Review | Stax Audio SRM-700T Amplifier / Energizer for Electrostatic Headphones

Stax Audio of Japan was founded in 1938 to make electronic equipment and in 1959, they released the first ever electrostatic (ES) stereo, high-fidelity headphones (Stax calls them “Earspeakers”), the SR-1. Since then, Stax has gone through a number of owners, and business models. They have made everything from amplifiers to CD players to DACs. In the 1980’s Stax made a fairly impressive line of electrostatic speakers and they were very highly regarded. Today, Stax is owned by a Hong-Kong based company, Edifier International Ltd, but the manufacturing of Stax equipment is still done in Japan. The new owners have refocused Stax exclusively towards Earspeakers, Amplifier/Energizers to power them and Earspeaker accessories. 

Recently, Stax has introduced two new amplifier/energizers for electrostatic ‘phones. These almost identical looking amps are called the SRM-700T (tube version) and the SRM-700S (solid-state version). Both of these amps are priced at US$3400, and the output stages are the only difference between them. While the “S” model uses FETs to drive the ‘phones and the “T” model uses a pair of 6SN7 dual triodes as output devices. Outwardly, the two new amps strongly resemble the older Stax amp/energizer called the SRM-007tII (still available at US$1780). Like the new SRM-700T, this amp uses a tube output, but in the case of the older design, the tube in question is a single 6FQ7/6CG7, a dual triode (half for the right channel and half for the left). 

The SRM-700T vs the SRM-007tII

The SRM-700T/S and the older SRM-007tII look almost identical, physically. The two new amps are roughly the same size and shape with the dimensions of 240mm (9.45 inches) W x 103mm (4.1 inches) H x 393mm (15.5 inches) D overall with a weight of 5.7 Kg (12.6 pounds) for the SRM-700T/S. The size of the SRM-007tII is only slightly different in overall dimensions and weight and the front panel layouts of the two are almost identical.

SRM-700T Layout, Front and Rear

From left to right the layout of the SRM-700T starts with a push on/push off switch. Next comes two 5-pin electrostatic headphone jacks which were, until fairly recently, a Stax proprietary design. Now, several electrostatic headphone manufacturers use it including HiFiMan for their Jade2, and Dan Clark’s (formerly Mr. Speakers) Voce model electrostatic phones. Stax calls this their “Pro” Earspeaker connection, and applies to the pinout as well as the bias voltage (a nominal 580 VDC in this case). Any headphone or headphone amplifier manufacturer wishing to use this connector and this spec may follow this protocol. There is even an adapter to allow the Koss ESP950 electrostatic phones to be used with the Stax “Pro” spec amplifiers such as the SRM-700T or S models. A combination connection adapter and 68 inch long extension cord, at US$180, it’s not cheap but I have an acquaintance who uses a pair of Koss ESP950s with the original Stax SRM-007t of 20 years ago and he says that the adapter, though expensive, is well worth the price because the Koss ‘phones sound so much better through the Stax amp than they do through the supplied Koss unit.

To the right of the two ‘phone connections is the concentric volume knob. This allows the user to vary the balance between the two channels by holding one knob section while turning the other. The two sections of the volume control are “clutched”, so that they will both turn together unless one is held while the other is turned. This allows any balance corrections to be maintained as the overall volume is adjusted. Above the controls are two LEDs, one is red and marked “External (Bypass)” and the other is green and located above the center of the volume control. It is marked “Internal”. This refers to the ability of the user to bypass the volume control of these amps in case the headphones are being driven by a variable volume control output on one’s system preamp or amplifier. This control is on the rear panel of both amps. Between the ‘phone jacks and the volume control is located another pair of LEDs marked “Source” a green LED denotes that the user is connected to the rest of his/her system via a balanced XLR pair. The blue LED indicates that one is using unbalanced RCA jacks to connect to one’s system.

The rear panel of the SRM-700T and S amps, again, from left to right, consists of a stacked pair of female XLR (sometimes called Cannon connectors) sockets for left and right balanced connections. To the immediate right of the two XLRs is a rotary two position switch with a small knob on it. In the left position, the knob points to the XLR connectors and indicates that they have been selected as the source for the headphone amplifier, and when switched to the right, the knob points to the pair of stacked RCA jacks indicating that the unbalanced connection is being used. Moving to the right again, there is a second pair of RCA jacks labeled “Parallel out” which means, ostensibly, that when using the amp in an unbalanced mode, the amp may be “daisy chained” with another component such as another headphone amp. To the right of the two pairs of RCA jacks is still another two position rotary switch. This one is marked “Internal” and “External (Bypass)”. These legends refer to the ability to use the Stax amps without the volume control. 

