Description
Vintage STAX SRA-125S Integrated Amplifier for Electrostatic Speakers 1976 The vintage Stax SRA-12S is an exotic integrated preamplifier combined with a dedicated Stax headphone amplifier. This item has only been tested by powering it on, pushing buttons, and turning knobs. I was unable to test the audio. Please read the condition section fully and ask any questions prior to bidding. WARNING: The line voltage selector is internal. Since you can be killed touching things inside this amp, even after its been unplugged, pay your certified technician to change it if needed. Deadly voltages present even when unplugged. Dont do this. Approximate measurements:7 x 13 x 4.5 Approximate weight:9 lbs 6 oz. Heres some more information and specs on this item: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 youre 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, youll need any ordinary power amplifier fed from its preamplifier outputs. It was featured on the cover of Audio magazine, October, 1978. InputsThere 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 ResponseRated DC - 1 MHz +0, -3 dB. DistortionRated 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. OutputsThe 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. SizeActual 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 PowerVOLTAGE WARNINGWhile the back panel is cheerfully marked 100 - 117 - 220 - 240 V 50/60 Hz, it doesnt 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 youre dead! Power Consumption: Rated at 29 Watts. Measured at 29 Watts at 120 V, 60 Hz. PriceCorrected 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. 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 oclock. 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 oclock, the ISA adds +22 dB of gain. Preamp Output Level Without the Intermediate Stage Amplifier, its 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 oclock (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 oclock (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, Ive 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; its all passively DC coupled! WIth the ISA out-of-circuit, a 200 mV rms input and the gain control at 2 oclock 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 oclock, SNR would be 93 + 6 = 99 dB. Sound For reasons I cant 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 dont 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. Its 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 youre sloppy with your grounding and power plan. The rear GROUND connection is for your turntable. If youre 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 its 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. Im too lazy to track it down; I cant hear it with the music playing, or with the SR-Lambda Pro phones. NoiseAs far as I can hear, its 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 cant 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 isnt attenuated by the pot. Weird. The SRA-12S has so much clean gain that its easy to crank it up and hear whatevers 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! ControlsNoise, dirt and crud Im impressed: this 1970s piece runs fine, without any noise when operating the switches, gain or balance controls. Gain (Volume) ControlThe gain control tracks (maintains stereo balance) just fine, except for the very last couple of degrees of maximum attenuation. Dont 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. Its as if the line-out section loads the potentiometers output significantly, but the ISA or headphone amp presents a much higher impedance to it. Balance ControlWhile 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 controls 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. GainThere is LOADS of gain without needing the Intermediate Stage Amplifiers 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. Its 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. MechanicsIts 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. Questions, comments, or concerns? Feel free to message me. Free local pickup is offered on all listings. Please note: We use quality, recycled boxes to ship your items. Part of our dedication to keeping our business as green as possible while providing you safe, and affordable shipping. If youre purchasing this item(s) to eat or drink out of or serve food on, we recommend you wash your item(s) first. Bidding is set up to block people with zero feedback to assist in protecting us from scammers. 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