Stax Interview and Factory tour 12/12/12

[eric65]

  Besides the economic reasons poined by others, one has to remember direct drive high voltage amplifiers have to conform to certain rules by the governing bodies regarding safety.  This is no joke - an amp really well designed to impeccably drive electrostatics, particularly in the treble, unfortunately doubles as an electr(ostat)ic
chair. Above so-and-so-much current an amp can spit out under worse conditions, it is banned - at least within EU. And that does have a perfectly understandable reason - unless you want to play "russian roulette" or have developed the notion of leaving this world while experiencing "the ultimate transient".
 
I do not know the exact limitation imposed by the EU - but it is way less than it would be required to really properly power the electrostatics. Stax certainly DO know the rules - and they build perhaps up to just slightly below in order to remain legit. They certainly can not afford being sued by the widow of the electrostatic-late sheik or anyone else - can they ?  
I have built my DIY amp decades ago - dwarfing anything ever seen to power the headphones by others by (almost) an order of magnitude - and certain to give the EU engineers an instant heart attack upon merely seeing the voltage/current figures. It does sound better, MUCH better - but ultimately I went with SRM1MK2/Lambda Pro, accepting the limitations, since *somehow* the plan is to roam this Earth for a while longer...
 
Please take this caution "do not try this at home" seriously - I did have one lucky escape to be able to tell about it - and am not exactly eager to repeat it. It happened more than two decades ago.
 
Yet... 

 
 
Sincerely, thank you very much.

Reading, very informative.

PS: my favorite amplifier (RKV-II) does not produce more than 100 volts (max) for an current (voltage) use under 10 Volt level. Not too much risk to my life with. .

 
Eric

 

[analogsurviver]

   
 
Sincerely, thank you very much.

Reading, very informative.

PS: my favorite amplifier (RKV-II) does not produce more than 100 volts (max) for an current (voltage) use under 10 Volt level. Not too much risk to my life with. .

 
Eric

RKV II is perfectly safe. I based my amp on the schematic of the same person that designed the RKV - it was published in Germany ages ago. It has anode power supply voltage of approx 1.5 kV - and nowhere enough current capability to make electrostatics really sing in the treble. Beside being over the EU limit already.
 
Then I "expanded" it ... - ultimately, 8 pcs of PL519 tubes at the output, 4 per channel.  -3dB @50 kHz , frequency response equals power response with the intended load.
It has been dismantled and is in storage ever since, just prior to the turn of the millenium. Who knows, one day I might be tempted to assemble it again - it was never housed in an enclosure, as it has detrimental effect to the performance. Think one table full of electronics - a nightmare to keep it (reasonable) clean and free of dust. No pets or, God forbid, children in THAT room ...
 
Most definitely not something one wants to play with unless perfectly aware what one is doing. 

 

[jgazal]

@analogsurviver how much current the german sennheiser HEV90 sinks?

 

[analogsurviver]

  @analogsurviver how much current the german sennheiser HEV90 sinks?

As with any ESL - as much as it can get.
 
ESLs are essentially capacitors - and their impedance drops with frequency. Sooner or later, the impedance gets too low to be supported by the current capability of the amplifier. It would take infinite current/power to have infinite frequency response with ESL from the electrical point of view alone. Mass of the diaphragm and the air
trapped between the stators and in the holes of the stators limits the high frequency response in mechanical sense - but that is usually far above the power response of the electrostatic amplifiers.
 
I do not know the exact capacitance ( you can calculate impedance from http://www.sengpielaudio.com/calculator-RC.htm ) of the HEV90, but in practice ESL  ( headphones+cable ) is around 100-200 pF. Than it is a simple matter to calculate the current required - for the given frequency. 
 
Power response of all commercially available electrostatic amplifiers is lower than their frequency response -
because they rely on negative feedback by forcing the front end to drive the output by the required ever increasing level of signal to cover for the losses due to ever falling impedance of the capacitive load. That means that above the power response, the output level will be falling at 6 dB/octave. The tubes in HEV amp are around 10W power each; similarly powered are Stax amps - and knowing output voltage swing is approx 1kV, it is easy to calculate what is the maximum current available. Stax amps do have specified what is their full or 0 dB level frequency limit - and although in theory 2.2 kHz at full output should be enough to cover the musical signals, in practice this is not enough. Just listen to a single uncompressed recording of soprano AT CONCERT LEVELS - and if it survives that, then listen to a female choir ... If it survives that - plug in parallel another set of ESLs - and you should definitely hear what I am talking about. Dynamic headphones may well have their problems, but the inability of the amp to drive the high frequencies is not one of them.
 
The reason why not beyond the power levels mentioned is simple - safety. Should any/all insulatiuon(s) fail, the amp should still not be able to kill you - as simple as that. 
 
