Stax SRM-1/MK2

After some years of seeing other headfiers rave about these amps, I finally got a B series model and found that indeed it had  advantages over my later model SRM3 amp.  I was mostly concerned with running a set of Sigma pros and Sigma/404's which are the least efficient Stax phones and they definitely sounded better with the SRM1Mk2 than my more modern SRM3. 

 

For some reason Stax reduced the voltage swing on the later amps compared to the SRM1Mk2, reducing it from 370 to 300 volts. Stax kept this lower voltage swing on most later transistor and tube amps although the most recent designs seem to be pushing it up again.  The 717 for example runs 450 volts and I believe some other new amps are back at about 340..   This may be why I hear better dynamics and air with the SRM1Mk2 than the SRM3.

 

I liked the SRM1Mk2 so much that I sold one of my 2 SRM3's and bought a second SRM1Mk2.  The second Mk2 was an A series model, meaning that it was even older. The C series being the most recent models.  I could hear no difference between the A and B models.

 

The Stax site shows that the original SRM1 dates back to 1979.   http://www.stax.co.jp/Export/History.html

 

It has both high and low bias sockets. The SRM1Mk2 was first released in 1982.  I do not know what differentiates  it from the non-Mk2.  The very last SRM1Mk2's dropped the low bias socket. and have 2 high bias sockets.   Some models are champagne colored, some are black.

 

To me the low bias socket  is a plus, since I have  some low bias phones.  If you really need 2 high bias outlets you can use a Stax 3-way adapter (but this needs an extension cord) or make can your own adapter.

 

Another advantage over later Stax amps is that it is easy to change the input voltage so you can take say a Japanese or British model and switch it over to 117 volts with a simple adjustment of an external plug.  Stax stopped offering this plug in later models, probably to close down the grey market of imported models coming in outside of the Stax dealers.  Subsequently they have cut wires around the power transformer so that it is even more difficult to make  internal modifications to the input voltage of their most recent amps..

 

I have heard some users contend that the Mk2 tends to harshness.  If so it's not obvious on the Sigma Pro, Lambda Signature  and Sigma/404  I normally use with these amps.  I rate this amp at 4 stars in audio quality where the $5,000.00 BHSE super amp would be 5 stars and a used 717 would be 4 1/2 stars. 

 

The only real drawback with these amps is their age.  Some suggest replacing the large capacitors on such old amps.  I haven't done that myself so I can't say if it would make them sound better. 

 

On balance, I think it is one of the better Stax  amps and I would recommend it over the more recent amps that give a swing of only 300 volts.   If you can get a decent one for $300.00 it is a good amp to have.

Info/specs:

You can find some info and specs and manual pages for the unit on:




http://wiki.faust3d.com/wiki//index.php?title=Stax_SRM-1/MK-2_Pro




I haven't heard the new series 3 amps, but that page also talks about srm-1 vs them, and SRM-1 driving not lambda frame ear-speakers.




From what I understand, that makes it good value from about 400 to $700 depending on version, condition and mods. (unlike the other reviewer suggesting $300).

Refreshing/restoring/modding one

I purchased my SRM-1/mk2 rev c (pro + normal sockets) from a fellow head-fier and it was fubar. Initially the unit went to the Sydney 'authorized Stax repairer' who claimed to have fixed the problem and 'tested it on both outs/everything worked' but somehow didn't notice the dead bias resistor...yea... To add insult to injury they changed only three out of four electrolytic caps....

Took me almost a month and a lot of help from Kevin and Birgir to fix it (mostly figuring out how to use a multimeter and a soldering iron without killing anything, waiting for parts and figuring out that my MD11 source was faulty and putting DC into line out). The end result was replacing the electrolytic power section caps (not good after 30 years/the aluminum bits at the top were cracked/etc) and diodes, a dead bias resistor and bias diode and some 650v film caps (4 power ones upgraded to 1KV as per schmatics) + 2pf 500v caps (upgraded to 1KV on spritzer's advice).




I've also talked to Birgir and he says the driver unit can be easily rewired for balanced ops (fit the minixlr and replace a couple of resistors), which is a plus for some. To top it off, the build quality is very solid. All the parts are discrete, and pretty user serviceable (and the schematics are floating around on head-fi), if you don't mind working with HV.

End-results

The results from a refurbished SRM1/mk2 are outstanding:




Clear audio across the whole spectrum - bass, mids, treble - from what I can tell - no roll-offs or bloat peaks - very neutral ss. Wonderful detail levels.
Enough power to easily drive my SR-404 to far beyond where my SRM-300 and SRM-252 were clipping (<=300v max out, clipping psu rails earlier). According to the faust3d page it's even suitable for Omegas in absence of enough cash for an SRM-7x7 or KGSS/Exstata .




I initially had a grounding issue with my DACs via USB->RCA, but then connected the amp via a jap 2 pin cable (probably what it's intended to be used with anyway) and now everything is good. Do note, the later models no longer have the voltage switch. you may need to get the unit rewired for your voltage (not hard, soldering 2-3 wires/plenty of help in the stax thread for that)




I'll soon have a chance to try SR-507 with it and compare it to Transistor-Amp-V3 and will update the impressions here. In the meantime, I still highly recommend the unit, but be sure to replace the electrolytic caps and get a non-fubar unit (if the transistors are dead, some of them seem to be pretty annoying to replace).

Pics:

here are some pics:




Front:





Back:


Board:




Replaced parts (less large power caps not shown and a bias resistor )