Low output impedence headphone amps

[FSonicSmith]

Let me start out by saying that I have no hidden agenda.
I am not a regular on this Board or any other pertaining to headphones.
Three of four years ago, I briefly got into headphones. My infatuation lasted a year or so.
I had read Sam Tellig's column in S'Phile raving about the Audio Technica ATH-AD700s for less than $100 and next thing I knew, I was in for a RSA Predator (which I love) and a Little Dot Mark IV SE (which I never liked much) and then came Denon AHD2000s and AKG 701s. At work I had our IT guys build me a custom computer with oversized PSU to support an Assus Xonar Essence sound card (which I love).
Much to my own surprise, right out of the blue, I compulsively ordered a pair of Audeze LCD-2s with Moon Silver Dragon V3 cables from Drew. They should be on their way soon.
I had no idea when I ordered them that only certain amps could optimally drive them.
Over the last few days, I have spent way too many hours lurking on this Board and others striving to figure out what head amp would best suit the Audeze's.
Last night I did a Google search using my subject line; "low output impedence headphone amps" and I discovered a blog that to me, makes a lot of sense. If you do your own Google search using that line, you will no doubt find it. The author says he was banned from this Board for unrelated reasons to this post.
My question is; is he correct that very few manufacturers choose to design and build amps with output impedences of zero to three?
Everything he says seems to ring true with what little I know. There are tons and tons of headphone amps out there and it seems precious few have low output impedence.
If he is correct, why is this? Frankly, his attempt to answer this question in his blog article is to my mind a bit lacking.
And related to that is this; now that so-called orthodynamic/planar cans are becoming popular, will manufacturers now start to meet market demand with suitable amps?
At this point, I have pretty much decided to buy the Violectric V200. I don't want to pay a grand for a headphone amp that looks to contain $50-$75 worth of parts (and most of us know such is not at all uncommon in the industry-the case is often more costly than the internals), but at least I can count on it being very well designed and very well built, and most importantly-capable of producing sound fairly faithful to the source.
 

 

[nikongod]

Quote:

If he is correct, why is this? Frankly, his attempt to answer this question in his blog article is to my mind a bit lacking.
 

Could you define "most" more concisely? There are tons of SS headphone amps with output impedance of 3ohms or less. 
Perhaps better than trying to find something that fits into an arbitrary rule, lets explore the rule. The rules first brigade will no doubt throw a fit, but thats OK.
 
The argument for low output impedance is based purely on rules created for multi-driver speakers or multi-driver headphones, with little real basis in conventional headphones or even single driver speakers. 
 
In a multi driver headphone or speaker there is a crossover. This crossover is sensitive to the output impedance of the amp driving it, and the crossover points (the frequency that the crossover works at) will change in undesirable ways if an inapropriate output impedance is used. In order to make life easy for speaker designers, 0ohms output impedance (maybe a LITTLE higher to account for cables and reality) was selected as the standard. In reality ANYTHING could have been used as the standard (4 ohms for example would have made life very easy for everyone) but the first people who could convincingly demonstrate the quality of their gear using measurements (and the lack of quality of everyone else's) were very good at making amps with very low output impedance so they pushed for a standard that favored themselves.
 
In headphones we dont have a crossover (multi-driver IEMs do, but thats an exception) so the worst effect of non-zero output impedance is eliminated.
 
Something neat with a high output impedance amp (120ohms or even more if your feeling frisky) is that it changes the frequency response of the headphones, particularly with higher impedance headphones. Quite a few people find this change preferable to the sound they get running the headphones out of an amp with a very low output impedance. 
 
Once we get past that, we could look at the IEC standard that says that headphones should be driven from an amp with a 120ohm output impedance, and what fun this generates. This standard does also allow for other output impedance, but it provides CLEAR evidence that 0ohms is not the only option.
 
At the end of the day, a 0-3ohm output impedance is not whats "right" its simply the lowest you can get. What is right is up to the user and unlike speakers the user has LOTS of say in how to run his/her headphones. If your looking for a hard and fast rule, there really isn't one here. Experimentation will yield the best results, and 2 people may even prefer to do it differently with the same headphones. 

