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Electrostatic Headphone Amplifier Review & Comparison

dense

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A friend of mine has this and always say this is the reference of a headphone. I am not sure what to say with the measurement I am seeing now. I don't want to break his heart.
 

trl

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I am not sure what to say with the measurement I am seeing now.
There is nothing to say, the amp simply doesn't measures very well, nothing else.

Our ears don't really love a flat & neutral output sound, this is why so many of us like the added harmonics introduced by the vacuum tubes and sometimes some treble roll-off or bass increase might help as well. This is why speakers amplifiers have tone controls and this is why Hi-Fi EQ devices exist.

In the case of SRM-313 I can see a bass increase for below 100Hz, which is probably a good thing when paired with KOSS electrostatic headphones, due to their known roll-off in this area.

In the end is about what sound you like most, unless you're working in a professional studio where the output sound really matters. I personally try to match at home the original sound, in respect with human ear's loudness curve too, so lower THD+N really matters for me.
 

Dealux

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I'd like to see some modern Stax amps too. I was considering getting a Stax headphone in the near future but not with those horrible energizers lol.
 

AudioJester

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i never said the keithley was on the same level as the audio-precision. What it does do is measure past 400vrms with an input impedance that
is electrostatic amplifier friendly. But if you really want to try and get .0000x numbers, then active high voltage fet probes or other similar items
are necessary. If you are adventuresome, then some of the victoreeen high voltage glass vacuum resistors work as input attenuators. I could post
pictures of these. Not sure where they can be purchased new anymore. Anything not vacuum glass enclosed is subject to arc-over and thats a fatal mistake. these things
https://resistorresisti.com/rx-4

i have measured many stax amps before and regardless of age they all measure better than .01% thd at 1khz at 100vrms. All of the stax amps except for the srm-001 and srm600 are fully dc coupled and when you add a low enough output resistance to ground you throw the entire amp out of balance and then all sorts of bad happens.

The koss does indeed measure much better. Reason is the open loop gain approaches 90db and the front end opamp uses 30db of that gain to reduce the distortion to low levels. And also the fact that its push pull. But the power supply kicks out at about 10 watts total power dissipation.
schematics have been posted elsewhere.

the ap technical note is 100% correct, other than its missing all the safety parts to prevent the ap from blowing up. Because at 1kv peak to peak at 20khz its real easy to generate a plasma. and plasmas have the ability to knock you ass across the room with relative ease.

Are you abe to share your measurements of these amps?
 

SpaceMonkey

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Too bad Amir didnt use 4 Ohm dummy load. That would please even more estat/stax/koss bashers.
 

Leporello

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I'd like to see some modern Stax amps too. I was considering getting a Stax headphone in the near future but not with those horrible energizers lol.
Is the SRM-313 actually horrible? Granted, it is nowhere near state of the art but as a SRM-313 / SR-303 owner I am more bothered by the response of the phones. Someone already noted the infamous "shout" that can be seen in practically every Stax Lambda measurement graph.
 

armani006

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I have NOS 006tII that has worked about 10 hours with 6CG7 lamps and CCS-mode, I would say that after installing of CCS-plate it became more like solid state amp, it means it has much less distortion for sure, I can hear it with my young ears )). It could be interesting to see measurements of Stax amp with CCS plate. Obvious that more stable current that gives this plate to tubes makes less of distortion.
 

Jimbob54

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Next 20 years : direct interface , civilan autonomous robots , havesting resorces from space , first humans on mars. Just to name a few things i expect to happen.

