This is a review and detailed measurements of the Stax SRM-Xh which powers ("energizes") the Stax Pro electrostatic headphones. It was kindly purchased used by a member and drop shipped to me. I don't know what it originally cost but I see them going for $300 or so on ebay right now. The unit I received did not come with a power supply so I used my 12 volt lab bench.
I must say, the Xh seems down right cheap and utilitarian compared to my SRM-313 next to it:
While mine has the nested dual volume controls, the Xh lacks that which did not allow me to compensate for the channel balance in my measurements.
As I noted, no power supply to power the unit. I went to make a connector for its 12 volt jack, only to realize that it uses reverse polarity of the shield being positive.
This should have been outlawed many years ago!
In the picture you see my SR-007MK1 Stax headphone. It is an old fellow and its headband is not as tight as it used to be so please don't pay super attention to the frequency response measurement. Purpose here is to compare the amps.
Stax SRM-Xh Measurements
I adjusted both units to deliver 94 dBSPL with 1 volt input and measured the frequency response of the SR-007:
Basically there is no difference in frequency response which is good. I later performed a more accurate stepped response and that showed the tiniest drop in high frequencies compared to SRM-313 which you can also see in the above graph with green peaking through.
What is notable is the lower distortion of the Xh in bass frequencies even though it has a worse spec than SRM-313:
One of my key complaints about Stax setup is lack of volume so I wanted to see where each one clipped. So I ran a traditional stepped sweep measuring THD+N versus measured level at 400 Hz:
We can clearly see that the SRM-313 pulls ahead of SRM-Xh delivering about 4 dB more acoustic power. At lower levels though as we saw in our previous test, the SRM-Xh does product less distortion. BTW, I could easily hear the clipping with the tone completely changing at that point so what you see, is what you get subjectively.
Specs show the max output of the SRM-Xh as 280 volts and the SRM-311 at 350 volts. Someone can do the math but seems consistent with me getting more output from the latter.
Stax SRM-Xh Listening Tests
I drove the Xh using my RME ADI-2 DAC V2 as the source at 0 dB. At lower volumes the sound was nice and I could not detect an issue. Turn up the volume past 12:00 o'clock though and peaks would get static. Drive it more and sound would just fall apart.
For comparison I then switched to SRM-313 and I could get a bit more volume before it too started to crackle. It is interesting that loudness which is a human perception requires tons more power as that 4 dB advantage was not that apparent. Still, I take this as even the max volume here, I found the overall loudness to be lacking.
SRM-Xh Teardown
I took a quick look inside in the process of verifying the polarity of the input voltage:
Conclusions
I imagine the SRM-Xh is much cheaper than my SRM-311. If so, it seems to serve that purpose of providing a much more economical amplification solution to drive the Stax headphones. Personally I like to see much better drive capability than just making things smaller and cheaper. Anyway, there is not much of a reference for me to judge the unit as good buy or not. You have the data; you decide.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
I must say, the Xh seems down right cheap and utilitarian compared to my SRM-313 next to it:
While mine has the nested dual volume controls, the Xh lacks that which did not allow me to compensate for the channel balance in my measurements.
As I noted, no power supply to power the unit. I went to make a connector for its 12 volt jack, only to realize that it uses reverse polarity of the shield being positive.
This should have been outlawed many years ago!In the picture you see my SR-007MK1 Stax headphone. It is an old fellow and its headband is not as tight as it used to be so please don't pay super attention to the frequency response measurement. Purpose here is to compare the amps.
Stax SRM-Xh Measurements
I adjusted both units to deliver 94 dBSPL with 1 volt input and measured the frequency response of the SR-007:
Basically there is no difference in frequency response which is good. I later performed a more accurate stepped response and that showed the tiniest drop in high frequencies compared to SRM-313 which you can also see in the above graph with green peaking through.
What is notable is the lower distortion of the Xh in bass frequencies even though it has a worse spec than SRM-313:
One of my key complaints about Stax setup is lack of volume so I wanted to see where each one clipped. So I ran a traditional stepped sweep measuring THD+N versus measured level at 400 Hz:
We can clearly see that the SRM-313 pulls ahead of SRM-Xh delivering about 4 dB more acoustic power. At lower levels though as we saw in our previous test, the SRM-Xh does product less distortion. BTW, I could easily hear the clipping with the tone completely changing at that point so what you see, is what you get subjectively.
Specs show the max output of the SRM-Xh as 280 volts and the SRM-311 at 350 volts. Someone can do the math but seems consistent with me getting more output from the latter.
Stax SRM-Xh Listening Tests
I drove the Xh using my RME ADI-2 DAC V2 as the source at 0 dB. At lower volumes the sound was nice and I could not detect an issue. Turn up the volume past 12:00 o'clock though and peaks would get static. Drive it more and sound would just fall apart.
For comparison I then switched to SRM-313 and I could get a bit more volume before it too started to crackle. It is interesting that loudness which is a human perception requires tons more power as that 4 dB advantage was not that apparent. Still, I take this as even the max volume here, I found the overall loudness to be lacking.
SRM-Xh Teardown
I took a quick look inside in the process of verifying the polarity of the input voltage:
Conclusions
I imagine the SRM-Xh is much cheaper than my SRM-311. If so, it seems to serve that purpose of providing a much more economical amplification solution to drive the Stax headphones. Personally I like to see much better drive capability than just making things smaller and cheaper. Anyway, there is not much of a reference for me to judge the unit as good buy or not. You have the data; you decide.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Any donations are much appreciated using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
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