Home  Donate  New  Search  Gallery  Reviews  How-To  Books  Links  Workshops  About  Contact

Stax SR-Lambda Professional
Electrostatic Headphones (1982-1995)
© 2011 KenRockwell.com. All rights reserved.

Intro   Specs   Performance   Compared   Usage   Recommendations

Please help KenRockwell..com

Stax SR-Lambda Professional

Stax SR-Lambda Professional. enlarge. This free website's biggest source of support is when you use these links, especially this link directly to them at eBay, where the sell for about $300 (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.

August 2011       All Reviews    Audio Reviews    Stax Reviews

Electrostatic Headphones: How They Work, and Why They Sound Better.

Audio-Technica   Beyer   Sennheiser   Sony   Stax   Ultrasone   Velodyne   Woo


Introduction         top

Intro   Specs   Performance   Compared   Usage   Recommendations


B&H Photo - Video - Pro Audio

Ritz Camera

I use Adorama, Amazon, eBay, Ritz, B&H, Calumet, J&R and ScanCafe. I can't vouch for ads below.

These Stax SR-Lambda Professional electrostatic headphones are the most perfect audio transducer I have ever heard, and I've grown up around daily music performance, the Quad ESL, Quad ESL-63 and B&W 801s.

The sound from these Lambda Pros is simply extraordinary. It is natural, clean, open, delicate, powerful, detailed, precise and revealing, all at the same time.

Stax electrostatic headphones win on subtlety. (Stax' marketing people have always called these "earspeakers" instead of headphones.) If you want to enjoy every nuance of your music across hours and hours of careful listening, these are your headphones. If you prefer something that boosts everything for a screeching 30-second demo spin, these aren't for you, although they do go extremely loud.

There is no distortion. Every instrument, voice and multitrack layer is heard distinctly, never mushed into anything else. Instead of loud walls of crud, one hears everything separately, regardless of how loud you enjoy your music.

Listening with these headphones, all the distortion and coloration of conventional speakers and headphones is stripped away, leaving only pure music. I hear no distortion at any level.

If choral masterpieces are your thing, you know these are up your alley, and likewise, if Metallica and Aerosmith are for you, you may for the first time hear every one of the hundreds of mixed, deliberately distorted sound layers individually. You can separate every voice in the chorus, and hear distinctly every one of the hundreds of layers of sound added in multitrack recording.

Whatever distortion was in the guitar amp is all you're getting; you won't get any more by turning up these headphones.

These are thirty years old, and way outperform brand-new conventional dynamic headphones like the Sennheiser HD 800 and Ultrasone Edition 8 in every way. The Stax SR-Lambda Professional costs a lot less, typically only $300 used.

These Stax headphones excel because they are electrostatic, which have numerous advantages over conventional headphones like most Sennheiser, Ultrasone, Beyer, Grado and AKG.

Unlike electrostatic speakers, these headphones play LOUD, easily hitting 118 dB SPL without distortion. They are rated at 0.007% distortion at 400 Hz at 100 DB SPL, which is live concert level. The sound quality is spectacular, and they crank out more sound than I'd ever want.

These headphones work only with Stax' dedicated amplifiers or adapters. The Stax SRM-1/MK-2 Professional amplifier, for example, was introduced in 1982 along with these headphones.

"Professional" means that these electrostatic headphones use a larger spacing between their electrodes, and use a higher polarizing voltage (580 V instead of 230 V) so that these headphones have a huge maximum output level, especially at low frequencies.

These Lambda Professionals and the SRM-1/MK-2 Professional amplifier were developed at the request of Mercedes-Benz, who asked Stax to develop a high-output version of the regular SR-Lambda (not Pro) that had been introduced in 1979. Mercedes-Benz needed a transducer that could reproduce extreme subsonic information loudly to help them reproduce the sounds of automobiles for design and analysis.

These Stax Lambda Professionals were the world's top headphones from their introduction in 1982 through 1995, when they were replaced by the similar Stax Lambda-Novas.

