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Old 14th April 2013, 11:33 AM   #1
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Default HP amp out impedance vs. HP impedance

Hi !

sorry if the issue has already been discussed
But in an old review of the Grace m901 headphone amp I read the following statement

Quote:
... The 901 uses a high-current output amplifier with an output impedance of 1 ohm and is capable of driving 8 ohm loads.
Perhaps because of its low output impedance, the 901 has the best low-frequency control of any headphone amp I have heard.
Is this statement correct ? I would like to get a nice low-frequency control on my low Z headphones so I would like to know if an amp with a very low Z out is mandatory.
Thanks a lot for any kind advice
Kind regards,
gino

Last edited by ginetto61; 15th April 2013 at 05:06 PM.
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Old 19th April 2013, 06:38 PM   #2
Rubin is offline Rubin  Canada
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The short answer is "yes": Low output impedance is essential for bass control on low impedance headphones.

A wonderful article on this topic recently appeared on Inner Fidelity:
Meridian Explorer: A Case Study of the Effects of Output Impedance on Headphone Listening | InnerFidelity
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Old 20th April 2013, 07:03 AM   #3
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Thank you very much indeed for your kind and valuable reply
What i do not understand is why low Z out amp are not very common while a lot of HPs around are indeed low Z
For instance I am listening to a HP of 55 ohm with an amp with a Z out of 10 ohm
The bass is very unsatisfactory in terms of control
The very same amp drives much better a 250 ohm HP by the way
Is it so difficult to build low Z out amp ?
This is extremely strange to me
Thanks for the very interesting article and regards,
gino

Last edited by ginetto61; 20th April 2013 at 07:28 AM.
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Old 20th April 2013, 10:21 AM   #4
sandyK is offline sandyK  Australia
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Some headphones are designed to comply with the old IEC61938 specification, that headphones should be driven from a 120 ohms source impedance irrespective of their impedance. The AKG 701 is one example. It sounds far more tonally balanced when driven from a 120 ohm source impedance, (although a few prefer 100 ohms.) You then need an amplifier with higher voltage drive capabilities for best results . Those using + and -15V ( or even + and - 20V) supply rails are a good choice.
SandyK
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Old 20th April 2013, 11:11 AM   #5
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Ok but i would very much liked a comment on the quoted statement
If the low frequencies are not controlled I am quite sure that they will blurred all the other frequencies
A low damping of the driver will cause the driver to have some kind of ringing
For me this is extremely clear
Nevertheless I would like to get a confirmation from experts
I think this is a fundamental issue
And sorry but to say
Quote:
The AKG 701 is one example. It sounds far more tonally balanced when driven from a 120 ohm source impedance,
is a little generic
Kind regards,
gino
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Old 20th April 2013, 11:40 AM   #6
sandyK is offline sandyK  Australia
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Quote:
Originally Posted by ginetto61 View Post
Ok but i would very much liked a comment on the quoted statement
If the low frequencies are not controlled I am quite sure that they will blurred all the other frequencies
A low damping of the driver will cause the driver to have some kind of ringing
For me this is extremely clear
Nevertheless I would like to get a confirmation from experts
I think this is a fundamental issue
And sorry but to say
is a little generic
Kind regards,
gino
Gino
There are quite a few members of a U.K. based forum that own K701s, and that is their general consensus. The K701s when driven from a much lower impedance source are far too detailed and fatiguing for long listening sessions. The same applies to the Audio Technica W1000 that I own.

Regards
Alex
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Old 20th April 2013, 11:57 AM   #7
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It would be nice to have more info
For instance a list of amps that make those HPs sound good
Saying that an amp with 120 ohm of Z out is needed for the best sound from that AKG is a little vague
And it would be useful also to know which low Z out amps are not good for the AKG
I say this because a friend of mine had a Melos Sha Gold HP amp, a vey low Z out unit but a very high quality one
It was just splendid with both Grado 32 ohm than Beyerdynamic 250 ohm
Fatiguing for me means usually distorting
Kind regards,
gino

Last edited by ginetto61; 20th April 2013 at 12:03 PM.
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Old 20th April 2013, 12:08 PM   #8
sandyK is offline sandyK  Australia
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Gino
Check out the impedance graph for the K701
Regards
Alex`

http://www.soundstagemagazine.com/me...impedance2.gif
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Old 20th April 2013, 12:26 PM   #9
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Thank you
But so do you exclude that a low Z out amp can make the K701 sound very good ?
I am quite obsessed about the danger of muddy bass that over imposes itself on the rest of the band
And actually i have found usually better the bass of high Z HPs, like 250 ohm HPs
Kind regards,
gino

Last edited by ginetto61; 20th April 2013 at 12:54 PM.
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Old 20th April 2013, 02:08 PM   #10
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Might want to consult Richard Marsh's article in Linear Audio Volume 3 -- an HP amplifier and equalization scheme.
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