[J Bones]

I have read a couple threads concerning the advantage(s) (if there are any) of higher impedance headphones. I am confused to if there really even is an answer. I hear that a higher impedance/load can lead to better damping factor, lower distortion (mostly with tube amps due to lower current needed), and a better frequency response. But on the other hand I hear that lower impedance headphones are easier to drive and don't rely so much on a stronger power supply.
 
I originally encountered this question when looking at the difference between a dedicated desktop amp and my stereo receiver. Given that the Q701's have an impedance of 62 ohms, I felt that a lower supply output impedance would work best with my headphones (hence a desktop amp vs. my Sony receiver). But at the same time, wouldn't a desktop amp not supply enough power to drive the headphones ? And when I constantly hear that the Q701's are typically hard to drive, wouldn't a stronger power supply be most ideal for listening with these cans (making a stereo receiver necessary)?
 
I guess what it really boils down to is why one would buy higher impedance headphones (Say Beyerdynamic 600-ohm series or anything in the hundereds of ohm resistance) vs. something lower like the Q701's or even lower in the 32-ohm range. And if someone did end up buying say the Q701's, why would you buy a desktop amp over a stereo receiver (which can also be used to power not just your headphones but a whole stereo system as well)? I feel like I have something backwards here or that I am missing one really important fact/point, but it just isn't coming to me.
 
Any help would be most appreciated!

 

[LinkPro]

High impedance requires more voltage, low impedance requires more current. 
 
It also depends on the sound pressure level (dB/mW or dB/V) of the paricular headphone. The higher the SPL and lower the impedance, the more efficient the driver is. That's why the K701 is hard to drive, its SPL is pretty abysmal for a dynamic driver, can't remember the exact figure though.
 
German brands tend to prefer high impedance design for their top cans, and in the same line-up, the model with the higher impedance generally sounds better (Beyerdynamic).

 

[Amorgan]

Quote:

 
 
German brands tend to prefer high impedance design for their top cans, and in the same line-up, the model with the higher impedance generally sounds better (Beyerdynamic).

 
Not always in my opinion. Some low impedance beyers sound better than the 600ohm models. 

 

[Kodhifi]

Quote:

 
Not always in my opinion. Some low impedance beyers sound better than the 600ohm models. 

Sacrilage!
 
 
The dampening factor is a major one. Also because impedance changes by frequency it can also effect the frequency response curve. 600ohm 880's have +4db more bass than the 250's at most frequencies for instance.
 
Regarding dampening factor and accuracy, I think this pic shows the difference.
 

 
 
600's don't go as much over or under, and it responds to the signal a bit quicker. Unfortunately since the bottom graph doesn't show the scale (presuming it's ms) it's difficult to say by how much.

 

[PurpleAngel]

Quote:

I have read a couple threads concerning the advantage(s) (if there are any) of higher impedance headphones. I am confused to if there really even is an answer. I hear that a higher impedance/load can lead to better damping factor, lower distortion (mostly with tube amps due to lower current needed), and a better frequency response. But on the other hand I hear that lower impedance headphones are easier to drive and don't rely so much on a stronger power supply.
 
I originally encountered this question when looking at the difference between a dedicated desktop amp and my stereo receiver. Given that the Q701's have an impedance of 62 ohms, I felt that a lower supply output impedance would work best with my headphones (hence a desktop amp vs. my Sony receiver). But at the same time, wouldn't a desktop amp not supply enough power to drive the headphones ? And when I constantly hear that the Q701's are typically hard to drive, wouldn't a stronger power supply be most ideal for listening with these cans (making a stereo receiver necessary)?
 
I guess what it really boils down to is why one would buy higher impedance headphones (Say Beyerdynamic 600-ohm series or anything in the hundereds of ohm resistance) vs. something lower like the Q701's or even lower in the 32-ohm range. And if someone did end up buying say the Q701's, why would you buy a desktop amp over a stereo receiver (which can also be used to power not just your headphones but a whole stereo system as well)? I feel like I have something backwards here or that I am missing one really important fact/point, but it just isn't coming to me.

My two cents and best guess.
Headphone manufacturers are going to make a headphone that people can use, so whatever audio playing device(s) people own dictacts what headphones they will buy.
Before portable quality audio devices came out, chances are that most (non professional audio) people that wanted quality audio would plug their headphones into there stereo receiver and/or record player or reel to reel tape player.
(I'm guessing in professional audio studios, 600-ohm headphone worked well with the equipment that was used)
I think it was fairly normal for those consumer devices to have a high impedance output (47-Ohm?) at the headphone jack, so you needed headphones above 100-Ohms, like 150-Ohm, 250-Ohm, 300-Ohm, or even 600-ohm headphone for decent damping.
So in the old days there would not be any mass market/consumer demand for low impedance headphones.
And Professional audio had no need for low impedance headphones.
Also in the really olden days, before solid state, amplifiers used tubes, no demand for low impedance headphones there.
 
With the old Walkman radios and portable cassette tape player, I doubt "quality" low impedance headphones would have made much difference at improving their audio quality anyway.
And any high quality low impedance headphone that were around I'm sure would have cost a lot more then most people would have been will to spend
 
Now when companies like Apple started come out with better quality portable audio and people were willing to pay hundreds of dollars for those devices.
I think people were will to start paying more cash for better quality low impedance headphones. so the demand for quality low impedance headphones has been growing.
As the first portable audio device to store songs in a (a very small amounts of) solid state memory, stored audio files (mp3s) had to be very small and I'm sure were of limited audio quality.
Using high quality headphone would have been more likey to show the imperfections in these recording. but now that you can store FLAC audio files, there is more market demand for quality headphones
 
Also another factor is the weight of the diaphragm in the headphone cups, you want the (metal) diaphragm to be as stiff and light as possible, so in the older days (before portable audio) you had access to lots of voltage to overcome the diaphragm weight.
With portable (battery powered) audio, you need to use power sparingly, so the less power you can use for driving headphones, the longer the battery power is going to last.
So if you lower the weight of the diaphragm, the less power you need to drive it. but in order to keep the stiffness of the lighter diaphragm, you have to use more expensive maternal, driving up the cost of the headphone.
Now a days they are starting to make the diaphragms out of non-metal (like Carbon nano-tubes), which I guess can be very light and still be very stiff. So as time goes on, I would think high impedance headphones, are gong to be in less and less demand, stuck in some high end nitch market.

