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

Headphone amps and amp/DACs
Stax SR-507 II,
Stax SRM-006tS 'Kimik'
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.

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