stax srm-d50 driVer/dac & sr-l500 earsPeakers
Warning! Once you have listened to your favourite music through a pair of electrostatic headphones, you will be spoiled for anything else.
Legendary. It’s a powerful and emotive word, and it is one that every audiophile I have ever spoken to uses when they are talking about Stax headphones… or, as Stax prefers to call them… ‘earspeakers’.
In fact, the unique design of nearly all Stax headphones is such that calling them earspeakers is actually highly appropriate, because in reality, they are essentially miniaturised full-sized loudspeakers… even the shape of the headpiece is rectangularly reminiscent of a loudspeaker cabinet, rather than the circular form usually employed for headphones. (I wrote ‘nearly all’ in that sentence because the three most expensive— and most recently designed—models in the Stax range have circular, rather than rectangular, earpads. Traditionally, however, Stax headphones had rectangular ear pads.)
But what makes Stax earphones truly unique is that whereas almost all headphones currently available for sale use one type of dynamic driver element or another (where a coil of wire interacts with a magnetic field in some way to move a diaphragm to create sound) Stax earspeakers instead use high-voltage electrical charges to move the diaphragm. You may well ask: ‘How high a voltage?’ To which the answer would be ‘ around 400 volts a.c. for the drive voltage as well as around 600 volts d.c. for the bias voltage’, according to Stax. The design is called ‘electrostatic.’
But how do you safely create such high voltages? With what Stax calls a ‘driver’, of which the SRM D50 Driver/DAC is the newest addition to the Stax driver line-up.
THE EQUIPMENT
Since it has been in the business of building electrostatic (ES) earspeakers since 1960, and all earspeakers require special electronics to ‘drive’ them, Stax has also been building ‘drivers’ since 1960. Of the models in the current range, the flagship is the SRM-T8000 ($7,999) and the entry-level model is the battery-powered SRM-D10 ($1,499) which is also the only other model in Stax’s range that incorporates an inbuilt digital-to-analogue converter. Indeed it appears the success of the SRM-D10 was one reason Stax developed the SRM-D50 in the first place, because, accord-
ing to this Japanese company, it was ‘ inspired by the battery-powered SRM-D10.’ Stax has obviously taken note of the huge resurgence in the popularity of headphone-only listening, particularly in such circumstances where the music source is a ‘non-traditional’ device for playing music, such as a computer, tablet or smart phone.
You can access the DAC inside the SRM-D50 only via physical means (that is, there is no Bluetooth or Airplay), but you can use copper cable (to a gold-plated RCA input) or optical fibre (to a Toslink input) to input S/PDIF, or you can also connect via USB (Type B) if you want to connect digital signals directly. If you’d prefer to connect analogue signals, there are two gold-plated RCA inputs on the rear panel for this purpose.
Switching between the inputs is totally manual, and accomplished via the rather interesting toggle switch at the left of the SRM-D50’s front panel, alongside which is a single five-pin DIN socket to which you connect the earspeakers (only ‘PRO-Bias’ Stax earspeakers can be used). Unlike many other headphone drivers in Stax’s range, this means that only a single pair of headphones can be used at a time. Given the target market for the SRM-D50 driver, I really don’t envisage that any potential buyers will be put off by this ‘single listener only’ limitation.
At the extreme right of the SRM-D50’s front panel is a small, smooth-rotation volume control and, to its left, an equivalently-sized VU meter with a rather old-fashioned look, right down to the ‘orangey’ colour of the meter illumination. The last time I saw a meter like this, it was on an über-expensive hi-fi component made in Switzerland by Nagra. Good company to keep!
VU stands for ‘Volume Unit’ and back in the days when almost every item of professional audio equipment had one (or more) of them, such meters were calibrated so that when the needle showed ‘0VU’, the input (or output) level was exactly 1.228 volts RMS across a 600Ω load. You weren’t supposed to use the meters to actually measure anything, but if the engineer made sure that the needles hovered not too far either side of the 0dB mark when music was playing, the circuit noise would be sufficiently far down so as not to be audible, and the transient peaks coming through would not be so high as to cause distortion. These days, the idea of ‘calibration’ has been lost, and so-called ‘VU’ meters are used solely in order to give an indication of signal level.
Stax’s ‘Owner’s Manual’ has a diagram with an arrow pointing at the VU meter and a caption saying it’s an ‘ Audio Source Indication Table’ and that ‘ when there is audio playing, the dial-plate of audio source indication will rotate.’ Needless to say, that’s not correct. The meter is not an audio source indication table, and it does not rotate. The meter is just a meter, and the audio source is indicated by green LEDs to the left of the mode switch. That the manual could be so wrong surprised me, but I was also disappointed to find it contains very little useful information (not even a link to a USB driver) and is also written in very poor English. Initially I thought that it might be a translation error, and that it was the colour of the meter’s lighting that cycled to indicate the audio source ( a la Chord), but no matter what input I used, or what digital format, the meter colour remained constant.
