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STAX SR-001 MK2 Portable Electrostatic Headphones

Review by

Aaron Kovics

Hey, I have a cool idea. What if someone were to take a piece of polymer or Mylar and place it in between two metal grids and apply voltages to them? They could use the grids as the charge and the polymer as the bias. Then they could send an analog signal to this concoction and see if they can get it to make sounds.

What if someone were to take it a step further and create large and small speakers that employ this methodology, and if they really wanted to push it, that someone could use this know-how in a pair of headphones. Okay, I see the men in the white coats coming but as long as I’m already off the deep end, we’re going to up the ante by making the headphones portable for people who enjoy large voltages very close to their brain whenever possible.

 It’s been done, you say? Hey, who stole my idea?


A Brief History

Electrostatic speakers became a reality in the 1920’s and were invented by two engineers who worked for Bell Telephone Labs. In 1957, Quad released the ESL-57, the world's first production electrostatic speaker. In 1969 the Magneplanar was invented and in 1970 the use of electrostatics in speakers was improved upon by two enthusiasts named Gayle Martin Sanders and Ron Logan Sutherland (hence the name Martin Logan) which set the standards that still exist today. As with all approaches to a result, there are pros and cons that may or may not have work-a-rounds.

In 1998, I was in a NYC stereo store auditioning speakers and there was a pair of Martin Logan’s with which I just fell in love. They had a rich and creamy mid range and highs that were slightly rolled off but encircled the soundstage in a seamless bubble. There were three problems that diminished my love affair into infatuation. The bass was truly lacking even though a cone woofer was used for the low end. You really can’t move air with electrostatics like you can with a dynamic driver so an independent woofer with a crossover network helps if it is incorporated properly. In this case, it wasn’t enough.

This brings up the second problem. Since you don’t have a sealed box enclosure, a good deal of the sound comes out of the back, so distancing the speakers from the back wall is imperative for creating the soundstage and eliminating any “slapping effect” issues.

The third problem is the limited sweet spot. Once you position the speakers the best way possible for your room and have found the perfect “sweet spot”, it is limited to one pair of ears. If you move three or four inches in any direction, the soundstage collapses. This is a serious problem for any application that involves more than one listener.

In 1960 Stax released their first electrostatic headphone model called the SR-01 introducing electrostatic technology into headphones. They stole my idea. I should sue. Back then they were basically two circular electrostat domes with a headband connecting them. They used a SRA4S amplifier that was designed to work with the SR-01. Flash forward to 1987 when they introduced the Signature Lambdas which are still popular today. This is when the earspeakers turned from circular to rectangular.

In 1994 they took the world by storm with the Lambda Nova Classic System which had the ampli fier included as a package and the cable became flat. The design had an open back that allowed the sound to be released through the back as do most open ended headphones. Each “earspeaker” plugs into an amp that is specifically designed to charge the metal grids or “plates” to stimulate the center material to create the correct vibrations which transpose into music or voice.

Unlike a dynamic headphone and dedicated headphone amp, an electrostat setup needs a certain amount of time to charge the stators to the proper voltage. I find thirty minutes is adequate for a critical listening session. Care needs to be taken not to touch the grids or allow dust to fall on them (the charge onto the dust or dirt can short the headphones) and obviously these shouldn’t be worn in the pool or shower or else you’d probably weld your cavities together.

In 1998 Stax moved up a notch to the Omega II’s which are circular, expensive and are still their top of the line offering. The phones include either a solid state amp labeled the 717t or a tubed version named the 007t for approximately $3800. I’ve owned a pair for a couple of years with the 007t tubed amp and it is one of my all time favorite headphones.





Today – The SRM-001 Mk2

Finally we get to the headphones under review in this article. The SRM-001 Mk2  is a package consisting of a driver unit called an “energizer” which can be run on two AA batteries for 4-5 hours or AC for home use. The energizer weighs 140 grams with the batteries and fits in a shirt pocket.

If you’d like you can remove the earpieces from the headband and use them as “buds” because they attach to your ear canal using small oval silicone rubber pads which also help to block out a small amount of external noise. Even though the package provides you with two different size pads, the devices would not stay in my ears without the headband. Your mileage may vary.

