Every hi-fi category has its legendary products, and for headphones few would deny a place in that pantheon to the Stax Lambda Signature ‘earspeakers’ (as Stax still calls its headphones). The Japanese company had, after all, developed the world’s first electrostatic headphones at the dawn of the 1960s; the original Lambda came 20 years later, and the Signature in 1987 — it became a reference product for a decade. This reviewer remembers being once saddled with a test of 70 types of blank tape cassettes for a UK hi-fi title; this potential nightmare of comparative star ratings could be reliably completed only once we had in place the spectacular audio resolution of a pair of Lambda Signatures, with their dedicated valve amplifier attached to another legend, a top-end Nakamichi cassette deck.

Many things have changed since those halcyon days of high fidelity, but we were delighted to discover that the extraordinary resolving powers of Stax electrostatic earspeakers are still wondrous to behold. 

Stax today is not quite the same company that released the Lambdas — it was revived from insolvency in the 1990s, and more recently came under the ownership of Beijing-based Edifier. Its top earspeakers now sport circular headshells, an outrageous departure from the company’s trademark rectangular designs.
So we were delighted to be offered a review loan of the new yet more traditionally hung SR-L700s, along with one of the company’s dedicated driver units, the SRM727II (pictured overleaf). Further delight to find that the ‘L’ in the name stands not only for Lambda, but for “advanced-Lambda”! And most delight of all when we burned them in and then put them on.   
In brief, then, the luscious tech of this open-backed electrostatic design includes thin-film diaphragms (hand-selected, we gather) sitting between the fixed electrodes that are machined through three-layer heat-diffusion etching, driven and provided with the required 580V bias voltage through their low-capacitance high-purity cables from the driver unit (or headphone amplifier, or “Energiser” as Stax sometimes calls them).
Connecting the two is the sturdy and usefully long (2.5 metres) cable, which uses conductors of very high purity — 99.99999% copper is used for the core wires, and silver-plated 99.9999% copper for the perimeter. It’s a flat format cable (the parallel structure is said to lower the capacitance between each wire) which is very easy to handle, and entirely devoid of the kinks and curls which can plague some of the thicker braided cables employed by other brands. The plugs at the amp end are Stax’s long-employed ‘pro-bias’ five-pin designs; two sockets are provided, should you wish to listen with a friend. 
To drive the SR-L700 earspeakers, we were supplied the SRM-727II headphone amplifier, although this is just one of Stax’s options for driving them. The SRM-727II was introduced back in 2007, and employs a semiconductor output drive stage that employs no negative feedback, the first such circuit ever employed in a Stax driver unit. A pure balance DC amplifier configuration is used for the large current emitter-follower Class-A output stage, said to optimise current gain while making it possible to achieve lower impedance at the output stage and significantly improve dynamics.
It’s a fine piece of electronics, and very Japanese in its impeccable build and aluminium casework, purposefully deep at 42cm and weighing a full 5.2kg, heavier than some full-sized stereo amplifiers designed to drive full-sized speakers. It offers the choice of unbalanced RCA or balanced XLR inputs, with the RCA phono sockets duped in parallel so you can chain them on. A switch selects between balanced or unbalanced inputs, but that aside, your only controls are the power switch and the lovely main potentiometer.  
As we said, there are other options for the headphone amplifier — there is, for example, a valve-based driver unit at a similar price. We can’t judge any differences that might become evident against the similarly-priced valve driver, or the less expensive options, but we can tell you that the sound which emerged from the lambs-leather earcups of this combined system was simply divine. Nothing creates a sense of soundstage space and detail like electrostatics, but the Stax deliver solid bass as well, and such tight fast response that unexpected transients had us physically jumping — one such was the start of Led Zeppelin’s rough mix (rough playing, but superior production) of ‘Since I’ve Been Loving You’, where Bonham’s snare quite knocked us back in our seat (we were listening loud enough to enjoy his squeaky bass pedal — the distortion here remains so low, that’s what you do).
Everything sounded wonderful (“EVERYTHING” screams our listening notes), from the tonally laid-back Diana Krall vocals on her Buble duet of ‘Alone Again Naturally’, to the complex layers of kd lang’s ‘The Air That I Breathe’; these latter are often flattened or congested with headphones of less resolving power, but here they were delightfully delivered with the crescendos of the choruses simply rising to a higher level rather than being clipped or compressed.
Those dynamic strengths were evident in the slam of instrumentation during jumpy jazz tracks, Chick Corea’s Australia Piano Concerto a rollicking ride with its bursts of orchestration breaking forth. And classical fans have long appreciated the delights of electrostatic sound, only enhanced by giving the Stax system high-resolution recordings — on our favourite Holst recording, an 88.2kHz delivery of the BBC National Orchestra of Wales performing the not-very-funny comic opera ‘The Perfect Fool’, the percussion held its own acoustic space even under the pummeling of strings and tympani in a way some very expensive loudspeakers simply cannot achieve. It’s a reminder that headphones are not just for privacy — they are also price effective, in that you can achieve the highs of headphonic joy rather more cheaply than an equivalently high-end loudspeaker-based hi-fi chain. 
After one long evening under the spell of the Stax system, we went off to hear a prototype of Sennheiser’s new HE 1, replacement for the Orpheus headphones, thinking they would have quite a job to sound better. The HE 1s were perhaps softer-sounding, while still achieving the same revelations, but really, the HE 1s are likely to retail around $75,000, and the Stax were delivering similar levels of pleasure. Their open leakage makes them useless in a shared room, of course (unless you plug a second pair into the driver). That aside, if you’re after a top-level headphone (or earspeaker, sorry), then you simply can’t NOT audition Stax. We’d recommend taking a nice long playlist with you. Maybe stay the night.
Stax SR-L700 earspeakers $2499 
tested with SRM-727 II driver $3399 
Type: push-pull open-back electrostatic  
Driver size: 90 x 50mm
Impedance: 145kΩ (including cable, at 10kHz)
Weight: 360g 
Warranty: One year
Product page: Audio Marketing