 

[davidsh]

High impedance headphones is better when you have a high impedance amp, or maybe a tube amp (as a rule of thumb)
Low impedance 'phones is easier to drive and sound just as good as high impedance 'phones when you have a low impedance amp, often solid state.
 
Tube amps are often good at putting out high voltage, while solid state is good at delivering current.
 
For portable audio devices you might want a high impedance and very good efficiency of the headphone. Portable devices are neither good at delivering voltage nor current and they typically have high output impedance.

 

[obobskivich]

I have read a couple threads concerning the advantage(s) (if there are any) of higher impedance headphones. I am confused to if there really even is an answer. I hear that a higher impedance/load can lead to better damping factor, lower distortion (mostly with tube amps due to lower current needed), and a better frequency response. But on the other hand I hear that lower impedance headphones are easier to drive and don't rely so much on a stronger power supply.

I originally encountered this question when looking at the difference between a dedicated desktop amp and my stereo receiver. Given that the Q701's have an impedance of 62 ohms, I felt that a lower supply output impedance would work best with my headphones (hence a desktop amp vs. my Sony receiver). But at the same time, wouldn't a desktop amp not supply enough power to drive the headphones ? And when I constantly hear that the Q701's are typically hard to drive, wouldn't a stronger power supply be most ideal for listening with these cans (making a stereo receiver necessary)?

I guess what it really boils down to is why one would buy higher impedance headphones (Say Beyerdynamic 600-ohm series or anything in the hundereds of ohm resistance) vs. something lower like the Q701's or even lower in the 32-ohm range. And if someone did end up buying say the Q701's, why would you buy a desktop amp over a stereo receiver (which can also be used to power not just your headphones but a whole stereo system as well)? I feel like I have something backwards here or that I am missing one really important fact/point, but it just isn't coming to me.

Any help would be most appreciated!

First, separate Beyerdynamic from the rest of the world in asking this question (because afaik Beyer is the only manufacturer that offers the same headphone in multiple impedances today) - Beyer does a great job of confusing everyone else (hah!). This doesn't mean they're "wrong" or "bad" - they just need to be treated somewhat differently (it's not like you can call up Sennheiser and order a 32, 250, or 600 ohm version of the HD 800 for example).

As far as higher Z vs lower Z - LinkPro is right; it just relates to demands placed on the amplifier. DF is unrelated here (I'll get to that). The primary argued advantages I've seen consistently stated for higher Z is noise rejection and compliance with a (basically) deprecated IEC standard (which specified 120R as a target goal for Zsource on headphone drivers).

High Z by itself doesn't really influence anything but demands on the amplifier, except when we come back to Beyer (where you're talking variations on the same driver and overall design) - where there are mixed opinions on improvements.

Regarding the 701 specifically, there's a lot of myths about their power requirements - the reality is that they're moderately insensitive compared to many dynamic headphones, but that any conventional headphone driver (receiver, headphone amp, etc) will provide substantially more power than they absolutely need. But there is no quality/judgment that can be made with the "701 vs 600 ohm Beyers" based on impedance (which is why I said we have to treat Beyer separately) - there's so many other things changing there as well. If we were talking 250R Beyer vs 600R Beyer that's another story.

High impedance requires more voltage, low impedance requires more current. 

It also depends on the sound pressure level (dB/mW or dB/V) of the paricular headphone. The higher the SPL and lower the impedance, the more efficient the driver is. That's why the K701 is hard to drive, its SPL is pretty abysmal for a dynamic driver, can't remember the exact figure though.

German brands tend to prefer high impedance design for their top cans, and in the same line-up, the model with the higher impedance generally sounds better (Beyerdynamic).

SPL can also be called sensitivity in some literature, and it should be noted that AKG always publishes their sensitivity/SPL values in dB/V (which "inflate" them - it's not wrong though, it just has to be kept in mind when comparing to other manufacturer's specs), not in dB/mW. The K701 require around 1.5 mW per channel to produce 90 dB output (based on Tyll's measurements) - it's been a while since I did the plug'n'chug, but they're around 92-93 dB/mW (AKG states something like 101 dB/V). Most dynamic headphones tend to be closer to 98-100 dB/mW, if not higher. Remember that a lot of desktop amps or receivers will usually provide at least 100 mW/ch (which is more than enough for the K701).

Sacrilage!

lol! (I read this in an booming, epic voice)

The dampening factor is a major one, as I just found out, because impedance changes by frequency, it can also effect the frequency response curve. 600ohm 880's have +4db more bass than the 250's at most frequencies for instance.

Yes and no. Impedance is complex resistance in an AC system, and CAN vary with frequency, but does not *always* (the Beyers DO, but not all headphones are that reactive; some are pretty stable and their impedance plots out like a map of Kansas). Generally greater than +/- 2 dB of difference will be audible (this is based on things written by Roger Russell and Floyd Toole), so if the change in FR is 1 dB or .5 dB or similar, it's probably not going to be something one would notice, but if it's +/- 4 dB as you're noting, that's going to be audible.

As far as how this comes into play, it's the relationship of the impedance of the driver and the amplifier's output impedance - higher output impedance will interact with reactive headphones to produce changes to the FR, stable headphones and/or low Zout generally won't see this occur. As far as where it influences the FR, it matters where the impedance has peaks or valleys or what the trend is like - with dynamic headphones it tends to be bass related, but with IEMs it can occur in the treble. GE has a great article about this (with IEMs):
http://en.goldenears.net/index.php?mid=KB_Columns&document_srl=1389

There's also a section on Roger Rusell's website that gets into this, but that site is hard to link directly (it uses frames and is composed as one huge wall of text) - if you're familiar with his page, go to the section on "wire impedance" for some examples with loudspeakers (this gets a lot more interesting with loudspeakers that employ passive crossovers, because you usually have multiple peaks and valleys in the impedance, which is why low Zout on speaker amplifiers is such a big deal (and/or why active/multi-way systems are becoming so popular)).

Regarding dampening factor and accuracy, I think this pic shows the difference.





600's don't go as much over or under, and it responds to the signal a bit quicker. Unfortunately since the bottom graph doesn't show the scale (presuming it's ms) it's difficult to say by how much.

That graph doesn't show what you think it does - that's a square wave response measurement (not impulse response), which lets us view (acoustic) phase information about the driver (because nothing is DC-to-light, you won't ever have perfect phase; square wave lets us look at the relationship between phase and frequency - a perfect square-wave would mean DC-to-light response, it could still ring though (which is what damping is meaning to correct)). Tyll has an article on InnerFidelity that goes into this in more depth - that plot can be generated from an FR plot, or vice versa (which was actually demonstrated a while ago in the Sound Science section with IIR filters too).