 

[Chris J]

Quote:

Could you define "most" more concisely? There are tons of SS headphone amps with output impedance of 3ohms or less. 
Perhaps better than trying to find something that fits into an arbitrary rule, lets explore the rule. The rules first brigade will no doubt throw a fit, but thats OK.
 
The argument for low output impedance is based purely on rules created for multi-driver speakers or multi-driver headphones, with little real basis in conventional headphones or even single driver speakers. 
 
In a multi driver headphone or speaker there is a crossover. This crossover is sensitive to the output impedance of the amp driving it, and the crossover points (the frequency that the crossover works at) will change in undesirable ways if an inapropriate output impedance is used. In order to make life easy for speaker designers, 0ohms output impedance (maybe a LITTLE higher to account for cables and reality) was selected as the standard. In reality ANYTHING could have been used as the standard (4 ohms for example would have made life very easy for everyone) but the first people who could convincingly demonstrate the quality of their gear using measurements (and the lack of quality of everyone else's) were very good at making amps with very low output impedance so they pushed for a standard that favored themselves.
 
In headphones we dont have a crossover (multi-driver IEMs do, but thats an exception) so the worst effect of non-zero output impedance is eliminated.
 
Something neat with a high output impedance amp (120ohms or even more if your feeling frisky) is that it changes the frequency response of the headphones, particularly with higher impedance headphones. Quite a few people find this change preferable to the sound they get running the headphones out of an amp with a very low output impedance. 
 
Once we get past that, we could look at the IEC standard that says that headphones should be driven from an amp with a 120ohm output impedance, and what fun this generates. This standard does also allow for other output impedance, but it provides CLEAR evidence that 0ohms is not the only option.
 
At the end of the day, a 0-3ohm output impedance is not whats "right" its simply the lowest you can get. What is right is up to the user and unlike speakers the user has LOTS of say in how to run his/her headphones. If your looking for a hard and fast rule, there really isn't one here. Experimentation will yield the best results, and 2 people may even prefer to do it differently with the same headphones. 

One word: efficiency.
 
 
 

 

[Rawrbington]

control?

 

[nikongod]

 
Quote:

One word: efficiency.
 

One word: Strawman. 

 

[fubar3]

It seems that recent DIY amps are leaning towards an engineering perspective with measured results. DIY amps such as "the Wire" and "O2" are not intended to tune or color audio for subjective preference.  I see nothing wrong with the subjective preferences, it is just difficult to design for.

 

[Chris J]

Quote:

 

One word: Strawman. 

I guess my original answer came off a bit terse.
Maybe I should expand a bit?
Looks like the OP found the article in NWAV Guy's blog.
Personally, I like his article.
But I don't see too many headphone amp manufacturers clearly stating the output impedance of thier product. Not sure why.
 
My thinking is:
Headphone amplifiers (like most audio amplifiers) are, technically speaking, voltage sources.
An ideal voltage source would have zero output impedance.
So it's not a standard, it's an ideal to shoot for.
 
In a solid state amp it is fairly easy to design in a very low output impedance.
As you point out, in reality it can't be zero due to cable impedance, the characteristics of the output devices in a solid state amp, etc.
The only reasons you may want to deliberately design in some output impedance are:
 -  for short circuit protection and
 - maybe some protection from the capacitance of the load (i.e. the headphones, the headphone cable) to keep the amp stable.
A vacuum tube amp with an output transformer would have a higher output impedance due to the output tubes & the transformer. The designers can't get it as low as a solid state design.  If you like this colouration, fine. Personally I like the colouration of tube amps. I just don't see the point of it in a solid state amp.
An OTL vacuum tube amp would have a still higher output impedance as tubes have a higher output impedance than transistors, generally speaking. The optimum place for OTLs is driving high impedance headphones.
 
So what's the point of having the lowest possible output impedance in your headphone amp?
1.    Efficiency:   the lower the output impedance the higher the efficiency of the amp.  For example, if you try to drive a 32 ohm Grado headphone with an amp having an output impedance of 32 ohms then the amp would have to output 2 volts for the headphone to receive 1 volt. So you are wasting voltage/power.  IEC's 120 ohm standard probably dates back from the time when most headphones had a very high impedance: 600-2000 ohms. In 2011 that standard just looks dated.
2.    Control:   the lower the output impedance the higher the damping factor, some folks argue that headphone drivers are so light that this is not as important in a headphone as it is in a loudspeaker, fair enough.
3.    Flatest possible frequncy respsonse from your head amp.  to avoid frequency response interaction between the output impedance of the amp and the headphone.  You may like this frequency response colouration in one headphone but not like it in another headphone, which doesn't strike me as being very versatile, but that's just my opinion. Why not use an EQ?
 