A reader's dad worked at Capitol Records, where the Stax Lambda Pros had been used for years. Capitol stopped using them around 2001, and threw them all away! That reader's dad grabbed a set from the trash along with the amplifier, and is still enjoying them to this day. Capitol downgraded to ordinary Sony headphones for the same reason they use Yamaha NS10s: to be sure mixes sound good on ordinary equipment, not just on the very best equipment. Popular music is done this way, while classical music is still done with the best monitors. (In practice, everyone checks their mixes on all sorts of gear to be sure it will sound great everywhere, and mastering is always done very carefully.)


Specifications         top

Intro   Specs   Performance   Compared   Usage   Recommendations



Ideally, connect to any Pro or Professional (580 V bias) 5-pin adapter, energizer or amplifier.

Also connects and works with any Stax standard (230 V bias) 6-pin adapter, energizer or amplifier, albeit with reduced sensitivity and reduced maximum output.



Inside, Stax SR-Lambda Professional

Cord and molded 5-pin plug, Stax SR-Lambda Professional. enlarge.


Flat six-conductor cord.

Molded 5-pin plug.

Raised black index.

PCOCC copper.

The dotted side is the right channel.

2.5 meters (8 feet) long.



1.5 microns thick.

For comparison, a very thin dry-cleaning plastic bag, the type that blows away just by breathing on it, measures about 20 microns!


Plate Spacing (electrode gap)

500 microns.



122 pF, including cord.



100 dB SPL with 100V RMS input at 1 kHz, 580 V bias.


Frequency Response

8 - 35,000 Hz, rated (tolerance not stated).



Rated at 0.007% at 400 Hz at 100 dB SPL.

Per this graph, distortion is about 0.03% (-70 dB) throughout most of the audio band at a screaming live-concert level of 100 dB SPL, dropping to about 0.007% around 400 Hz.

The world's finest dynamic loudspeakers, like the $24,000-the-pair B&W 800 Diamond, look like garbage by comparison. They are lucky if they can keep distortion below 0.5% from 80Hz - 100 kHz at only a mere 90dB.

Even at 20 Hz and an insane 100 dB SPL, the SR-Lambda Pro's distortion is rated at a vanishing 0.1%. By comparison, distortion is so ugly in dynamic speakers at 20 Hz that even the best of them, like the B&W 800 Diamond, don't publish the figures.


Maximum Output Level

118 dB SPL at 400 Hz, 580 V bias.

Elsewhere it's been rated at an insane 114dB at 20 Hz, and an unfathomable 111 dB at 10 Hz. Daimler-Benz needed this for analyzing car noise and vibration.


Weight (actual measured)

11.9 oz. (338g), without cord.

16.135 oz. (457.4g), with original cord and plug.

Stax specifies 340g without cord, and 460g with cord.



1984: $1,700, corrected for inflation in 2011, including including Stax SRM-1/MK-2 Professional amplifier ($799 in 1990 dollars).

1990: $2,000, corrected for inflation in 2011, including including Stax SRM-1/MK-2 Professional amplifier ($1,199 in 1990 dollars).

2011: About $300 used, or about $700 usedwith Stax SRM-1/MK-2 Professionalamplifier, used at eBay today (see How to Win at eBay).


Performance         top

Intro   Specs   Performance   Compared   Usage   Recommendations


Sound Quality         top



See also Electrostatic Headphones: How They Work, and Why They Sound Better.

As I said above, this is the cleanest and best sound I've ever heard outside a concert hall.

Personally, I prefer headphones to loudspeakers, so it sounds better than anything I've heard over monitors working in recording studios, too.

One caveat from the professional world is that these headphones are often used to monitor classical recordings, but be careful. Since the sound is reproduced so cleanly, your brain thinks that it's in the original performance space, and it sounds great even if the microphones aren't well placed. In other words, these headphones may trick your brain into thinking that your recording is better than it really is. Yes, technical problems like distortion will show up first over these headphones, but more subtle things like sloppy mic placement might not become apparent as you get sucked into enjoying the performance.

The ultralight diaphragm ensures no acoustic energy storage, meaning that there is none of the time-smearing and resonances of other headphones to muddy the sound.

Since everything is so clearly rendered, and bass is effortlessly and completely reproduced, these need not be played loud. They sound great at soft levels. There are no resonances or short-term echoes hiding any of the music, so it's all audible at any level.