The DAC Stax is using inside the SRM-D50 to convert digital signals to analogue is one of ESS’s earlier-generation DACs (ES9018) but it can handle all 16-bit or 24-bit PCM file formats up to 384kHz as well as DSD 2.8/5.6MHz (using DoP), and its performance is excellent, as it uses ESS’s patented 32-bit Hyperstream DAC architecture and Time Domain Jitter Eliminator, which ESS claims delivers a signal-to-noise ratio of 135dB and THD+N of just 0.0001%. It also has a filter that can be programmed for any type of rolloff, though Stax doesn’t give any indication of what type of filter it is using.
The output from the ES9018 is sent to a Texas Instruments OPA 1642 dual JFET operational amplifier for which TI claims ultra-low noise (5.1nV/√Hz at 1 kHz), ultra-low distortion (0.00005% at 1kHz) and a very high slew rate (20V/μs).
Given the high quality of the DAC and the TI device, I was surprised that Stax has not fitted a line output to the SRM-D50 to enable owners to use it as both as a headphone DAC and as a conventional DAC. This would have been a great ‘value-add’ and tempted many more audiophiles into buying it.
If you’re using the digital input you won’t need a USB driver for the SRM-D50 if you’re using an Apple device, or one with USB OTG (On The Go) functionality, but if you’re using Windows (7, 8 or 10) you will have to download a driver from the internet. I personally would prefer to have everything I need to get a component up-and-running supplied in the box with it, so I think Stax should have provided the driver either on a disc or on a USB stick. Stax says the driver is here: www. staxaudio.com/driver/stax-srm-d50 but when you click on the link on this page that says ‘SRM-D50 Windows Driver 30A0 V4.11.0’ the download is actually a USB Windows Driver for the Stax SRM-D10 (but it apparently works for the SRM-D50 as well). I did eventually find one specifically for the SRM-D50 on Stax’s Chinese website at: www.stax.com.cn/ stax2/contact_us.htm
Interestingly, when Stax (Japan) announced the SRM-D50 in 2018, it stated that the product would not be available for sale in Japan, specifically emphasising in its press release: ‘ This product will be a new product for overseas, and we will not sell it in Japan at present.’ [https://stax.co.jp/2018/06/26/nr_ srm_d50/] I am not sure if the company made this statement because the SRM-D50 was one of the first Stax products to be manufactured in China, rather than in Japan, where traditionally almost all previous Stax products have been built, or whether it is because Stax is wholly owned by Chinese manufacturer Edifier and, at the time it purchased Stax, in 2011, Edifier’s president and CEO, Wendong Zhang, specifically stated that Stax would
Once you have listened to a pair of electrostatic headphones— sorry… earspeakers!—you will be spoiled for anything else…
continue to operate independently in Japan as a separate entity. [www.edifier.com/int/ en/blog-news-press/edifier-acquisition-headphone-brand-stax]
SR-L500 EARSPEAKERS
In order to evaluate the SRM-D50, Stax’s Australian distributor, Audio Marketing, also loaned us one of the most popular Stax Earspeakers, the SR-L500, which sell for $999 (RRP). You could, however, use the SRM-D50 to drive any ‘PRO-Bias’ Stax earspeakers.
The SR-L500 design has fixed perforated stainless steel electrodes between which is a high-polymer 45×90 thin-film diaphragm that’s less than two microns thick. The four stainless steel electrodes (two per ear-piece) mean that these Stax SR-L500s are rather weighty on the head, tipping the scales at close to half a kilo. Thankfully, the headband has been made wide enough and comfortable enough to distribute this weight over a larger surface area of your head, so the weight isn’t as noticeable as it might otherwise have been. The band is also large enough to accommodate a so-called 10-step slider mechanism (ARC) by which you can adjust the position of the ‘ear-speakers’ over your ears for maximum comfort and best sound. It’s exactly the same mechanism Stax developed for its model SR-009 ear-speakers. I say ‘so-called’ 10-step slider, because it actually offers eleven different settings, so one more than its descriptor would seem to indicate!