Personally, I don't see the point of a solid state amp with a high output impedance.

 

[nikongod]

 
Quote:

I guess my original answer came off a bit terse.

Isnt that the point of a single word reply?
 
Quote:

But I don't see too many headphone amp manufacturers clearly stating the output impedance of thier product. Not sure why.
 
My thinking is:
Headphone amplifiers (like most audio amplifiers) are, technically speaking, voltage sources.
An ideal voltage source would have zero output impedance.
So it's not a standard, it's an ideal to shoot for.

I'd say its a few other parts.
To start with most people dont even think about this stuff. I guess at the very least that is something nice about this, although in the end it still sucks. What sucks here is that rather than thinking about what output impedance does and where messing with that can benefit the user a simple rule is being followed. Sacred cows are too fat to fit in the room with quality sound. Kill them. 
 
As for why MFR's dont specify output impedance, my guesses go to 3 groups. 
The first group is a few talented individuals who cant make an SS amp that does not hiss without adding output impedance. Considering that the headphones which are the most sensitive to output impedance are the multi-drivers, which are also the most likely to hiss this crowd is really scummy. 
 
The second group is MFR's who do actually specify output impedance correctly. Maybe you need to ask them, but they readily supply it. 
 
The third group is MFR's who know that the few people who understand output impedance could probably measure it if they really cared. Alternately for this group, with the overwhelming trend towards "0 ohms or just give up" advice that is mindlessly repeated their numbers would be misinterpreted. 
 
Quote:

In a solid state amp it is fairly easy to design in a very low output impedance.
As you point out, in reality it can't be zero due to cable impedance, the characteristics of the output devices in a solid state amp, etc.
The only reasons you may want to deliberately design in some output impedance are:
 -  for short circuit protection and
 - maybe some protection from the capacitance of the load (i.e. the headphones, the headphone cable) to keep the amp stable.
A vacuum tube amp with an output transformer would have a higher output impedance due to the output tubes & the transformer. The designers can't get it as low as a solid state design.  If you like this colouration, fine. Personally I like the colouration of tube amps. I just don't see the point of it in a solid state amp.
An OTL vacuum tube amp would have a still higher output impedance as tubes have a higher output impedance than transistors, generally speaking. The optimum place for OTLs is driving high impedance headphones.

 
Its not much harder to design an SS amp for a high output impedance without resorting to the cop-out of output resistors. Any difficulty beyond what one would experience when designing a "pure" voltage source amp are simply a result of the design process being so unusual. 
 
Short circuit protection and isolation from cable capacitance are great reasons to have a little output resistance (or even an output resistor!) but you keep skipping the change in  frequency response that a high output impedance amp causes in some headphones
 
You should isolate how much of the sound you like in tubes is the tube & distortions it generates, and how much is the output impedance. You will probably find the experience very enlightening.  
 
It is not much harder to design a tube amp with very low output impedance than it is to design an SS amp. It just requires a boatload of feedback, which is not really very different from an SS amp, just out of fashion in tubes.
 
Quote:

 
So what's the point of having the lowest possible output impedance in your headphone amp?
 

 
Thats what one of the rules says to do, obey. 
 
Quote:

 
1.    Efficiency:   the lower the output impedance the higher the efficiency of the amp.  For example, if you try to drive a 32 ohm Grado headphone with an amp having an output impedance of 32 ohms then the amp would have to output 2 volts for the headphone to receive 1 volt. So you are wasting voltage/power.  IEC's 120 ohm standard probably dates back from the time when most headphones had a very high impedance: 600-2000 ohms. In 2011 that standard just looks dated.
 

