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Electrostatic Headphone Amplifier Review & Comparison

amirm

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#1
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:

Stax Electrostatic Headphone Tube Energizer Amplifier Review.jpg


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:

Stax SRM-313 Headphone Amp Audio Measurements.png


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:

Stax SRM-313 Headphone Amp Frequency Response Audio Measurements.png


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:

Stax SRM-313 Headphone Amp Max Power Audio Measurements.png


Let's jump to the other extreme and test the SRM-007t which is a differential tube amplifier (balanced in, balanced out):
Stax SRM-007t Electrostatic Headphone Amplifier Audio measurements.png


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:

Stax SRM-007t Electrostatic Headphone Amplifier Power into 100k Audio measurements.png


No wonder I always heard this amp distorting like heck at higher volumes.

Its frequency response is more flat though:
Stax SRM-007t Electrostatic Headphone Amplifier Frequency Response Audio measurements.png


Here is the SMR-006t which is also tube based but its output is not differential:

Stax SRM-006t Electrostatic Earphone Headphone Amplifier Audio Measurements.png


Frequency response is the same as SRM-007t:

Stax SRM-006t Electrostatic Earphone Headphone Amplifier Frequency Response Audio Measurements.png


It has the same distortion as well although noise floor is higher yet:

Stax SRM-006t Electrostatic Earphone Headphone Amplifier Power into 100K Audio Measurements.png


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:

Koss E 90X Energizer Electrostatic Headphone Amplifier Stereo  Measurements.png


Running our sweep we see that the KOSS is much more well behaved albeit, with much lower output voltage:

Koss E 90X Energizer Electrostatic Headphone Amplifier Power into 100k  Measurements.png


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:

Koss E 90X Energizer Electrostatic Headphone Amplifier Frequency Response Measurements.png


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:

Persimmon.jpg


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/
 

Blumlein 88

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#2
Wow really surprised at the tube Stax. I had the really old Lambda where you ran it thru the interface and connected your own power amp. Wish you had that to test. Who knows how the step up transformers would fare.

I used it at times with large tube amps and solid state. It seemed happier with amps that had + and - outputs like modern class D has. I used it for a long time with a Sumo Model 9 which functioned this way.
 

YSC

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#3
The Stax Tube amp is expected to have high distortion for me as most tube followers are after those sort of harmonic distortion sound as in a lot of high end speaker tube amps... but still a bit surprising for me for the distortion to be that high though.. I am really curious about the highly praised Sennheiser Orpheus and Blue Hawaii amps for the sake of knowing the SINAD of amps that can literally buy me a new car
 

mshenay

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#4
Cool post! Happy to see some one measure these

How ever you need to get a hold of one with the CCS Mod, it helps to reduce the clipping!
 

JohnYang1997

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#5
Those distortions are caused by tube input stage. Would love to see how BHSE measure silicon input tube output. I guess it wouldn't measure too well but should be better.
 

wwenze

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#7
I wonder if I can just use a high power speaker amp and still get less power supply noise than these...
 

bigjacko

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#8
Thank you for doing the review on electrostatic energizer, not much people on internet does this. The measurement explains why after following oratory1990's eq my ESP/95X sound bassier than other headphone eqed. There were a lot of people talking about koss energizer sounds worse than Stax's, interested to see their opinions now.
 

EchoChamber

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#10
15 years ago I was building tube amps, DACs and preamps looking for that “magic” tube sound... Distortion was the thing (of course no one talked about it that way).
 
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amirm

amirm

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Thread Starter #11
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amirm

amirm

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Thread Starter #12
I wonder if I can just use a high power speaker amp and still get less power supply noise than these...
It would need to be super high power amp. Probably as high as 2000 watts or more.
 

Blumlein 88

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#13
800 watts at 8 ohms should do 80 volts. Probably not really enough to enjoy.
 

restorer-john

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#14
@amirm The input sensitivity for 100V out is 100mV (60dB) on the SRM-006T and SRM-007T and the SRM-313. Your plot for the SRM-006T shows 2V in, 20 times the rated sensitivity. So, if you are feeding 2V in, the volume control on the Stax could not set at maximum (as it is up front of the gain stages) if you only got 80V (32dB gain) out. So the input impedance was high as a result and the performance, both the frequency response, the noise level and distortion will all be affected.

Could you run the Dashboard THD+N/SINAD plot at rated input (100mV- with the vol pot wide open) as below on say the 007t, so we can see how close to rated spec it actually is?

006
1604116403830.png

007
1604116605991.png

313
1604117396437.png


It seems strange Stax would have held onto the old definition of line level even for products made well into the 2000s.
 
Last edited:

restorer-john

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#15
I wonder if I can just use a high power speaker amp and still get less power supply noise than these...
You'd be better off to just put a 100V PA line transformer and a couple of flyback diodes on a decent 50wpc amplifier and call it done.
 

001

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#16
"....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. ..."

Fruit & Veg. are supposed to be good for you. Pretty well accepted truth universally, but I so clearly remember the first time I took my first bite out of a not-yet ripe persimmon. I thought someone had filled my mouth with slivers of glass. Youch! Talk about 'astringent'.
Amirm, what variety of persimmon are you growing? [yes, I realise I haven't uttered a single technical term on this forum. hehehe]
 

levimax

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#18
Hey Amir.... how about a real level matched blind ABX test between the tube and SS Stax amps.... do you think you could tell the difference with music? It would be an interesting test I think.
 

RayDunzl

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#19
@amirm

What SPL level does the 80V test signal level create at the headphones?
 
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amirm

amirm

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Thread Starter #20
Fruit & Veg. are supposed to be good for you. Pretty well accepted truth universally, but I so clearly remember the first time I took my first bite out of a not-yet ripe persimmon. I thought someone had filled my mouth with slivers of glass. Youch! Talk about 'astringent'.
Amirm, what variety of persimmon are you growing? [yes, I realise I haven't uttered a single technical term on this forum. hehehe]
These are Japanese variety meaning you can eat them when they are hard. Indeed they don't ever get as soft as western version. It is tricky though as too early and they are astringent although not nearly as bad as the western variety in that phase. I will probably wait a month or two before eating these to make sure they are ripe enough. Wish I could leave them on the tree to ripen.....
 
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