Damping factor as a "thing" however is a marketing construction - it's an easier way for an advertiser to put Zout on a specsheet (and it's not even done "right" these days - it needs a frequency and load bound (e.g. DF: 240 @ 1khz 8R)). Numerous data exists that show electrical damping by and large is irrelevant to a system's overall Q - both because overall system DF will always be a relatively low value (again, the bounding concerns in action), and because the amplifier has relatively little "control" to exert (if you want to try an experiment, take a woofer and short it (no amp involved) - it shouldn't be very mobile according to DF marketing (because it's seeing nearly 0R); or try it hooked up to a "high DF" amplifier, powered on, with no signal - again, the amplifier shouldn't allow it to move).

There's a few good articles on this, like this one:
http://www.butleraudio.com/damping1.php (note the author, and the table about half-way down the page; these values will be different with your headphones but the same concept applies)

A high load from the headphone will make the spec look better, but it doesn't actually change the amplifier's Zout, and given the DT 880's fairly reactive nature, they'll still respond to high Zout amplifiers just as the lower impedance variations do. InnerFidelity has measurements of all three, with impedance plots:
http://www.innerfidelity.com/images/BeyerdynamicDT880600ohm.pdf
http://www.innerfidelity.com/images/BeyerdynamicDT880250ohm.pdf
http://www.innerfidelity.com/images/BeyerdynamicDT88032ohm.pdf

To me (and I have not personally compared all three), the 33R model's impulse response looks noisier, and THD figures look best (especially power handling) for the 600R model. Driver matching looks better on the 250R and 600R models, and the slight changes in FR are what account for the changes in SR response.


EDIT:
Found Tyll's post on "Interpreting Square Waves":
http://www.innerfidelity.com/content/headphone-measurements-explained-square-wave-response

I have a longer document on it that I have locally, I think it's out of a book but I don't remember where I got it - no idea if it can be uploaded, but I'd certainly share it with you if there was a way (it's like 60 pages long iirc though, so I'm not pasting the entire thing in here h34r; it gets into phase in more depth though.

High impedance headphones is better when you have a high impedance amp, or maybe a tube amp (as a rule of thumb)
Low impedance 'phones is easier to drive and sound just as good as high impedance 'phones when you have a low impedance amp, often solid state.

Tube amps are often good at putting out high voltage, while solid state is good at delivering current.

For portable audio devices you might want a high impedance and very good efficiency of the headphone. Portable devices are neither good at delivering voltage nor current and they typically have high output impedance.

I agree except for the last part - which is sort of backwards. Portable devices (I'm assuming you mean like an iPod) tend to have fairly low output impedances (GE has a table of values as an example), and low voltage delivery (limited by their batteries), so they tend to be a good match for low impedance, high sensitivity headphones (an example would be the Beats Pro). High Z tends to be a bad match with portable players.

 

[J Bones]

Quote:

High impedance headphones is better when you have a high impedance amp, or maybe a tube amp (as a rule of thumb)
Low impedance 'phones is easier to drive and sound just as good as high impedance 'phones when you have a low impedance amp, often solid state.
 
Tube amps are often good at putting out high voltage, while solid state is good at delivering current.
 
For portable audio devices you might want a high impedance and very good efficiency of the headphone. Portable devices are neither good at delivering voltage nor current and they typically have high output impedance.

 
     Why is it again that a tube amp is better designed for a set of cans with much higher resistance than another pair of cans? I really don't think the Q701's (which by the way, if I didn't mention it earlier, are the set I own) are all that resistant...but either way, I've been looking into something other than my Digizoid, and the Schiit Lyr has thus caught my attention. Now, I actually decided on it well before I started hearing that a tube amp sometimes works best with a pair of cans with a higher impedance but never why. Would this for some reason not work with the Q701's? The way I originally looked at it is that the output impedence of the Lyr is much lower than some others and would still work with the Q701 no matter what (not concerning other factors like THD or power output etc).
     I'm assuming this is also why you mention that lower impedance headphones are typically driven better by low impedance amps (often those that are solid state too)...
 
Quote:

As far as how this comes into play, it's the relationship of the impedance of the driver and the amplifier's output impedance - higher output impedance will interact with reactive headphones to produce changes to the FR, stable headphones and/or low Zout generally won't see this occur. As far as where it influences the FR, it matters where the impedance has peaks or valleys or what the trend is like - with dynamic headphones it tends to be bass related, but with IEMs it can occur in the treble. GE has a great article about this (with IEMs):
http://en.goldenears.net/index.php?mid=KB_Columns&document_srl=1389
 

I'm assuming you are saying that the Q701's aren't that reactive and thus an amp with a higher output impedance wouldn't have so much of an effect on the FR of the Q701 versus a pair of cans that are more reactive (say your example, the Beyers)?

 

That graph doesn't show what you think it does - that's a square wave response measurement (not impulse response), which lets us view (acoustic) phase information about the driver (because nothing is DC-to-light, you won't ever have perfect phase; square wave lets us look at the relationship between phase and frequency - a perfect square-wave would mean DC-to-light response, it could still ring though (which is what damping is meaning to correct)). Tyll has an article on InnerFidelity that goes into this in more depth - that plot can be generated from an FR plot, or vice versa (which was actually demonstrated a while ago in the Sound Science section with IIR filters too).

Damping factor as a "thing" however is a marketing construction - it's an easier way for an advertiser to put Zout on a specsheet (and it's not even done "right" these days - it needs a frequency and load bound (e.g. DF: 240 @ 1khz 8R)). Numerous data exists that show electrical damping by and large is irrelevant to a system's overall Q - both because overall system DF will always be a relatively low value (again, the bounding concerns in action), and because the amplifier has relatively little "control" to exert (if you want to try an experiment, take a woofer and short it (no amp involved) - it shouldn't be very mobile according to DF marketing (because it's seeing nearly 0R); or try it hooked up to a "high DF" amplifier, powered on, with no signal - again, the amplifier shouldn't allow it to move).

 

I still don't see your point though with the 'control' the amp has over dampening factor...are you trying to prove the negligible effect of higher/lower impedance on DF?