Every new speaker or headphone usually lets us hear something we haven't before in our favorite music, mostly because new transducers have different colorations and will emphasize something that hasn't been emphasized before. That doesn't make them better, just different. With these Lambda Professionals, I've heard far more inside of every recording than I ever have. This is because there is no distortion and no short-term internal energy storage to smear the sound. Since everything is presented exactly as recorded, with no extra crud, one can hear inside, around and behind many other sounds and hear many, many things one wouldn't have heard unless one is already listening with other electrostatic or extremely high-quality transducers devoid of cabinet reflections and internal energy storage.

The Lambda Pro's reproduction is so uncolored and unpolluted that even a dB of equalization makes a huge difference. Through normal speakers, it takes a few dB of adjustment to hear any change in the sound, and to my surprise, adjusting an octave equalizer just a dB or fraction is clearly audible through these.



With great recordings, these are exceptional, however, the Lambda-Pro's matter-of-fact presentation is completely unforgiving of harsh recordings. Sweet recordings become beyond four-dimensional, while harsh recordings are harsh.

If a recording is even a little bright, it's obvious, so sometimes I set an octave equalizer to lose about 2dB at 4 kHz, 1 dB at 8 kHz and 16 kHz to sweeten.

The Ultrasone Edition 8 is much more pleasant with the general run of recordings, never sounding harsh, even with bright recordings, however the Edition 8 never gets as sweet and open as the Stax, and the Stax sound great with harsh recordings after a little EQ.

With flat recordings, like those from Telarc or this binaural recording made with the Neumann KU-100 dummy head, they are perfect.

Many very fine recording microphones emphasize midrange to make things a little brighter through normal speakers, and recording processes often emphasize this further to make things louder, so if any of this is going on, the frank honesty of the Stax Lambda Pro makes it as obvious as it makes everything else.


Output Levels

Unlike electrostatic speakers, these headphones, with any Pro series amplifier like the Stax SRM-1/MK-2 Professional, easily plays far, far louder than I can stand.

It has loads of MOL, more than my Beyer DT-990s!

If you were expecting something weak, guess again. These play LOUD if you want them to.

I get plenty of output even with a standard (230 V) amplifier like the SRA-12S. I don't play these too loud. If you want loud, stick with a 580 V (pro) output, which is all Stax has made since the 1990s.



Bass is perfect. Unlike every other transducer to which I apply some low-bass (c. 20 Hz) equalization, these sound great, and start sounding to heavy if I add any EQ.

Bass is never distorted, and never rings, hangs-over, resonates, doubles, or does anything other than reproduce the sound as recorded.

Therefore, since it doesn't boom or distort and get muddled into everything else, the bass remains completely separate from the rest of ensemble. The bass simply does its own thing like every other instrument, the same way it does live. Unlike most transducers, these headphones aren't working hard to play even very loud and deep bass.

Unlike my DT-990s, no matter how loud I crank these with low bass, I can't get them to buzz or to rattle. The only way I can get anything funny out of them is to feed them subsonic (16 cps) sine waves, and deliberately play them way too loud, listening for audible artifacts.



Soundstage is what we expect from headphones.

I love it, and prefer it to speakers.

On-center things sound solid, centered and coherent.


Comfort         top

These are circumaural, meaning they sit around the outside of my ears.

They are lightweight, and comfortable for hours and hours. Head pressure is light, and the floppy top band molds itself to any shaped head, distributing the minimal weight across your entire head.

These are the most comfortable headphones I've ever worn for hours and hours.

The pressure is light, so if you lean forward or back or move around, they'll fall off.

As open headphones, some sound leaks to the environment.

Earpads are premium Kobe vinyl, and feel great. As of 2011, Stax makes an SR-507 that looks about the same, and has real leather pads, so it might not be that big a deal to change them, if you really want germs growing in them (nothing grows in vinyl; it's not porous).


Mechanical Quality         top

These are mechanically very delicate. Everything is perforated plastic, which is why they are so light and comfortable for hours and hours, but I always set them down on a desk very carefully.

If you're taking them on the road, give them their own Pelican case.


