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Topic: Amp for Sennheiser HD800 headphone (Read 38712 times) previous topic - next topic
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Amp for Sennheiser HD800 headphone

Hi,

I recently ordered a set of Sennheiser HD800 headphones.  To me, analyzing requirements for headphone amplification seems very difficult.  With loudspeakers it seems simple - match a speaker of the correct impedance with an amplifier capable of handling that impedance load; match the rms output of the amp with the rms power handling of the speaker.  I know it's really a lot more complicated than that, but for a non-engineer, end user level grunt such as myself, that method works and generally keeps me from blowing up expensive things.

I've been referencing this list:  http://www.head-fi.org/t/476345/headphone-...ements-compared that says in order to drive the HD800 to 120 db I need an amp that can provide 210 milliwatts, 26 milliamps, 7.94 volts rms, presumably into the HD800's nominal impedence of 300 ohms; although the impedence on the headphone varies with signal frequency. 

So I guess my question is, how do I know if a give headphone output or amplifier is capable of providing that sort of power, or am I overthinking things?  Should I just plug it in to my receiver's headphone out, and if it's not loud enough, consider a separate headphone amp?

Obviously 210 milliwatts doesn't seem like much, then again 300 ohms seems high for someone used to freestanding speaker impedences at 8 or 4 ohms typically. 

I don't get much useful feedback to this sort of question on audiophile headphone sites because they start recommending tube amps, which I'm not really interested in (don't they just add distortion and color the sound? I'm interested in faithful, accurate sound reproduction). 

I was considering an amp with the following specifications and was wondering if it would be adequate (costs about $200):

Frequency Response: 10–50k, ±0.3
THD at 1V output: <.002%
Input Impedance at 1kHz (ohms): 70k
Output Impedance: 0.5 ohm
Output Voltage: 10Vrms


Thanks in advance, maybe an EE or two on here could elighten me.

Amp for Sennheiser HD800 headphone

Reply #1
Doesn't what's been said on this rather recent thread fit your bill somehow?

http://www.hydrogenaudio.org/forums/index....howtopic=103231

Edit: it wasn't meant to quote anything. Oh yeah, and the link for the referred thread itself.
Listen to the music, not the media it's on.
União e reconstrução

Amp for Sennheiser HD800 headphone

Reply #2
Obviously 210 milliwatts doesn't seem like much


That's actually quite a bit. Your 120dB SPL peak requirement is also excessive, and can result in permanent hearing damage. A peak value of just 110dB SPL is much more realistic, and the power requirement drops significantly: 20mW! And that value just so happens to be the rated output power of the E-MU 0404 USB. That's what IgorC uses to drive his HD800s, and he seems quite happy about it.

Other than that, I can only vouch of the Objective 2 headphone amplifier, which performs very well with a wide range of headphones, including much more power-hungry ones.

Edit: the other parameter you should look for is the output impedance of the amp, which should be lower than 300 / 8 = 37.5?. The 0404 USB is 22?, and the O2 is 0.5?.

Amp for Sennheiser HD800 headphone

Reply #3
Doesn't what's been said on this rather recent thread fit your bill somehow?

http://www.hydrogenaudio.org/forums/index....howtopic=103231

Edit: it wasn't meant to quote anything. Oh yeah, and the link for the referred thread itself.


I did read through that thread in my searches and was a bit concerned with the specified 20 mw maximum output power on the 0404, if the hd800 indeed needs 210mw to get to 120 db.  Granted 20mw should still drive them to 110, which is probably pretty loud.

I'm not really clear on how the specified 10 volt rms spec on the amp I'm considring (headroom micro) translates to power.  According to ohms law calculators, 10 volts into 300 ohms should yeild 333 mw, but I don't know if the amp would deliver that into 300 ohms or not since it's not specified.

Amp for Sennheiser HD800 headphone

Reply #4
Obviously 210 milliwatts doesn't seem like much


That's actually quite a bit. Your 120dB SPL peak requirement is also excessive, and can result in permanent hearing damage. A peak value of just 110dB SPL is much more realistic, and the power requirement drops significantly: 20mW! And that value just so happens to be the rated output power of the E-MU 0404 USB. That's what IgorC uses to drive his HD800s, and he seems quite happy about it.

Other than that, I can only vouch of the Objective 2 headphone amplifier, which performs very well with a wide range of headphones, including much more power-hungry ones.

Edit: the other parameter you should look for is the output impedance of the amp, which should be lower than 300 / 8 = 37.5?. The 0404 USB is 22?, and the O2 is 0.5?.