 

I agree except for the last part - which is sort of backwards. Portable devices (I'm assuming you mean like an iPod) tend to have fairly low output impedances (GE has a table of values as an example), and low voltage delivery (limited by their batteries), so they tend to be a good match for low impedance, high sensitivity headphones (an example would be the Beats Pro). High Z tends to be a bad match with portable players.
 

For the last part, does this then sort of show a demand for higher voltage (non-portable and most likely on your desktop) style amps for higher impedance cans?

 

 

[whoelse]

We all understand the efficiency part, is it correct:
 
Low grain amps are potentially less sibilant and likewise, higher impedance headphones/speakers are potentially less sibilant?
This was what i always thought, am I wrong?

 

[whoelse]

According to Beyerdynamic's FAQ:
 
"If you take a look at the offerings of dynamic headphones, you will find a very wide range of impedances. The spectrum ranges from 16 ohm headphones to 600 ohm headphones. Where does this wide range come from? And which headphones are suitable for what applications?
In order to get to the bottom of these questions, a power evaluation method must first be established. The basic task of the headphone is to convert the arriving electrical signal into sound pressure. The extent that this succeeds is described by the nominal sound pressure level of the headphones. This value (specified in the unit dB SPL) describes how high the generated sound pressure is when 1 mW of electrical power is supplied. If you take a look at the nominal sound pressure level of similar headphones with differing impedances, you will find that the value is roughly the same. That means that you need to consider the electrical power converted in the headphones in order to make a statement about the sound pressure attained. 

For the following considerations, the impedance of the headphones is assumed to be real (described as resistance with the formula symbol R). This is not completely correct, but is sufficient for our purposes here.

You can calculate the electrical power converted in a resistor from the applied voltage:

P = V²/R

As an alternative, you can also use the current:

P = I² x R

Since we are talking about AC voltage and AC current, the RMS value must be used in both cases.

Low impedance (up to 100 ohms): connection to mobile devices (MP3 players, laptops, etc.). 

Medium impedance (between 100 and 300 ohms): connection to stationary installations (HiFi amplifiers etc.).

High impedance (over 300 ohms): connection to high-quality headphone amplifiers. Higher impedance = better sound = higher power requirements."

 

[tomb]

Quote:

 
     Why is it again that a tube amp is better designed for a set of cans with much higher resistance than another pair of cans? I really don't think the Q701's (which by the way, if I didn't mention it earlier, are the set I own) are all that resistant...but either way, I've been looking into something other than my Digizoid, and the Schiit Lyr has thus caught my attention. Now, I actually decided on it well before I started hearing that a tube amp sometimes works best with a pair of cans with a higher impedance but never why. Would this for some reason not work with the Q701's? The way I originally looked at it is that the output impedence of the Lyr is much lower than some others and would still work with the Q701 no matter what (not concerning other factors like THD or power output etc).
     I'm assuming this is also why you mention that lower impedance headphones are typically driven better by low impedance amps (often those that are solid state too)...
 

There are a couple of reasons tube amps are better for high-impedance phones.  One is Ohm's Law, or rather, a variation of it.  Higher-impedance loads require more voltage for the same amount of power than low-impedance ones.  Without digressing into a lot of Ohm's Law, algebra, and an explanation of the relationships, let's consider some real world examples.
 
Let's say for a given efficiency headphone, you want to be able to produce music peaks or transients that take 100mW of power.  BTW, everything is reduced to "power."  It matters little to try and figure out how much current an amp can supply or how much voltage and then relate that to the headphone separately.  The fact is that the headphone needs power, period.  Its efficiency determines how much and its impedance what combination of voltage and current it will pull.
 
At 100mW and with 32 ohm headphones, you need 0.056 Amps.  Using a variation of Ohm's Law, we can find out that at the same time and with 32 ohms impedance, the voltage for a music peak to produce that 100mW excursion is 1.79 V.  That's too much for a single AA battery, but two AA batteries can handle it without an issue (2 x 1.5V = 3V).  So, a little portable amp with a couple of AA batteries could handle things just fine.  (There are penalties in many circuits - minimum voltage for opamps, etc. that will burn some of the available voltage, but this is close enough for an example.)  This is why you see so many headphones touted for portable devices by stating that they're low impedance.  Some are down to 25 and 18 ohms these days, not just 32.
 
On the other hand - assuming we have a 300 ohm headphone that is as efficient as a 32 ohm headphone, you only need 0.018 Amps.  However, the voltage required for that power goes up to 5.4V.  That's more voltage than can be produced by a couple of batteries - it takes 4, at least.  Or worse, it's more voltage than is available from a PC's USB power supply.  It's one reason that the CMoy was always built with a 9V battery, not a AA, AAA, or multiples of those batteries - it wasn't just for low weight.
 
None of this is absolute, averaged, or RMS power - it's musical peak power (in our example).  This means that if the voltage supply in the amp is unable to produce the actual voltage difference, it will clip, period - and added distortion most likely occurs well before that.  Things get much worse as you go up in power - for 300 ohms impedance, 7.8V are needed for 200mW, 9.5V for 300mW, etc.  Meanwhile, only 5.7V are needed for 32 ohms at a full watt (17V are needed for 300 ohms).  Remember, for every 3dB of sound level increase, a doubling of power is required.  That may sound like a lot of dB's when looking at typical headphone efficiencies, but these are transient peaks that we want to retain, not overall listening volume.
 
Anyway, the high voltage required for most tubes means that all that extra voltage swing is readily available for high-impedance phones.  It's also a reason you see so many tubes for electrostatics - it's much easier to produce all that high voltage that's needed (many solid-state parts are well-beyond their safety ratings).
 
The second reason tubes may be better for high-impedance (and by corollary, worse for low-impedance phones) is the issue of blocking caps.  In tube amps, transformers are often quite expensive (and very heavy).  Therefore, you often see tube amps at the low end (and some high-end) with some variation of OTL (Output-Transformer-Less) or hybrid design.  These amps have what's called a DC offset at the output.  For some OTL tube amps, this voltage may be as much as what's seen at the tubes' plates - a couple of hundred volts or more.  Your headphones would be fried instantly if you plugged them in without protection.  So, capacitors are used on the amplifer output to block that DC voltage.  BTW, if insufficient time is allowed for these caps to charge on power-up: THUMP!  It's why relay-delays are often used.  The same offset issues and blocking cap requirements can be true of many solid-state amps that don't use feedback or have some other discrete buffer output that develops offset.
 
