Listening to the Stax SR-009 and Great Headphone Amplifiers

Hearing the SR-009 and seeing all the high-end e-stat amps at RMAF in October gave me the idea that it would be fun to do comparative listening session for a holiday season post. For most here (and that includes me) this stuff is pretty much unobtanium, so I thought a dreamy little vicarious listening session through my words here might fit in nicely with all the other sugar plum fantasies we get to experience this time of year.

My Christmas present to you lucky few who might actually be thinking of getting some SR-009 headphones is just a taste of what you may hear, and a simple starting point for your personal journey into audio ecstasy. (Lucky bastards!)

As I said, the idea started at RMAF when I realized there were four really high-end electrostatic amplifiers that were worthy of pairing with the Stax SR-009 ($5250) electrostatic headphones---a headphone that many consider the world's best, I know I do. The four headphone amps that I brought in for the comparison are: Ray Samuels A-10 Thunderbolt ($6500); Cavalli Audio Liquid Lightning (around $5000, a revised version is expected in April); Woo Audio WES ($4990 base price, upgrades available); and HeadAmp Blue Hawaii SE ($4980, $5980 w/Alps RK-1 pot). Yama's Enterprises loaned me a set of Stax SR-009 ($5250) headphones for the test, but also shipped off the Stax SRM-727II ($2200) and SRM-007tII ($2400) electrostatic headphone amps to use a baseline for testing the above high-end amps.

I also had to figure out what I was going to use as a source, and how to set-up all the equipment in the space available. Physically, that's a lot of big amplifiers, and just getting the signal around to all the amps would be a bit tricky. I contacted Jon Iverson (Stereophile's DAC guru) for a USB DAC recommendation. He really enjoyed the Ayre QB-9 ($2500), and had one on hand that he could send for the review. After playing around with this DAC for a little while now, I can heartily echo his sentiments. This is a very nice DAC, sounding far better than it's price would indicate, IMHO.

The last little bit was getting my hands on some 10 foot balanced and unbalanced interconnects to reach all the amps spread out on my demo room counter. Now, I'm not a cable fanatic, but I do hear subtle differences between cables. A ten foot run of poor cable could certainly color the listening tests. (All you objectivists out there can just put you fingers in your ears and go "la la la la la" for this paragraph, okay?) I've always liked Cardas cables in situations like this. Their cables, it seems to me, always deliver a nice, smooth sound. As most readers know by now, I like things on the smooth side, and I was worried that the speedy nature of the SR-009 headphone might grate on my nerves for the listening tests. I gave my contact at Cardas a call, and asked for ten feet each of the best balanced and unbalanced interconnects they had...not really knowing the current line-up and costs. Two days later I received the long balanced and unbalanced top-of-the-line Cardas Clear interconnects. Valued at $4220 and $3920 respectively! These had better be good...and after playing around with some gear and interconnects I'm very familiar with, I'd say they're damned good...and damned smooth.

So, for those of you following along at home, that's about $45,000+ worth of gear once you throw in my MacBook, Amarra, and some power conditioners. Merry Christmas to me!

Listening Tests
With everything set-up, it's time for the listening tests to begin. Evaluating and describing the differences between headphones is much easier than doing the same for amplifiers. Headphone enthusiasts have long argued whether the source or the headphones are the most important link in the audio reproduction chain, but there's pretty universal agreement that the amp is least important. A decent amp is a decent amp, to a first approximation, and all the amps in this test are quite competent at doing the fundamental job of being a wire with gain.

For the first day of listening tests I decided to simply relax, be non-judgemental, and run my basic test tracks (find information on these tracks here) through each amp listening with the SR-009. First time through my impression was that the amps all sound fairly similar, with one glaring outlier. The one exception was the Stax SRM-727II, which seemed somewhat thick and inarticulate compared with the rest. But first listen is far too early for snap judgements, and I made a second listening round of the amps.

This time differences started to become apparent to me when changing from amp to amp, but again, I resisted coming to any conclusions. Rather than endlessly looping through the entire range of amps, I now began to switch back and forth between two amps listening to differences on a particular track. Once I got familiar with the changes, I would switch one of the amps out and repeat. This proved fairly revealing of the differences between amps, but it was quite time consuming as well.