Thanks, we cross posted and you kind of read my mind a bit there.  I realize that 120 db is way too high for normal sustained listening, but it's nice to have the power on tap should it be needed I guess.  For instance, I usually just listen to my Etymotic ER4p IEM's through my laptop and have to turn the master volume to about 20% to listen comfortably. I have no idea what that translates to in terms of power demand, but I can't imagine it'd be much, granted the ER4p is a 27 ohm headphone and comparing it to an HD800 is like comparing a moped to a boeing 747. 

I'm probably overthinking it and should just try the HD800 out of a reciever jack first, then get something like the EMU if it won't go loud enough.

Amp for Sennheiser HD800 headphone

Reply #5
If you're going to use your balanced armature IEMs in addition to the HD800s, I strongly recommend a low output impedance headphone amplifier such as the O2. Buy that, and you'll be able to use it with pretty much any type of headphones or IEMs.

Amp for Sennheiser HD800 headphone

Reply #6
Bear in mind the EMU-404 has a ~22 ohm output impedance, and a receiver is likely to be even worse. This is not ideal for driving headphones (damping factor problems). I would recommend considering spending a little more on something like the O2.

EDIT: Beaten to it, partially, with the exception that I wouldn't like to drive the HD800 from a higher output impedance either, ideally.

Amp for Sennheiser HD800 headphone

Reply #7
Bear in mind the EMU-404 has a ~22 ohm output impedance, and a receiver is likely to be even worse. This is not ideal for driving headphones (damping factor problems). I would recommend considering spending a little more on something like the O2.

EDIT: Beaten to it, partially, with the exception that I wouldn't like to drive the HD800 from a higher output impedance either, ideally.


Thanks, I do have a 75 ohm resistor cable that I can use with my Etymotic IEMs if needed.  I have been looking at the O2 amplifier and am intrigued by it.  I really enjoyed reading through the blog associated with it as well.  I wish he gave a specification for 300 ohm power output though.  337 mw at 15 ohms, 613 at 33 ohms, and 355 at 150 ohms.  I guess I could presume about 180 mw at 300 ohms, which is pretty good for the price, although the hd800's impedence gets as high as 600 ohms at 100 hz.

Amp for Sennheiser HD800 headphone

Reply #8
The O2 will do an excellent job with your cans, don't worry about it. Also, don't judge its power ratings as maximums; the O2 has current limiting in order to prevent headphones from getting driven too loud, to the point of damaging them.

Amp for Sennheiser HD800 headphone

Reply #9
although the hd800's impedence gets as high as 600 ohms at 100 hz.


That does not increase the voltage requirement at 100 Hz, however. That is, contrary to popular belief, you do not need "more power" to compensate for the increased impedance (that would in fact be equivalent to a high output impedance). An amplifier with a low output impedance is basically a voltage source, and therefore it has to output the same voltage, but less current (so less power, which makes sense, as the headphone becomes more efficient at its resonance frequency) at 100 Hz than for example at 2 kHz.

Amp for Sennheiser HD800 headphone

Reply #10
I don't get much useful feedback to this sort of question on audiophile headphone sites because they start recommending tube amps, which I'm not really interested in (don't they just add distortion and color the sound? I'm interested in faithful, accurate sound reproduction).


Despite my source was technically able to drive the hd800 (a xonar stx) , I was never completely satisfied with the hd800 , except with particular music genres (classical , dark ambient, field recordings ...) .  My (last) theory to explain such dissatisfaction, is that the hd800 have an unusually low decay when playing transients . By decay, I  mean, how fast the hd800 is able to restore silence, when a sound stop to be played (this is what people seems to have in mind, when they describe the hd800 as "fast").  One explanation for such low decay, is because of the ring driver, that eliminate some "momentum from air"  (compared to regular drivers).  This is in principle "more accurate" (low noise/distortion)  , but imho music (especially electronic music)  is not usually mastered for this kind of presentation (or we aren't prepared psychologically) .  Anyways this makes a somehow exhausting presentation , where you actually "hear everything", but still don't enjoy it (some call this "analytical" presentation, others just find it "unbearable" without being able to explain why) . Hence one possible  reason,  people are actually looking for non transparent sources : I  guess to mask a bit this unusually small decay (I've personally experimented with different kind of dsp , and also a mod).  Imho we have with the hd800 a counter-intuitive example, where more accuracy could be less enjoyable.