Well, the use of blocking capacitors at the output of an amp sets up an RC circuit along with the impedance of the headphone.  An RC circuit forms what is otherwise known as a high-bandpass circuit.  The frequency output is dependent on the circuit characteristics - low frequencies will be blocked.
 
Figuring out which low frequencies will be blocked usually involves determining the cutoff frequency of the RC circuit that results.  This tells you where the frequency drops by 3dB given a certain output capacitor value and headphone impedance.  Common large capacitor sizes might be 220uf, 470uf, 680uf, or 1000uf.  For 220uf and a 300ohm headphone, the cutoff frequency is 2.4Hz - no problem.  However, for a 32 ohm headphone, the cutoff frequency with 220uf is 22Hz - now we're getting into audibility, because phase distortions are introduced at frequencies much higher than that.  Plus, this is the -3dB point - the bass actually begins to fall off between 50 and 100 Hz - maybe not so bad for a pair of headphones, but horrible for an amplifier.
 
So what does this mean, in practical terms, for a tube amp with low impedance vs. high impedance phones?  Well, at the higher voltages for many tube amps, only electrolytic caps are available in these sizes and even then - totally unavailable or practical at the high capacitor uf ratings.  This gets momumentally worse for low impedance cans than for high ones.  So, expect to lose bass with OTL tube amps and low-impedance phones.  It depends on the actual circuits with tube hybrids and whether or not feedback is introduced, but many may have the same issue.  As for feedback, many people think feedback sounds bad, so it could have diminishing returns if used for reducing offset (typically in solid-state circuits).
 
So, bottom line: tube amps produce much more voltage (much easier to produce power at high impedances) and OTL tube amps will have DC blocking caps that may cut the bass at lower headphone impedances.  Both facts combine to make tube amps often more suitable for high-impedance phones.  There are exceptions, to be sure.  Many hybrids perform outstandingly well with low impedance phones and of course, a transformer-coupled tube amp is going to be optimized for a range of impedances (why many of them have high and low-Z switches).
 
 
EDIT: P.S. I see the posting of Beyer's marketing explanation for low-impedance vs. high-impedance headphones.  They make great headphones, but there's no consistency between quality and impedance in headphones and amplifiers.  One does not imply a higher quality than another - although with Beyer, it seems to work out that way.

 
BTW, Beyer is of course correct when they state real power with AC (the music signal) must be measured in RMS.  However, I wasn't trying to digress that far.  Rather, I was trying to convey that the DC voltage in an amp (short of buck-boost circuits which add distortion) can't be created out of nothing.  If the peak of the amplified signal wave wants to hit + and - 5V, the power supply at those points in the circuit have to have +and- 5V, period.  If the power supply doesn't supply that voltage, it can't hit that peak, no matter what.

 

[obobskivich]

     Why is it again that a tube amp is better designed for a set of cans with much higher resistance than another pair of cans? I really don't think the Q701's (which by the way, if I didn't mention it earlier, are the set I own) are all that resistant...but either way, I've been looking into something other than my Digizoid, and the Schiit Lyr has thus caught my attention. Now, I actually decided on it well before I started hearing that a tube amp sometimes works best with a pair of cans with a higher impedance but never why. Would this for some reason not work with the Q701's? The way I originally looked at it is that the output impedence of the Lyr is much lower than some others and would still work with the Q701 no matter what (not concerning other factors like THD or power output etc).
     I'm assuming this is also why you mention that lower impedance headphones are typically driven better by low impedance amps (often those that are solid state too)...

Tube amps, if we're talking OTLs or something, generally have a higher output impedance (which knocks a lot of low impedance headphones off the table for even being safe to connect - unless you're dealing with a very robust OTL amp (some of the Moth designs, for example (these can run into the multiple thousands of dollars)), and usually don't have a problem spitting out the voltage that a high Z headphone wants. Of course if transformers are involved, or we're dealing with a hybrid, that entire scenario changes, and there are certainly people who run lower impedance headphones off of their OTLs and like it just fine, but there is a point where it becomes a safety/compatibility concern (some manufacturers will flat out state that their amplifier is not stable into a given "low Z" load, and this exists with SS amps too (for example Creek has flat-out said the OBH-11 and OBH-14 will not remain stable into <16 ohms and that it's "do so at your own risk")) - just something to keep in mind.

The K701 (or whichever version of the 701 you have) are 62R, which is in that weird "middle impedance" area - it isn't down there with low Z headphones like you'd find from Grado, Audio-Technica, Denon, and some Sony and Ultrasone models, but it isn't up there at 300R like the "typical German" headphones (e.g. the HD 600). They aren't a nightmare for an amplifier as long as it can provide them with that ~2 mW/ch that they want (they're insensitive too) - the Lyr probably will do fine with them given that it's a hybrid (so it shouldn't be at any risk hooked up to them); but you could certainly email Schiit and ask if you want to be triple sure (I know that Jason has said their OTL is a no-go with 16-30R cans, and usually suggests the Asgard there, but afaik Lyr won't have a problem either).

The way you originally looked at it is correct (again, my understanding is that the Lyr won't have a problem with a 62R load and the 701 are insensitive enough that you should have usable range on the volume dial).

I'm assuming you are saying that the Q701's aren't that reactive and thus an amp with a higher output impedance wouldn't have so much of an effect on the FR of the Q701 versus a pair of cans that are more reactive (say your example, the Beyers)?

Yep. Basically as long as the amplifier is stable and can provide the power they need without going into clipping, it should be good to go. Any coloration you want will have to come upstream - e.g. tubes - because it would take very dramatic changes to Zout to get their FR to start moving around in a noticeable manner (I've had them on upto 1k and you will hear that, but it still isn't very dramatic - they still sound pretty much like 701s, they don't get "flabby" or "mushy" down low like a lot of marketing would insist).

I still don't see your point though with the 'control' the amp has over dampening factor...are you trying to prove the negligible effect of higher/lower impedance on DF?

DF is load impedance over output impedance (and therefore is frequency and load dependent); it's dictated by the amplifier's Zsource and the load. Increasing the impedance of the load will give you even higher DF numbers, but none of that means anything - because DF is a nonsense specification (it's seriously just there to make consumers feel good). And like I said, modern amp specs don't even post their BS spec correctly - they just arbitrarily say "this amp has a DF of 300" - well that doesn't work (it has to be stated more like "DF = 300 @ 1khz with an 8R load"). Check the links for more information on *why* DF is nonsense, or basically just ignore the entire thing as marketing shenanigans.