Compared         top

Intro   Specs   Performance   Compared   Usage   Recommendations

Older Stax earspeakers from the 1960s and 1970s, like the New SR-3 and SR-X Mark 3, tend to be less bright, with less extended bass. Older Stax headphones are fantastic, clean and open, but not quite as clean and open as these Lambda Professionals.

I have not heard them, however today's newest iterations of this Lambda, the SR-507, SR-407, SR-307 and SR-207, still look the same and probably sound about the same.

The earlier SR-Lambda (non-pro, 1979-1982, 230 V bias), doesn't go as loud or as deep. I have not heard them either.

The new Stax Omega SR-007 MK2 has about 4dB less response above 2kHz, eliminating the occasionally excess brightness of these Lambda Pros.


Usage         top

Intro   Specs   Performance   Compared   Usage   Recommendations



Inside, Stax SR-Lambda Professional

Cord and molded 5-pin plug, Stax SR-Lambda Professional. enlarge.

As you can see by the funny connector, the Stax SR-Lambda Professional must be used with one of Stax' dedicated energizers or amplifiers.

The SR-Lambda Professional is intended for use with any "pro" (580 V bias) adapter, energizer or amplifier, however it also works with any previous standard (230 V bias) system.

The old 230 V devices use 6 pins, and the pro systems use only 5 pins, so this can fit into any of them, but the lower-voltage headphones can't plug into a Pro output by accident and get damaged.

Using it on an older 230 V system lowers its sensitivity and maximum output. Stax cautions that if you like it loud and use it with an older amplifier or energizer, that you may be tempted to push it too far and damage something. I certainly hope that nothing would be damaged by clipping, but let's face it: Stax stopped making the 230 V amplifiers back in in the 1980s, so anyone getting one of these will certainly want a pro-bias system to drive it.

All Stax amps, tube or transistor, battery or wall-powered, put out about 350-400 V of audio; it's only the bias voltage that varies significantly.


Amplifiers         top

With the Stax SRM-1/MK-2 Professional, these Lambda Pro are usually a bit too bright for my taste.

The standard-bias only Stax SRA-12S doesn't have enough output for more dynamic music with the Lambda Pros.

These Lambda Pros sound absolutely fantastic with the Stax SRM-T1, with which the treble is less forward, and while still a very little bit bright, sounds spectacular.

With the SRM-T1, these Lambda Pros sound even a bit better than the Stax Omega SR-007 MK2, because on the SMR-T1, the Omegas have a slightly depressed upper midrange and treble. Your preferences will depend on the recording, and your preference for tonal balance.


Safety         top

Watch for High Voltage when unplugging the headphones from an adapter, energizer or amplifier.

It seems easy to grab the pins by accident as you insert or remove the plug from a socket, and if you touch them while energized, you may get a jolt of 580 Volts.

Beware: these voltages stay in this equipment for a long time, even when unplugged. Be careful!


Charge Time         top

These use static electricity, meaning non-moving (static) electric charges.

The static charge, a.k.a. polarizing voltage, is supposed to take a few minutes to spread around the headphone's diaphragm. If the charge was free to move at will, it would move under the forces that are supposed to move the diaphragm when subjected to voltage fields from the plates. If the charge was free to move (not static), electrostatic headphones and speakers wouldn't work.

Therefore, it takes time for the static charge to spread across the entire diaphragm.

As the charge spreads, it is normal for the audio to increase in level for the first few minutes after you've plugged the adapter into the wall. It's also normal for each channel to come up at slightly different rates, meaning it is expected that for the first few moments the channels often seem unbalanced.

Personally, I leave this thing plugged in and energized all the time, so my headphones are always ready to go. It's hard to measure, but electrostatic systems love to be left alone — that's why they're called static, and not dynamic. Some people say, and I wouldn't disagree with them, that electrostatic systems sound best if they're left plugged in for at least a day.

I felt that these were a little brittle at first, and that went away after stabilizing a few days.


Man-Handling         top

The diaphragms are virtually weightless, and float around in the air between two perforated metal plates.

Stax cautions that if you press these against your head, the diaphragm might move to one side of the 500 micron space between the perforated plates in which it operates, and discharge its polarizing voltage through that plate.