With the lasted hd700, Sennheiser has restored a "regular driver", and lot of people are happy, because suddenly they find it "easy" to drive (i.e they find it enjoyable with a regular transparent source, that is not insanely priced).


Amp for Sennheiser HD800 headphone

Reply #11
I don't get much useful feedback to this sort of question on audiophile headphone sites because they start recommending tube amps, which I'm not really interested in (don't they just add distortion and color the sound? I'm interested in faithful, accurate sound reproduction).


Despite my source was technically able to drive the hd800 (a xonar stx) , I was never completely satisfied with the hd800 , except with particular music genres (classical , dark ambient, field recordings ...) .  My (last) theory to explain such dissatisfaction, is that the hd800 have an unusually low decay when playing transients . By decay, I  mean, how fast the hd800 is able to restore silence, when a sound stop to be played (this is what people seems to have in mind, when they describe the hd800 as "fast").  One explanation for such low decay, is because of the ring driver, that eliminate some "momentum from air"  (compared to regular drivers).  This is in principle "more accurate" (low noise/distortion)  , but imho music (especially electronic music)  is not usually mastered for this kind of presentation (or we aren't prepared psychologically) .  Anyways this makes a somehow exhausting presentation , where you actually "hear everything", but still don't enjoy it (some call this "analytical" presentation, others just find it "unbearable" without being able to explain why) . Hence one possible  reason,  people are actually looking for non transparent sources : I  guess to mask a bit this unusually small decay (I've personally experimented with different kind of dsp , and also a mod).  Imho we have with the hd800 a counter-intuitive example, where more accuracy could be less enjoyable.

With the lasted hd700, Sennheiser has restored a "regular driver", and lot of people are happy, because suddenly they find it "easy" to drive (i.e they find it enjoyable with a regular transparent source, that is not insanely priced).


Thanks.  I'm looking forward to listening to it.  I suspect I won't be completely impressed, but I wanted to try a true top of the line headphone, and for once in my life I have some money, some time, and nothing better to do.  I've never listened to Sennheisers at all, I only have the Etymotic ER4P/S, Alessandro/Grado MS2, Beyerdynamic DT250-80, Sony MDR7506, and Koss KSC75.  Of those, the sound of the Alessandro is probably my favorite fore pure enjoyability, but for critical listening or long listening sessions, I prefer the Etymotic. 

On the amplifier front I went ahead and purchased the Headroom Micro Amp for about $250.  It's the one rated at 10v rms at .5 ohm output impedence.  What that means in terms of driving the HD800 I'm not quite sure, but I suspect it will be adequate.  I probably should've gone with the O2, and maybe still buy one of those in the future since it is portable, but I always wanted a headroom amp and was disappointed to learn they discontinued them, so I purchased one while I still could.

Amp for Sennheiser HD800 headphone

Reply #12
Thanks.  I'm looking forward to listening to it.  I suspect I won't be completely impressed


Not necessarily ... but it might take you some time to get used to their particular sound signature ( Or you might never get used to it). I'm thinking that  the crossfeed of your "Headroom Micro Amp" could help.


Amp for Sennheiser HD800 headphone

Reply #13
Quote
It's the one rated at 10v rms at .5 ohm output impedence. What that means in terms of driving the HD800 I'm not quite sure...
The higher voltage means you can "go loud" with any headphone, including high-impedance headphones which tend to require more voltage.*  It also gives you more headroom, if you want to boost the bass, etc.

The low output-impedance means that it will work with low-impedance headphones (or high impedance headphones).    That spec is not important with your high-impedance headphones.  It doesn't mean it will work with 1/2 Ohm headphones (which don't exist) or regular speakers....  The 1/2 Ohm is the internal source impedance.  The lower the source impedance (relative to the headphone impedance), the less it interacts with the headphone impedance to muck-up frequency response.      If headphone impedance was constant across the frequency band (it never is), a higher source impedance would simply reduce the output signal.    But when the load impedance (headphone) varies with frequency, a (relatively) high source-impedance will result in output-voltage variations across the frequency range.

In short, the goal is NOT to "match" the impedance...  The goal is (relatively) low source impedance and (relatively) high load impedance.

It's hard to compare headphone amp specs, because sometimes the headphone amp's specified output impedance rating is the recommend headphone impedance.  But with 1/2 Ohm, you know that's the true internal source impedance.