For the last part, does this then sort of show a demand for higher voltage (non-portable and most likely on your desktop) style amps for higher impedance cans?

Yep. Most desktop amps have no problem driving higher Z cans (say, >100R) - honestly you don't have to worry about voltage or current swing, because you can black-box this entire discussion and just look at power specs. For example if your amp says it provides 1500mW into 32R, and 80mW into 600R, and you have headphones that are say, 300R Znom, and 97 dB/mW (these are all roughly real numbers based on real products btw) - you should have no problems whatsoever. But if your amplifier says it provides 15 mW into 600R and you have headphones that require 5000 mW at 600R Znom (these are not reflective of any product I'm aware of) - you should probably look at another amp or another pair of cans. By and large desktop amplifiers (if they're well done), receivers, etc ("amps") won't have issues with the bulk majority of headphones, which is why manufacturers like CI Audio and Musical Fidelity can get away claiming their amps drive "everything" - not something they can do with speaker amps. Portable devices you gotta watch (because they'll have limited voltage swing) if you want to get into higher Z cans, but generally portable cans (lets use the Beats Pro as an example) are sensitive and low impedance, because the powers that be understand the limitations of an iPod or Walkman or whatever else.

So the short and sweet is: DF is nonsense (so ignore it), and impedance is really only a compatibility concern *unless* you're talking about specific Beyerdynamic headphones, in which case find a Beyer fan to tell you what the differences are between the various models. With something like the 701 you really shouldn't have too many problems with what you can and can't plug it into, in the world of desktop amps, but as always - contact the manufacturer if you want a definite "yes" or "no" in terms of stability (and I'd only worry about this with OTLs and extremely high gain parts - the Lyr fits into the later).

We all understand the efficiency part, is it correct:

Low grain amps are potentially less sibilant and likewise, higher impedance headphones/speakers are potentially less sibilant?
This was what i always thought, am I wrong?

I think that's probably over-general. There's plenty of high impedance headphones that'll torch your ears off, and plenty of VERY low impedance headphones that are smooth as butter.

 

[J Bones]

Quote:

There are a couple of reasons tube amps are better for high-impedance phones.  One is Ohm's Law, or rather, a variation of it.  Higher-impedance loads require more voltage for the same amount of power than low-impedance ones.  Without digressing into a lot of Ohm's Law, algebra, and an explanation of the relationships, let's consider some real world examples.
 
Let's say for a given efficiency headphone, you want to be able to produce music peaks or transients that take 100mW of power.  BTW, everything is reduced to "power."  It matters little to try and figure out how much current an amp can supply or how much voltage and then relate that to the headphone separately.  The fact is that the headphone needs power, period.  Its efficiency determines how much and its impedance what combination of voltage and current it will pull.
 
 
 

Would that then be more of a reason to buy a tube amp for a pair of cans with a lower SPL, like the Q701s (which are like 92-93db/mW I believe), regardless of the resistance of the cans?

 
 
 
At 100mW and with 32 ohm headphones, you need 0.056 Amps.  Using a variation of Ohm's Law, we can find out that at the same time and with 32 ohms impedance, the voltage for a music peak to produce that 100mW excursion is 1.79 V.  That's too much for a single AA battery, but two AA batteries can handle it without an issue (2 x 1.5V = 3V).  So, a little portable amp with a couple of AA batteries could handle things just fine.  (There are penalties in many circuits - minimum voltage for opamps, etc. that will burn some of the available voltage, but this is close enough for an example.)  This is why you see so many headphones touted for portable devices by stating that they're low impedance.  Some are down to 25 and 18 ohms these days, not just 32.
 
 

For the Q701's, they would then require 2.493V to produce the same 100mW peak, which is still manageable with two AA batteries. Would this suffice then to say that most portable headphone amps will compare in quality/supply with other desktop amps since anything above the AA batteries power would be unnecessary and lost?

 
 
On the other hand - assuming we have a 300 ohm headphone that is as efficient as a 32 ohm headphone, you only need 0.018 Amps.  However, the voltage required for that power goes up to 5.4V.  That's more voltage than can be produced by a couple of batteries - it takes 4, at least.  Or worse, it's more voltage than is available from a PC's USB power supply.  It's one reason that the CMoy was always built with a 9V battery, not a AA, AAA, or multiples of those batteries - it wasn't just for low weight.
 
None of this is absolute, averaged, or RMS power - it's musical peak power (in our example).  This means that if the voltage supply in the amp is unable to produce the actual voltage difference, it will clip, period - and added distortion most likely occurs well before that.  Things get much worse as you go up in power - for 300 ohms impedance, 7.8V are needed for 200mW, 9.5V for 300mW, etc.  Meanwhile, only 5.7V are needed for 32 ohms at a full watt (17V are needed for 300 ohms).  Remember, for every 3dB of sound level increase, a doubling of power is required.  That may sound like a lot of dB's when looking at typical headphone efficiencies, but these are transient peaks that we want to retain, not overall listening volume.
 
 

Would this though be a worry for headphones who require power just less than that of 2 AA batteries? Since having a more powerful source like a desktop amp could guarantee you that the music doesn't clip this I feel is the flip side of using a desktop amp versus portable (clipping vs voltage required to reach desired power peaks?)

 
 
 
Anyway, the high voltage required for most tubes means that all that extra voltage swing is readily available for high-impedance phones.  It's also a reason you see so many tubes for electrostatics - it's much easier to produce all that high voltage that's needed (many solid-state parts are well-beyond their safety ratings).
 
The second reason tubes may be better for high-impedance (and by corollary, worse for low-impedance phones) is the issue of blocking caps.  In tube amps, transformers are often quite expensive (and very heavy).  Therefore, you often see tube amps at the low end (and some high-end) with some variation of OTL (Output-Transformer-Less) or hybrid design.  These amps have what's called a DC offset at the output.  For some OTL tube amps, this voltage may be as much as what's seen at the tubes' plates - a couple of hundred volts or more.  Your headphones would be fried instantly if you plugged them in without protection.  So, capacitors are used on the amplifer output to block that DC voltage.  BTW, if insufficient time is allowed for these caps to charge on power-up: THUMP!  It's why relay-delays are often used.  The same offset issues and blocking cap requirements can be true of many solid-state amps that don't use feedback or have some other discrete buffer output that develops offset.
 
Well, the use of blocking capacitors at the output of an amp sets up an RC circuit along with the impedance of the headphone.  An RC circuit forms what is otherwise known as a high-bandpass circuit.  The frequency output is dependent on the circuit characteristics - low frequencies will be blocked.
 
