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Effect of output impedance on headphone with a flat impedance-freq curve

sergeauckland

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I remember that years ago, a standard was proposed for headphone drivers to have a 300 ohm output impedance, and that headphones should be designed with that in mind. The idea was that integrated amplifiers' headphone outputs were pretty much always done with a resistor of a few hundred ohms in series with the headphones, driven from the power amp's main loudspeaker output. The standard was intended to regularise the de-facto practice. Whether anyone ever took any notice I don't know. Just like the standard that loudspeakers' minimum load impedance should be no less than 80% of nominal. Widely ignored by most? loudspeaker manufacturers.

All the headphones I've measured, which I accept isn't too many, have shown a very even impedance characteristic, with no peaks in the bass, just a gentle rise at HF, reflecting the inductance of the speech coil. I'm inclined to the view that with most headphones, the difference between a low impedance and medium impedance drive is just one of level, as I've not measured any significant change in frequency response. As to distortion, I don't know, but not been conscious of any audible change.

S.
 

solderdude

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I currently use my DT880 600 Ωs from an integrated amp with an output impedance of 68 Ω. I've been considering trying them from the speaker terminals and was intrigued by what you said. If I drive them with close to zero source impedance, there will be more distortion?

DT880-600 from 0.2ohm and 120ohm. As can be seen no audible influence so also not with 68ohm.
r120-1.6db.png


Distortion will not change either, at least not in my measurements when compensated for SPL level.
Only distortion measured at the output plug of an amplifier under load will appear to be higher with higher output R amplifiers.

When you can drive it to the levels you want using the current amp there is no reason to switch to another amp unless you want separate volume control and have a smaller device.
You will need an amp with gain >12dB
 

Sharur

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DT880-600 from 0.2ohm and 120ohm. As can be seen no audible influence so also not with 68ohm.
r120-1.6db.png


Distortion will not change either, at least not in my measurements when compensated for SPL level.
Only distortion measured at the output plug of an amplifier under load will appear to be higher with higher output R amplifiers.

When you can drive it to the levels you want using the current amp there is no reason to switch to another amp unless you want separate volume control and have a smaller device.
You will need an amp with gain >12dB
Why is an amp with >12dB needed? If i recall correctly, the JDS Labs Atom provides 125 mW of power but am unable to find the gain increase.
 

JohnYang1997

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Why is an amp with >12dB needed? If i recall correctly, the JDS Labs Atom provides 125 mW of power but am unable to find the gain increase.
That was based on assumption that the output level of the DACs are 2V unbalanced. To me you don't really need that much unless you want to apply EQ.
 

solderdude

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Why is an amp with >12dB needed? If i recall correctly, the JDS Labs Atom provides 125 mW of power but am unable to find the gain increase.

Because the DT880-600 is very voltage insensitive (96dB/V) where most other headphones are between 105dB/V and 120dB/V.
You thus need a lot of gain and at least 7 to 8Vrms output voltage for this particular headphone.
When you are going to apply EQ then you will need even more gain to compensate for the negative gain needed for EQ. (see JohnYang's reply above)
That's why a lot of folks complain about amplifiers not having enough 'power'.
That, however, is almost never the issue. Lack of gain is.

Your current amp has 14dB gain (5x) for the headphone out.

JDS labs (standard = 1 and 4.5x gain) it is listed under configuration.
4.5x gain = 13dB.
The fun part of Atom is that you can order it in other gains as well.
Suppose you want to use a phone + amp you will only have 1V at your disposal which is -6dB opposite a DAC. In that case you will need more gain.
 
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KSTR

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I currently use my DT880 600 Ωs from an integrated amp with an output impedance of 68 Ω. I've been considering trying them from the speaker terminals and was intrigued by what you said. If I drive them with close to zero source impedance, there will be more distortion?
That's difficult to say, in your case I think any distortion change will be rather small. The voice coil sees 600Ohm (it's own impedance) as its termating impedance when amp outputs are used, while on the 68Ohm of the phones jack it is ~670Ohms. That's only 10% change and will have little effect on distortion. Frequency response change will be the largest effect, expect the DT880 to have a tad less bass (100Hz region) on the amp output. I'd personally stick with the phones output if you don't have any real issues. For a 600Ohm phones the 68Ohm is not a big difference.
 

Sharur

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DT880-600 from 0.2ohm and 120ohm. As can be seen no audible influence so also not with 68ohm.
r120-1.6db.png


Distortion will not change either, at least not in my measurements when compensated for SPL level.
Only distortion measured at the output plug of an amplifier under load will appear to be higher with higher output R amplifiers.

When you can drive it to the levels you want using the current amp there is no reason to switch to another amp unless you want separate volume control and have a smaller device.
You will need an amp with gain >12dB
I decided to test this myself expecting to hear no difference. I ordered speaker terminal adapters on amazon. Aaaaanddddddd aside from volume, there is no difference as far as i can tell. At least I can brag about having a 6000+ damping factor now :)
speaker tapped.jpg
 
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Sharur

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@solderdude do you know if there are any disadvantages to using the speaker terminals, like noise? Or would noise floor be same at the same power output?
 

solderdude

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Noise floor using direct speaker out will be 10dB 'worse' but because the DT880-600 is very voltage insensitive it may not be audible.
It is possible to accidentally blow the DT880 with direct speaker connection.

30Vrms is 1.5W in 600ohm and is farr beyond the maximum power rating of the driver using the NAD.
 
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Sharur

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Noise floor using direct speaker out will be 10dB 'worse' but because the DT880-600 is very voltage insensitive it may not be audible.
It is possible to accidentally blow the DT880 with direct speaker connection.

30Vrms is 1.5W in 600ohm and is farr beyond the maximum power rating of the driver using the NAD.
This is only from peak power right?
 

solderdude

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The amp can provide this continuous.
 

MRC01

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... I remember that years ago, a standard was proposed for headphone drivers to have a 300 ohm output impedance, and that headphones should be designed with that in mind. ...
I recall the same, but it was 120 ohm, not 300. FWIW, this Wikipedia article says: The 1996 IEC 61938 standard recommended an output impedance of 120 Ohms, but in practice this is rarely used and not recommended with modern headphones.

That said, it's common for headphones to be voiced brighter than neutral, and to have max impedance in the bass, in which case a high output impedance can "tame" the bright/aggressive sound.

My preamp has dual headphone outputs: one at 0 ohm, one at 120 ohm. On the 120 ohm output, the LCD-F (planar) sounds the same just quieter. The HD-580 sounds just a touch less bright.
 

solderdude

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Indeed a touch less bright (or warmer) 0.2 ohm 120 ohm (= HD600 but is close if not similar to HD580)

hd600-120-ohm.png
 

MRC01

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Indeed a touch less bright (or warmer) 0.2 ohm 120 ohm (= HD600 but is close if not similar to HD580)...
This reminds me of a tube amp I had years ago, a Wheatfield HA-2 by Pete Millet. It was a SET, OTL and had that magically tubulicious sound on the HD-580 headphones. Compared to a good solid state amp it was clean, not obviously euphonic, but just slightly warmer and smoooottttth mids and treble. Now I looked back in the specs and see it had a 100 ohm output impedance.
 
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