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Help with headphone compatibility
MagR
post Apr 14 2013, 10:28
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Hi

Ive ordered an O2 amp with ODAC which I should get in a couple of weeks. This is the open source amp and DAC by NWAVGuy who recommends this forum for objective advice.

I already own Sennheiser HD650s which this amp will have no problem driving looking at the specs and reading user reports. I want to know your thoughts on this amps abilities to drive two other headphones I am interested in to experience their different sound signatures.

Firstly the AKG K701 I keep reading this headphone is hard to drive but the specs Ive found dont seem to bear this out. The impedance is 62 ohms which is quite low compared to the HD650 at 300ohms. The sensitivity is 105 db per volt which is similar to the HD650s 103db per volt and equates to about 97db per milliwatt. I dont understand why people are saying the AKGs are more diicult to drive than the Sennheisers as these specs suggest it should be the other way round. Has anyone got any experience with this headphone and the O2 amp?

The second headphone I am interested in is the Beyerdynamic DT880 600ohm. NWAVGuy says this needs at least 5 volts to drive and the O2 can put a maximum of 7 volts into 600ohms. It looks like this is ample to handle this headphone on paper but I want to know if anyone has any experience of actually using this combination rather than relying on hearsay. What I dont want is to regularly run the amp right up to its limits in case it gets a bit ragged. The music (and games in particular) I own vary quite considerably in volume hence amp requirements. Can anyone with experience confirm whether this amp has enough power for this amp in practical use?

My final question relates to the Beyerdynamic DT880 above. Im drawn to the 600ohm version as the reports Ive read suggest this sounds the best and has the best treble. There is also a 250ohm version (the original impedance I think) which is much easier to drive. Does anyone have any direct experience of the actual sonic difference between the 250ohm and 600ohm versions? If it is marginal then the 250ohm seems a safer bet as the O2 should handle this more easily.

Thanks in advance

Mag
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Arnold B. Kruege...
post Apr 14 2013, 13:31
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QUOTE (MagR @ Apr 14 2013, 05:28) *
Hi

Ive ordered an O2 amp with ODAC which I should get in a couple of weeks. This is the open source amp and DAC by NWAVGuy who recommends this forum for objective advice.

I already own Sennheiser HD650s which this amp will have no problem driving looking at the specs and reading user reports. I want to know your thoughts on this amps abilities to drive two other headphones I am interested in to experience their different sound signatures.

Firstly the AKG K701 I keep reading this headphone is hard to drive but the specs Ive found dont seem to bear this out. The impedance is 62 ohms which is quite low compared to the HD650 at 300ohms. The sensitivity is 105 db per volt which is similar to the HD650s 103db per volt and equates to about 97db per milliwatt. I dont understand why people are saying the AKGs are more diicult to drive than the Sennheisers as these specs suggest it should be the other way round. Has anyone got any experience with this headphone and the O2 amp?

The second headphone I am interested in is the Beyerdynamic DT880 600ohm. NWAVGuy says this needs at least 5 volts to drive and the O2 can put a maximum of 7 volts into 600ohms. It looks like this is ample to handle this headphone on paper but I want to know if anyone has any experience of actually using this combination rather than relying on hearsay. What I dont want is to regularly run the amp right up to its limits in case it gets a bit ragged. The music (and games in particular) I own vary quite considerably in volume hence amp requirements. Can anyone with experience confirm whether this amp has enough power for this amp in practical use?

My final question relates to the Beyerdynamic DT880 above. Im drawn to the 600ohm version as the reports Ive read suggest this sounds the best and has the best treble. There is also a 250ohm version (the original impedance I think) which is much easier to drive. Does anyone have any direct experience of the actual sonic difference between the 250ohm and 600ohm versions? If it is marginal then the 250ohm seems a safer bet as the O2 should handle this more easily.


As long as you keep high linearity, high NFB amps like the 02 from clipping, there will be no audible distortion.

So this statement: "That I dont want is to regularly run the amp right up to its limits in case it gets a bit ragged." we be more about paranoia than actual SQ problems as long as clipping is not involved.

OTOH, I find 7 volts peak output into high imepdance loads to be a bit modest.

The O2 amp itself seems like it could be hacked to produce more power by simply raising the power supply voltage and using IC regulators with higher voltages (e.g. 18 bolts) than the modest 12 volt units in the original design.
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skamp
post Apr 14 2013, 14:56
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QUOTE (MagR @ Apr 14 2013, 11:28) *
Firstly the AKG K701 – I keep reading this headphone is hard to drive but the specs I’ve found don’t seem to bear this out. The impedance is 62 ohms which is quite low compared to the HD650 at 300ohms. The sensitivity is 105 db per volt which is similar to the HD650’s 103db per volt and equates to about 97db per milliwatt. I don’t understand why people are saying the AKG’s are more diicult to drive than the Sennheisers as these specs suggest it should be the other way round.


