Stax SR-009S Electrostatic Headphone Review

[amirm]

This is a review, listening test, EQ and measurements of the Stax SR-009S electrostatic headphone. It is on kind loan from a member and costs US $4545.

The SR-009S has the look of feel of typical Stax headphone using nice mix of leather and metal. It fit loosely on my head even at its tightest setting. So if you have a small head, it may not work for you. Comfort was very good.

Stax SR-009S Measurements
The owner didn't send me an amplifier so I dug into my stash of vintage Stax amplifiers and landed on SRM-313. It is solid state and I feel that it has less distortion than its two other tube brethren. Here is our standard frequency response:

Starting from left, we have flat bass with a bit of a shelf down. We then ahve some kind of peaking around 1.1 kHz and then a much sharper one around 5 kHz. There is a sharp cancellation around 6 kHz which is likely due to internal reflections. In terms of relative deviation from our target we have:

The deviation is not easy to fit into parametric EQ but I had good luck doing so (see next section).

This is the graph that made my jaw drop:

The thing is distortion-less even at 104 dBSPL! I did have to back off to 110 dBSPL as anything above that caused severe distortion. But even that looks very good. I don't know why distortion is higher at 94 dBSPL than higher levels below 300 Hz.

Here is the same but in absolute levels:

Group delay is extremely messy, partly caused by the internal reflections/resonances noted in frequency response measurements:

I don't have sensitivity measurements for you since this is an "active" headphone.

Stax SR-009S Headphone Listening Test and Equalization
I expected out of box experience to not be great. In reality it wasn't bad and had something special I could not put my finger on until I equalized it:

I started with Bass enhancement thinking it would be easy -- it was not. A shelving filter caused severe distortion at anything but moderate levels. I tried a few things and eventually just slapped that one PEQ at 47 Hz. It added extra warmth/bass without distortion. The other three filters worked the first time so I can't complain.

I was stunned at the sound quality post EQ. There was this melt in your chair spatial quality that was beyond just about any headphone I have heard. It seemed to so nicely separate instruments with uncanny precision. I know, I am talking like a subjectivist but I can't help it. Track after track sounded just amazing. The sound was detailed, spacious and just beautiful. I wished for a bit more bass but I could not get it. Volume would go up to medium loud after which I would hear static. This is not a headphone you take to hip hop party but for a more refined level of music listening.

Conclusions
Out of box the response of the SR-009S is OK (not offensive) but the headphone comes to life with a handful of filters to deliver one of the best sounds I have heard, at moderate levels. And with moderate level of bass. It is the kind of experience that gets your friend to stop laughing after you had told him how much you spent on it. It is something you may prepare your evening listening session around. After a great meal, and with a drink in hand, you sit back, put on the SR-009S on your head, hit play, and congratulate yourself for having worked hard enough to have the money to enjoy this level of musical experience.

All of these superlatives aside, company should have focused more on getting a better out of box response out of the headphone. Sources of resonances should have been found and fixed. I know it will never happen but inclusion of filtering in the amplifier would go a long way toward eliminated the need for external EQ. If they don't want to do it in digital domain, they can do so in analog.

I can't recommend the Stax SR-009S without EQ. With EQ, it is a wonderful headphone and highly recommended for low to medium level listening.

Manufacturer SR-009S SPECIFICATIONS:

Type: Push-Pull Open Back Circular Electrostatic Headphone

Frequency Response: 5 - 42,000Hz

Electrostatic Capacitance: 110pF (including cable)

Impedance: 145k Ω (including cable, at 10kHz)

Sound Pressure Sensitivity: 101dB / 100V r.m.s. 1 kHz

Maximum Sound Pressure: 118dB / 400Hz

Bias Voltage: 580V DC

Left & Right Identification: "L" and "R" indicated on the arc assembly (inside head spring) Left channel cable is marked with a solid Line. Right channel is marked with a dotted Line.

Ear Pads: Genuine Lamb Leather (direct skin contact), high-quality synthetic leather (surrounding portion)

Cable: Silver-coated 6N (99.9999%) OFC parallel 6-strand, low-capacity special wide cable, 2.5m full length

Weight: 1lb without cable / 1.3lb with cable

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As always, questions, comments, recommendations, etc. are welcome. Click here if you have some audio gear you want me to test.

Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/

 

[Doodski]

It is the kind of experience that gets your friend to stop laughing after you had told him how much you spent on it.

RFLMOA... chuckle.

 

[ROOSKIE]

Hate to say it, I voted poor.
@this price it has to excell beyond measure and superlative directly out of the box to justify the cost @ $4545. That price has swagger and it simply must be superb or I find it insulting.

