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Stax SRA-12S
Integrated Amplifier
(1976-1982)
© 2011 KenRockwell.com. All rights reserved.

Intro   Specs   Performance   Compared   Usage   Recommendations

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Stax SRA-12S

Stax SRA-12S Integrated Amplifier. 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 $500 (see How to Win at eBay). It helps me keep reviewing these oldies when you get yours through these links, thanks! Ken.

 

May 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

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The Stax SRA-12S is an exotic integrated preamplifier combined with a dedicated Stax headphone amplifier.

It runs pure class-A. It is DC-coupled, uses FETs in the first amplification stage, and four 2SC1167 bipolar transistors in the final stage. Stax advertised this as all-FET (page 47, Audio magazine, October 1975).

In addition to the high-voltage bias and audio output specific to Stax electrostatic headphones, an unusual feature is a switchable + 20 dB Intermediate Stage Amplifier to give extra gain. It switches out of the audio path for normal use. This lets one operate this amplifier at the optimum gain at all times.

This is a very handy amplifier: simply add any source or sources, a pair of Stax headphones and/or a power amplifier and speakers, and you're all set. Better, this compact unit can sit on your desk, controlling the entire show while all the ugly stuff stays hidden.

Its power output amplifier is for driving Stax headphones; to drive loudspeakers, you'll need any ordinary power amplifier fed from its preamplifier outputs.

It was featured on the cover of Audio magazine, October, 1978.

 

Specifications         top

Intro   Specs   Performance   Compared   Usage   Recommendations

 

Rear, Stax SRA-12S

Rear, Stax SRA-12S Integrated Amplifier. enlarge.

 

Inputs

There is one conventional phono input, and four line-level inputs.

They are labeled thusly:

 

MAG PU: Magnetic phono pickup, RIAA (1mV, 47k Ohms).

COND PU: Condenser phono pickup, standard flat (unequalized) line-level (100 mV, 100 k Ohms) to be fed from an outboard Stax condenser phono preamplifier-equalizer like the Stax ECP-1. In actual use, this is a regular line-input just like the rest; all the funny business for driving and processing the signals from Stax' condenser direct phono cartridges is done in an external unit, like the ECP-1.

TUNER: Standard line-level (100 mV, 100 k Ohms).

AUX 1: Standard line-level (100 mV, 100 k Ohms).

AUX 2: Standard line-level (100 mV, 100 k Ohms).

 

There is also a full tape-monitor loop (100 mV 100 k Ohms), which may be used for a tape or digital recorder, or as a sixth input, or as an external processor loop.

 

Frequency Response

Rated DC - 1 MHz +0, -3 dB.

 

Distortion

Rated 0.05% THD at 1 kHz at full output.

Rated 0.05% intermodulation distortion at 250 Hz/8 kHz at 4:1 at full output.

 

Outputs

The two 230 V bias (standard) headphone outputs are connected in parallel.

They are rated for 350 V RMS output.

There are two preamp outputs, marked 1 and 2.

Output 1 goes live when the Loudspeaker button is pressed.

Output 2 is a freebie: it only goes live when the Headphones (Earspeakers, or ES) are activated.

There one 300 W switched power outlet.

 

Size

Actual measured values:

7 x 4.3 x 13.7 inches, WHD, including feet and protrusions.

177 x 110 x 304 millimeters, WHD, including feet and protrusions.

 

Line Voltage and Power

VOLTAGE WARNING

While the back panel is cheerfully marked "100 - 117 - 220 - 240 V 50/60 Hz," it doesn't make it clear that there is an internal connector that must be moved only by a technician to select one of these voltages. Plug it into 220 without having a technical readjust it, and you're dead!

 

Power Consumption:

Rated at 29 Watts.

Measured at 29 Watts at 120 V, 60 Hz.

 

Price

Corrected for inflation in 2011, it had a catalog price of $2,000 ($500 in 1975 dollars).

In 2011, they sell used for about $500, when you can find them at this link to them at eBay. (see How to Win at eBay).

Stax SRA-12S Internal View

Stax SRA-12S Integrated Amplifier. enlarge.