While I'm very familiar with dynamic headphones, I've not done a lot of listening to the various electro-static headphones and amplifiers available, so I thought it might be a good idea to get some testing advice from a few long-time e-stat enthusiasts I know. With a few PMs and emails and a day's wait, I had some very interesting comments. One common comment was that some electrostatic amplifiers vary significantly in the way they sound between low volume and high volume listening. So I started to add some high volume listening in to my amp testing. Well now! That really began to bring out some changes. Some of the amps did indeed vary quite a bit under those tortuous conditions. I typically listen at a level that's probably lower than most, and I certainly don't recommend high-volume listening, but a good amp should have plenty of headroom to handle dynamic passages without becoming confused. For those of you who might find themselves doing some e-stat amp auditioning in the future, I highly recommend spending some time doing high-volume listening tests. It really is quite revealing.

After a week of listening tests, I did come to some conclusions about the amps. Let's take them one at a time.

timmyw's picture

Thanks for this Tyll. I have always been curious about these. I have never tried them though despite having the opportunity to do so. I just know deep down that I would be disappointed in them on some level, always trying to nit pick something or other. Your impressions certainly do nothing but buffer that opinion.

Perhaps I will grow a pair at the next meet I go to and try and get some decent time with these. I doubt any of the 5k amps in your article will be on show though. 

Funny, even though I have the cash I don't think I would ever buy them and a quality amp like those in this article, regardless of how they sound or my personal imrpessions. I would rather buy a BMW. Or put a deposit down on a house. 

But there is that little niggly feeling in the back of my mind that says I might just like them too much.

I think it's that niggly feeling that scares me.

Still fun to read about them though!

P.S. Merry Christmas!'s picture

I'd like to first say thank you for taking the time to articulate your listening experiences even when the last thing on your mind is having to elaborate on what you're perceiving or having to analyze such an understandably self-indulgent experience.

I just have to mention that Eddie Current's Electra is coming out soon, and as a proud owner of the Balancing Act and having briefly previewed the Electra, the Electra is an amp to watch as a high-end e-stat option.

It might not have to be mentioned, but for the sake of being complete, if you're really serious about e-stats and know quite a bit about engineering amps, then should consider building your own T2.

Excellent review!

Tyll Hertsens's picture

...I guess that's a good enough reason to think about doing it again next year.

Currawong's picture

I reckon you nailed the one issue that people tend to forget about when doing reviews, how loud we listen.  Nice to have someone do a comparison of all the well-known top 'stat amps though. Pity there's no time to do some tube rolling with them.

Tyll Hertsens's picture

I'd have loved to have the time to do just that.

I would very much like to do an article on the Liquid Glass amp and a bunch of tubes. Maybe throw in the Bottle head Crack and/or Woo WA3 into the mix as well. I think a general article on the experience of rolling tubes would be very cool. While having to replace tubes on a tube amp is a bit of a bother, the opportunity to tweak and amp with various tubes is just good fun.

donunus's picture

I just wished I could afford a 009 and a Blue Hawaii!

Tyll Hertsens's picture

...both, man!

spritzer's picture

Thanks for a great comparison Tyll.  It's a mammoth task to nail down the sound signature of so many amps in a short period of time but it is a much needed service.  I've had all these amps in my own system over the last year and I concur with his findings and the BHSE even holds its own against our own Tour de Force, the DIY take on the Stax SRM-T2.  That amp costs about the same as the BHSE in parts alone.  

I'd like to expand on what Tyll said about pushing amps to their limits when reviewing.  With electrostatics most amps are just idling at normal listening levels but as the volume is pushed upwards the amp needs to conduct more voltage and is thus approaching the limits of its range.  This is where issues crop up such as tubes are less linear and start to compress, the amp runs out of steam so the voltage starts to sag into the load presented, lack of headroom in the amps due to limited gain shows further compression. 

Another issue is the wild impedance swing of electrostatic transducers.  The amps have the easiest time with the midrange as it's the most benign load but the bass and treble present much more of a challange.  Here is where the power delivery of the amp really comes into play plus high slew rate and low output capacitance.  The bass is often loose and non precise with the top end rolled off and veiled/distorted due to this.  The problem is just confounded by increasing the volume level.