It is quite remarkable what results heve been achieved despite the understandable limitations.

 

[bcschmerker4]

I reckon that, for a tube amplifier for electrostats, a balanced PO stage on each channel, with dual 6V6GTs or EL84's or similar (4 total beam power tubes) driving toroidal step-up transformers with powdered-metal cores rated at 100 kHz, might do the job of keeping the frequency response consistent through not more than 50 kHz.  How much voltage and current can the Stax® harnesses tolerate?  That puts an absolute limit on the sound pressure levels possible.

 

[analogsurviver]

  I reckon that, for a tube amplifier for electrostats, a balanced PO stage on each channel, with dual 6V6GTs or EL84's or similar (4 total beam power tubes) driving toroidal step-up transformers with powdered-metal cores rated at 100 kHz, might do the job of keeping the frequency response consistent through not more than 50 kHz.  How much voltage and current can the Stax® harnesses tolerate?  That puts an absolute limit on the sound pressure levels possible.

It is voltage that is the limiting factor as far as electrostatic drivers are concerned - NOT the current. Stax amps have approx 1800 V peak to peak ( or approx 600 V RMS ) - and the drivers have no problem with this kind of drive voltage. Drivers do not dissiapate any power, being for all practical purposes pure capacitance load. That means it can accept practically infinite current - far more than any real amp can provide. Absolute sound pressure in ESL is entirely the function of the voltage.
Current is only needed to charge the capacitance - and it of course increases linearly with frequency. The inability to charge the capacitive load will ultimately set the high frequency limit as far as electrical drive is concerned.
 
I have been commenting on direct drive electrostratic amps exclusively. You can call them OTL - or whatever - and this kind of ESL amp is in theory suoperior to anything using a transformer. Usual restrictions in the bass due to saturation and serial LC resonance due to interaction of output transforme's inductance with capacitance of (headphone + cable) spring to mind. Another disadvantage of transformer coupled amp is the fact that most often the transformer is in the package of the headphones - to be powered by a regular low voltage low impedance amplifier normally used for dynamic speakers. HOW MUCH is the maximum voltage swing of the said amp+transformer - that is the main problem. It is easy to reach more than allowed - resulting in arcing or completely frying the ESL.
 
One way to guard against overload ( a single scratch on an analog record is all that it takes ... ) is by connecting two powerful zener diodes back to back at the input of the transformer / output of the amp. If you never heard a REALLY HARD clipping - that is as bad as it gets. Plus your amp has to be immune to practically shorted output at (next to ) full output. Commercially used in transformer for the Sennheiser Unipolar 2000 and 2002.
 
The other way - and much better way - would be arranging the amplifier so that it under any circumstances can not exceed the maximum voltage at the output of the transformer that is still safe for the ESL . That would most likely require both amp and output transformer(s) to be in one box - or dedicated amp to power the appropriate transformer arranged so to be safe FOR THE ESL load.
 
But please do not forget - it IS possible to design such an (amp+transformer) that will safely drive ESL of choice - while providing enough current to send you to everlasting hounting grounds - should insulation fail for any reason.To say that electrostatics are perfectly safe because polarizing voltage/bias, although anything from 200 to 1500 V DC is safe due to large value series resistor limitting the current to totally harmless pA ( or thereabouts ) WHILE keeping quiet what is the actual voltage/current delivered to the drivers themselves ( regardless of type of the amplifier/transformer ) is misleading - to stay on the polite side. And it is outlawed and banned, at least within the EU.
 
Transformers capable of swinging required voltage in the bass ( where usually the largest amplitudes occur in music ) AND capable of high frequency response with LOW distortion are hard to do > expensive. Recent attempt is for the ressurected Jecklin Float - prior to that, there were no transformer boxes for ESLs in a long time - because the "usual" transformer of the approximate size we grew used to accompany the electrostatic headphones is so poor in the bass ( distortion usually trough the roof due to saturation ) that direct drive amps prevailed. They also have built in "voltage limitting" and as strange it may sound, are actually more economical way to reasonably well power the electrostatic headphones than amp+transformer combo. Ultimately, it boils down to execution/money WHILE staying within legally imposed limits regarding voltage/CURRENT limits that are still safe to humans.
 
It still holds true that what drives the electrostatic headphones or speakers is ultimately more important than headsphones/speakers themselves - once these are free from the most glaring defects.
 
What you propose ( tubes + relatively low step up ratio step up transformers ) is perhaps the best practical solution to the problem of driving the ESLs. Requirements for such transformers are MUCH more manageable than of standard variety. BUT it is an entirely different dedicated kind of amplifier ( it can only power the dedicated ESLs ) and audiophiles are known to be quite conservative in their approach - sometimes it takes decades before something new is "accepted". 