*  Similarly, a power amplifier typically puts-out twice the power with a 4-Ohm speaker as it does at 8-Ohm speaker....  If you put two 8-Ohm speakers in parallel, you get 4-Ohms total...  Or, if you are running an 8-Ohm speaker at 100 Watts, and you put a 2nd 8-Ohm speaker in parallel, they both get 100W, for a total of 200W.  If you push that too far and exceed the amplifier's specs, you can burn it up.  (Too low of a load impedance usually won't "blow" a headphone amp, but it's a bad idea to connect an 8-Ohm speaker.  )

Amp for Sennheiser HD800 headphone

Reply #14
Just a few thoughts:

- a headphone with a higher impedance but same sensitivity is easier to drive (infinitely high impedance would mean the headphone draws no current from the source)
- high output impedance will mean a part of the voltage will drop across the internal resistance, so a rise in the impedance of the headphone will change the ratio of this voltage split often causing a bass boost with dynamic headphones
- all bets are off with high output impedance in combination with multi-driver IEMs

- 355 mW at 150 ohms means V = sqrt(0.355*150) = 7.3 V
- 7.3V into 300 ohms = at least 178 mW (at least because higher load impedance usually means higher output voltage)
- 2V get you to about 110 dB SPL with the HD800 and a full-scale sine wave, so adding a bit of excess gain and you should be fine
"I hear it when I see it."

Amp for Sennheiser HD800 headphone

Reply #15
Just a few thoughts:

- a headphone with a higher impedance but same sensitivity is easier to drive (infinitely high impedance would mean the headphone draws no current from the source)
- high output impedance will mean a part of the voltage will drop across the internal resistance, so a rise in the impedance of the headphone will change the ratio of this voltage split often causing a bass boost with dynamic headphones
- all bets are off with high output impedance in combination with multi-driver IEMs

- 355 mW at 150 ohms means V = sqrt(0.355*150) = 7.3 V
- 7.3V into 300 ohms = at least 178 mW (at least because higher load impedance usually means higher output voltage)
- 2V get you to about 110 dB SPL with the HD800 and a full-scale sine wave, so adding a bit of excess gain and you should be fine


Got the HD800 and the amp today and am really quite happy with them.  The amp drives them to intolerably loud levels at about 30% volume on high gain.  Staright out of my laptop's headphone jack, they leave a lot to be desired at maximum volume though, so I'm glad I have the amp.

Back to the topic, what's the typical "output impedence" of a mid fi level home audio reciever, such as a denon, marantz, or premier?  How about a typical car audio amplifier that's designed to push full range speakers into 4 ohm stereo or subwoofers at 4 ohms mono?  Are those sorts of amplifiers also designed with very low (< 1 ohm) output impedences?  Are headphone jacks typically the only "amps" with high output impedence (as I've read, it was because older receivers just used big resistors to reduce the power from the main amp for head use, to avoid the necessity for seperate headphone electronics). 


Amp for Sennheiser HD800 headphone

Reply #16
Back to the topic, what's the typical "output impedence" of a mid fi level home audio reciever, such as a denon, marantz, or premier?

You'd have to look up the specs or measure it, but usually in the hundreds! Yamaha for example specifies many of their headphone outputs with 470 ohm.
Your guess with internal resistors connecting the headphone jack to the speaker taps internally is right and still a common practice today.

How about a typical car audio amplifier that's designed to push full range speakers into 4 ohm stereo or subwoofers at 4 ohms mono?  Are those sorts of amplifiers also designed with very low (< 1 ohm) output impedences?

Yes, usually very close to zero ohm. This is true for most receiver's speaker taps, solid state power amps.

Are headphone jacks typically the only "amps" with high output impedence

There are some power amps, especially tube ones, that may have an output impedance up to a few ohms.
This causes some peaks in the speakers frequency response, bass boost, flabby bass etc. which audiophiles that are into tube amps praise as <insert flowery BS here>.

Powering a headphone directly with an opamp can lead to instability. The easy (but imo wrong) way to fix this is by adding output resistors that raise the output impedance.
Such resistors are also used to limit output current.
Another use is to limit the output level depending on the load: a high impedance headphone may get 50% of the voltage, but a low impedance IEM that is highly sensitive may only get 10%. So when you switch from headphone to IEM your IEM won't blow up.

That's why you see high output impedance with headphone jacks. The standard really should be 0 though...
"I hear it when I see it."

Amp for Sennheiser HD800 headphone

Reply #17
Back to the topic, what's the typical "output impedence" of a mid fi level home audio reciever, such as a denon, marantz, or premier?