Your calculation is incorrect: the AKG K701s are indeed 105dB/V, but you have to take their impedance into account (62Ω):
dBV - (10 * log10(1000/Z)) = 105 - (10 * log10(1000/62)) = 93dB/mW

The Sennheiser HD 650s are 103dB/V, 300Ω = 98dB/mW
The Beyerdynamic DT 880s are (reportedly) 96dB/mW.

NwAvGuy says the O2 can drive all those cans just fine.

This post has been edited by skamp: Apr 14 2013, 14:57


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stephan_g
post Apr 14 2013, 15:06
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While the NJM4556 is specified for supplies up to +/-18V, it would be increasingly unhappy driving low-impedance loads, potentially becoming uncomfortably warm.

Really, 7 Vrms is just fine for most anything short of K240M/DFs. It still is good for 110+ dB with DT880-600s.

Incidentally, K701s measure about 100 dB / 1 V, so they are in fact a little quieter than the usual Senns, at lower impedance, which makes them harder to drive.

Oh, and when used with the ODAC, the standard 2.5x low gain of the O2 would be fine. That would give you a nominal 5 Vrms output, with a bit of headroom for intersample-overs.

This post has been edited by stephan_g: Apr 14 2013, 15:36
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Propheticus
post Apr 14 2013, 15:10
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When desired atainable SPL is 115dB (peaks for dynamic range taken into account, you wouldn't want to listen to an average volume this high) you get:

antilog((115-96)/10)= 79.43 mW
The O2 can deliver 88 mW @ 600 Ω, so you're good.
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MagR
post Apr 15 2013, 10:51
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QUOTE (skamp @ Apr 14 2013, 13:56) *
QUOTE (MagR @ Apr 14 2013, 11:28) *
Firstly the AKG K701 I keep reading this headphone is hard to drive but the specs Ive found dont seem to bear this out. The impedance is 62 ohms which is quite low compared to the HD650 at 300ohms. The sensitivity is 105 db per volt which is similar to the HD650s 103db per volt and equates to about 97db per milliwatt. I dont understand why people are saying the AKGs are more diicult to drive than the Sennheisers as these specs suggest it should be the other way round.


Your calculation is incorrect: the AKG K701s are indeed 105dB/V, but you have to take their impedance into account (62Ω):
dBV - (10 * log10(1000/Z)) = 105 - (10 * log10(1000/62)) = 93dB/mW

The Sennheiser HD 650s are 103dB/V, 300Ω = 98dB/mW
The Beyerdynamic DT 880s are (reportedly) 96dB/mW.

NwAvGuy says the O2 can drive all those cans just fine.

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MagR
post Apr 15 2013, 10:58
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QUOTE (skamp @ Apr 14 2013, 13:56) *
QUOTE (MagR @ Apr 14 2013, 11:28) *
Firstly the AKG K701 I keep reading this headphone is hard to drive but the specs Ive found dont seem to bear this out. The impedance is 62 ohms which is quite low compared to the HD650 at 300ohms. The sensitivity is 105 db per volt which is similar to the HD650s 103db per volt and equates to about 97db per milliwatt. I dont understand why people are saying the AKGs are more diicult to drive than the Sennheisers as these specs suggest it should be the other way round.


Your calculation is incorrect: the AKG K701s are indeed 105dB/V, but you have to take their impedance into account (62Ω):
dBV - (10 * log10(1000/Z)) = 105 - (10 * log10(1000/62)) = 93dB/mW

The Sennheiser HD 650s are 103dB/V, 300Ω = 98dB/mW
The Beyerdynamic DT 880s are (reportedly) 96dB/mW.

NwAvGuy says the O2 can drive all those cans just fine.

Thanks for reply. I didn't know impedance affected the sensitivity calculation. I know HD600's (and presumably HD650's) are 97 db/mW so I thought I could just substitute this for the AKG's. That makes more sense now as 93 is considerably lower.

Thanks again

Mag
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MagR
post Apr 15 2013, 11:04
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Hi

Thanks for all your replies chaps - very helpful. Has anyone got any thoughts on sonic differences between Beyer DT880 600ohm and 250ohm versions. I've ordered a pair of AKG K701's so it is just the Beyers I need to make a decision over.