In other words, if you hear a Pop! while moving these around, that's normal.

This can happen if I they are put down too quickly on a hard table: the diaphragm wiggles too much, touches one of the plates, and discharges.

When either of the diaphragms discharges this way, it doesn't hurt anything, but you will lose most of your bass response for a minute while the static charge builds back up across the diaphragm.


Recommendations         top

Intro   Specs   Performance   Compared   Usage   Recommendations

With headphones this good, they deserve one of Stax' dedicated amplifiers. They work with the passive transformer adapters, but if you're going this far, I'd suggest a real Stax amplifier. The Stax amplifiers are all painstaking class-A designs, and are very well made. It seems silly to screw with a passive adapter and a power amplifier with this level of headphone.

For a total of about $700, half the cost of a Sennheiser HD 800 or Ultrasone Edition 8 , you can get this and a Stax SRM-1/MK-2 Professional amplifier, and its audio quality will be in a completely different dimension. Better, even if you bought the HD-800 or Edition 8, you'd still need to drop another $1,800 for a decent headphone amplifier like the Grace Design M903.

I bought the $1,500 Sennheiser HD 800 in December 2010 expecting them to make everything sound great. They sounded worse than the Beyer DT-990s I'd been using since 1988, so the Sennheisers were promptly returned.

These Stax SR-Lambda Professionals really do make every recording I have suddenly sound better than they ever have. It took a few months, but I finally got what I wanted, for a fraction of the price of the inferior Sennheisers.

The Ultrasone Edition 8 sound much, much less open and clean than these Stax, but at least they plug right into an iPod or iPad, and I can throw them in my bag without having to baby them.

Therefore, if you want fantastic sound, this Stax is it. I'd suggest these original classic SR-Lambda Professionals, or any of the newer SR-Lambdas, Novas and I forget what else they've been called.

I sometimes drop a dB or two around 5 kHz with an equalizer to sweeten harsh recordings, which are always rendered with the same unflinching accuracy as everything else.

The transducer is everything, and as Stax so humbly and truthfully has advertised, these are the most accurate audio transducers known to man. Once you get these, they'll makes everything that sounds good sound fantastic.

Want incredible sound from your iPad or iPod? Easy: presuming you've made good transfers, any 99¢ 3.5mm to RCA cord between your iPod and the back of any Stax amplifier will quickly send you to goosebump territory. Transducers this good quickly make you recognize just how misguided are audiophiles who waste their time chasing cables or magic power cords instead of just getting some good electrostatics.

Today, the current Stax models are compatible with all the same amplifiers and seeming pretty much the same as these. Today they are called the SR-507, SR-407, SR-307 and SR-207. They all seem to be the same thing, differing in cables and earpad material. Good luck buying these new anyplace other than eBay; I know of no retail dealers in California — or anywhere.

If you find this research helpful, this free website's biggest source of support is when you use these links, especially this link directly to these at eBay (see How to Win at eBay). This helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.


Help me help you         top

I support my growing family through this website, as crazy as it might seem.

The biggest help is when you use any of these links to Adorama, Amazon, eBay, Ritz, Calumet and J&R and when you get anything. It costs you nothing, and is this site's, and thus my family's, biggest source of support. eBay is always a gamble, but all the other places always have the best prices and service, which is why I've used them since before this website existed. I recommend them all personally.

If you find this page as helpful as a book you might have had to buy or a workshop you may have had to take, feel free to help me continue helping everyone.

If you've gotten your gear through one of my links or helped otherwise, you're family. It's great people like you who allow me to keep adding to this site full-time. Thanks!

If you haven't helped yet, please do, and consider helping me with a gift of $5.00.

As this page is copyrighted and formally registered, it is unlawful to make copies, especially in the form of printouts for personal use. If you wish to make a printout for personal use, you are granted one-time permission only if you PayPal me $5.00 per printout or part thereof. Thank you!

Thanks for reading!

Mr. & Mrs. Ken Rockwell, Ryan and Katie.

Home  Donate  New  Search  Gallery  Reviews  How-To  Books  Links  Workshops  About  Contact