This is a review and detailed measurements of a number of amplifiers/energizers/drivers for electrostatic headphones. I own three of them which are rather vintage Stax units: the SRM-311, SRM-006t and SRM-007t. The first one is solid state and the other tube. The last unit for comparison is the Koss E/90X energizer that is on kind loan from a member.
Here is the gang:
The Stax units harken back to the great days of Japanese hi-fi with nice enclosures, switches and volume pots. The Koss in sharp contrast could be mistaken for a give-away kids toy in McDonald's happy meal.
I did not have the plug for any of these so made a twisted wire probe that I simply stuck in the Left+/- connection. For this reason, the noise level may be higher than it would be with a more proper connector.
Stax Electrostatic Headphone Amp Measurements
I had to modify my headphone amp dashboard as it uses a 600 ohm load which doesn't work for electrostatic headphones which essentially have infinite resistance. The highest my Audio Precision goes to is 100,000 ohm so that is what I used. The other major difference is that electrostatic headphones require far higher voltages to produce sound. So instead of my usual 2 or 4 volt output, I used a whopping 80 volts! I thought this would be within the comfort range of all the headphone amps as it is in the middle setting of the Stax headphone amps. Alas, this did not turn out to be the case but we are getting ahead of ourselves. Let's start with the Stax SRM-311 where I developed the tests:
Second harmonic is at -88 dB which more or less sets our SINAD since distortion is more dominant than noise (SINAD is a relative sum of noise and distortion).
Frequency response had a slight low frequency boost:
And droop at 20 kHz. The variation probably gets lost in the frequency response of the headphone itself.
Last test that I prepped was the power test but now into 100K ohm:
Let's jump to the other extreme and test the SRM-007t which is a differential tube amplifier (balanced in, balanced out):
Now this is a bummer. Distortion at around 80 volts was huge as you see above. I thought it might be clipping or something but that is not the case:
No wonder I always heard this amp distorting like heck at higher volumes.
Its frequency response is more flat though:
Here is the SMR-006t which is also tube based but its output is not differential:
Frequency response is the same as SRM-007t:
It has the same distortion as well although noise floor is higher yet:
So this rules out anything being broken in the tube energizers other maybe higher noise of the SRM-006t.
KOSS E/90X Energizer Measurements
The E/90X lit its LED in red in protest well before 80 volt out. So I decided to take it easy on it and turn up the level to just before LED turned red:
Running our sweep we see that the KOSS is much more well behaved albeit, with much lower output voltage:
Both distortion and noise are low but then we run out juice at nearly 10% of what the Stax amps can produce. I noted that the output level was not that high in my review of the KOSS ESP 95X headphone about this issue.
Frequency response test was interesting for two reasons:
Notice that the low frequency response changes with output level! Is this some kind of loudness compression? If so, the KOSS headphone measurements will be level sensitive.
The second issue was instability at frequencies below 20 Hz. I set the frequency to 16 Hz and as I turned up the volume gradually the output suddenly jumped from something like 10 volts to 160 volts! The LED would then flicker between red and green. Suggest using a high pass filter with this headphone/amp to make sure it doesn't see anything below 20 Hz.
Conclusions
Anyone want to buy a pair of hardly used Stax tube headphone amps? 'Cause after seeing how poorly they perform, I don't think I want to be seen in the same house with them! The heck were they thinking? A tube amp mates well to electrostat headphones because it naturally have a high voltage output. Why screw that up with tons and tons of distortion? I guess they decided to use no feedback.
I was surprised that the KOSS E/90X had very low distortion. Alas, it must use much lower voltage rails/output transistors so not able to generate nearly as high a voltage as the Stax units can. Alas, if it is providing adaptive bass boost, using a different amp without that feature (the norm) would cause different audible results.
Will be interesting to test other after-market amps to see how they compare to these.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Dropped by the garden (what's left of it) to see if the persimmons are ripe enough to pick and my heart fell to the ground. The one fruit I was eyeballing everyday was gone! Fearing the worst, I searched for the rest and found a few remaining ones:
We waited nearly 10 years for this little tree to bear fruit and it finally bore fruit a couple of years ago. Alas, last year some varmint at them all.
So despite losing some, I am happy to have rescued the above. They last a long time since they are the Japanese variety.