As pointed out above, this is useful when one is connected to one’s system’s amp or preamp and wants to use the main system volume control on one’s Earspeakers. The switch is accompanied by a warning label to the effect that in the external mode, the front panel mounted volume control does not work. This means that if the amp is connected to an un controlled line-level source, that the Stax amps will deliver full volume to the Earspeakers, possibly damaging them. Finally, there is an IEC connector for the mains (household power) cable. 

SRM-700T Evaluation Process

To evaluate the SRT-700T I used four electrostatic headphone types. From Stax, I used both the new flagship SR-009S and the SR-L300. From HiFiman I used the Jade2 headset, and from Dan Clark, I used the Voce electrostatic headphones. All of these phones conform to the Stax “Pro” spec and are therefore fully compatible with the SRM-700T amplifier/energizer. Although I briefly listened to the SRM-700T using the unbalanced (RCA) inputs connected directly to my Schiit Yggdrasil DAC, I did the majority of my listening via a 25 foot (7.62m) long pair of Mogami balanced cables with XLRs.

The Sound of the SRM-700T

I’ve never heard the solid state version of this amplifier. So my comments apply only to the tube version. I was able to compare this unit to the $7000 Linear Tube Audio (LTA) Z10e OTL amp and the HiFiMan Jade 2 solid-state amplifier. I also compared the SRM-700T with the SR-009S headphones to the LTA amp with the same SR-009S. I was also able to compare the Stax setup to the combination of the Headamp GS-X MkII headphone amp driving a pair of Abyss AB-1266 Phi TC phones in the balanced configuration (review in progress).

There are certain things that one expects from an electrostatic headphone setup, and indeed one does not expect that any dynamic phones would stand up to the large diaphragm area of the ES phones or the speed, transient response and high-frequency extension, coupled with the low distortion inherent in the ES push-pull drive. But lately, with the advent of new materials and techniques, many dynamic phones approach the sound of a good pair of ES phones with amazing results.

However, none, in my experience, come as close as the Abyss AB-1266 phones. While these very expensive planar dynamic phones through the Headamp amplifier come close to matching the sound quality and presentation of the Stax SR-009S through the SRM-700T, the electrostatics still have the edge in terms of speed and overall intimacy of sound. The Stax throws a wider, deeper soundstage and this is mostly due to the amplifier. When plugged into the HiFiMan Jade2 solid state amp, that wide, deep soundstage diminishes considerably. The Voce ‘phones exhibit some of this uncanny imaging performance when connected to the SRM700T and the Stax SR-L300s (see review posted February 21st) are very close to the SR-009S in this regard. Clearly getting the soundstage correct is a Stax characteristic, it seems, and one they share with the LTA Z10e.

Both of these throw the widest deepest soundstage that I have ever heard from headphones, and do it with pinpoint image specificity with the sound appearing to wrap from left to right outside of the listener’s head! To say that this effect is somewhat spooky would be an understatement. I’ve never heard a pair of dynamic phones even come close except for the Abyss with the Headamp GSX-mk2, which together, come close to the SR-009S in this regard. The Abyss/GSX-mk2 duo lists for around US$8,000. The combination of the Stax SRM-700T and the SR-009S phones is US$7795, so price-wise, I’d say that the difference in cost between the two is negligible.

One of my favorite recordings is the Angel Cabrera plays Debussy album from our own Mario Martinez’ Playclassics.com catalog. This is without a doubt the best, most realistic recording of a single grand piano that I have ever heard (Cabrera is a first class concert pianist, by the way. Not flashy like your modern young turks who win piano competitions, but rather he conveys a sense of style, coupled with a reverence for the material and a seriousness of purpose that makes me think of a Rubinstein or a Horowitz). Through the Stax duo, this 24/96 performance ceased to be a recording.  I completely “forgot” that I was listening to a recorded piano and for all intents and purposes, Cabrera’s piano was sitting right in front of me out there in the dark! I was totally enthralled by the performance (as always) but now I found the sound that I was hearing to be an equally immersive experience.

