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Stax SR-507 II earspeakers and SRM-006tS ‘Kimik’ energiser

Stax SR-507 II earspeakers and SRM-006tS ‘Kimik’ energiser

When it comes to personal audio, the audiophile choice for the longest time has been Stax. The Japanese company has been making audio electronics since 1938, originally concentrating on cartridges and electrostatic tweeters. Stax has been making electrostatic ‘earspeakers’ since the SR‑1 of 1960 and – although Stax continued to develop other audio products into the early 1990s – electrostatic headphones have been the brand’s primary focus for more than 50 years. Put this into perspective: Stax was making ‘earspeakers’ years before the designers of many of its rivals were born.

Of all the designs in the Stax back-catalogue, few can have been as distinctive or as successful as the SR‑Lambda. Not as ‘out there’ as the SR‑Sigma (which looked like you were wearing a pair of small loudspeakers, and perhaps the most legitimate reason why you can’t call a pair of Stax earspeakers ‘headphones’), the unique flat rectangular electrostatic block design is surprisingly comfortable and durable (it’s also arguably the ideal shape for an electrostatic panel). The SR‑507 II is a direct descendent of that 1979 design.

As an electrostatic driver energises the whole diaphragm surface uniformly, the original design concept was to give priority to the exact response to signals over rigidity – although greater rigidity makes the system less prone to environmental problems from high humidity or temperature (not necessarily a UK problem), older materials meant the benefits of a more robust assembly were often outweighed by increased resonance and reduced high and low frequency extension. However, the latest and best iteration of the Lambda, the SR‑507 II uses a new GRP shell, which gives the whole assembly more rigidity and robustness, because it is now coupled with a thinner diaphragm made of a new variant on Mylar, which is itself more robust than predecessors. All of which adds up to a design with even less resonance and distortion than its predecessors, which is saying a lot.

Elsewhere on the Stax SR‑507, the connector cable is also now made from silver-plated high-purity PC‑OCC copper, and the wide, flat spacing of the conductors also helps to lower capacitance. The new Lambda also features a more comfy headband, with more adjustment, goat skin cushions, and better moulded ear shapes for better pressure to the rear of the earspeaker.

An electrostatic headphone/earspeaker cannot just plug into any headphone amplifier, as it needs power to energise its panels: this is why they use a large five‑pin connector that looks like a valve seat instead of a jack or XLR. The SRM‑006tS is the top energiser in Stax’ Lambda series (with the valve SRM‑007tII and solid-state SRM‑727II destined for the top SR‑007Mk2 and SR‑009 earspeakers). This is a high-voltage, low current amplifier designed specifically to drive electrostatics; it uses two 6FQ7/6GC7 double triodes and sports two sets of RCA inputs and one set of XLRs. There is also a solid-state SRM‑323S, which undercuts the 006tS, but remains untested at this time. It has its fans, but many also suggest the money spent on the 006tS is money well spent.

This is, however, in its standard guise. Fortunately, Nigel Crump of Symmetry the UK distributor hands the SRM‑006tS over to his engineer (Mark Dolbear of High End Workshop, who also runs Electromod and knows a thing or two about headphones) for what is known as ‘Kimik’ modifications. Dolbear spent a considerable amount of time learning how to drive Stax’s own test equipment, but in becoming an expert in the testing, also learned how he could improve the performance of the energiser in several key areas. First, he replaces the standard tubes with cryogenically-treated, matched valves, and fits these with EAT tube dampers. Then he replaces the standard case fuse with one from Synergistic Research (Electromod is the UK distributor for Synergistic)… and then spends the better part of a week precisely setting up bias and offset, essentially ‘blueprinting’ the energiser.