Appreciate any kind donations using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
Here is the gang:
The Stax units harken back to the great days of Japanese hi-fi with nice enclosures, switches and volume pots. The Koss in sharp contrast could be mistaken for a give-away kids toy in McDonald's happy meal.
I did not have the plug for any of these so made a twisted wire probe that I simply stuck in the Left+/- connection. For this reason, the noise level may be higher than it would be with a more proper connector.
Stax Electrostatic Headphone Amp Measurements
I had to modify my headphone amp dashboard as it uses a 600 ohm load which doesn't work for electrostatic headphones which essentially have infinite resistance. The highest my Audio Precision goes to is 100,000 ohm so that is what I used. The other major difference is that electrostatic headphones require far higher voltages to produce sound. So instead of my usual 2 or 4 volt output, I used a whopping 80 volts! I thought this would be within the comfort range of all the headphone amps as it is in the middle setting of the Stax headphone amps. Alas, this did not turn out to be the case but we are getting ahead of ourselves. Let's start with the Stax SRM-311 where I developed the tests:
Second harmonic is at -88 dB which more or less sets our SINAD since distortion is more dominant than noise (SINAD is a relative sum of noise and distortion).
Frequency response had a slight low frequency boost:
And droop at 20 kHz. The variation probably gets lost in the frequency response of the headphone itself.
Last test that I prepped was the power test but now into 100K ohm:
Let's jump to the other extreme and test the SRM-007t which is a differential tube amplifier (balanced in, balanced out):
Now this is a bummer. Distortion at around 80 volts was huge as you see above. I thought it might be clipping or something but that is not the case:
No wonder I always heard this amp distorting like heck at higher volumes.
Its frequency response is more flat though:
Here is the SMR-006t which is also tube based but its output is not differential:
Frequency response is the same as SRM-007t:
It has the same distortion as well although noise floor is higher yet:
So this rules out anything being broken in the tube energizers other maybe higher noise of the SRM-006t.
KOSS E/90X Energizer Measurements
The E/90X lit its LED in red in protest well before 80 volt out. So I decided to take it easy on it and turn up the level to just before LED turned red:
Running our sweep we see that the KOSS is much more well behaved albeit, with much lower output voltage:
Both distortion and noise are low but then we run out juice at nearly 10% of what the Stax amps can produce. I noted that the output level was not that high in my review of the KOSS ESP 95X headphone about this issue.
Frequency response test was interesting for two reasons:
Notice that the low frequency response changes with output level! Is this some kind of loudness compression? If so, the KOSS headphone measurements will be level sensitive.
The second issue was instability at frequencies below 20 Hz. I set the frequency to 16 Hz and as I turned up the volume gradually the output suddenly jumped from something like 10 volts to 160 volts! The LED would then flicker between red and green. Suggest using a high pass filter with this headphone/amp to make sure it doesn't see anything below 20 Hz.
Conclusions
Anyone want to buy a pair of hardly used Stax tube headphone amps? 'Cause after seeing how poorly they perform, I don't think I want to be seen in the same house with them! The heck were they thinking? A tube amp mates well to electrostat headphones because it naturally have a high voltage output. Why screw that up with tons and tons of distortion? I guess they decided to use no feedback.
I was surprised that the KOSS E/90X had very low distortion. Alas, it must use much lower voltage rails/output transistors so not able to generate nearly as high a voltage as the Stax units can. Alas, if it is providing adaptive bass boost, using a different amp without that feature (the norm) would cause different audible results.
Will be interesting to test other after-market amps to see how they compare to these.
-----------
As always, questions, comments, recommendations, etc. are welcome.
Dropped by the garden (what's left of it) to see if the persimmons are ripe enough to pick and my heart fell to the ground. The one fruit I was eyeballing everyday was gone! Fearing the worst, I searched for the rest and found a few remaining ones:
We waited nearly 10 years for this little tree to bear fruit and it finally bore fruit a couple of years ago. Alas, last year some varmint at them all.
So despite losing some, I am happy to have rescued the above. They last a long time since they are the Japanese variety. Appreciate any kind donations using: https://www.audiosciencereview.com/forum/index.php?threads/how-to-support-audio-science-review.8150/