 
Nope. Efficiency is a strawman and this is why. 
Headphone systems are inherently inefficient. We as a group regularly pull in excess of 50w from the wall to put less than 3mw into our headphones. Maybe we get a very efficient amp that only sucks down 2W to put out the same 3mw. wow, (3mw*2)/2000mw=0.3% If you cant sacrifice a few more volts in the name of improved sound quality I'm unimpressed. 
 
Regarding the fact that it does take extra voltage swing to get the same power into the load when a high output impedance is used. BIG DEAL! How many people are really running their headphone systems with *only* 20db of voltage or current overhead? Measure a few systems, most people have 40db to play with. 40db of overhead, and you say efficiency. 
 
The only thing efficient about a headphone system is that it might pull fewer watts from the wall than a speaker system.  
 
Quote:

2.    Control:   the lower the output impedance the higher the damping factor, some folks argue that headphone drivers are so light that this is not as important in a headphone as it is in a loudspeaker, fair enough.
 

 
I'd argue against this, even in a loudspeaker. electrical damping factor over the actual drivers is BS. It simply doesnt work. If the DRIVER is going to overshoot it is going to overshoot. It makes no difference if the amp has a damping factor of 1000 or 1/2. 
 
What people call "poor control" is the crossover points no longer lining up properly when the output impedance of the amp is not correct for the crossovers. 
 
Food for thought: what is the damping factor on a speaker driver after you take the crossover into account?  
 
Quote:

Originally Posted by Chris J /img/forum/go_quote.gif
 
3.    Flatest possible frequncy respsonse from your head amp.  to avoid frequency response interaction between the output impedance of the amp and the headphone.  You may like this frequency response colouration in one headphone but not like it in another headphone, which doesn't strike me as being very versatile, but that's just my opinion. Why not use an EQ?
 

 
This one is actually quite interesting, and brings out a great point. 
What if you like the frequency response coloration with one headphone driven from an amp with a 0ohm output impedance, but not with another? This does not strike me as very versatile, but thats just my opinion. Why not own multiple amps?
 
I should note that I EQ everything, although I use the same curve for all of my headphones. 

 

[Chris J]

Well I guess the OP has lost interest in his thread!
 
I'm going to agree with NwAv Guy's headphone amp impedance blog. Call me crazy.

 
I agree with your point about having multiple head amps for different sounds, which is why I have a couple of SS amps and a couple of tube head amps.
But still don't see the point in any of the designers designing in a higher output impedance than necessary.
 
NwAvGuy brings up an interesting point: you don't see loudspeaker amp manufacturers designing in high output impedances.
 
The whole voltage swing thing is important if you're using a little battery powered head amp with very limited possible output voltage with a fairly low efficiency headphone, e.g. an iBasso D10 or D12 with AKG K70x series.

 

[FSonicSmith]

I have not lost interest. I have been monitoring this thread like a hawk. I very much appreciate both your perspective Chris J and Nikongod's. 
While I am not EE, I tend to agree with your statements Chris more than Nikongod's. Nikongod, in the first line of his first response said;
 
Quote:

 There are tons of SS headphone amps with output impedance of 3ohms or less. 

 
Where are these "tons" of SS headphone amps? Yes, I was referring to NWAV Guy's blog and I accept his premise-very FEW amp manufacturers design or implement low impedence because up until recently, most headphone manufacturers were making high impedance phones for a variety of reasons. If there were truly "tons" of low output impedance SS amps out there, the manufacturers would be touting such in their specs, particularly in light of the popularity of the new planar phones. However Chris, I am not sure about your statement that;
 
 
 
Quote:

Headphone amplifiers (like most audio amplifiers) are, technically speaking, voltage sources.

 
 
Maybe I am quoting you out of context but it seems to me that such is true with high impendance phones like AKG701s and the various Senns, but it is not true with low impedance phones. With low impedance phones, the amp must serve as a current source. But no, I am by no means bored or disinterested. My Audeze LCD-2s are on their way and right now I need to make a decision between the Violectric V200 and the Bryston BHA-1 unles something else comes to my attention. I am looking for something decidedly high-end with great build quality and few compromises in terms of parts or design for the sake of ultimate cost. Nagra is going a bit far but the Violectric and Bryston are up there. I understand that a tube amp can be made with low output impedance and I would love to find a deal on a Leben CS300SX even though it is an integrated amp.

