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Stax SRM-T1
Vacuum Tube Output (1987-1993)

© 2011 KenRockwell.com. All rights reserved.

Intro   Specs   Performance   Compared   Usage   Recommendations

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Stax SRM-T1

Stax SRM-T1. enlarge.


Stax SRM-T1

Rear, Stax SRM-T1. enlarge. This free website's biggest source of support is when you use these links, especially this link directly to them at eBay, where they sell for about $600 (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

Adorama pays top dollar for your used gear.

B&H Photo - Video - Pro Audio

The Stax SRM-T1 is a high-voltage amplifier and bias source for Stax' electrostatic headphones.

The SRM-T1 has two Professional (580 V) and a third Standard (230 V) output, so any Stax earspeaker ever made since 1959, from the original SR-1 to today's SR-009, works perfectly. (Stax calls its headphones "earspeakers.")

The SRM-T1 is an FET-input, tube-output, pure class-A, DC-coupled amplifier. It has no capacitors or transformers in the audio path. It's housed in a completely non-magnetic case to ensure no inadvertent magnetic coupling happens. It uses two 6FQ7/6CG7 vacuum tubes in its output stage.

It has enough output to deafen us with any of Stax' headphones, without any distortion.

It is fed from any device with 1/4" RCA jacks, or even directly from iPods and iPads with any 99¢ 3.5mm to RCA cord. If you do your iPod transfers correctly, this and a set of SR-Lambda Professionals sounds indescribably incredible driven directly from an iPod, if you'll just listen.

This SRM-T1 was Stax' first tube-output amplifier. It was replaced in 1993 by the SRM-T1S which debuted with the first SR-Omega. The SRM-T1S is the same as ths SRM-T1, except for a switchable input selector.


Specifications         top

Intro   Specs   Performance   Compared   Usage   Recommendations



Stax SRM-T1.

Unlike the SRM-1/MK-2, I've only seen "SRM-T1" punctuated one way.


Output Tubes

Two 6FQ7/6CG7.



There is one loop-through line-level input.

Loop-through means they are wired together; you can insert this in between any two other components, and even if it's turned-off, the signal goes through.

100 mV, 50 k Ω.



60 dB (1,000x).

In other words, a 100 mV input can produce a 100 V output at maximum gain.


Frequency Response

DC-20 kHz ± 1dB at 100 V RMS driving an SR-Λ Signature 1.

Appreciate how no other amplifier rates its performance actually driving a real, reactive load as Stax has rated this amplifier.



0.02% at 100 V RMS driving an SR-Λ Signature 1.

Again, this is real performance driving a real, reactive load.



The three headphone audio (plate) outputs are connected in parallel.

Bias voltages: 230 V DC (Normal) and 580 V DC (Pro).

Maximum output: 300 V RMS at 1 kHz.


Size (actual measured)

195 x 87.5 x 346 millimeters (7.68 x 3.44 x 13.62 inches) WHD, rectangular case only, excluding tube humps, feet, knobs and connectors.

195 x 103 x 377 millimeters (7.68 x 4.06 x 14.84 inches) WHD, including tube humps, feet, knobs and connectors. Leave more room for the plugs when installing.

Stax specifies 195 × 103 × 375 millimeters (7.68 x 4.06 x 14.76 inches).



3.9 kg (8.6 pounds).


Line Voltage and Power


Rear-panel selectable 100 V, 117 V, 220 V or 240 V.



50 - 60 Hz.


Rated Power Consumption

45 Watts.

With the main amplifier off (red pilot light) but the pre-heater left on (green pilot light only), it's rated 12W.


Measured Power Consumption

35.5 Watts.

With the main amplifier off (red pilot light) but the pre-heater left on (green pilot light only), it consumes 8.4 W.



2011: about $600 used at this link to them at eBay. (see How to Win at eBay).

Stax SRM-T1

Stax SRM-T1. enlarge.


Stax SRM-T1

Rear, Stax SRM-T1. enlarge.


Performance         top

Intro   Specs   Performance   Compared   Usage   Recommendations



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


It sounds smooth, perfect and detailed, exactly as I had dreamed. I thought those people at SAPPHIRE's tube table were making all this up, until I heard this for myself.

The Stax SRM-T1 sounds absolutely awesome. Music sounds as music should; nothing more, nothing less.

With any of the Stax Lambda Pro or ancient Stax New SR-3, it just sounds great. I plugged it in, and it just grabbed me to shut up and listen to the music, not the gear, for hours and hours and hours. Bravo!

Seriously, even the 1970's Stax New SR-3 with this amplifier sounds so much better than the Ultrasone Edition 8 or Sennheiser HD800 that it's amazing.

With the Stax Lambda Pro, it's audiophile death: once an audiophile gives a listen to this setup, he'll be converted into a music lover and be cured of ever wanting any more equipment. He'll finally be able to stop researching about all this on his computer and start actually enjoying music for a change!


Channel Balance

The potentiometer is ganged and clutched. This means balance is set by holding the sleeve and turning the top of the LEVEL knob.

The potentiometer on this sample tracked extremely well. Only below about 70 or 80 dB attenuation was there even the slightest imbalance between channels. One slight adjustment of the two knobs, and the darn thing is now perfect all the way smoothly down into silence. This is extraordinary performance.



It sounds great, without any noise or distortion even at deafening levels.