WARNING: Deadly voltages present even when unplugged. Don't do this.

 

Performance         top

Intro   Specs   Performance   Compared   Usage   Recommendations

 

Overall

The Stax SRA-12S is a handy, hot amplifier, especially if you run a system with or around Stax Headphones, and/or need a state-of-the-art audiophile reference desktop system. For desktop use, your monoblock power amps go under your desk, controlled by the SRA-12S at your fingertips.

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

 

Measurements

Preamp Gain

With the Intermediate Stage Amplifier (ISA) in-circuit, gain is unity to the preamp outputs with the gain control at 12 o'clock.

With the Intermediate Stage Amplifier (ISA) bypassed, gain is unity to the preamp outputs with the gain control full clockwise.

With the gain control fully clockwise, the ISA adds +20.4 dB of gain.

With the gain control at 12 o'clock, the ISA adds +22 dB of gain.

 

Preamp Output Level

Without the Intermediate Stage Amplifier, it's passively coupled to the input, so there is no practical limit to the output.

With the ISA in-circuit, I measure 8.2 V RMS (18.27 dBV or 20.49 dBu) at 0.1% distortion at 1 kHz.

 

THD + N

With 30 kHz measurement bandwidth, 1 kHz tone and the ISA in-circuit:

THD + N is 0.03% or less from 4.5V RMS down to 80 mV.

THD + N is 0.01% or less from 1.4V RMS down to 260 mV.

At 1 V RMS output, THD + N is 0.007% (-83 dB).

 

Preamp Frequency Response

With Intermediate Stage Amplifier (ISA)

With the ISA in-circuit and the gain control at 12 o'clock (unity gain) and a 200 mV RMS signal fed into the AUX 2 input:

5 Hz: -1.0 dB

8 Hz: -0.5dB

10 Hz: -0.3 dB

20 Hz: -0.1 dB

50 Hz: 0.0 dB

100 Hz: 0.0 dB

200 Hz: 0.0 dB

500 Hz: 0.0 dB

1 kHz: 0.0 dB

2 kHz: 0.0 dB

5 kHz: 0.0 dB

10 kHz: 0.0 dB

20 kHz: 0.0 dB

50 kHz: -0.1 dB

100 kHz: -0.3 dB

150 kHz: -0.3 dB

 

With the ISA in-circuit and the gain control at 12 o'clock (unity gain) and a 200 mV RMS signal fed into the COND PU input:

5 Hz: -1.0 dB

8 Hz: -0.5dB

10 Hz: -0.3 dB

20 Hz: -0.1 dB

50 Hz: 0.0 dB

100 Hz: 0.0 dB

200 Hz: 0.0 dB

500 Hz: 0.0 dB

1 kHz: 0.0 dB

2 kHz: 0.0 dB

5 kHz: 0.0 dB

10 kHz: 0.0 dB

20 kHz: 0.0 dB

50 kHz: -0.1 dB

100 kHz: -0.3 dB

150 kHz: -0.3 dB

Therefore, I've confirmed that the COND PU is the same as the other inputs. THD was also 0.007% at 1 V RMS at 30 kHz bandwidth.

 

 

Without Intermediate Stage Amplifier (ISA)

WIth the ISA out-of-circuit, a 200 mV rms input and the gain control fully clockwise for a 200 mV RMS output:

5 Hz to 50 kHz ±0 dB.

-0.1 dB at 100 kHz.

Performance is the same with power off; it's all passively DC coupled!

 

WIth the ISA out-of-circuit, a 200 mV rms input and the gain control at 2 o'clock for 31.6 mv (-30 dBV) output, high frequencies roll off a bit because of the high output impedance from the passive circuitry after the gain-control pot. There is no active circuitry or amplification between input and preamp output with the ISA out-of-circuit:

5 Hz to 3 kHz ±0 dB.

-0.2 dB at 4 kHz.

-0.25 dB at 5 kHz.

-0.6 dB at 7.5 kHz.

-1 dB at 10 kHz.

-2 dB at 15 kHz.

-3 dB at 20 kHz.