The shape of the electrostatic electrodes means that the shape of the ear-pieces is completely different to dynamic (or other) headphones that use round drivers, as you can see from our photograph, and as I’ve already mentioned. This means that there’s plenty of room for even the largest ears, but also means that the sound is arriving at the ear from above and below the external ear (pinna), which gives a more natural sound than if the sound waves are only fired directly into the ear canal, which is effectively what happens with conventional dynamic drivers, no matter whether they’re around-ear, on-ear or in-ear types. The high-tech electrostatic driver elements Stax uses are, as you’d imagine, quite delicate, with Stax providing two warnings about the need to avoid ‘external shock’, so you should make sure you never drop the SR-L500s, or handle them roughly, for example by placing them down quickly onto a hard surface. Stax also recommends that you: ‘ Avoid using earspeaker in high-temperature and humid conditions.’
However the one thing you should NEVER do with your SR-L500s is push against the electrode that is closest to the ear, because this could damage it irreparably. (The outer element is protected by a hard plastic grid, which means you can’t get anywhere near it.) In these litigious times, we’ve all become quite used to reading warnings on our takeaway coffees—‘ Caution! May be hot!’— and on our packets of peanuts—‘ Warning! May Contain Nuts!’— so I wasn’t sure if Stax was being overly cautious by including a separate ‘ Warnings and Cautions’ sheet along with its Owner’s Manual which, in addition to all the usual cautions, recommended that (amongst others I won’t mention) that you should not use the SR-L500s ‘ immediately after bathing’ or ‘ when your hair is wet’ or ‘ when driving a car or motorcycle’ or ‘ when wearing pierced earrings’. One can only wonder how Stax imagines you’d ever fit a pair of SR-L500s under a motorcycle helmet! Nonetheless, Stax does sell replacement electrostatic diaphragms for all its models. A replacement pair to suit the model SR-L500 retails for $800.
The headphone cable is also completely different from that of conventional dynamic drivers, due to the need for the additional electrical conductors required to provide the bias voltage to the electrodes. The six cables
The purity of the sound issuing from the Stax SRM-D50/SR-L500 combo was simply (and literally) jaw-dropping!
are arrayed alongside each other in a flat configuration in order to reduce the capacitance between them, so the cord looks for all the world like a long strip of liquorice in terms of shape, colour… and even texture!
As for the conductors inside the cable, they’re made from high-purity copper, purified to 99.99999 per cent pure using a technique patented by Hitachi.
Unlike the SRM-D50 which, as I previously stated, is manufactured in China, the Stax SR-L500s are manufactured by Stax in Fujimi-shi, Saitama-ken, Japan, a small city that’s situated around 25km to the northwest of Tokyo.
IN USE AND LISTENING SESSIONS
It was only after I started listening to the SRM-D50/SR-L500 combination that I realised that the VU meter on the front panel shows the level of the incoming signal, rather than the level going to the headphones, which is what I might otherwise have expected. No problems with this except that the Stax does not provide a method whereby you can adjust the level of the input signal, and in situations where the incoming signal is digital, this means you will sometimes have no means of adjusting the input level at all. When I connected the digital output of my CD player to the coaxial input of the SRM-D50, the VU meter’s needle spent a disconcerting amount of time in the ‘red’ zone at the right-hand end of the meter, and of course I couldn’t do anything about it. At the same time, despite this, I heard absolutely no distortion at all, so despite the SRM-D50’s meter indication being well over 0VU, its input was not being overdriven. It was also immediately obvious that the meter did not have true VU ballistics, because the needle’s motion was completely undamped. It flicked back and forth at such high speed when playing music that it was impossible to make any real judgement of ‘average’ level… all of which means the meter is really little more than a show pony.
The same could not be said of the sound issuing from the Stax SR-L500s, which was pure champion thoroughbred, right up there with Winx. ( For readers not acquainted with the ‘sport of kings’, Winx is Australia’s most famous racehorse, a seven-year old mare that recently became the first Australian horse in history to post 31 consecutive wins and by the time you read this will likely have posted several more… Editor.)
The purity of the sound issuing from the SRM-D50/SR-L500 combo was simply (and literally!) jaw-dropping… I personally found myself listening open-mouthed in wonderment, nearly unable to believe what I was hearing. The reproduction of the sound of the flapamba that introduces Rikki Don’t Lose That Number, from Steely Dan’s ‘Pretzel Logic’, is absolutely perfect. If you listen to this track on ordinary headphones (or loudspeakers) you’d be forgiven for thinking you were hearing a marimba, but if you listen using this Stax combo, you’ll immediately know it’s not a marimba at all, but some other similar-sounding percussion instrument (even if you’ve never heard the sound of a flapamba).
But it isn’t only the tonal nature of the flapamba’s sound, but the spatial quality of it, where the sound starts high up and to the left, then partially pans down and across for a brief foray to the right, all the while not quite maintaining focus, before returning right and disappearing totally… which means that when the piano riff (famous, but actually knocked off from jazz composer and pianist Horace Silver’s Song For My Father) kicks in totally focused at stage centre, the sheer dramatic effect is stunning. BTW, I love Jeff ‘Skunk’ Baxter’s guitar solo on this track. Listening to it, you can’t help but wonder how he started out as a drummer, then played guitar with both Dan and the Doobies, and is now Chairman of the US Congressional Advisory Board for missile defence! (Can’t make any drummer jokes about that one!)