The gain structure is perfect: most of the time, I have the gain control set to the 12 o'clock position.

Left and right channels are friction coupled in one knob. To shift the channel balance, just hold both and turn each part of the knob in different directions.



As far as I can hear, it's silent, so long as you have it installed correctly to avoid hum or buzz.



I never heard any.



Everything should be this simple: two power switches, and one gain (volume) control.

The gain control feels great: it's slick, smooth and undamped. It flicks up and down with ease, and the big knob makes changing levels a joy.

It's easy to find and grab the knob and turn it from any angle, even in the pitch dark, for fast and precise adjustment. I love real volume control knobs; you can take digital up/down buttons and shaft encoders and put them in the doo-dah (garbage) can.

Stax uses a premium semi-sealed Alps 50 k Ohm potentiometer, and it's as smooth as the glassy feel that Penny + Giles used to advertise for their professional audio controls.

The power LEDs flicker with the 60 Hz power, not driven by DC.



WARNING: DO NOT OPEN unless you are a licensed technician. Deadly voltages can be present even after being unplugged.

Output stage: two 6FQ7/6CG7 vacuum tubes.

Stax SRM-T1, interior

Stax SRM-T1, Interior. enlarge.


Stax SRM-T1, interior

Stax SRM-T1, Interior. enlarge.

The SRM-T1 is very well made.

The wiring is neat and clean, and they use fancy super oxygen-free copper wire with giant crystals that I wouldn't know where to buy (I build my own gear with military teflon-insulated silver wire).

Pure DC coupled: no transformers and no capacitors in the audio path.

It runs pure class-A, with FETs in the first amplification stage, and vacuum tubes in the output stage.

It's got plenty of power supply capacitance; when turned off, it keeps playing for quite a while.



As a pure class-A amplifier, and as a tube amplifier, it runs hot.

This amplifier is supposed to get hot, even sitting idle.

Imagine a 45W bulb in a small box like this, and that's exactly as hot as this is supposed to get.



The SRM-T1 is built internally more like a piece of military or industrial hardware than almost anything from what passes for professional audio gear today.

The big volume control knob just glides. In fact, the Alps potentiometer is so smooth and the knob so big that you can spin it and keeps going by inertia.

The amp is built on a heavy inner aluminum chassis with nonmagnetic top and bottom covers, and a solid aluminum front panel.

Stax deliberately made everything nonmagnetic to eliminate unintended audio interactions.

The nonmetallic metal covers are held in place by machine screws screwed into solid metal inserts. Stax never got lazy and tried to use self-tapping bung screws like we lazy Americans; they took this very seriously.


Compared         top

Intro   Specs   Performance   Compared   Usage   Recommendations

With the Stax Lambda Pro, compared directly with the Stax SRM1-MK2, I was amazed that it really sounded as those folks at SAPPHIRE's tube table keep telling us: it's ever so slightly smoother, with less high treble. Since the Stax Lambda Pro tend to be on the bright side, I really like the subtle difference I hear with this SRM-T1 compared to the all solid-state SRM1-MK2.

I'm not doing this double-blind, so it could be my ears playing tricks on me as so often happens in audio, but I really do prefer this over the Stax SRM1-MK2 and Stax SRA-12S for use with the Stax Lambda Pro.

Likewise, this SRM-T1 sounded just a little bit better even with the ancient Stax New SR-3.

Darn. Now I want a tubed iPod. I thought those tube people were crazy, except for mics and mic preamps.

Oddly, probably because its bigger and has even more ventilation and a perforated circuit board, even though it draw more power, it feels like it runs a little cooler than the Stax SRM1-MK2.

I don't know that this is an issue of tubes versus bipolar transistors in the output section, or simply that this SRM-T1 sounds like it has a few less dB of gain at 16 kHz compared to the SRM1-MK2. This couple of dB make all the difference between smooth and too bright; Stax headphones make even the slightest imbalances obvious.


Usage         top

Intro   Specs   Performance   Compared   Usage   Recommendations


DANGER: Feedback

If a headphone's cord is resting on your body, and if you pull a plug out of another piece of equipment and touch the tip, at volume settings above about 12 o'clock you may get positive feedback that results in a 120 dB howl at about 7 kHz, enough to deafen you temporally or cause permanent hearing loss.

The signal can couple through your body from the cord and back into the amplifier, and at high gain settings, cause the whole system to oscillate at full output.

ALWAYS turn down the volume before changing any connections!



This amplifier is a class-A tube power amplifier, so it gets warm.

Don't cover any of the top or bottom convective cooling holes.

Never put anything on top of this amplifier. Not only are you likely to damage the amplifier, you may burn whatever you put on top.

Proper airflow is critical. No fan is used for noise reasons. Air flows naturally with convection from bottom to top, so leave all the slots and holes free.



Watch for High Voltage when plugging and unplugging headphones: even when off, it's easy to touch the connector pins and potentially (hee hee) get a shock.

Never, ever open this thing. Lethal voltages are present, even if unplugged. The capacitors can store enough energy to kill a man for quite some time.


Recommendations         top

Intro   Specs   Performance   Compared   Usage   Recommendations

The only place to find these limited-production amplifiers is here at eBay, where they usually sell for about $600 if you know How to Win at eBay.

If you've found my efforts in documenting this classic equipment helpful, this free website's biggest source of support is when you use these links, especially this link directly to them at eBay (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.


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


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