-9 dB at 50 kHz.

-14 dB at 100 kHz.

 

Preamp Output Noise

With the Intermediate Stage Amplifier (ISA) bypassed, the preamp is passive and has no noise.

With the ISA in-circuit, the output noise measures, regardless of gain control setting or signal output level:

-93 dBV (22 microvolts) RMS, 30 kHz bandwidth.

-89 dBV (36 microvolts) RMS, 80 kHz bandwidth.

-80 dBV (100 microvolts) RMS, unweighted, 700 kHz bandwidth.

Therefore, with a 2V RMS (6 dBV) output from a CD player at 0 dB FS with the gain control at 12 o'clock, SNR would be 93 + 6 = 99 dB.

 

Sound

For reasons I can't fathom, the New SR-3 sounds much better with this amp than it does with the SRM-1/MK-2 Professional or the SRD-5. The SR-X Mark III sounds great; this amp is optimized for standard-bias headphones.

The stereo volume control tracks well, so long as you don't use the Intermediate Stage Amplifier when not needed.

 

Heat

As a pure class-A device, everything runs as hot, or hotter, at idle as it does at full output.

The four power output transistors are always sinking about 5 Watts each, and since they have no heat sink and sit close together, there is a very hot spot in the top middle of the amplifier. It's supposed to get hot, so be careful!

The SRA-12S gets so hot that it often even smells hot. This is normal; just ask yourself how hot would a regular 30 W light bulb get if you crammed it in a steel box this small.

 

Hum

Think tubes. Even though this is a solid-state amp, everything is high impedance and high voltage, just like tube amps.

Unlike the 10 k Ohm audio equipment most popular today, this 100 k Ohm input-impedance equipment is much more susceptible to hum pick-up if you're sloppy with your grounding and power plan.

The rear GROUND connection is for your turntable.

If you're getting hum, try modifying your ground design, or reverse the power plug. If using an iPod or iPad, remove it from the charger or computer, and let it run on its own battery.

I get an almost audible bit of what sounds like power supply hum in one headphone channel with no signal present, but it's virtually inaudible, and only audible with 230 V phones and only with nothing playing. With Pro headphones like the Lambda Pros, their lower sensitivity (at 230 V) makes it go away. I'm too lazy to track it down; I can't hear it with the music playing, or with the SR-Lambda Pro phones.

 

Noise

As far as I can hear, it's silent in its normal mode.

If you kick in the Intermediate Stage Amplifier for 20 dB more gain, there is a tiny bit of barely audible noise in very quiet environments. I can't hear it when music plays.

Oddly, the Intermediate Stage Amplifier is after the gain-control pot and the last thing before the output stage, so its noise isn't attenuated by the pot. Weird.

The SRA-12S has so much clean gain that it's easy to crank it up and hear whatever's in your source, be it noise or music.

Impressively, if you plug your iPod directly into the SRA-12S, it is astounding how quiet is its output. I can hear everything in the original CD. DDD 20-bit noise-shaped disc? I can still hear the room noise in the original performance space pot-down and pot-up between selections. Ha!

 

Controls

Noise, dirt and crud

I'm impressed: this 1970s piece runs fine, without any noise when operating the switches, gain or balance controls.

 

Gain (Volume) Control

The gain control tracks (maintains stereo balance) just fine, except for the very last couple of degrees of maximum attenuation. Don't go there; turn off the Intermediate Stage Amplifier (ISA).

With the ISA out and using the Preamp 1 output, the very top of the gain control (loudest volumes) change the gain a great deal with a small amount of rotation. This goes away with the ISA in the circuit, or using the headphone outputs. It's as if the line-out section loads the potentiometer's output significantly, but the ISA or headphone amp presents a much higher impedance to it.

 

Balance Control

While the headphone outputs seem unaffected, the effect of the balance control on the PRE OUT 1 becomes very touchy at the very top of the gain control's range.

With the gain all the way up, the balance control suddenly has a very strong effect on PRE OUT 1 as soon as one moves it off-center.

If you engage the Intermediate-Stage Amplifier, the balance control operates normally at all gain control settings.