You'd have to look up the specs or measure it, but usually in the hundreds! Yamaha for example specifies many of their headphone outputs with 470 ohm.
Your guess with internal resistors connecting the headphone jack to the speaker taps internally is right and still a common practice today.

How about a typical car audio amplifier that's designed to push full range speakers into 4 ohm stereo or subwoofers at 4 ohms mono?  Are those sorts of amplifiers also designed with very low (< 1 ohm) output impedences?

Yes, usually very close to zero ohm. This is true for most receiver's speaker taps, solid state power amps.

Are headphone jacks typically the only "amps" with high output impedence

There are some power amps, especially tube ones, that may have an output impedance up to a few ohms.
This causes some peaks in the speakers frequency response, bass boost, flabby bass etc. which audiophiles that are into tube amps praise as <insert flowery BS here>.

Powering a headphone directly with an opamp can lead to instability. The easy (but imo wrong) way to fix this is by adding output resistors that raise the output impedance.
Such resistors are also used to limit output current.
Another use is to limit the output level depending on the load: a high impedance headphone may get 50% of the voltage, but a low impedance IEM that is highly sensitive may only get 10%. So when you switch from headphone to IEM your IEM won't blow up.

That's why you see high output impedance with headphone jacks. The standard really should be 0 though...


Thanks for that. 

In addition to standardizing headphone amplifier output impedance, is there an engineering reason why headphone driver impedance isn't somewhat standardized as well; for instance the vast majority of freestanding home audio speakers seem to fall somewhere in the relatively narrow realm of 4-8 ohm nominal.  Headphones, on the other hand, as we've discussed fall anywhere from about 20 ohms to 600 ohms, although admittedly the vast majority probably of mainstream consumer level headphones probably fall around 30 ohms.  The HD800's for instance are 300 ohms nominal and sound pretty nice.

Someone mentioned that utilizing a high impedence voice coil on an efficient headphone has the advantage of being less demanding on an amplifier and also more suitable for headphone amps with high output impedance. 

I also read the following benefits of high impedance headphones on another forum:
Quote
1.) The displacement (amount of movement) of the diaphragm (the thing that actually vibrates and produces sound) is better controlled via a more accurate flux (magnetic field to pull and push the diaphragm). This gives better accuracy and more performance at lower frequencies.

2.) The impedance is increased by using a different wire for the voice coil and most importantly more turns. More turns or loops creates a larger field (area of magnetic influence), in lay mans terms bigger magnetic area for the coil to move more efficiently.

3.) It also allows SS amps to work more efficiently with lesser distortion have a look at data sheets and a graph of distortion vs output impedance for most audio op-amps and you'll get the idea.


I'm wondering why these same principles don't apply to full sized loudspeakers.  Or perhaps they do, but the tradeoff in efficiency just isn't worth it when you consider the amount of amplifier power you'd need to drive high impedance, full sized loudspeakers.

I've read the arguments on that other forum as to why 8 ohm became the loudspeaker standard - mostly because it's simply what receivers and amplifiers are designed to work best at, not because it's optimal from a speaker design standpoint.  I would be interested in hydrogen's take on that.

Amp for Sennheiser HD800 headphone

Reply #18
In addition to standardizing headphone amplifier output impedance, is there an engineering reason why headphone driver impedance isn't somewhat standardized as well; for instance the vast majority of freestanding home audio speakers seem to fall somewhere in the relatively narrow realm of 4-8 ohm nominal.  Headphones, on the other hand, as we've discussed fall anywhere from about 20 ohms to 600 ohms, although admittedly the vast majority probably of mainstream consumer level headphones probably fall around 30 ohms.


Yeah, almost the entire market is 16 or 32 ohm headphones these days.  Higher impedances are mostly limited to pro, audiophile, boutique, etc parts.  In that sense its not too different from speakers where most are 4 or 8 ohms, but all kinds of weird stuff exists out there. 

As for why, different designs tend to have different impedances.  I don't think it would make sense to have every IEM have the same impedance as a large over-ear pair of headphones. 

Someone mentioned that utilizing a high impedence voice coil on an efficient headphone has the advantage of being less demanding on an amplifier and also more suitable for headphone amps with high output impedance.


I can't comment on the details of driver design, but increasing the coil area will increase the field generated at constant current, but will decrease the current and hence counteract that gain to at least some degree.  So its not so simple as higher impedance == more wire == linearly more B-field. 

I'm wondering why these same principals don't apply to full sized loudspeakers.  Or perhaps they do, but the tradeoff in efficiency just isn't worth it when you consider the amount of amplifier power you'd need to drive high impedance, full sized loudspeakers.