 

[nikongod]

Quote:

Where are these "tons" of SS headphone amps? Yes, I was referring to NWAV Guy's blog and I accept his premise-very FEW amp manufacturers design or implement low impedence because up until recently, most headphone manufacturers were making high impedance phones for a variety of reasons. If there were truly "tons" of low output impedance SS amps out there, the manufacturers would be touting such in their specs, particularly in light of the popularity of the new planar phones. However Chris, I am not sure about your statement that;

Tons of DIY amps: 
Pimeta, the wire, M3, B22, ppa, gilmore SS amps (dynawhatever), and all the commercial variants of them. 0ohm output impedance (or close enough) is not a difficult thing to find with SS amps. 
 
Headamp specifies output impedance for all of their dynamic amps. Even the portables. 
 
 
Quote:

Maybe I am quoting you out of context but it seems to me that such is true with high impendance phones like AKG701s and the various Senns, but it is not true with low impedance phones. With low impedance phones, the amp must serve as a current source. But no, I am by no means bored or disinterested. My Audeze LCD-2s are on their way and right now I need to make a decision between the Violectric V200 and the Bryston BHA-1 unles something else comes to my attention. I am looking for something decidedly high-end with great build quality and few compromises in terms of parts or design for the sake of ultimate cost. Nagra is going a bit far but the Violectric and Bryston are up there. I understand that a tube amp can be made with low output impedance and I would love to find a deal on a Leben CS300SX even though it is an integrated amp.

You are mis-using the term current source. No worries. 
 
The term "voltage source" means that we attempt to control the output voltage and we get whatever current we need to do that. A perfect voltage source has an output impedance of 0ohms.
 
The term "current source" means that we attempt to control the output current and we get whatever voltage we need to do that. A perfect current source has an output impedance of infinity ohms.
 
Those are the ideal definitions, but most people throw in the towel once they pass a factor of 10 and say good enough.
 
As an aside, there are some very good applications for current source amplifiers, but they are somewhat niche. Nelson Pass has a couple good articles on the whole idea that are not tooooooo techy, just enough to be interesting. 
 
I think you meant to say "can supply lots of current" instead of serve as a current source, although if you have the opportunity try a set of Grados on a current source amp I would highly suggest it, I hear its very nice from a DIYer I trust greatly. 
 
Last bit of nitpicking: 
The Leben amp is an integrated, but so are the 2 others you specifically called out. All integrated amp means is that the amplifier has a volume control built in. The term "integrated amp" is a pejorative in audiophile circles because it implies you didn't care enough to buy a dedicated preamp to hook it to a dedicated speaker amp. Almost all headphone amps are integrated amps, with the few exceptions being funky DIY contraptions. There are some benefits to separate pre and power amps, but it is the last 0.2%
 
Anyways, nit-picking aside. For an ortho we have different problems to solve than conventional dynamics. 
The problem with orthos is not getting output impedance "right" (by whichever definition you choose). Most orthodynamic drivers have REALLY flat frequency VS impedance curves. The output impedance of the amp interacts with the bumps and dips in impedance at various frequencies with a conventional dynamic to cause a change in audible response. With an ortho these bumps and dips simply arent there! Output impedance is not the issue with an ortho. 
 
The problem to solve with orthos is that they need a lot of power compared to a conventional headphone. This is where they get killed, or correctly where headphone amps get killed driving orthos. 
Look for something that can put at least 3W into 8ohms. More if you listen loud. Dont worry about output impedance. There are a few headphone amps that can do it, but mneh. I would lean towards small high quality speaker amps rather than sticking with headphone amps. older low-power krell amps, Nelson Pass/firstwatt amps, small tube amps (the leben), etc. 

 

[Rawrbington]

Quote:

I'd argue against this, even in a loudspeaker. electrical damping factor over the actual drivers is BS. It simply doesnt work. If the DRIVER is going to overshoot it is going to overshoot. It makes no difference if the amp has a damping factor of 1000 or 1/2. 
 
What people call "poor control" is the crossover points no longer lining up properly when the output impedance of the amp is not correct for the crossovers. 
 

help me out here, im still learning.
i have recently sat down with a pair of 32 ohm beyers and 600 ohm beyers and played both side by side out of a head amp with an output impedance of approx. 100 ohms.
the 600 ohm sounded incredible, while the 32 ohm sounded bad and poorly controlled. 
what else would cause such a stark difference in control and sound?  im not highly educated in this area.
 