If you are not used to portables, you may find that after initial use, the buds may leave your ears a bit tender. But as with me, most people report that their ears adjust rather quickly and the tenderness goes away. Even though the pads are inserted and cover your whole ear canal, they do not block out many dB’s of ambient noise due to the open back design.


If you are a lover of electrostatic sound and you like it in your headphones, and you would like it in a doggy bag to go, then the SM-001 Mk2 system is unquestionably the choice for you. Or, if you have never experienced what an electrostat can do for music, you should give these a try.

First, let’s start out with the price. This system can be purchased for $239.00. That’s two tanks of gas to fill an Escalade. Second, let’s point out convenience. If you can lift a toothbrush with tooth paste on it, then you can carry this unit around with you. 4-5 hours of battery life is not something to shout off the rooftops about, but carry an extra set of 2 rechargeable AA’s with you on long trips and you can travel from NYC to North Carolina in electrostat heaven. This system weighs less than six ounces..


If you hold them up to the light you can see right through the screens and yet the bass that is produced by these is something that has to be heard to be believed There isn’t anything in there to push air, but as long as you position the silicone pads correctly the bass is solid and without muddle or boom. There is a slight mid-bass hump and the treble is rolled off slightly, but these are small sacrifices to make when it comes to being able to walk through the streets with electrostatic speaker technology on your ears.



The Sound

I used two different types of music to evaluate these. Harmonic male vocals from Ladysmith Black Mambazo and a Big Band tribute of Jaco Pastorius songs. I used the vinyl format on my VPI HRX.

You might remember Ladysmith Black Mambazo from Paul Simon’s Graceland. “Diamonds on the Souls of Her Shoes” was the radio hit. They do their own version of this on their album ” Long Walk to Freedom” and it is stellar. This performance comprises of eight men doing multiple harmonies from bass to soprano and it is a journey through vocal sound heaven. Melissa Etheridge joins them on this cut and she blends right in as an equal part.

Can you say midrange? This album was made for headphones. This album was made for electrostats. I have listened to it many times in 5.1 surround sound and if you want to hear everything that this recording has to offer, the Stax SM-001Mk2 system is the way to go.

However, to really put these headphones through the wringer, I decided to broaden the range. I knew listening to “Word of Mouth Revisited” would make or break these. This recording invites some of the world’s best bass players to play on each track revisiting Jaco’s music during his short but impressive bass playing career.

The bass was nowhere near what is portrayed on my speaker system, but you would be hard pressed to find any system that includes amp and headphones under $300 to create bass that is as clean and true although not visceral.

The horns and cymbals don’t find their way to the highest highs, but the soundstage that is produced by these headphones and the bargain price wins hands down as one of the finest portable setups that you could possibly own.

Remember those three problems I mentioned earlier? In these headphones, the ultimate bass is not achieved, but it is much better than early room speakers. The second problem of speaker placement within a room is eliminated because the speakers are placed in your ear. There’s not much room for music to bounce around in there unless you are Mickey Mouse. There is nothing Mickey Mouse about eliminating the narrow sweet spot. The sweet spot with these is your cerebral cortex.


Recommended for lovers of jazz, classical, male and female vocal, alternative, blues and country. If you listen mainly to Urban, Hip Hop or other genres that are driven by ultra low, ultra loud, throbbing bass lines where soundstage and midrange transparency is not as great an prerequisite, you should probably stick with a dynamic-type head phone. Remember, the Stax SM-001Mk2 is a system – it includes the amplifier.


SRM-001 Specifications


Driver unit for exclusive use with the S-001MK2 earspeaker. All-Stage semiconductors, Class A operation, Portable earspeaker driver unit.

Frequency Response

:5 to 20,000Hz

High Frequency Distortion

:Max. 0.01% (with 1kHz, 100V r.m.s. output)



Input Terminal

:1 (1/8in. stereo mini jack)

Rated Input Level

:100mV (with 50V output)

Input Impedance


Max. Output Voltage

:240V r.m.s. (1kHz)

Standard Bias Voltage

:580V (with use of AC adapter)

Power Consumption

:0.8W with batteries, 1.3W with AC adapter

External Dimensions

:2.4 (W) x 0.9 (H) x 4.7 (D) in


:3.6oz / 4.9oz with 2 AA standard batteries