Stax has been making electrostatic earspeakers (please don't call them headphones) since 1960, when the company introduced the SR-1. Stax still makes a range of earspeaker systems beginning with the $565 SRS-002 on up to the $1,775 SRS-4170. Stax also has several more expensive "separates," including the flagship $4,450 SR-009 (earspeakers and $2,150 SRM-007tII amplifier. This review will concentrate on the SRS-4170 system, which is situated near the middle of the Stax lineup.
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The SRS-4170 system is made up of two components: an earspeaker and an amplifier to power it. Electrostatic headphones, unlike conventional "dynamic" headphones, require a constant bias voltage charge to operate. This charge can be supplied by a specialized amplifier, or a conventional amplifier coupled with a Stax adapter box. Stax ceased making the adapter units years ago (but they are still readily available in the used market.) The SRS-4170 system couples the $520 SR-407 earspeaker with the $1,325 SRM-006ts amplifier. If you do the math, the combination price for the two components together is exactly the same as if you purchased the two separately. Obviously, Stax doesn't offer a package discount.
As the earspeaker part of the system, the SR-407 looks very much like some earlier Stax models, including the venerable Lambda Signature Nova and Lambda Nova Classic. The SR-407 even shares the same headband, driver yokes, earpads, and outer casing as the earlier models, but inside, it's very different. Since first introduced in 1979, the Lambda series has had four revisions. The original Lambda signature had a one-micron-thick Mylar panel, but shortly after introduction, the thickness changed to 1.5 microns, due to the addition of adhesive resin. When Stax introduced the SR-404, the panel's thickness was reduced to 1.35 microns. The SR-407 has that same 1.35 panel thickness, but the film material has been improved with what Stax calls "super engineering plastics" that has less sensitivity to temperature and humidity than the older material. Using this new material also allows the SR-407's driver housing to be factory-rebuildable, which will make the SR-407 more readily repairable than earlier models.
One final improvement in the SR-407 over its predecessors is that, instead of protective foam, the SR-407 uses a cloth material in front of the drivers that will not deteriorate with age. Older-model Stax earspeakers' foam dries up and flakes or powders away, often damaging the drivers in the process. This can't happen on the SR-407 earspeakers.
The SRM-006t amplifier uses circuitry originally developed on the Stax SRM-T1 amplifier, which was introduced in 1993. According to Stax, the SRM-006t is an "all class-A" design that is direct-coupled throughout, with no capacitors in the signal chain. It uses a high-voltage dual triode 6FQ7/6CG7 vacuum tube in its output stage, coupled with a simple two-stage FET (field effect transistor) input stage. The SRM-006t has three inputs: one balanced XLR and two unbalanced RCA. One of the two unbalanced inputs also has a fixed-output single-ended pass-through with RCA outputs. The SRM-006t front panel has two "Pro only" earspeaker outputs (earlier Stax earspeakers used a lower bias voltage and are not compatible with the current 580-volt bias voltage). The front panel also has an on/off button, three input buttons, and a large volume knob.
Hooking up the Stax SRS-4170 system is simple, as long as you have a fixed-level line output from your preamp or receiver. Don't even think of connecting the Stax system to a variable-level output, such as the one labeled "preamplifier output," unless you are very careful. If the volume control on your preamp or receiver is too high, a variable-output connection can damage your Stax system. The right choice would be a "tape out" or "recorder out," both of which are almost always fixed-level line outputs. When you turn on the SRM-006t amplifier, it will take almost a minute for the tubes to warm up and stabilize. During that time, the output is muted. Once up and running, the SRM-006t operation is simple: select an input and adjust the volume.
The SR-407 earspeakers have an adjustable leather headband that should fit 99 percent of the population. Even on my small, pointy head, there was still some room to further shorten the headband if necessary. The Stax Lambda design is among the most comfortable headphones ever made, due in large part to its light weight and well-stuffed, carefully-shaped ear cushions. Compared with my own 20-year-old Stax Lambda Nova Signature earspeakers, the SR-407s were slightly less comfortable, principally because the SR-407's earpads were a bit stiffer and thicker than the Lambda Nova Signature's. I suspect that, with some wearing time, the SR-407s earpads will break in and be as comfortable as those on the older Stax.
Read more about the Stax SRS-4170 earspeaker system on Page 2.