A sealed-back Stax earspeaker? The arrival of such a beastie would shock traditionalists as would a Ferrari SUV. But as Stax tells us, 'For a number of years, we have been approached by numerous recording studios to develop a 'closed back' type of earspeaker. [It] would allow engineers to monitor in environments that have high ambient noise levels.'

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Let's face it: the only thing that keeps Stax headphones from being regarded as 'perfect' is sound leakage in both directions. This is not debilitating, though, and Stax has made, exclusively, open-back electrostatic headphones for 40 years. But if there's a demand for sealed Staxes (e.g. my son gets cheesed off when I use open-back headphones in the same room where he's watching TV), then why not meet it?

Stax's dilemma? Doing it without compromising the very airy, open sound that makes their earspeakers so coveted.

Stax's 4070 uses a special enclosure to house the high purity copper electrodes and ultra-thin electrostatic diaphragm, a cup assembly employing an original '"Bass Reflex" ventilation air structure' and sporting large cushions that completely cover the ears. Reminiscent of rectangular open-back Staxes, such as the SR-404, the cushion forms a seal against outside noise. The sealing effect endows the 4070 with exceptional bass response, though the closed-back cup means that the 4070 is automatically at less of a disadvantage than the open models.

Stax earspeakers need energising just like MartinLogan and Quad electrostatic loudspeakers, and do not work via a conventional headphone socket. Because the energisers - which also act as the headphone's amplifier - are dedicated to Stax headphones (ditto for other makes of electrostatic headphones), one cannot compare the sound of the energisers to anything other than the various Stax models. At present, there are four available - two each of valve and solid-state - offering a variety of features.

Supplied with the 4070 was the SRM-717, the better of the two solid-state models, retailing for £1295. It only works with the PRO-bias earspeakers, not the earliest models, and connects to a pre-amp as if it were an ordinary power amplifier, so it's also shorn of input selectors. For the review, I used it entirely as a stand-alone system with sources with variable output, ensuring that I auditioned just the headphones and energiser, without a pre-amp in-between the Stax and the source.

In the SRM-717, an all-stage semiconductor, 'Pure balance' DC amplifier amplifies the balanced input, and delivers this through high-grade, dual-shaft, quad-unit volume control. A low-noise dual-FET is used at the first stage, and a large current emitter follower at the output stage to deliver the necessary low impedance. Non-magnetic material is used for the chassis, which measures 195x103x420mm (WHD). Gain is 60dB, the frequency response is stated as DC to 100,000Hz and high frequency distortion is a maximum of 0.01 percent. As operation is true Class A, the SRM-717 runs very hot. Which brings me to the valve energisers.

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