Yes, cool that eq fixes much of it. So drop the 1st 4, charge $545 and we are mostly good.

By the way why $4545? Strange price.

Interesting review. Thank you.

 

[VietnamRadioEnjoyer]

@amirm If you liked EST sound, maybe its time to ask DCA for a Corina sample to review? apparently it has proper tuning and it is one of the few ESTs that can do bass (alongside Audeze CRBN)

 

[amirm]

By the way why $4545? Strange price.

It may be translation from Japanese Yen which was rounder number.....

 

[amirm]

@amirm If you liked EST sound, maybe its time to ask DCA for a Corina sample to review? apparently it has proper tuning and it is one of the few ESTs that can do bass (alongside Audeze CRBN)

I asked Audeze once and I think their answer was that I wouldn't like it because it has flat bass or something like that....

 

[PenguinMusic]

Hi,

Must say I loved this review. And all in all, I love the reviewsz of Headphones and Speakers.
Why ?
Because it seems to me that maybe, numbers do not say it all...

Looking at the numbers, it would seem these headphones are OK.
That they would match any other HP that has been through PEQ.

From Aminr's experience, it seems this is not the case.
That they are much better than that...

And as such, that you can have something that does sound better than what numbers would allow to expect.

Don't get me wrong : I am not saying that measures are useless.
They probably tell almost everything when it comes to "digital".
But maybe there is still room for subjective perception in the "analog" field ?
Which is nice because else, we would all end up with the same device at some point...

 

[solderdude]

I don't know why distortion is higher at 94 dBSPL than higher levels below 300 Hz.

I suspect because these headphones are very open you are seeing relatively (to the signal) more room noise.

What does the GD look like at 104 and 110dB. ?
IME the 'messy' GD is caused by room noises as well. I only see it on open headphones never on closed headphones so does seem to have to do something with either ambient sounds or something to do with the grille.



The forward/open sound is seen in this STAX too (the hump at 1kHz).
I liked the bass on this headphone and would easily use it without EQ. It did not sound 'special' to me but rather like one of the better planar magnetics.
That is.... with well made recordings that actually have deep lows in them. For most pop and rock recordings bass is a bit thin. For me that is the fault of the recording though.
Harman bass correction will make sure the bass is fine on those recordings too.
Every headphone amp (or headphone) should have a 'bass lift' button (just 4 to 5dB).

The sound is 'flawless' (other than the tonality a bit) and it sounds quite 'effortless'. Any 'things' you hear in the music are in the recording.

The HE1 and Sonoma model one was 'better' but both have dedicated active EQ. Because of that, sadly, the Sonoma clipped quite early. The HE1 was better in that aspect but the one controlling the volume control was knowing where to 'stop'.

The 110dB limit is typical for electrostatics and where planar magnetics have them beat. They can reach more impressive levels.

 

[Doodski]

Because it seems to me that maybe, numbers do not say it all...

The numbers do say a lot. They say that anything within boundaries is possible and that even with a small boundary delta the sound can vary considerably.

That they are much better than that...

Yes, and it's fun finding another diamond in the rough.

 

[Rja4000]

A review of a Stax !
Wonderful !
Thanks !

 

[Rja4000]

SRM-313

Would the TOPPING EHA5 amp work ?

Such low distortion with a transducer is something special.
And very much desirable, IMO.

As you say, if they could fix the issues...

We want more Electrostatics to be reviewed.
Please...

 

[solderdude]

As you say, if they could fix the issues...

Just like the SRM717 it can go a little louder and maybe treble extension could differ a bit.
Ultimately there is a limit to the plate voltage (also limited by the bias voltage).

 

[amirm]

Would the TOPPING EHA5 amp work ?

I don't have one but I assume so. In hindsight, I could have asked for a review sample and test both together. Oh well.

 

[martijn86]

There was this melt in your chair spatial quality that was beyond just about any headphone I have heard.

We all know that there are qualities to a headphone or IEM's that surpass good tonal compliance to a target and low distortion. You hear things about the difference between dynamic, planar, open or closed back, electrostatic etc. but I never see it backed up with data. Is it in group delay? Step response? Harmonics? I refuse to believe that it is a magic sauce that can't be measured.

It is the kind of experience that gets your friend to stop laughing after you had told him how much you spent on it. It is something you may prepare your evening listening session around.

I think many people are looking for these qualities but we are left to trust subjective reviews to find them. Which I think is quite frustrating.

 

[phoenixdogfan]

The fact that it can't be played all that loud just keeps it out of the top category for me. And I fully agree, it should have had a more conformant FR right out of the box, or at least had some vendor curated EQ a la Audeze.