 

[eric65]

  It is voltage that is the limiting factor as far as electrostatic drivers are concerned - NOT the current. Stax amps have approx 1800 V peak to peak ( or approx 600 V RMS ) - and the drivers have no problem with this kind of drive voltage. Drivers do not dissiapate any power, being for all practical purposes pure capacitance load. That means it can accept practically infinite current - far more than any real amp can provide. Absolute sound pressure in ESL is entirely the function of the voltage.
Current is only needed to charge the capacitance - and it of course increases linearly with frequency. The inability to charge the capacitive load will ultimately set the high frequency limit as far as electrical drive is concerned.
 
I have been commenting on direct drive electrostratic amps exclusively. You can call them OTL - or whatever - and this kind of ESL amp is in theory suoperior to anything using a transformer. Usual restrictions in the bass due to saturation and serial LC resonance due to interaction of output transforme's inductance with capacitance of (headphone + cable) spring to mind. Another disadvantage of transformer coupled amp is the fact that most often the transformer is in the package of the headphones - to be powered by a regular low voltage low impedance amplifier normally used for dynamic speakers. HOW MUCH is the maximum voltage swing of the said amp+transformer - that is the main problem. It is easy to reach more than allowed - resulting in arcing or completely frying the ESL.
 
One way to guard against overload ( a single scratch on an analog record is all that it takes ... ) is by connecting two powerful zener diodes back to back at the input of the transformer / output of the amp. If you never heard a REALLY HARD clipping - that is as bad as it gets. Plus your amp has to be immune to practically shorted output at (next to ) full output. Commercially used in transformer for the Sennheiser Unipolar 2000 and 2002.
 
The other way - and much better way - would be arranging the amplifier so that it under any circumstances can not exceed the maximum voltage at the output of the transformer that is still safe for the ESL . That would most likely require both amp and output transformer(s) to be in one box - or dedicated amp to power the appropriate transformer arranged so to be safe FOR THE ESL load.
 
But please do not forget - it IS possible to design such an (amp+transformer) that will safely drive ESL of choice - while providing enough current to send you to everlasting hounting grounds - should insulation fail for any reason.To say that electrostatics are perfectly safe because polarizing voltage/bias, although anything from 200 to 1500 V DC is safe due to large value series resistor limitting the current to totally harmless pA ( or thereabouts ) WHILE keeping quiet what is the actual voltage/current delivered to the drivers themselves ( regardless of type of the amplifier/transformer ) is misleading - to stay on the polite side. And it is outlawed and banned, at least within the EU.
 
Transformers capable of swinging required voltage in the bass ( where usually the largest amplitudes occur in music ) AND capable of high frequency response with LOW distortion are hard to do > expensive. Recent attempt is for the ressurected Jecklin Float - prior to that, there were no transformer boxes for ESLs in a long time - because the "usual" transformer of the approximate size we grew used to accompany the electrostatic headphones is so poor in the bass ( distortion usually trough the roof due to saturation ) that direct drive amps prevailed. They also have built in "voltage limitting" and as strange it may sound, are actually more economical way to reasonably well power the electrostatic headphones than amp+transformer combo. Ultimately, it boils down to execution/money WHILE staying within legally imposed limits regarding voltage/CURRENT limits that are still safe to humans.
 
It still holds true that what drives the electrostatic headphones or speakers is ultimately more important than headsphones/speakers themselves - once these are free from the most glaring defects.
 
What you propose ( tubes + relatively low step up ratio step up transformers ) is perhaps the best practical solution to the problem of driving the ESLs. Requirements for such transformers are MUCH more manageable than of standard variety. BUT it is an entirely different dedicated kind of amplifier ( it can only power the dedicated ESLs ) and audiophiles are known to be quite conservative in their approach - sometimes it takes decades before something new is "accepted". 

 
Hello, again thank you for all these explanations.
 
Finally, the idea of Mr. Helmut Becker in Germany to use a low ratio (cran) for his transformers (Verto box) with a ratio 1:5.9 and a self biasing (SB), with its amplifier tube OTL RKV-II (output maximally 80-100 Volt on 2000 Ohm) may be considered a not bad idea, both economically (Nb: the AudioValve Verto box and RKV-II amp are worth less expensive in France than the STAX SRM 727 amps or 007 t) and safe: the output voltage is limited excursions to 80-100 Volt x 5.9 = 470-590 Volt.

Otherwise, for having listened to this combo, as well as another person, listening to the RKV-II-Verto combo (with headphone Stax SR-009) is very nice, the same global qualitative level as that of the STAX SRM 727 amplifier, see a little better on some audio criteria, but it is subjective.