Thanks in advance

Mag
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DVDdoug
post Apr 15 2013, 22:08
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QUOTE
Has anyone got any thoughts on sonic differences between Beyer DT880 600ohm and 250ohm versions.
Of course, the 250 Ohm version should be slightly louder (with the same voltage). Otherwise if there are any audible differences, I would assume the differences are subtle and it would come down to personal preference. I also assume the frequency response specs are the same?

QUOTE
Thanks for reply. I didn't know impedance affected the sensitivity calculation.
It usually doesn't... It depends on how sensitivity is defined. If you see XdB @ Y Volts, impedance is not a factor as long as you know the output voltage rating from the amp and that it's capable of driving the headphone's impedance. If you see XdB @ Y mW (efficiency) you'll need to know the impedance to calculate the required voltage.

Most amplifiers are "constant voltage" devices. That doesn't really mean the voltage is constant, since it obviously varies with the music and the volume control. It means that the internal source impedance is low, so the output voltage doesn't vary (much) when load impedance varies (as long as the load impedance is within spec).

If you cut the impedance in half and keep the voltage constant, you get twice the power, and all else being equal, twice the acoustic power (+3dB more volume). In cases where a manufacturer offers the same model in two different impedances, everything else is nearly equal and the headphones will have nearly the same efficiency dB/mW, and the lower impedance headphone will have higher sensitivity (dB/V).

Since it takes more voltage to get the same power (mW) out of a higher impedance headphone, these higher impedance headphones typically require higher voltage, and someone might say that a 250 or 600 Ohm headphone is "hard to drive", compared to a 32 Ohm headphone.

If you have a poorly designed headphone amp (or a soundcard that's not optimized for driving headphones), it may very from the "constant voltage" ideal. Since the impedance of all headphones varies over the frequency range, the frequency response of these headphone amps can be different with different headphones. Or if there is an output capacitor, and it's on the small size, you can lose bass on low impedance headphones.

These interactions (that only occur with badly designed headphone amps) are minimized (or virtually eliminated) with higher impedance headphones (250 or 600 Ohms). But of course, the reduced sensitivity often means that the cheap headphone amp can't put out the voltage needed for the desired volume.
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MagR
post Apr 17 2013, 15:51
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QUOTE (DVDdoug @ Apr 15 2013, 21:08) *
QUOTE
Has anyone got any thoughts on sonic differences between Beyer DT880 600ohm and 250ohm versions.
Of course, the 250 Ohm version should be slightly louder (with the same voltage). Otherwise if there are any audible differences, I would assume the differences are subtle and it would come down to personal preference. I also assume the frequency response specs are the same?

QUOTE
Thanks for reply. I didn't know impedance affected the sensitivity calculation.
It usually doesn't... It depends on how sensitivity is defined. If you see XdB @ Y Volts, impedance is not a factor as long as you know the output voltage rating from the amp and that it's capable of driving the headphone's impedance. If you see XdB @ Y mW (efficiency) you'll need to know the impedance to calculate the required voltage.

Most amplifiers are "constant voltage" devices. That doesn't really mean the voltage is constant, since it obviously varies with the music and the volume control. It means that the internal source impedance is low, so the output voltage doesn't vary (much) when load impedance varies (as long as the load impedance is within spec).

If you cut the impedance in half and keep the voltage constant, you get twice the power, and all else being equal, twice the acoustic power (+3dB more volume). In cases where a manufacturer offers the same model in two different impedances, everything else is nearly equal and the headphones will have nearly the same efficiency dB/mW, and the lower impedance headphone will have higher sensitivity (dB/V).

Since it takes more voltage to get the same power (mW) out of a higher impedance headphone, these higher impedance headphones typically require higher voltage, and someone might say that a 250 or 600 Ohm headphone is "hard to drive", compared to a 32 Ohm headphone.

If you have a poorly designed headphone amp (or a soundcard that's not optimized for driving headphones), it may very from the "constant voltage" ideal. Since the impedance of all headphones varies over the frequency range, the frequency response of these headphone amps can be different with different headphones. Or if there is an output capacitor, and it's on the small size, you can lose bass on low impedance headphones.

These interactions (that only occur with badly designed headphone amps) are minimized (or virtually eliminated) with higher impedance headphones (250 or 600 Ohms). But of course, the reduced sensitivity often means that the cheap headphone amp can't put out the voltage needed for the desired volume.


Thanks for your reply.

Can I just clarify something? When a headphone is described as say 97 dB/V is this correctly described as a measure of its sensitivity? When a headphone is described as say 100 dB/mW is this correctly described as a measure of its efficiency?

I think I was incorrect in assuming both dB/V and dB/mW were measures of sensitivity.

Thanks in advance

Mag

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pdq
post Apr 17 2013, 16:05
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Both are measures of efficiency/sensitivity, and their values are related by the impedance.
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