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Old 10th February 2013, 05:59 AM   #21
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LDRs aren't very linear as resistors IIRC. Why not just use a pot?

Any headphone which has an almost perfectly resistive impedance is basically operating in current-drive due to its own internal resistance swamping out the coil reactance. So those AKG's probably tolerate high source impedances better than most headphones.

On that note, current drive is supposed to be better for speakers, although somewhat more difficult to implement. So perhaps the same could be true of headphones? Just EQ out the impedance curve If you can measure the impedance of your headphones this should be easy. Unless your source impedance changes with volume, then the EQ would wander.

The serious flaws of voltage drive - Current-Drive - The Natural Way of Loudspeaker Operation
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Old 10th February 2013, 12:15 PM   #22
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Almost any dynamic headphone I've seen is under-damped, so any increase in output impedance makes this just worse.

Ideal output impedance = 0, but the rule of thumb is less than 1/8th of nominal impedance of the headphones.

Some dynamic headphones have a fairly flat impedance response, but that's the exception...
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Old 10th February 2013, 01:54 PM   #23
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Quote:
Originally Posted by keantoken View Post
...
On that note, current drive is supposed to be better for speakers, although somewhat more difficult to implement. So perhaps the same could be true of headphones? Just EQ out the impedance curve If you can measure the impedance of your headphones this should be easy. Unless your source impedance changes with volume, then the EQ would wander.

The serious flaws of voltage drive - Current-Drive - The Natural Way of Loudspeaker Operation
It reminds me of current drive too. I've played with currentclones and loudspeakers but ... I've been keeping that way of looking at it quiet because it would be confusing to people with less technical knowledge.
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Old 10th February 2013, 02:05 PM   #24
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Quote:
Originally Posted by xnor View Post
...
Ideal output impedance = 0, but the rule of thumb is less than 1/8th of nominal impedance of the headphones.
...
***Those with less technical knowledge, please note the 1/8th rule is good technical practice and should not be deviated from unless you know what you are doing.***

NwAvGuy summarizes and gives a practical example:

THE SHORT VERSION: All you really need to know is most headphones work best when the output impedance is less than 1/8th the headphone impedance. So, for example, with 32 ohm Grados the output impedance can be, at most, 32/8 = 4 ohms. The Etymotic HF5s are 16 ohms so the max output impedance is 16/8 = 2 ohms. If you want to be assured a source will work well with just about any headphone, simply make sure the output impedance is under 2 ohms.
http://nwavguy.blogspot.co.uk/2011/0...impedance.html

Quote:
Originally Posted by xnor View Post
...
Almost any dynamic headphone I've seen is under-damped ...
Can you point me towards some data?

Thanks,
Jeff

Last edited by AudioLapDance; 10th February 2013 at 02:10 PM.
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Old 10th February 2013, 02:37 PM   #25
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comparison-beyerdynamic-dt-880-32-ohm-dt-880-250-ohm-and-dt-880-600-ohm-headphones

.. and the countless headphone datasheets you can find on that site. (If the page is empty try another browser.)
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Old 11th February 2013, 02:32 AM   #26
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Originally Posted by xnor View Post
comparison-beyerdynamic-dt-880-32-ohm-dt-880-250-ohm-and-dt-880-600-ohm-headphones

.. and the countless headphone datasheets you can find on that site. (If the page is empty try another browser.)

Perfect, thanks xnor. I'll get to step response and damping soon, first I want to explore frequency response.



I tried to model my AKG K301 as 300 ohm res peak at 80Hz, dipping to 120 at 2kHz and up to 200 at 20kHz.

(Not in this case because I have no graph but ... what's cool here is that if you produce an electrical circuit that follows the impedance of measured headphones then the electrical circuit is modelling both the electrical and mechanical properties of the phones!)

So ... you start with the nominal resistance 120ohm, then you add a resonator circuit that produces the peak at 80Hz [RLC res, 120+180=300], then you add an inductor to cause the hf lift:

AKG K301 R series test circuit.png

I just opened a Tina simulation and cut out the middle, here's the frequency response for a variety of R series

Basic 2 ohm out for source then totals of 2, 7 [ipod], 10 and 20 ohm [standards], 50, 120 [standards], 500, and 909 ohm.

AKG K301 R series 2 to 909 ohm freq resp.png


Nothing significant for the lower output impedances but, of course, my crazy circuit produces a 6.5 dB hump at resonance (assumed 80ish) as predicted by xnor. Also interesting is that crazy old standard of Rout=120ohm (dark grey) produces noticeable boost and phase shift.

Here's TINA
SPICE-Based Analog Simulation Program - TINA-TI - TI Software Folder
And the circuit if you want to play:
No Headphone Amp simulation.zip

Cheers,
Jeff

Last edited by AudioLapDance; 11th February 2013 at 02:41 AM.
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Old 11th February 2013, 05:54 AM   #27
Blues is offline Blues  United States
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Quote:
Originally Posted by keantoken View Post
...On that note, current drive is supposed to be better for speakers, although somewhat more difficult to implement. So perhaps the same could be true of headphones? ...The serious flaws of voltage drive - Current-Drive - The Natural Way of Loudspeaker Operation
Thanks for the link keantoken...

Some posts of Hifiman HE-6 owners using transconductance (current drive) amps as headamp with great results.
HifiMAN HE-6 Planar Magnetic Headphone

First Watt F1, F1J, F2, and F2J are transconductance amps from NP
FIRST WATT PRODUCTS

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Old 11th February 2013, 01:39 PM   #28
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Here's a great book from 3 years ago and an inverted current clone from a decade ago!

current drive, inverted currentclone.jpg

Current drive is confusing and turns everything 'upside-down'. For those trying to get used to seeing things from a voltage perspective (even if idealized and inaccurate) ...

Such confusions could lead to calls of reprehension preceded by explicit advisions!

Please, let's start another thread!

Last edited by AudioLapDance; 11th February 2013 at 01:51 PM.
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Old 11th February 2013, 01:42 PM   #29
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So does this curve remind anyone of anything:

AKG K301 R series 2 to 909 ohm freq resp.png

I mean besides bad design and throwing explicit advisions to the wind.
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Old 11th February 2013, 02:28 PM   #30
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Quote:
Originally Posted by xnor View Post
Almost any dynamic headphone I've seen is under-damped, so any increase in output impedance makes this just worse.

Ideal output impedance = 0, but the rule of thumb is less than 1/8th of nominal impedance of the headphones.

Some dynamic headphones have a fairly flat impedance response, but that's the exception...
I read somewhere that the standard source resistance for a dynamic headphone output is 120 ohms. They are designed for this specific source resistance.

http://en.wikipedia.org/wiki/Headpho...tput_Impedance

Last edited by oshifis; 11th February 2013 at 02:31 PM. Reason: Found the reference
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