STAX SRA-12S

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.

Inputs: 

- 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:
7 x 4.3 x 13.7 inches, WHD, including feet and protrusions.
177 x 110 x 304 millimeters, WHD, including feet and protrusions.