 

Gain

There is LOADS of gain without needing the Intermediate Stage Amplifier's extra 20 dB of boost for standard headphones, but not that much at the preamp out.

Even though there is plenty of gain, be careful with Pro-bias phones because of their lower sensitivity with the 230 V bias of this amp. It's not hard to clip the amp with pro phones with particularly dynamic music, so watch it.

 

Intermediate-Stage Amplifier (ISA)

The ISA comes after the volume-control pot.

I measure 20 dB of gain, as expected.

While the complete amplifier is advertised as DC-coupled, oddly the ISA seems to be -1 dB at 20 cps, -3 dB at 10 cps and -6.5 dB at 5 cps off from its +20 dB of gain.

 

Mechanics

It's built on a stamped-steel chassis with steel top cover and aluminum front panel.

The screws on the side holding the top cover seem to be stainless steel.

Stax SRA-12S

Stax SRA-12S Integrated Amplifier (front on left). enlarge.

WARNING: Deadly voltages present even when unplugged. Don't do this.

 

Electronics

The four-section combined gain and balance control looks fairly dinky, compared to the luxurious sealed potentiometers of the Stax SRM-1/MK-2 Professional, but it works fine, and to my pleasant surprise, wasn't dirty at all on this 35-year-old amplifier.

The power transistor are four 2SC1167, mounted in free-air with no extra heat sink.

The large capacitors are Elna 250V 47uF.

The pilot light seems driven by the output of the power supply; it's slow to turn on and off.

The line voltage selector is internal. Since you can be killed touching things inside this amp, even after it's been unplugged, pay your certified technician to change it if needed.

Stax SRA-12S

Top view, Stax SRA-12S Integrated Amplifier. enlarge.

WARNING: Deadly voltages present even when unplugged. Don't do this.

 

Compared         top

Intro   Specs   Performance   Compared   Usage   Recommendations

Unlike simple amplifiers like the SRM-1/MK-2 Professional, this fully integrated amplifier can be the center of a fine music system, replacing a conventional preamplifier, control center and Stax headphone amplifier with just this one unit.

Since it only has standard 230 V bias outputs, it's not the best choice for use with Pro phones. It works, but reduced sensitivity and output level.

It works great with standard-bias (older) Stax headphones.

Compared to any passive adapters, it's a huge step up.

 

Usage         top

Intro   Specs   Performance   Compared   Usage   Recommendations

 

Pro-Bias Headphones

Pro-bias (580V bias) headphones like Stax Lambda Pro sound swell, but with less maximum output level than with a dedicated Pro-bias driver like the SRM-1/MK-2 Professional.

They sound great, and usually have plenty of MOL for me, but if you like to hurt your ears, use a dedicated 580V amp along with this control center.

To do this, simply connect a 580V-bias amp like the SRM-1/MK-2 Pro to the REC output of the SRA-12S. The REC output is passive; even with the SRA-12A turned-off, this output is active and has no noise or distortion. Now you'll have four Stax outputs, each pair with its own volume control, as well as the superb master-control functions of the SRA-12S.

 

Heat

This amplifier is a class-A power amplifier, and it gets HOT.

If you cover any of the top or bottom convective cooling holes, it probably will catch fire.

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 you must leave all the slots and holes free.

 

Safety

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.

 

Connections

 

Switched power outlet

Stax says only connect low-power devices like tuners, equalizers and tape decks to this outlet, but since it's rated for 300 W, I see no reason to plug a sensibly-sized power amp into it, even if Stax cautions against it.

By plugging the power amp and the rest of your system into this, everything is controlled from the power switch of your SRA-12S.

 

Inputs

The line inputs and tape monitor loop are obvious.

The spring-loaded ground terminal is for your turntable.

The COND PU input seems bizarre, but it's just another regular line input.

 

PRE OUT 1

Connect PRE OUT 1 to your power amplifier.

PRE OUT 1 turns on only when the Output: LS/ES (Loudspeaker/Earspeaker) switch is pressed in to the LS (Loudspeaker) position.

