When discussing the high-end Head-Fi revolution, it’s rather difficult to not include STAX in the conversation; the manufacturer has been the most important player in the electrostatic headphone market since 1960 and some of their models are considered to be the benchmark that all others are judged by.
Unlike other headphones that use magnetic fields to help create sound, electrostatic headphones instead use electric fields; there are several advantages to using electric fields including less obstruction of sound waves as they don’t have to traverse around large numbers of magnets, and faster response time compared to dynamic drivers.
There are some disadvantages too, the need for a constant power supply to produce the electric field, and the need for a warm-up period to allow the electric field to reach full strength.
Failure to wait for the field to reach full strength can result in poor sound quality whilst failure to provide enough current to generate the electric field will result in no sound at all. Because of this need for power to create the electric fields in addition to the signal amplification, standard amplifiers won’t work with electrostatic headphones.
Instead manufacturers refer to electrostatic amplifiers as energizers or drivers with STAX using the later terminology. Another term unique to electrostatic headphones is bias which references the voltage/current needed to generate the electric fields and it is important to know that not all electrostatic headphones require the same bias.
For that reason, when STAX sent us their new SR-X1 ear speaker for review, they also sent along the SRM-400S Driver Unit as well as a SRM-D10 MKII portable driver unit which I reviewed previously.
The SRM-400S was created to be the foundation of the STAX line for the next decade just as the SRM-353X it replaced was for the previous generation of headphones.
The SRM-353X arrived in 2015 as the most cost effective desktop driver unit for STAX’s ear speakers and quickly became a common site alongside the various headphones at shows and retailers.
It would be fair for some to point out the SRM-252 as a more cost-effective option, but it also only offered 250V of output compared to the SRM-353X that offers 400V making the less expensive model less useful.
Build Quality
The SRM-400S is a rather large unit for those considering it as the hub of their desktop headphone system; the chassis is 7.66″ W x 4″ H x 14″ D and its all-metal construction and large transformer makes it a rather hefty 7.5 pounds.
The front face is brushed aluminum with matching power button at left and volume knob at right split by a pair of 5 pin DIN ports both marked “Earspeakers – Pro Type Only.”
The volume knob could easily be overlooked in this design as it works as a single volume knob like any other but if the forward or rear portion are held and the other turned, it adjusts each ear independently as a method of balance control.
The sides and top are a dark gray metal that appears to be powder coated whilst the rear face is black with a pair of XLR inputs at left, a selector swtich for XLR/RCA inputs, a pair of RCA inputs and a pair of RCA parallel outputs followed by a ground connector and finally a Nema type C13 outlet for power cable at far right.
Overall, it is a well-crafted unit with a solid feel; buttons and switches provide a tactile response, XLR cables click firmly into place, and the volume knob moves easily without feeling like it might fly away in the process.
Before moving on to its internal layout, there are a couple of items mentioned in the build that likely need require greater explanation. The first is the “Pro type Only” label on the front panel. This references a change made by STAX in the mid-1980s (My first exposure to pro bias was the Sigma Pro in 1987) that raised the polarizing voltage from the earlier 230V standard to a much higher 580V.
Stax has since quit making normal bias ear speakers with all currently cataloged models and driver units operating at the newer standard. Still, if you have an old pair of STAX Earspeakers, make sure you know the bias they require before plugging them in.
The second is the concept of parallel RCA outputs. This is not a pre-out with volume control like some headphone amplifiers provide, but instead operates as a simple pass-through to allow for daisy chaining an additional headphone amplifier from the same source for those with both electrostatic and dynamic/planar magnetic headphones.
How Does This Thing Actually Work?
The outward appearance of the SRM-400S seems to draw a lot of industrial design cues from the 700 series driver units and internally that trend continues with a substantial degree of trickle-down technology from the 700S.
It should be noted that the “s” designation at the end of the model number refers to the solid-state topology of the unit. Tube models have a “t” following the series number.
Internally, the unit is all DC circuitry so the first order of business is converting AC power. For this, the SRM-400S uses both a standard high power transformer along with a newly designed winding for the low power circuitry that reduces noise and improves efficiency.
The big change though is that the SRM-400S is an all FET design with dual FETs on the first stage and FETs also on the output stage; FETs are field-effect transistors which is a type of electronic gate that can control how much current is allowed to pass and can be thought of as a high precision volume control.
This may seem odd but a lot of noise, distortion, and imbalance in amplifiers traces its roots back to volume controls. By hand-selecting very tightly matched FETs and using those, STAX is able to create very tightly matched levels on the left and right channels while not diminishing sound quality in the process.
The 700s was the first model in STAX line to use FETs in all stages with the SRM-400S now offering that same basic design at a more “budget” friendly price.
The result is an amplifier with 60dB gain while maintaining a harmonic distortion of less than 0.01% (1kHz/100Vrms). Power consumption is 30 watts matching the previous 353 with a 400V maximum output at (1kHz).
Listening
To give the new SRM-400S unit a proper workout, I paired it with the SR-X1 that STAX had sent for review along with my personal STAX 009S, and the Audeze CRBN2.
These three models cover multiple price points of the electrostatic headphone category with the SR-X1 coming it at $535.00 USD, the Audeze CRBN2 retailing at $5,995 and the STAX SR-009S priced at $4,545.
The first thing I noticed is how different the low-end is on these three models. The SR-X1 has what many would call typical electrostatic bass; very clean but not particularly impactful.
The SR-009s takes a solid step forward in bass quantity while still not rivalling some of the better planar magnetic or dynamic models for overall impact and the CRBN2 delivers both quantity and quality that can compete with anything I’ve heard at any price point.
This pattern was repeated at several steps along the path with all three delivering excellent clarity and detail, but the nuances and dynamics improve as I moved upward.
The SRM-400S worked admirably with all three headphones and the only complaint I had is one that is likely more common among reviewers than home users; I wish the RCA/XLR switch was on the front panel because when switching between my main system which includes dCS and RME ADI-2 DACs, I had to turn the unit around so I could reset the switch before I could resume listening.
The Bottom Line
The new SRM-400S is a cross between the previous generation SRM-353X and the current SRM-700S and inherits a lot of technology from the 700S while retaining a price much closer to the 353X.
I found that the SRM-400S performed well with a variety of pro bias electrostatic headphones and for the price, the 400S is a good compromise between price and performance for those looking to purchase an electrostatic driver unit (energizer).
You can spend a lot more, but it’s hard to imagine a unit coaxing that much more out of the CRBN2 or the SR-009S than I was able to hear using the SRM-400S.
I did some A/B testing vs my own BHSE and while I’m a sucker for tube sound, I couldn’t tell that the 400s lost a step anywhere to the more expensive Headamp unit and those who prefer a little dryer signature may find the SRM-400S to be a better match overall.
Pros:
- Very capable unit, will drive any pro-bias headphone easily
- Capable of driving two electrostatic headphones simultaneously
- Provides pass through for those using both Electrostatics and other headphones
Cons
- Only function is to drive electrostatic headphones, no pre-amp, non-electrified outputs, or DAC
- Unit isn’t standard component dimensions so stacking may present issues
Where to buy:
Related Reading:
- STAX SR-003 MK2 Electrostatic Earspeakers with SRM-D10 II Energizer Review
- STAX SR-L700 MK2 Open-Back Electrostatic Earspeaker Review
- Edifier STAX Spirit S5 Planar Magnetic Headphones Review