 

[analogsurviver]

   
Hello, again thank you for all these explanations.
 
Finally, the idea of Mr. Helmut Becker in Germany to use a low ratio (cran) for his transformers (Verto box) with a ratio 1:5.9 and a self biasing (SB), with its amplifier tube OTL RKV-II (output maximally 80-100 Volt on 2000 Ohm) may be considered a not bad idea, both economically (Nb: the AudioValve Verto box and RKV-II amp are worth less expensive in France than the STAX SRM 727 amps or 007 t) and safe: the output voltage is limited excursions to 80-100 Volt x 5.9 = 470-590 Volt.

Otherwise, for having listened to this combo, as well as another person, listening to the RKV-II-Verto combo (with headphone Stax SR-009) is very nice, the same global qualitative level as that of the STAX SRM 727 amplifier, see a little better on some audio criteria, but it is subjective.

Yes, this pairing of tubes with reasonable voltage and low step up ratio transformers is a good and sensible idea.
 
What you are refering too in voltages is RMS - and the levels achievable are just what the Stax ESL drivers need. 590 V RMS is at the upper limit for safe driving of Stax drivers. The SPLs should match those of better Stax amps.
 
It would be nice to see some measurements ( square wave 20 Hz, distortion levels, frequency response, power response, ) - if sensibly optimized, it could well turn out as a a viable less expensive alternative. Ultimately, anything involving a transformer will suffer in the bass - but ESLs do not exactly excel in the bass to begin with. I can imagine the use of an amplifier of appreoximately mentioned voltage, both tube and semiconductor, plus low step up ratio transformers to sound quite good indeed. I did not have the pleasure to audition it yet.

 

[bcschmerker4]

Looks as though, per analogsurviver, the Stax® harnesses can sustain 1.8 kV p-p (meaning Vbd > 14.4 kV).  I asked about the current, as electrostats' transducer impedance is inversely proportional to frequency:
 
Zl = ( Rs^2 + ( (2πvLp + (2πvCl)^(-1) )^2 )^(0.5)
 
Meaning that the amplifier's design powerband has to reach the near-ultrasonic range for sufficiently flat response from the transducers, as the capacitive reactance bottoms out, and the output current therefore peaks, at max frequency.  This situation favors powdered-metal cores, which are commonly used for LF and MF radio, over the laminated-iron cores typically used for audio frequencies with amplifiers for dynamic and planar-magnetic speakers and headsets.

 

[ToroFiestaSol]

Sorry for the necro-bump, but...
 
SR-Omega: introduced in 1993
SR-007 MK1: introduced in 1998 (5 years later)
SR-007 MK2: introduced in 2007 (9 years later)
SR-009: introduced in 2011 (4 years later)
 
We are ending 2016, more than 5 years after 009 launch...maybe we'll see a successor this year?

 

[arnaud]

That's absolutely not out of question, but I don't think anyone has the slightest clue atm... I definitely could imagine an SR-009mk2.

 

[ToroFiestaSol]

  That's absolutely not out of question, but I don't think anyone has the slightest clue atm... I definitely could imagine an SR-009mk2.

 
Or a SR-010
 
I hope they listen to customer's feedback, because 009 and 007 are very polarising products.
 
I think that 009 is a step up in build quality, but a step backwards sonically from 007.
The problem is that 007 is too fussy to get right and sound superior to 009 (port mod, massive aftermarket amplifier, proper seal and quality source).
Another thing that I don't like about 009, it has the most confortable earpads ever, but they're too flat and need to be deeper (my opinion, of course).

If they could do something with:
- 009 build quality and industrial design of similar beauty.
- Neutral frequency response but organic and musical sounding, with good bass impact. 
- Better sounding than 009 and properly set up and 007. 
- Efficient. 
- No need for mods to release it full potential. 
- No tricky seal like with 007.
- No Lambda's plastic headband like 009 please.
- At a realistic price point (being more or less 009 price a realistic price point)
- Please, adjust price variations, no more than 10-15% price variation between Japan and the rest of the world.
...boom, a new legend is born. But I don't know, is always a hit or miss with Stax.
 
I'm happy as a pig with my modded 007 and KGSSHV Mini but I would love to see some action from Stax.

 

[Pokemonn]

We need T2 successor too!

 

[purk]

We need T2 successor too!

If you are willing to pay serious cash then they may do it.

 

[Pokemonn]

  If you are willing to pay serious cash then they may do it.

Yes i agree! I hope so too.
 
FYI...
 
from post #1 of this thread...
 

13. Is there a plan for a flagship amp to go along with the 009?

 

[Mr. Sasaki] Yes, we have a plan for this. But it will take some more time, as design is done one product at a time.