Stax SRS-2020 Electrostatic headphone and amplifier

Since the mid-90’s, STAX has introduced their new lambda earspeakers (STAX way of calling their headphones) in three performance grades, Signature, Classic and Basic. The earspeakers are mated with appropriate amplifiers and sold as systems. The SRS-2020, introduced in 1999, is a Basic system consisting of the SR-202 earspeakers and the SRM-212 amplifier. Although it is an entry level STAX system, its performance is excellent. All the fine qualities of electrostatic reproduction of music, such as speed, low distortion, timbrel accuracy and ability to delineate instruments and voices in space, are all there in spades. Although it is not as good as high end STAX systems, this Basic system still sets a very high bar of performance that I think few, if any, headphones of other technologies in its price range can match. Of course, electrostatic technology has its own limitation. It cannot move as much air to produce the bone shaking bass that dynamic and orthodynamic technologies can. So electrostatic headphones are definitely not for bassheads, although this Basic system has decent bass that I think is adequate for many genre of music, such as jazz and most vocals.    
The physical condition of this set is very good to excellent. The earspeakers have seen limited use as there is very little wear on the earpads and the headpad. However, the glue on one edge of each pad has loosened due to age and the pad has lifted up a bit on that edge. But it has no effect on the sound as there is no break in the seal. The amplifier has some minor scratches and scuff mark on the metal cover but the front and back plates are perfect.   
The power supply of this amplifier consists of a wall wart which outputs 12VDC nominal to a regulated switching up-converter on the amplifier board which up converts it to the high voltage supply that the amplifier needs. However, this amp was made for the Japanese market so the wall wart is rated for 100VAC input. To operate this amplifier on north American power outlets of about 120VAC, one can do one of two things. First, the stock wall wart can be replaced with a local wall wart rated for 120V input and outputs the same 12VDC. However, this is complicated by the fact that the STAX wall wart feeds the negative side of the 12VDC to the center pin of the plug whereas most north American units feed the positive side to the center pin. Also, the STAX wall wart is very high quality and it may be difficult to get a similar quality replacement. Another approach, which is what I adopted, is to take advantage of the fact that the STAX wall wart is linear, which means that it uses a transformer that steps down the AC voltage before rectifying it to DC. So it will work just fine if it is plugged into a 120VAC wall outlet, except that the output voltage will be raised up proportionally, to about 14.4VDC nominal. If you operate the amplifier at this higher voltage, I bet it will work just fine, at least for a while. But the excessively high voltage would likely over stress the components of the up-converter and shorten its operating life. So it is definitely not advised. However, the higher DC voltage can be lowered by putting a zener diode in series with the input DC supply line. I used a 2.7V zener which drops the input supply voltage to a safe level before feeding it to the up-converter. With this modification, this Basic system from Japan will work reliably on US domestic power and there is no difference in performance. The addition of a zener diode operating well within its dissipation rating should have no impact on longevity, which is important as STAX equipment are known to last decades. Interestingly, the modified system will also work just fine on 100VAC which I had confirmed by plugging it back into my 120V→100V step-down transformer. This is because the regulated switching converter in the SRM-212 has a wide operating range and will work even if its supply voltage drops substantially below 12VDC, with no stress issue.    
I have listened to this modified system for an extended period of time, and enjoyed my favorite music played through this system. I have always been amazed at how relatively small the sonic difference is between STAX systems at different price levels. So this entry level system would be a great introduction to the high end world of electrostatic sound, if your taste in music is a good match to the strengths of this technology.  I have a 100% rating on eBay(peterye116), so you can buy with confidence.