It's a transformer amplifier, like what Eddie Current did relatively recently. These things tend to be decent if very heavy and tend to have terrible output impedance. This one uses FETs as transimpedance amplifiers. The sound and distortion characteristics depend a lot on the specific transformer - and very low output impedance might be problematic to achieve. They also tend to have bit nonlinear frequency response with rolloffs on both ends as well as worsened THD at lowest end, but most have nicely decreasing harmonic distortion vs order. The use of FETs as transimpedance can actually break that last part though...

In other words, I will not believe their marketing mumbo-jumbo until someone measures it. "Subsonic protection" sounds like mumbo-jumbo too, it is built-in in the transformer - I'd be more afraid it has some kind of subbass roll-off. The power drop at low impedances suggests high output impedance, if I were to wager, about 120 Ohm.

Oh, and it does have high THD at low load impedance. They already say 0,5% at 33 Ohm (which SPL and frequency though?), which can toss the headphone distortion over the audibility threshold.

Metrum Acoustics will be releasing their new Aurix headphone amp soon: http://www.metrum-acoustics.nl/Aurix_english.html

I thought this line was funny:

"When we were requested to develop a headphone amplifier, for sure that had to be something special."

Well...if you say so? But give us more details:

"Only two FET transistors are in the signal path as an impedance converter ( source follower, no amplification )  and it is this property were a  FET transistor can do an extremely good job and delivers a very high bandwidth having low noise and low distortion . The first Fet is used as an impedance converter to drive the step-up transformer. The second FET is used to drive the headphone. The circuits  are running  in full  " Class A " so that a relatively large heatsink is required to flow the heat . Besides the amplifier part we have used a lot of electronics just for creating the right conditions for the amplifier part and protecting your headphone against subsonics , offsets , pops and clicks. The result is a sound that is mainly determined by the connected source."

This falls outside my realm of understanding, so can folks more knowledgeable on this topic provide insight on this? Do similarly designed headphone amps exist (i.e. is this actually a unique design or just marketing)? What are the benefits, limitations, and potential risks of a design like this? What are your predictions on how it will sound? Or, perhaps one of you out there has actually heard it? I think I've seen internal pictures floating around on the internet. I'm not looking to purchase this by any means, but I am genuinely curious to learn about it as best as I can.

Sounds a LOT like this design...
http://www.mediafire.com/view/?c2zqvacgq079dpw

If I were to describe the design it would say.
Only two MOSFET transistors are in the signal path as an impedance converter ( source follower, no amplification )  and it is this property were a  MOSFET transistor can do an extremely good job and delivers a very high bandwidth having low noise and low distortion . The first MOSFET is used as an impedance converter to drive the step-up transformer. The second FET is used to drive the headphone. The circuits  are running  in full  " Class A " so that a relatively large heatsink is required to flow the heat . Besides the amplifier part we have used a lot of electronics just for creating the right conditions for the amplifier part and protecting your headphone against offsets , pops and clicks.

The design is public though.

That description reminded me of a certain First Watt amp that Papa came up with.



Except that one is obviously push-pull instead of SE. Also that's an autoformer, not a transformer. But maybe they use the same thing.

I think such a design could have the potential to sound good. The real limitation would be the quality of the transformer. Specs of the Metrum seem typical of any Class-A amp w/o feedback, so you can take that as you will.

"Sounds better than measured" means "I don't like blind testing" or "I have nothing that measures better to compare" alternatively "everything sounds about as good to me". The last point is possible, this kind of amplifier can reach all known psychoacoustic thresholds easily - but a botched implementation can also fail to do so.

Solderdude, where did you dig it out? Is that yours? RMAA gives incorrect results quite often.

Well it was an idea I had early 2010 when I joined RG:
http://rockgrotto.proboards.com/thread/5100/class-single-variation-on-theme?page=4
Am not a member there any more though ....

At one point I thought I would just build an amplifier that did the amplification with a trafo, just for laughs, not expecting too much.
Own home made electrostatic speakers (with non-tensioned membranes) that sounds awesome, yet the audio part is powered by a handful of power transformers when I worked at Panasonic service centre.
I was always amazed how a few mV from a cartridge could end up with well over thousand volts and still sound THAT good despite of those VCR power trafos.
This idea made me think of that design and built one and made the article.
Not a serious attempt, just trying to do something different than usual.
Indeed for the sake of being different, not thinking it is a 'better concept'.

It doesn't measure that well, but surprisingly enough well enough to meet some basic specs.
It surprised me how well it sounded, expected it to sound less than other amps... but it doesn't.
Didn't get it right from the start though and cost me quite some time to come up with some practical issues.
But this is the fun part for me... not using it right now though.

I don't like blind testing but do it anyway IF I want to find out if something is really different.
I don't bother t.b.h. if it sounds good, it does but do like to measure things and now my way around measurement setups (measure and design a lot for a living, just not in audio field) and their limitations and how to work around it.

Well it does 'function' as a kind of brickwall and isn't really bothered by HF energy so for his DAC's it isn't that bad an idea.
I believe in my article I even specifically mentioned NOS DAC's without a filter.
On the other hand as it is bandwidth limited on both sides of the spectrum you can say bye bye to sharp risetimes (needle pulses) and perfect high- and low-frequency squarewaves. But these do not exist in audio anyway.

It kind of defeats the purpose (his purpose) of NOS DAC's t.b.h. but does some reconstruction/HF filtering.

With normal designs he should use an input filter when selling them with his DAC's.