 at 07:15 PM.
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Old 13th September 2014, 07:37 PM   #10
jcx is offline jcx  United States
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the old DIN standard is never used today with so many 32 Ohm headphones for DAP running from a single LiIon cell

the high flex cables for headphones may have the wires very close - nF of cable C is possible even in typical length manufacturer supplied headphone cables

some op amps like a isolating impedance for that much C load - but few 10s of Ohms often works
lossy ferrite bead or series R||L as used in audio power amps can give lower audio Z and still isolate the output from the cable C at MHz feedback gain intercept
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