 
Quote:

Anyways, nit-picking aside. For an ortho we have different problems to solve than conventional dynamics. 
The problem with orthos is not getting output impedance "right" (by whichever definition you choose). Most orthodynamic drivers have REALLY flat frequency VS impedance curves. The output impedance of the amp interacts with the bumps and dips in impedance at various frequencies with a conventional dynamic to cause a change in audible response. With an ortho these bumps and dips simply arent there! Output impedance is not the issue with an ortho. 
 
The problem to solve with orthos is that they need a lot of power compared to a conventional headphone. This is where they get killed, or correctly where headphone amps get killed driving orthos. 
Look for something that can put at least 3W into 8ohms. More if you listen loud. Dont worry about output impedance. There are a few headphone amps that can do it, but mneh. I would lean towards small high quality speaker amps rather than sticking with headphone amps. older low-power krell amps, Nelson Pass/firstwatt amps, small tube amps (the leben), etc. 

does this mean my Woo with high output impedance could possibly power the LCD2?
or would i still be better off sticking with a high z can like the T1?
 

 

[nikongod]

 
Quote:

help me out here, im still learning.
i have recently sat down with a pair of 32 ohm beyers and 600 ohm beyers and played both side by side out of a head amp with an output impedance of approx. 100 ohms.
the 600 ohm sounded incredible, while the 32 ohm sounded bad and poorly controlled. 
what else would cause such a stark difference in control and sound?  im not highly educated in this area.
 

 
There are a couple things here. 
I'm assuming you are talking about the Woo2 as the amp based on your signature. Based on that assumption the most likely reason is that the amp simply works better into high impedance loads than low. OTL tube amps dont generally do well with low output impedance loads (without lots of global feedback) due to lots of distortion. This problem is not isolated to that amp, it applies similarly to most OTL tubes. The good news is that so many people like the sound of OTL tube amps with high impedance headphones
 
Slightly less likely reason:
The 32 ohm headphones just don't sound right with a high output impedance from any amp. It does happen, but I still think its a great idea to experiment with high output impedance amps - you may find something really nice that way. 
 
 
Quote:

does this mean my Woo with high output impedance could possibly power the LCD2?
or would i still be better off sticking with a high z can like the T1?
 

For the woo 2, I'd say eeeeeh while leaning away. I'd certainly try it (maybe it sounds cool?) but I don't think that amp will really work right with an ortho.
 
On a positive note, the Woo2 will make an awesome preamp if you do decide to go with a dedicated power amp for the LCD2. 
 
I really liked the T1, particularly out of an OTL tube amp. I mostly liked the way they throw a nice wide soundstage and the frequency response is quite good to my ears - the highs generally stay behind the vocals (I really cant stand it when cymbal crashes jump in front of the singer and slap you in the face), and the bass is quite satisfying too. They are great headphones its just that the ortho-love drowns out the dynamic cans. 

 

[SobbingWallet]

Quote:

This one is actually quite interesting, and brings out a great point.  What if you like the frequency response coloration with one headphone driven from an amp with a 0ohm output impedance, but not with another? This does not strike me as very versatile, but thats just my opinion. Why not own multiple amps?

Could you explain what you mean by this?  Other than noise/hiss (due to poor design), is there a reason why a 0 ohm output impedance amp would color the signal with any headphones, on account of its low output impedance alone?  Or are you just referring to the "synergy" problem of liking/disliking the total combined frequency response coloration of amp + headphones?
 

 

[nikongod]

 
Quote:

Could you explain what you mean by this?  Other than noise/hiss (due to poor design), is there a reason why a 0 ohm output impedance amp would color the signal with any headphones, on account of its low output impedance alone?  Or are you just referring to the "synergy" problem of liking/disliking the total combined frequency response coloration of amp + headphones?
 

If the headphones were designed around a high output impedance they may sound pretty bad from a 0ohm output. Beyer DT880 springs to mind as the best example - with harsh and grating highs when run from an amp with a 0ohm output impedance, but they calm down and really sound quite nice driven from 120.