The SRM-727 is the least favored amp here but it can indeed be modified to greatly enhance its performance.  It is simply a matter of extending the feedback loop to include the output stage as well.  The amp is built up in two sections with most of the stages on a small plug in daughter board and Stax were kind enough to have taps present on the plug-in card to extend the feedback loop so it's just a matter of desoldering two resistors per channel and adding two new axial ones of the same value.  There are full details of this mod online.  The 007T can also be modified to take 6S4A tubes but this is an extensive mod and not for the timid.  Well worth it though as if fixes the core problem of this amp.  

Happy holidays to all and I'll get back to listening to my BHSE. 

Tyll Hertsens's picture

Very much apreciate the comments, mate. 

For those of you who don't know, Spritzer is one of the very few experts in the arena of electrostatic headphones that I know. If you're rummaging through the HEad-Fi threads on these products and see one of his posts, weigh the contents heavily. He knows his stuff.

Makes great bread, on e-stats though, that's where he spends the bread from his bread. :)

arnaud's picture

... you always find the right topic to exactly address the needs? There couldn't be a more useful article except for including the Electra. However, being still a prototype stage, it's understandable you couldn't have it tag along.

One thing I am wondering about, with you latest foray into headphone amp measurements, any change you did try to correlate your listening impressions with some objective results?

Many thanks again for the write-up,


Tyll Hertsens's picture

"Tyll, how comes ... you always find the right topic to exactly address the needs?"

Thanks. I try to keep my ear to the ground at the various headphone fora to see what interests people.  I also listen to myself and what seems to interest me. 

Still to early too relate amp measurements to subjective experience.  I reckon it'll be much harder than with headphones...if possible at all.  

anguish's picture

Many thanks for this great review Tyll!

I have been dreaming for years of wandering into the land of Stax and electrostatic headphones. Now I realize how even more costly the journey is than I once thought. I think I'll need to win the lottery or rob a bank...

This review certainly is a great starting point! Blue Hawaii seem to have nailed it, but I would be still be curious to try the various amps :) 

On a side note: The SR009s weigh about 600g, is that comfortable enough for longer listening sessions? I wonder how heavy that feels on one's head. Or is the sound so fabulous you become unaware of everything else and barely remember to breathe? I'm using HD650s which weigh about half the weight of SR009s. 

spritzer's picture

The suspended arc design means the weight of the headphone is evenly spread and they are one of the most comfortable headsets out there.  Going from a HD650 you will feel the difference but it is never bothersome. 

Another alternative would be a used Sennheiser HE60 which is very similar to the HD650 in terms of construction and weight.  Rather differnent sound to the SR-009 though. 

anguish's picture

Thanks for your comment spritzer. I have never tried a headphone above 300-something grams so I really wonder how it feels. 

I think it's a matter of time and saving up until I will pull the trigger for Stax audio nirvanna :) I might try with a cheaper one first, because I won't be able to be patient enough for an SR009 right away as I'm deadly curious about electrostatic headphones. 

paul's picture

Dear Tyll:

Would you mind talking about the source material. Does a standard Apple Lossless rip sound good? What kind of software were you using?

Happy New Year,


Tyll Hertsens's picture

...very good recordings on 16/44.1  .WAV files in iTunes with Amarra. 

Had some hi-rez FLACs that I listened to a bit, but I'm not as familiar with them as my standard test tracks.

Jazz Casual's picture

Thanks Tyll for another enjoyable read. You have a way of making head-fi accessible even when it falls into the category of unobtanium for most of us.

I've been flirting with the idea of moving into Stax electrostats for a long time, and yet I've continued to avoid it while increasing my collection of dynamic headphones instead. I suppose it's just been easier to do. 

My reluctance to take the plunge is probably similar to a lot of people who are interested but hold-off. It's nigh on impossible to audition them. Even the specialist headphone store that I frequent which is an authorised Stax dealer, does not have any models available for audition. So getting into Stax electrostats involves a leap of faith and a financial commitment that I am still reluctant to make.