In use, the rectangular shape of the Lambda sits extremely well over the ears, although it can feel a little ‘clamped on’ if you are a big-headed reviewer. There is a lot of adjustment possible, but the overall feeling is one of always being aware there is an earspeaker in position. It’s not uncomfortable, though, and certainly not claustrophobic in the manner of the old Jecklin Floats (which always made me feel like I was being fitted for a particularly tight crash helmet). And, as with any electrostatic system, there is a lot of sound leakage in and out of the headphone – this is not a headphone to wear while someone else is listening to the TV, because you’ll only upset each other as the TV sound leaks into your listening and vice versa. In a quiet room, though, and the Stax combo is an incredibly different beast.

We had one of Symmetry’s demonstrator sets, which arrived fully run in, but the sound it makes is very Stax indeed. It seems like a paradox to say something is at once ‘warm’ sounding and ‘neutral’ sounding, but when you hear the SR‑507 II/SRM‑006tS ‘Kimik’ package, it becomes an obvious and natural description of things. Although it’s a lot less warm than it used to be.


I remember listening to a pair of Stax Lambda headphones some years back, and they made a lasting impression. The overall performance was one of incredible clarity in the midrange, coupled with a slightly soft, but hugely enjoyable bass and a treble that was detailed and informative, but slightly laid back. The overall impression was one of warm, satisfying detail, not unlike the sound one might hear from a pair of original Quad Electrostatics. The latest SR‑507 II/SRM‑006tS ‘Kimik’ are very different beasts, in the way more modern Quads are different to those original ‘57s. A deeper, tighter, and more dynamic bottom end has replaced the lush, soft bass and the treble has been extended, while retaining its informative and detailed demeanour. I think this makes for a better overall balance.

The SR‑507 II is a mature listening choice. That doesn’t mean you need to be in your 60s and it doesn’t mean you only listen to ‘old geezer’ music. It means it’s the kind of headphone you buy when you are done with the fireworks and the glitz, the bright and the instantly impressive. Your listening is refined and focused, and you make a discerning choice of equipment based on those criteria. You will want an earspeaker that creates excellent soundstaging, effortless and real dynamic range, detail, good frequency extension, and an absence of strong character, preferring instead to let the music sing for its own supper. And the SRM‑006tS ‘Kimik’ completes the package brilliantly.

Jazz is an obvious partner here: Pure Desmond by Paul Desmond [CTI] is a very laid-back collection from the composer of ‘Take Five’, and ‘Nuages’ sees the interplay between Desmond’s alto sax and Canadian guitarist Ed Bickert rendered beautifully, with delicate, almost non-existent underpinning from bassist Ron Carter and sticksman Connie Kay. It’s easy for this to slip into sounding like bland dinner jazz, but instead the Stax pairing helps show this to be one of Desmond’s more sophisticated solo offerings. In particular, it shows that if Bickert had stepped into the limelight more, he’d be as well known in jazz circles as someone like Kenny Burrell. I think jazz is an obvious partner for the Stax earspeakers in part because they both seem like late night travellers. This kind of jazz isn’t the kind of thing you would play early in the morning, and the refinement of the Stax combo isn’t like a musical jolt of coffee – more like a brandy at the end of a fine evening: something to be relished, not rushed.

As we moved through the musical repertoire, so the Stax rose to most challenges. Vocals, especially the purest of female vocals, were intelligible to an excellent degree – ‘My Husband’s Got No Courage in Him’, by Silly Sisters (Maddy Prior and June Tabor) from the Chrysalis album of the same name, is a fine piece of English folk-rock (with the accent on the ‘folk’), and the two pure voices singing harmony are just made for the clarity and soundstage separation of the SR‑507 II/SRM-006tS ‘Kimik’ combination. The dynamic range and bass depth gave them an uncanny sense of two people being in the room with you. The same happened with orchestral music, although here you can hear a slight foreshortening of soundstage width and depth next to the very best in breed. Then again, you can own I think it’s about two and a half sets of this Stax combination for the cost of the best of breed, so I’ll trade a slightly smaller soundstage! Piano in particular though is outstanding – and it makes dynamic drivers sound like they are dividing a piano up into its component parts, where here, it’s just a piano.