Higher impedances don't necessarily require more power, just more voltage.  In the old days before solid state amps, higher impedances were widely used, because it was easier to make higher voltages than higher currents.  Now we have CMOS FETs running at 1.8,  2.5 or 3.3v with almost arbitrarily high current and so you see more low impedance designs. 

I've read the arguments on that other forum as to why 8 ohm became the loudspeaker standard - mostly because it's simply what receivers and amplifiers are designed to work best at, not because it's optimal from a speaker design standpoint.  I would be interested in hydrogen's take on that.


If the argument was that higher impedances should be used, keep in mind that doubling the impedance from 8 to 16 ohms and using an amp made for 8 ohms only reduces your volume by 6dB, and perhaps not even that depending on the output impedance.  If a 16 ohm design works well, its not very hard to use it with existing equipment.  Lower impedance is much harder though.

Edit:  fix quote

Amp for Sennheiser HD800 headphone

Reply #19
The amp drives them to intolerably loud levels at about 30% volume on high gain.


Perhaps you mentionned that fact just as a reference as to how loud you amp can get with your HD800s, but just in case: don't use gain unless you have to (i.e. when your source is so low that the amp at full volume can't get loud enough without increasing gain).

Amp for Sennheiser HD800 headphone

Reply #20
The amp drives them to intolerably loud levels at about 30% volume on high gain.


Perhaps you mentionned that fact just as a reference as to how loud you amp can get with your HD800s, but just in case: don't use gain unless you have to (i.e. when your source is so low that the amp at full volume can't get loud enough without increasing gain).


The gain on the amp is a 3 setting switch, high/low/medium.  It definitely needs to be on at least medium with the HD800.  I'd say comfortable listening level is 20% volume on the high gain, 30% volume on the medium gain, and 100%+ on the low gain.

Amp for Sennheiser HD800 headphone

Reply #21
If the argument was that higher impedances should be used, keep in mind that doubling the impedance from 8 to 16 ohms and using an amp made for 8 ohms only reduces your volume by 6dB, and perhaps not even that depending on the output impedance.  If a 16 ohm design works well, its not very hard to use it with existing equipment.  Lower impedance is much harder though.


Doubling the speaker impedance at identical efficiency actually only reduces the volume by 3 dB, because it halves the power output in the worst case.

Amp for Sennheiser HD800 headphone

Reply #22
You can design drivers with an arbitrary impedance within reasonable limits. Beyerdynamic has some headphone models with 32, 80, 250, 600 ohm "systems" for example.
The low impedance ones are gonna be louder with voltage limited portable devices. Multiple high impedance headphones can be connected in parallel, for example 10x 600 ohm in parallel results in a 60 ohm load.

IEMs are a different matter because of the space constraints and wire cannot be infinitely thin. I guess that's why most are around 16 ohms.
"I hear it when I see it."

Amp for Sennheiser HD800 headphone

Reply #23
If the argument was that higher impedances should be used, keep in mind that doubling the impedance from 8 to 16 ohms and using an amp made for 8 ohms only reduces your volume by 6dB, and perhaps not even that depending on the output impedance.  If a 16 ohm design works well, its not very hard to use it with existing equipment.  Lower impedance is much harder though.


Doubling the speaker impedance at identical efficiency actually only reduces the volume by 3 dB, because it halves the power output in the worst case.


My mistake.  It is indeed 3 dB.

Amp for Sennheiser HD800 headphone

Reply #24
I would like to share my experience with my devices which have different output impedances (Zo). In my opinion the rule of thumb  "Zo should 1/8 of headphone impedance" works well. 
My devices are Xonar STX (Zo 10 ohms), EMU Pre-Tracker (Zo 22 ohms) and fiio 17 ( Zo less than 1 ohm).  All 3 of them drive my pair of HD 800 well as the their respective Zo-s are lower than 1/8 of headphone impedance.

fiio17's Zo is <1 ohm which is it >22x less than EMU-Pre tracker Zo (22 ohms).  Has it any benefits driving high impedance phone (300 ohms)?
No, it hasn't in my experience. 22 ohms is already low enough to not have impact, no benefit going lower (1 ohm or even 0.1 ohm). 

In fact my preference are: Xonar > EMU Pre-Tracker >> fiio E17.  Despite that  E17 has the lowest Zo, EMU and Xonar  have better distortion characteristic.   

I can't find now a link to one source where EMU USB devices were tested with 300 ohms load. The measurements were excellent.