Still, I'm sure it sounds magical at more moderate volumes if properly Eq'd, and I don't doubt it does some things uniquely well which makes it justifiably prized for those who can afford the tariff. It's just that at the end of the day I know that tariff involves paying more than $5k (phones plus electrostatic driver) for a very nice head stage (not sound stage) coming from a very luxurious looking personal sound system. Personally, I blanche at that knowing I could get any number of excellent phones for $1000-1500, and then go out and get myself an endgame pair studio monitors like the Neumann KH-150's, for about the same all-in cost.

Others, may see that differently, of course. Here;s hoping they enjoy their splurge.

 

[elberoth]

Great review Amir. This has been my experience with Stax 009 as well. At first you wonder "OMG, how much?" then you put them on and ... can't stop smiling.

One thing you missed to mention is how comfortable they are for long listening sessions.

 

[kemmler3D]

I am not saying that measures are useless.
They probably tell almost everything when it comes to "digital".
But maybe there is still room for subjective perception in the "analog" field ?

With headphones... sort of.

Unlike with electronics, with speakers and headphones, the direction from which the sound hits your ear matters, likewise dispersion characteristics. With speakers this is fairly easy to measure, the Klippel does off-axis measurements really well and you can see the results in any review that uses one.

With headphones directionality is basically impossible to measure or predict with standard techniques. The best we can do within reason is get a somewhat decent measurement of FR and distortion.

That doesn't mean directionality inside a headphone can't be measured and even controlled in principle. But the sound also interacts with the unique shape of your ear... so in practice headphone directionality is both hard to measure and not very informative, even if we did measure it.

There's also the concept of acoustic crossfeed, (i.e. sound crosses between ears via open backs) which I think is iffy, but some sites measure it, like RTINGS. In very special circumstances this could increase the sense of space.

 

[Michael Fidler]

I guess Stax have the advantage of making custom amplifiers for these cans so it wouldn't perhaps be too difficult to build a suitable analogue equaliser with a few op-amps to do away with those frequency response anomalies without causing too much distortion? 81dB is certainly impressive!

 

[Maiky76]

Here are some thoughts about the EQ.

Notes about the EQ design:

• The average L/R is used to calculate the score.
• The resolution is 12 points per octave interpolated from the raw data (provided by @amirm)
• A Genetic Algorithm is used to optimize the EQ.
• The EQ Score is designed to MAXIMIZE the Score WHILE fitting the Harman target curve (and other constrains) with a fixed complexity.
  This will avoid weird results if one only optimizes for the Score.
  It will probably flatten the Error regression doing so, the tonal balance should be therefore more neutral.
• The EQs are starting point and may require tuning (certainly at LF and maybe at HF).
• The range around and above 10kHz is usually not EQed unless smooth enough to do so.
• I am using PEQ (PK) as from my experience the definition is more consistent across different DSP/platform implementations than shelves.
• With some HP/amp combo, the boosts and preamp gain (loss of Dynamic range) need to be carefully considered to avoid issues with, amongst other things, too low a Max SPL or damaging your device. You have beed warned.
• Not all units of the same product are made equal. The EQ is based on the measurements of a single unit. YMMV with regards to the very unit you are trying this EQ on.
• I sometimes use variations of the Harman curve for some reasons. See rational here: https://www.audiosciencereview.com/...pro-review-headphone.28244/page-5#post-989169
• https://www.audiosciencereview.com/...pro-review-headphone.28244/page-6#post-992119
• NOTE: the score then calculated is not comparable to the scores derived from the default Harman target curve if not otherwise noted.

Good L/R match.

I have generated one EQ, the APO config file is attached.

Score no EQ: 68.0
Score Amirm: 71.0
Score with EQ: 77.0

Stax SR-009S APO Score Full EQ Flat@HF 96000Hz
August242023-160624

Preamp: -8.5 dB

Filter 1: ON PK Fc 26.47 Hz Gain 8.50 dB Q 0.36
Filter 2: ON PK Fc 1092.31 Hz Gain -4.88 dB Q 2.24
Filter 3: ON PK Fc 2594.12 Hz Gain 3.58 dB Q 0.90
Filter 4: ON PK Fc 4958.45 Hz Gain -7.37 dB Q 6.00
Filter 5: ON PK Fc 6039.98 Hz Gain 5.98 dB Q 2.76
Filter 6: ON PK Fc 1769.48 Hz Gain -2.52 dB Q 5.01

 

[Chester]

Great to see this reviewed, I have heard it a few times and always thought it sounded a little special, but put it down to expectation bias as it was “different tech”.

If they were a little cheaper and getting a suitable amp to drive them wasn’t such a minefield, I think I would go for it.

Thanks for the review!