This is my fist posting for this forum. I hope it’s to your liking !
Apart from the audio qualities, I am also interested in the technical qualities of equipment. So my posts sometimes will be a bit technical. Just skip the uninteresting bits. If that is almost all: so much the better. That way you have more time left for enjoying the music ;-)
Quite recently I finally managed to obtain a Stax SRA-12S pre-amplifier with built-in ES headphone amplifier.
According to the previous (first) owner the amp got very hot.
Well, since it is supposed to run quite hot, I was not alarmed and bought the SRA-12S unseen (well…. just a photo).
Unfortunately one channel of the ES-output was not functioning properly.
Spritser was so kind to mail me a photo of the schematics, and having a schematic and/or service manual certainly always makes repair a lot easier.
First I checked all electrolytics, and I only needed to replace 3 of them.
All the others were in excellent condition. Quite an unexpectedly good result, given the age of the SRA-12S.
As you all know, you can consider the power supply to be in series with the output (for the audio signal), so the supply quality has to be as good as the amps themselves.
Two of the defective electrolytics were at the final filtering of the + and – 15V regulator board, so I replaced these with MKP’s.
Not sure if it really gives an audible improvement, but it won’t hurt either. And film capacitors will outlive me (if I manage not to touch the 650V too often).
The defect in the HighVoltage amp turned out to be a bad 2SC1167 power transistor.
The fault was intermittant, drawing more current and blocking the audio signal. But in between blocking, the signal was not steady and gave a rustling, noisy, clicking sound. So a replacement transistor was to be found. To be absolutely sure I checked all (old) transistors I had, but alas…. no 2SC1167 amongst them.
It is a HV transistor in TO-3 package.
I considered using transistors in a different package and adding a heat sink, but decided againt it for aesthetic reasons. If I could find myself a pair of suitable TO-3 substitutes, that is.
I measured all HV transistors that I have, on my Tesla transistor tester, and compared them with the 2SC1167s in the amp.
Max current specifications of the transistor is not an issue, because all these large HV transistors are made for several Amps. And in this application the quiescent current is about 7.5mA. So the remaining important spec points are max C-E voltage and amplification factor at very low currents. For this I measured the DC collector current at a DC base current of 1mA.
A logical transistor would have been the BU208, but the Hfe measured way too low.
Despite the fact that I only have a limited number of transistors at my disposal, I was very happy to find 2 transistors (type Philips BUX88) that match and perform very well, and that can handle the high voltage. I am sure there are a lot of other types that will perform equaly well. These transistors have a large spread in the Hfe at such a low collector current, so it will probabply always be advanteous to select them with a tester.
Mounting and soldering them in the amp proved that they are working like a treat!
Problem solved, now there is room for some subtle improvements.
The original output capacitors of the HV amps had been replaced by the previous owner (or the importer) a long time ago.
The original value is 4.7nF. Since other STAX amps have higher value output capacitors (often 50nF), I decided that a somewhat higher value could not hurt. So I replaced all four of them with a high quality Philips MKP 10nF-2000V.
These are quite a bit larger than the originals, but can be fit in just about.
These are important capacitors, and only very good quality types will get you the very best performance and reliability.
The SRA-12S that I have is of the first version, all be it of a highish serial number. The later version looks (and probably performs) identical, but has input protection diodes (from input to +15V and from input to –15V) for all amplifier circuits. I added this to my SRA-12S as well, just to be on the safe side.
The later version also has higher capacity filter capacitors for the + and – 15V. So I added additional smoothing capacitors to the originals.
An MKP (polypropyene) capacitor across the +650V power line should possibly improve the quality of the HV, so I put a 220nF capacitor across the 650V.
The later version SRA-12S also has a 100nF-250V cap across the bias supply, so I added this capacitor also.
I am not decided yet if that is actually to be considered an improvement or not. Given the fact that the coating on the diafragm has a very high resistance, it should make no real difference except maybe filtering out that last bit of 100/120Hz ripple. The charge on the diafragm should be constant for low distortion, and due to the high surface resistance, this will be the case. If the surface resistance would be quite low, I would prefer not to include the 100nF cap. In that case the series resistor (4.7M) is in the charge circuit, acting with the capacitance of the speaker (approx 100pF) as a time constant, to prevent the charge on the membranes from changing to fast.
I did some measurements on the SRA-12S, and this amp proves to be excellent.
The mains voltage in this country is 230V, so I set the voltage selector for 240VAC (the nearest higher value). It was still on the 220V setting, which corresponds with the local mains voltage when the unit was sold.
The maximum output voltage (without load) measured about 1080Vpp or 380Vrms.
This is dependant of the setting of the variable resistors on the output amplifier boards. I adjusted for max output voltage swing, and equal collector voltage of the 2SC1167s. (The collector voltage for max output turnes out to be 280VDC; with higer mains voltage, probable 290V will be best.)
The output with connected phones is a few percent lower.
Not bad results at all.
At this (lower than nominal) mains voltage, the high voltage supply is 630VDC. At 240VAC mains, the max output will be about 4% higher. Which, of course, is of no audible advantage whatsoever (<0.4dB).
I was not able to measure the distortion of the HV amp (because of the balanced output).
The I.S. (inter stage) amplifier gave the following results (measured @1kHz with HP339A with 30kHz and 80kHz filters engaged):
@ Vout = 0,5Vrms: d=0.009% (left and right channels). The distortion residue is all noise.
@ Vout = 1Vrms: d=0.0085% (left) / 0.007% (right). Noise and 2nd harmonic.
@ Vout = 2Vrms: d=0.0145% (left) / 0.011% (right). Predominantly second harmonic.
First class, certainly since the distortion is very low and almost only 2nd harmonic.
Bias voltage is 197V (@ Vmains = 230VAC), which is very close to the specified 200V.
The “normal bias” later changed to 230V.
Input sensitivity with balance control in mid position and volume max, is 95mV for 1V out. Both channels have identical sensitivity.
The sound is absolutely first class (on my SR-X Mk-III).
I woundn’t be surprised if the SRA-12S proves to be one of the better STAX units.
I will do some comparison with my SRD-7, which I have always liked.
I would be delighted to hear any experiences with the SRA-12S from the forum members!
Does anyone still have a SRA-12S and how does it sound to your ears?
Maybe there is someone out there having the user manual? A copy or high resolution scan certainly would not go amiss ;-) Thanks!
A test report from one of the magazines of the time would also be appreciated!
At the time (end of the ‘70s) it certainly must have been state of the art.
In the attached pictures you can see:
* the output voltage swing, just before clipping (200V/div)
* the inside of the SRA-12S
* the outside of this fantastic amplifier