Best case- worst case is stagnation. Pandemic plus lazyitis and fear in the first world. Pandemic, fear and resource shortages and resulting war in the 2nd and 3rd worlds
 

Vini darko

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Best case- worst case is stagnation. Pandemic plus lazyitis and fear in the first world. Pandemic, fear and resource shortages and resulting war in the 2nd and 3rd worlds
Nah worst case is solar micro nova. With our weakened magnetic field. Back to the Stone age :oops:
 
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Francis Vaughan

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A friend of mine has this and always say this is the reference of a headphone. I am not sure what to say with the measurement I am seeing now. I don't want to break his heart.
If you are referring to the tube amps, I would hold my breath. The current numbers do not hold water from a basic scientific point of view. A trivial circuit analysis is quite clear on this point. The output stages are being run way outside of their design parameters when loaded by 100kΩ. Vigorous hand-waving not withstanding.
 

srsxmi

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Been waiting for this kind of information for a long time, and we get four measurements in one!! Many thanks Amir :)
I sure wonder about the entry-level Stax SRM-252S and "mid" range SRM-353X now! Hope you get hands on them some day.
Oh and since you measured the iFi iDSD already, perhaps the iFi iESL some day :D

Hmm, I have an SRM-353X just sitting in a cabinet unused.
 

srsxmi

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..... The oft-repeated audiophile 'wisdom' that the E/90X is the limiting factor for the ESP/95X and you need to spend hundreds to thousands of dollars on a Stax amp for the headphones to 'achieve their full potential' is evidently just another audiphool myth.

Good point. Until seeing this review, I believed the "myth" to be true because I did a similar test, and to my ears to an ESP/95X with the Koss E/90X is inferior to a Stax SRM-353X.

That said, it will be interesting to see if this pattern holds across other Stax gear. And to see how trained listeners respond (e.g., Amir).
 

kevin gilmore

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the koss amp is a good example of something that tests great and sounds awful. If you look at the schematic there are lots of hints as to why.
ac coupling between the opamp and the output section causes significant phase shift that varies over the frequency range. minimal bias current in the output stage does not help. poorly matched p and n channel mosfets cause additional issues. etc.

the koss amp is a class AB amp, mostly class B. All stax amps are class A.
 
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amirm

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If you are referring to the tube amps, I would hold my breath. The current numbers do not hold water from a basic scientific point of view. A trivial circuit analysis is quite clear on this point. The output stages are being run way outside of their design parameters when loaded by 100kΩ. Vigorous hand-waving not withstanding.
??? I call this hand-waving:

So, the 006t tube amp will have its wings noticeably clipped running into 100kΩ. Probably has a hundred or so volts stripped from its output swing.
Not sure about the 007t. But what maybe worse, the load line of the tubes in both amps will be shifted driving the lower impedance, possibly enough to make a real difference to operating conditions and distortion levels. There isn't room for a great deal of forward gain to allow feedback to reign this in, so I do suspect that the load is probably making life hard for the tube amps. The defined gain of at least the 006t is 100. I don't see a huge amount of forward voltage gain in the circuit, so feedback looks limited.

Distortion in these tube amps starts early and hard as I showed:

index.php


"100 volt clipped from its wings" would only impact higher output levels, not the entire graph. Distortion is setting in as low as 1 volt output.

So no, the setup doesn't cause such a transfer function, the lack of feedback does.

This is on top of clear observation of listening to the amp and hearing distortion become quite audible at higher volumes.

So if you have measurements that show otherwise, let's see it. Otherwise reading the tea leaves from schematic of other units no less is the definition of hand-waving, not hard comparative measurements. Science would have to be re-written if a no feedback amp is linear.
 

pozz

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kevin gilmore

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First, all stax amps have feedback. More than 30db of feedback at 100hz, 10db of feedback at 20khz. From the output stage directly to the input stage.
But a DC coupled load like a 100k resistor throws off the current source driving the front end differential amp. Because the feedback really wants to be at -4v.

There are a number of highly linear ZERO feedback amplifiers. Audio-gd, Ayre, D'agostino and Bakkon are just a few of the better ones.
all are better than .01% thd, some much better. highly accurate current mirrors as vas stages are a wonderful thing.
 
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amirm

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There are a number of highly linear ZERO feedback amplifiers. Audio-gd, Ayre, D'agostino and Bakkon are just a few of the better ones.
I wouldn't put "better" and Audio-gd in the same sentence based on testing I have done. :) They are distortion factories:

index.php
 

pozz

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