Figuring out which low frequencies will be blocked usually involves determining the cutoff frequency of the RC circuit that results.  This tells you where the frequency drops by 3dB given a certain output capacitor value and headphone impedance.  Common large capacitor sizes might be 220uf, 470uf, 680uf, or 1000uf.  For 220uf and a 300ohm headphone, the cutoff frequency is 2.4Hz - no problem.  However, for a 32 ohm headphone, the cutoff frequency with 220uf is 22Hz - now we're getting into audibility, because phase distortions are introduced at frequencies much higher than that.  Plus, this is the -3dB point - the bass actually begins to fall off between 50 and 100 Hz - maybe not so bad for a pair of headphones, but horrible for an amplifier.
 
 

I'm not exactly sure how you got to the conclusion that low frequencies will be blocked, but I understand that OTL tube amps with blocking capacitors will block low frequencies (the bass I'm assuming). How would this work though a Lyr? I can't figure out how to know whether or not the hybrid amp has blocking capacitors and of what size (to know whether amping the Q701's with the Lyr would restrict the bass level frequencies).

 
 
 
So what does this mean, in practical terms, for a tube amp with low impedance vs. high impedance phones?  Well, at the higher voltages for many tube amps, only electrolytic caps are available in these sizes and even then - totally unavailable or practical at the high capacitor uf ratings.  This gets momumentally worse for low impedance cans than for high ones.  So, expect to lose bass with OTL tube amps and low-impedance phones.  It depends on the actual circuits with tube hybrids and whether or not feedback is introduced, but many may have the same issue.  As for feedback, many people think feedback sounds bad, so it could have diminishing returns if used for reducing offset (typically in solid-state circuits).
 
So, bottom line: tube amps produce much more voltage (much easier to produce power at high impedances) and OTL tube amps will have DC blocking caps that may cut the bass at lower headphone impedances.  Both facts combine to make tube amps often more suitable for high-impedance phones.  There are exceptions, to be sure.  Many hybrids perform outstandingly well with low impedance phones and of course, a transformer-coupled tube amp is going to be optimized for a range of impedances (why many of them have high and low-Z switches).
 
 

This may be why then I still do hear that the hybrid amp such as the Lyr is typically not a problem with the whole K/Q7XX line...and is is actually of benefit?

 
 
 
EDIT: P.S. I see the posting of Beyer's marketing explanation for low-impedance vs. high-impedance headphones.  They make great headphones, but there's no consistency between quality and impedance in headphones and amplifiers.  One does not imply a higher quality than another - although with Beyer, it seems to work out that way.

 
BTW, Beyer is of course correct when they state real power with AC (the music signal) must be measured in RMS.  However, I wasn't trying to digress that far.  Rather, I was trying to convey that the DC voltage in an amp (short of buck-boost circuits which add distortion) can't be created out of nothing.  If the peak of the amplified signal wave wants to hit + and - 5V, the power supply at those points in the circuit have to have +and- 5V, period.  If the power supply doesn't supply that voltage, it can't hit that peak, no matter what.

 
Quote:

Tube amps, if we're talking OTLs or something, generally have a higher output impedance (which knocks a lot of low impedance headphones off the table for even being safe to connect - unless you're dealing with a very robust OTL amp (some of the Moth designs, for example (these can run into the multiple thousands of dollars)), and usually don't have a problem spitting out the voltage that a high Z headphone wants. Of course if transformers are involved, or we're dealing with a hybrid, that entire scenario changes, and there are certainly people who run lower impedance headphones off of their OTLs and like it just fine, but there is a point where it becomes a safety/compatibility concern (some manufacturers will flat out state that their amplifier is not stable into a given "low Z" load, and this exists with SS amps too (for example Creek has flat-out said the OBH-11 and OBH-14 will not remain stable into <16 ohms and that it's "do so at your own risk")) - just something to keep in mind.

 

This may be a question for Jason to answer then too, but how does the Lyr then produce such a low output impedance given it's (probably expensive) transformer it needs?

 

The K701 (or whichever version of the 701 you have) are 62R, which is in that weird "middle impedance" area - it isn't down there with low Z headphones like you'd find from Grado, Audio-Technica, Denon, and some Sony and Ultrasone models, but it isn't up there at 300R like the "typical German" headphones (e.g. the HD 600). They aren't a nightmare for an amplifier as long as it can provide them with that ~2 mW/ch that they want (they're insensitive too) - the Lyr probably will do fine with them given that it's a hybrid (so it shouldn't be at any risk hooked up to them); but you could certainly email Schiit and ask if you want to be triple sure (I know that Jason has said their OTL is a no-go with 16-30R cans, and usually suggests the Asgard there, but afaik Lyr won't have a problem either).

The way you originally looked at it is correct (again, my understanding is that the Lyr won't have a problem with a 62R load and the 701 are insensitive enough that you should have usable range on the volume dial).

 

This is what really has got me hung up the most, is that the resistance of the phones really is in such a grey area as to whether or not it is considered high or lower impedance. I'm thinking, and correct me if I'm wrong, but I feel that with these phones I will most likely just have to test out and see what amp works best with the Q701's since facts and figures may not always be that helpful in choosing the right answer for me given their 'middle-of-the-road' resistance...

 

 

[obobskivich]

If you don't embed your replies in the quotes, it's much easier to read.

Anyways:

This may be a question for Jason to answer then too, but how does the Lyr then produce such a low output impedance given it's (probably expensive) transformer it needs?

Jason will be able to give you a far more detailed answer (like actual part specs), but the Lyr is not an OTC/WOT - it's a hybrid. It uses tubes in its gain/buffer stages, and has solid-state output devices. No output transformer is involved or required; it's roughly similar to putting a tube buffer between your source and a fully SS amplifier (but it's contained within the amp, and likely better matched).

An example of an OTC/WOT amp would be the Woo WA6, and the transformer does put the price up (my understanding is the WA6 is just the WA3 with an OT). I'm not sure what Zout actually is on that amplifier, but with the transformer involved, it's suitable down to 16R (and Jack has confirmed this). Output impedance doesn't absolutely dictate what you can plug into though (for example many receivers and full-size power amps will have an output impedance of ~400-600R on the headphone jack, as a result of series resistors between the power amp and the jack (to protect both sides), and they will get along fine with more or less anything (but you might get some hiss (from some amps) with extremely sensitive and low impedance headphones that also offer some isolation (like IEMs or cans like the ESW9)).