Sticking with the Dan, I played Kid Charlemagne not only to hear Larry Carlton’s guitar solo (listed as one of the 100 greatest guitar songs by Rolling Stone magazine) but also to hear Chuck Rainey’s fabulous bass lines on this track and Bernard ‘Pretty’ Purdie’s percussion (he was indicted into the drumming hall of fame in 2013). His kit sounded incredibly realistic through the Stax combo, and it’s not only the superb transient ability of the electrostatic diaphragms, which means they start and stop instantly, so the speed of the response is second to none, but just listen to the sound of Purdie’s high hats—sizzling without ever sounding over-bright.
Listening to Purdie made me long to hear his signature ‘Purdie Shuffle’ (playing triplets against a half-time backbeat) so it was onto Babylon Sisters from the band’s last album where it’s a feature, plus I also got to hear the way the Stax combo delivered the detailing and complexity of the intro, only to be not at all surprised to hear every single instrument, and every individual note, more clearly than I’ve ever heard them before, though either headphones or loudspeakers. (Jeff Porcaro also played drums on Gaucho, but much of the drumming was actually a drum machine.) Gaucho was Steely Dan’s last album for 20-years, most probably because they’d run out of original ideas, proved by the fact that Keith Jarrett sued Becker and Fagen for copyright infringement over the title track, claiming it plagiarised his song Long As You Know You’re Living Yours. And if you listen carefully to Babylon Sisters, especially at around 1:18 in, you’ll wonder why Billy Joel didn’t sue them as well!
The clarity and ‘open-ness’ of the sound from the SRM-D50/SR-L500 combo is so amazing that it’s easily possible to completely forget you’re wearing headphones, so complete is the illusion of space and depth. The ‘sound from the centre of your head’ feeling that you often get when listening to headphones, particularly when there’s a strong central stage presence, was completely absent. A part of this is no doubt due to the design, because sound is issuing from both sides of the diaphragm, so anyone in the same room as you is going to hear a rather tinny, low-volume version of whatever you’re listening to. (My daughter didn’t mind the sonic ‘spill’ when I was listening in the same room as she was studying, but she did complain about a ‘clicking’ sound which I couldn’t hear at all. It turned out the clicking
was coming from the VU meter needle hitting the top and bottom stops. I couldn’t hear it because I was wearing the headphones.)
Speaking of volume, the sound of the SRM-D50/SR-L500 combo is so clean and so free from distortion that you’ll be tempted to turn the volume up. Don’t. Firstly, you could damage your hearing, and secondly you could damage the electrostatic diaphragms, with Stax providing yet another warning that if the electrode and the diaphragm are ‘brought into strong contact’, that they could ‘ easily deteriorate’. I personally doubt that anyone could possibly turn the volume up high enough to do this whilst wearing them, and so does Stax, it seems, because its warning involves turning the volume up while you’re not wearing the headphones… though why you’d want to do this is beyond me.
Given the lack of distortion and the spatial clarity of the Stax SRM-D50/SR-L500 you might well imagine that they’d be perfect for listening to complex full-scale orchestral works… and you’d be perfectly right: they are perfect for this purpose. But in fact the Stax SRM-D50/SR-L500 combo will deliver the sound of ANY musical genre to an exceptionally high standard… the very highest standard, in fact. Listening to Beethoven’s ninth (and last) symphony first raised the hairs on the back of my neck and then, when the chorus enters in the fourth movement, tears to my eyes. The emotional delivery made possible by these headphones is all-encompassing. Whereas bass response is often seen as a limitation with electrostatic loudspeakers, it is not a limitation with electrostatic headphones. The proximity of the membranes to the ears—and the fact that the earcup around the membranes totally encloses the ear—means that the bass is not rolled off at all, so it’s always fast, free of distortion, tonally accurate and reproduced at the correct volume for spectral accuracy.
Given that the SR-L500’s are rather more ‘thorough-bred’ and sensitive than normal dynamic headphones, I would definitely recommend investing in a good headphone stand for safe storage, plus Stax’s HPS protective covers ($49) to minimise the chance of the electrostatic diaphragms becoming dusty.
CONCLUSION
Once you have listened to your favourite music through a pair of electrostatic headphones—sorry… earspeakers!—you will be spoiled for anything else. And by
The Stax SRM-D50/ SR-L500 combo will deliver the sound of ANY musical genre to an exceptionally high standard