When enjoying headphones, it's muted. Clever!

 

PRE OUT 2

PRE OUT 2 is a freebie: it follows the input to the headphone amp, meaning it is only active when the LS/ES (Loudspeaker/Earspeaker) switch is left out in the ES (Earspeaker) position.

 

REC OUT

The REC OUT connection is passively coupled from the output of the input selector. Even with the SRA-12S turned off, the REC OUT is a noise and distortion-free direct connection from the selected input.

The Intermediate Stage Amplifier (ISA), stereo mode, volume and balance controls have no effect.

 

Stereo Mode Selector

Just like classic tube amplifiers of the late 1950s and early 1960s when stereo was fresh and new, there is a full-featured stereo mode selector.

It's the middle knob between the input and volume controls.

Here's what it does:

 

Reverse

Flips the stereo image from left to right. Basses will be on the left, and violins on the right. This is weird.

In the early days of stereo (late 1950s and 1960s), sometimes people would get their connections crossed. This control saved you the trouble of giving your amp a reach-behind to fix the problem.

 

Stereo

Normal stereo, in other words, this is what you get with other pieces of stereo gear that lack this advanced mode selector.

 

L + R (Left + Right)

This is a fancy way to say "mono."

The stereo image is collapsed to a point in the middle.

 

L (Left)

Only the left channel is heard.

It's piped to both output channels at the same time.

Thus the sound is mono, and you only hear whatever's coming in on the left channel.

The reason for this switch was for playing mono tapes on a stereo tape deck. You could select the channel on which the mono recording lies, and hear it through both channels. If you didn't have this control, you'd have to turn down the other channel to turn off the second mono program, and listen out of one ear.

With a stereo input, this flattens the stereo image to a point source, and since you're only getting the left side, you'll hear more violins and violas, but less bass and cello.

 

R (Right)

Only the right channel is heard.

It's piped to both output channels at the same time.

Thus the sound is mono, and you only hear whatever's coming in on the right channel.

This flattens the stereo image to a point, and since you're only getting the right side, you'll hear more bass and cello, but less violins and violas.

What we really need is a switch to suck out the saxophone. Who ever let those into the orchestra, anyway?

 

Balance Control

Youngsters may not remember these.

The balance control adjusts the relative level of the left and right channels.

Back in the old days of tape, sometimes things would be misaligned, and the balance control would let one get the stereo image back on-center.

 

Intermediate Stage Amplifier (ISA)

You kick this into the circuit when the I.S. Amp button is out. I leave this pressed in, eliminating this extra amplification.

I'd not use the Intermediate Stage Amplifier unless I really needed more gain.

The Intermediate Stage Amplifier is after the front-panel gain control.

The gain pot may not track well in stereo if you leave the Intermediate Stage Amplifier active, because you'd have the gain (volume) control set almost to minimum most of the time.

Be careful of the I.S. Amp button. Hit it by accident if you're not using it and you'll get blasted-out with an instant +20 dB of extra gain, equivalent to cranking the volume control by 180 degrees clockwise.

 

Charge Time

This system uses static electricity, meaning non-moving (static) electric charges.

The static charge, AKA polarizing voltage, is supposed to take a few minutes to spread around the diaphragm in the headphones. 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 charge to spread out and even out around the diaphragm.

As the charge spreads, it is normal for the audio to increase in level for the first few minutes that 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 seconds the channels may 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 better if they're left plugged in for at least a day.

 

Recommendations         top

Intro   Specs   Performance   Compared   Usage   Recommendations

The only place anyone will ever find these limited-production amplifiers is here at eBay, and they don't appear often.

If you use traditional Stax headphones (6-pin, 230V bias) and want a desktop master control center, the SRA-12S is superb.

If all you want is an amplifier section for Stax headphones, it's much easier to find other amps, like the SRM-1/MK-2 Pro, which sell for the same price, and offer the advantage of both Pro (5-pin 580 V) and standard outputs, and better heat distribution and possibly slightly higher audio performance.

If you want a master control to save you from having an additional preamplifier on your desk, the SRA-12S is awesome.

 

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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