This is what really has got me hung up the most, is that the resistance of the phones really is in such a grey area as to whether or not it is considered high or lower impedance. I'm thinking, and correct me if I'm wrong, but I feel that with these phones I will most likely just have to test out and see what amp works best with the Q701's since facts and figures may not always be that helpful in choosing the right answer for me given their 'middle-of-the-road' resistance...

Yeah, by and large trial and error is probably your best bet (especially if you subscribe to "all amps are inherently different sounding" - no harm if you do, it's just that if you do, all the charts and graphs in the world won't tell you what you want to know). Personally I found the 701 to be fairly unpicky, as long as the amplifier in question could provide enough power (so basically any dedicated headphone amplifier, stereo receiver, etc was suitable, but portable devices, PC soundcards, etc tended to be hit and miss). With something like Lyr you should not want for power, and the same would be true if you had a good stereo receiver, or even a "lesser" desktop headphone amp (like the FiiO E9); so it comes down to other considerations, like aesthetics, ergonomics, price, etc. Aside from a select few amplifiers that might be right at the lower bound of their stable range, and amplifiers that cannot provide enough power, I don't think you're going to run into any legitimate compatibility concerns though. If you're looking at true OTL I'd say ask the manufacturer before you buy, but otherwise it's pretty much free-range (I honestly can't think of a production SS amp that isn't going to handle 62R). You'll probably have a better time of finding tubes in a hybrid (like Lyr) than in an OTL for these cans though - but in that $400-$600 range there's some OTC/WOT amps (like WA6) that you could also consider. If that's a direction you want to go. There's plenty of hybrids around that price too, and plenty of pure SS amps (and some that cost less than that range and are good performers as well). Plus various full-size components. Really you have a lot of options.

 

[tomb]

Quote:

If you don't embed your replies in the quotes, it's much easier to read.

Geez!  Amen to that! Much easier to copy/reply to, too!

  Although, in his defense, a lot of it is Head-Fi's current forum software.
Manually copied quotes follow:
 
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Would that then be more of a reason to buy a tube amp for a pair of cans with a lower SPL, like the Q701s (which are like 92-93db/mW I believe), regardless of the resistance of the cans?

No.  SPL has really nothing to do with current/voltage ratios.  It's simply a measurement of sensitivity - how much power to produce a certain sound level.  The impedance of the cans determine the ratio of voltage vs. current, and therefore, the best match with an amp.
 
Quote:

For the Q701's, they would then require 2.493V to produce the same 100mW peak, which is still manageable with two AA batteries. Would this suffice then to say that most portable headphone amps will compare in quality/supply with other desktop amps since anything above the AA batteries power would be unnecessary and lost?

No, not necessarily.  The Q701's actually have a quite low sensitivity.  If you combine that with the relatively low impedance (62 is much, much closer to 32 than 300), then the conclusion is that Q701's need current the most with current amplifier topologies.  In the case I cited of the AA batteries, the voltage threshold might be met, but the current would be sorely lacking.  You also have to consider the topology of a typical portable amp.  Battery usage is paramount.  So, things like Class A current design, where current would be readily available without switching distortion, are almost completely absent in portable devices.  This is because simply flipping the power switch would mean that they would be burning significant amounts of current.  So, no - portable is probably not a good match for K/Q-701's.
 
Quote:

Would this though be a worry for headphones who require power just less than that of 2 AA batteries? Since having a more powerful source like a desktop amp could guarantee you that the music doesn't clip this I feel is the flip side of using a desktop amp versus portable (clipping vs voltage required to reach desired power peaks?)

All I cited as an example was a voltage threshold (and how you could easily dismiss portables and solid-state devices with high-impedance headphones).  It does not follow at all that a portable amp would be able to meet the power requirements.  Voltage is only one variable in the power equation.
 
 
Quote:

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I'm not exactly sure how you got to the conclusion that low frequencies will be blocked, but I understand that OTL tube amps with blocking capacitors will block low frequencies (the bass I'm assuming). How would this work though a Lyr? I can't figure out how to know whether or not the hybrid amp has blocking capacitors and of what size (to know whether amping the Q701's with the Lyr would restrict the bass level frequencies).

 
Simple.  You need capacitors so large on the output of OTL amps for low impedance phones that they're simply impractical, not even manufactured in those sizes, even.  (All because of the RC circuit and the resulting cutoff frequency.)  Most mfrs don't even try.  I submit that's a more common reason of limiting low headphone impedance on OTL amplifiers, rather than claiming any instability in low impedance loads.  The mfrs simply don't want to be accused of having an amplifier without bass, which would happen when paired with most low impedance phones.

 
I can't figure out whether the Lyr has blocking capacitors or how big they are, either.  Just looking at it, it appears they have a solid-state output buffer whose offset is zeroed out with an opamp.  If that's the case, then they may not have any blocking capacitors at all.
 
They seem like good guys, though, why not just send them an e-mail or PM and ask them directly?
 
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This may be why then I still do hear that the hybrid amp such as the Lyr is typically not a problem with the whole K/Q7XX line...and is is actually of benefit?

 

 
This could very well be the case.
 
 
Quote:

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This may be a question for Jason to answer then too, but how does the Lyr then produce such a low output impedance given it's (probably expensive) transformer it needs?

 
Low output impedance can be achieved a couple of ways - no output capacitors and/or lots of feedback.  As for the transformer(s) (it has two in the pics), it appears to me that one is the PT (power transformer), while the other is a transformer for the heaters.  Many times, those functions are combined into one transformer.  However, to keep things simple and cheap, it might've been easier to use two transformers.  This is because the high-voltage transformation needed for the tubes are in contrast to the typically low-voltage and high-current heaters in the tubes.  It appears the Lyr is designed around the 6DJ8/6922 family of tubes.  Those tubes might have 200V on the plates, while the heaters need only 6.3V but at 300ma each.  IOW, it's two different optimizations in the way the transformers are wound and it may have worked out cheaper to have two separate ones, despite the increased cost in iron.
 
This is just my personal opinion, but I have had a pair of K701's for many years.  I think perhaps the tube hybrid is among the best for proper feeding of such headphones - although I would limit the feedback involved, if possible.  The high current and somewhat higher than normal output impedance from a typical tube hybrid tends to give the K701's ample current while not emphasizing the harshness that can occur with SS, super-low-output impedance amps.