Stax earspeakers have one design limitation: they do not go loud, and the SR‑507 is a fine example of just how much volume they deliver to your ears. It’s worth putting this into perspective. These are not whisper quiet designs. They go more than loud enough for most listeners, especially those who tend to know their way around audio systems and like their ears undimmed by excessive SPLs. But if you are looking to find a headphone that can play Metallica at volume, you need to be looking at headphones, not earspeakers. The SR‑507 II/SRM-006tS ‘Kimik’ is an elegantly dynamic and informative transducer, but it’s not for headbangers.

In many respects, the SR‑009 flagship we tested (and seriously loved) is almost ‘too’ good. It’s incredibly detailed, demanding, and – ultimately – rewarding. It will tell you if the mastering engineer was not playing their ‘A’ game, and it will tell you when not all your audiophile ducks are in a row. Things sound outstanding through the SR‑009, but there are times when you want to just relax into the music a little more. You still want the insight, the definition, and that sense of supreme fidelity. But, and this is important, you don’t want to be so wrapped up in the fidelity that you lose the enjoyment. Put simply, the SR‑009 is so revealing it can leave you sometimes wanting less!


In fact the two are very close in tonal terms. The SR‑507 II/SRM‑006tS ‘Kimik’ is every bit as detailed, but not as expansive, as focused, or as ‘profound’ in the bass. However, the SR‑507 II is also revealing of source and system, and that often means longer listening sessions to a wider range of music. Don’t view this as a negative in either direction: both are hugely entertaining, hugely revealing earspeakers delivering a quality of performance that you will struggle to achieve in the room with loudspeakers at virtually any price. The SR‑009 justifies its place at the top of the Stax stack by virtue of uncanny levels of information, while the SR‑507 II sweetens the pill slightly. Another way of putting this is the SR‑009 is close to perfection, and the SR‑507 II is close to close to perfection. On balance, I’d recommend going for the SR‑009 if you can possibly justify it, but I’d still hanker after the SR‑507 II for some ‘end of the evening’ chilling out.

The Stax SR‑507 II/SRM‑006tS ‘Kimik’ would be highly recommended if it came at the separate price of £2,240, but Symmetry prices the package very aggressively at £1,895. I think this makes the Stax SR‑507 II/SRM‑006tS ‘Kimik’ the best overall balance of performance and price in the high-end headphone world, if you are prepared to trade volume for detail and refinement. Very highly recommended.

Technical Specifications

SR-507 II

  • Type: Push-pull oval sound element, open-back, electrostatic headphone
  • Frequency response: 7Hz–41kHz
  • Electrostatic Capacitance: 110pF (including cable)
  • Impedance: 145kOhms nominal (including cable)
  • Sensitivity: 100dB/100V rms @ 1kHz
  • Maximum Sound Pressure: 118dB/400Hz
  • Bias Voltage: 580V DC
  • Weight: 533g (with signal cables)
  • SRM-006tS ‘Kimik’
  • Type: Vacuum tube output stage Low noise dual FET input Class A operation, Pure balance DC amplifier configuration Earspeaker driver unit
  • Vacuum Tubes: 2× 6QF7/6CG7
  • Inputs: two stereo single-ended (via RCA jacks), one stereo balanced (via dual 3-pin XLR jacks)
  • Outputs: one RCA parallel output, five pin balanced headphone socket (×2)
    Frequency response: DC – 80kHz, +0dB, –3dB
  • THD+N: Max. 0.01%, (1kHz, 100V rms)
  • Gain: 60dB (×1000)
  • Rated Input Level: 200mV/100V outputs
  • Maximum Input Level: 30V rms at min. volume
  • Maximum Output Level: 300V rms (1kHz)
  • Dimensions (H×W×D): 104 × 195 × 420mm
  • Weight: 3.4kg
  • Price: £1,895 (package price. Sold separately, £2,240)

Manufacturer: Stax Ltd


UK Distributor: Symmetry

Tel: +44 (0)1727 865488



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