What I do know is that unless I win the lottery, I will never own the SR-009 or a Blue Hawaii, and it's highly unlikely that I will ever hear them; more's the pity. But if I ever do win that lottery, then that's what I'd go for. My personal summit-fi would be reached.  

In the meantime, I'll continue to contemplate investing in a SR-507 and SRM-323S or the SR-007MKII. I'm drawn to the SR-007MKII despite its supposed flaws but God knows what amp I'd get to drive it, as the prevailing view is that neither the SRM-727tII or the SRM-727II are adequate. 

Dinny FitzPatrick's picture

I suggest keeping your eye out for a KGSS.  That will do a great job driving the O2.

alexandrov's picture

"...Now, I'm not a cable fanatic, but I do hear subtle differences between cables.."

I don't want to start a cable discussion. I'm not a cable fanatic too!

But I think reviewing too many headphones and amps sometimes may distract you from profound understanding the nature of the components (and systems). Cables are the fourth most important component of a system alongside the transducers, the amp and the source. Cables CAN change the whole sound signature of the system. Using "wrong" cables (as well as "wrong" amps and sources) may lead to wrong idea of what we have for reviewing. Yes, MEASURING cables is a tricky thing but headphone LISTENING is largely infruenced by cables.

Thank you for the useful comparative review!

Best regards


Matt's picture

$45.000 of audio equipment and a $25 chair...

joelha's picture

While I'm perfectly prepared to hear (or read) a resounding "No way" to my question, I travel a ton. Is there such a thing as carry-on-friendly amp which would properly power the SR009's or am I just dreaming at this point?

The SR009's are the one headphone I've heard which I could imagine listening to for long periods of time when away from my home system.

I understand the (hopefuly) soon to be released Abyss headphones are supposed to be easier to drive (and I've heard them) although I don't know if they'll have a headphone amp I could travel with either.




Tyll Hertsens's picture


The need for a bias of 580 Volts, and the need to swing a 1000 volt audio signal pretty much rules out a portable battery powered amp. Pretty much...there's always some nut-job audiophile willing to try anything. 

*raises hand*

joelha's picture

Thanks for the quick reply, Tyll.

Just to be clear, the amp would not have to be battery powered, just transportable. I just returned from yet another overseas trip and having a great system I could use in my hotel room would be a huge bonus.

Just for the record, I'm prepared for yet another "No way".


Tyll Hertsens's picture


...(more like out of my butt) I'd guess the real problem is heat dissapation. These amps tend to run hot and you need to have a certain amount of heat sink or space to radiate away all the heat. The Stax amps are sort of compact, you might be able to pack one in a suitcase.

Spritzer would have to answer your question to know the real deal.

Frankly, at some point I'd just have to say JH13 Freqphase and a HeadAmp Pico with your source and you're done.

joelha's picture

Personally, I prefer off the top of your head, but then that's just me. smiley

I've never heard an IEM that I liked, but I do appreciate your quick replies.

The ongoing search for audio nirvana continues.

Thanks a lot.


Justin@HeadAmp's picture

people wanted a portable Stax amp ~2006 and I considered it.  Problem was, those were the days when Altoids tin portable amps were considered "big" and everyone wanted a iPod shuffle sized amp with a 500 hour battery life.  Times have changed again, and now the hot thing is brick sized portables with full size connectors and 5 hour battery life.  Which is what a decent Stax portable would have to be.

spritzer's picture

It is possible to build a battery powered amp to drive them but it all relies on the transformer used for the internal powersupply.  This has been our hurdle for a DIY approach as nobody can build one with good enough specs.  It's not good to loose 25% of the energy in the battery just to heat up the transformer. 

That said, to properly drive the SR-009's you need power and and that means a lot of batteries and a very short run time.  It would also not be small and portable. 

joelha's picture

Thanks Spritzer.

What is the smallest Stax amp which you would recommend with the SR009's?


Justin@HeadAmp's picture

He will say Stax SRM-323S ;)

spritzer's picture

Well it is the best Stax amp in current production.  cheeky  If people are open to vintage amps then the SRM-1 Mk2 is also excellent and the SRA-12S is the best one I've found.  These amps are all more than 20 years old so they need to be refurbished to sound as they should.