The attack on the keys by Cabrera comes across with the right amount of speed and a subtle, but clear impression of the mechanism of the piano working to create this sound – just as it would do in a real recital venue. Many modern piano recordings place microphones too close to the open grand piano lid or in some cases, even inside of the piano, both of which exaggerate the sound of the mechanism (and produce a piano image that’s 10 feet wide!). The transient response of the Earspeakers and the warm, clean sound of the amplifier coupled with the almost distortionless presentation of the push-pull drive on these phones removes so many layers of veil from the presentation that are normally there with other headphone systems that if Salome were in the room, she would be down to, at most, only one of her seven veils! 

Moving on to a recent acquisition (or I should say re-acquisition), a vinyl album by a Nashville pick-up jazz ensemble headed by Ferrell Morris with Stan Getz on tenor sax, Ron Carter on bass, Craig Nelson on both acoustic and electric bass (gotta wonder. Is this Craig T. Nelson, the actor? Nah! Couldn’t be) and many more well known sidemen. It is called Bits of Percussion, and this recording, never released on a CD, was made by a now defunct organization calling itself “Audio Directions."  It’s catalog number is AD-102 if you want to search for a copy. It was recorded in 1979 on a Sony PCM-1600. Now, I know this piece of equipment intimately because I once had access to one (it belonged to a friend) and we made many a recording together with it. I was not impressed with the sound at the time, and was convinced that digital would never eclipse a good analog tape recorder with half-track heads and a recording speed of 38cm/sec (15ips). Boy was I wrong! But somehow, this particular recording exhibits none of the hardness and digital glare that I normally associate with this piece of equipment. Perhaps transferring it to vinyl tamed the digital gremlins; I don’t know. What I do know is that this is one of the best sounding and most approachable jazz recordings I’ve heard.

On The cut, The Lamp Is Low (based on Debussy’s Pavane pour une infante défunte) the exotic percussion just sparkles through the SRM-700T. It doesn’t matter in this regard whether one is listening with either of the Stax headsets or the Dan Clark Voce or the HiFiMan Jade2 ‘phones, the highs have a bite and a transient quickness that no dynamic phone of my knowledge can match. I don’t even think that the aforementioned Abyss/Headamp combo can match the transient response of the Stax amp and either the SR-009S or the SR-L300 ‘phones (in spite of the fact that this dynamic headphone ensemble is among the top two or three headphone setups I’ve ever heard). But I’m going by past experience with the Abyss/GSX-mk2. I don’t have it in my possession at this time to be able to directly compare on this recording. Delineation of line between the various instruments with the SRM-700T is uncanny and something I’ve never heard before with this recording. The Stax amp allows me to hear deeper into the ensemble than ever before and this is the second copy of this record that I have owned (the first was “loaned” to someone and I forgot to whom I loaned it – and apparently, so did they!). The first copy I had I listened to often for about 30 years. I was happy to find another copy on Amazon.
 
In closing, I’d like to say, that if you enjoy the virtues of electrostatic headphone listening, you’re going to need a good amplifier/energizer for the task. Skimping here is simply not an option. While there are a number of such amps available to the modern audio enthusiast, one would do well to choose one that conforms to the Stax “Pro” standard for both connector pinout and biasing voltage. Solutions are available from the aforementioned Headamp brand (although the GSX-mk2 model, mentioned above, is not designed for electrostatic phones, Headamp makes, as I recall, two models that are specifically for Stax-type headphones), Woo Audio, IfI and several others, also make Stax compatible ES headphone amps. In my experience it comes down to the two top-tier models that I have lived with for some extended amount of time: the very expensive LTA Z10e or the more reasonable Stax SRM-700T (or it’s sibling Solid State version). You can’t go wrong with either, although the Stax, while being the less expensive of the two will power a second set of ‘phones, the LTA has only one ES jack, but it does have the option of two dynamic headphone ports in addition, and it will drive a pair of very efficient speakers with about 12 Watts RMS/channel. That probably accounts for, and  surely justifies, the almost twice the price of the Stax SRM-700T/S. 

By the way, of the ES phones used in this evaluation, I have ranked them with regard to my opinion of their sound quality. (1) Stax SR-009S, (2) Stax SR-L300, (3) HiFiMan Jade2, (4) Dan Clark Voce. The Abyss AB1266 Phi TC are not ES phones, so they aren’t in the ranking, but they are every bit the equal of the Stax flagship, the SR-009S. But understand that these two phones could not be more different in their presentation and which anyone would choose over the other is a matter of personal taste. I know which one I would choose!

Cheers!

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• Stax Audio SRM-700T Amplifier / Energizer ($3,400)
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