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The Stax Thread III

Discussion in 'High-end Audio Forum' started by currawong, Aug 20, 2013.
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  1. JimL11

    Nope.  The SRX is originally a Stax design from the 1970s that was given to the DIY community.  I named my version the SRX Plus to honor  the original Stax circuit, and to indicate that it was an optimized version of it.  The SRX Plus circuit is almost exactly the same as the SRX, but updated with the addition of constant current sources or loads to improve its function and drive capability.  The MOSFET constant current sources/loads set the circuit parameters but the active devices are tubes.  The fact that such a "simple" circuit, when updated, is still sonically competitive with some of the best designs today is a testament to the talent of the Stax engineers who designed it.
    The T2 shares the skeleton of the SRX, as described in the previous post,  To say that all the intervening transistor circuitry merely replaced the two coupling capacitors is somewhat of an oversimplification, as the output stage of the T2 is a common grid rather than the SRX's common cathode configuration, and the former requires an extra driver stage.  In fact the BHSE uses a similar output stage configuration which was inspired by the T2.
  2. Thaudiophile
    I have never heard Sr omegas.I would really like some comparison and impression of Stax SR Omega with Sr 009,007 and other flagships.
  3. deuter
    People with Omega 007 and BHSE, what volume level do you listen at.
  4. hpeter
    I meant tube endstage driving a transformer/choke, where the stax are connected to it.
  5. JimL11

    The Woo WES has choke loaded outputs, but has a somewhat mixed sonic reputation (see InnerFidelity review).  From a technical point of view, I personally don't see a benefit to choke-loaded outputs for electrostatic headphones, as the very high impedance of estat phones means that most of the signal current is wasted in the choke rather than driving the headphones.  
    AFAIK, there has never been a commercial transformer output amp for electrostatic headphones.  Stax used to sell transformer and bias boxes for their headphones through the 1980s, that were designed to be connected to speaker amplifiers.  There are a couple single-ended designs with transformer outputs floating around the internet but all strictly DIY. 
  6. joseph69
    You're a world of information, Jim.
    What impedance are the 007/009 anyway?
  7. JimL11

    The impedance of an electrostatic headphone varies with frequency, because it partly resembles a capacitor, although not completely, because, unlike a capacitor, the headphone makes sound.  In the case of the 007 and 009, they have a capacitance close to 100 pf, so  at 1 kHz the 007 has an impedance of about 170 kilohms, and the 009 has an impedance around 145 kilohms, both very high.  The impedance varies as the inverse of frequency, so at 10 kHz the impedance is 10-fold lower, at 100 Hz the impedance is 10-fold higher.  However, since the headphone makes sound, there is another term to the impedance which can be approximated by a high value resistor, say around 100-200k. In combination, they still result in a high value impedance.  
    The output stage active device (tube or transistor) sees this high impedance in parallel with the output stage load, whether it is a resistor, choke or constant current load.  In order to have sufficient current drive, the resistor must be significantly lower impedance than the headphone, however, this also means that the majority of the signal current is sucked up by the plate resistor due to Ohm's law.  In the case of an output choke, the impedance of the choke varies directly with frequency, which means at low frequencies where much of the power demands in music lie, again most of the signal current goes to driving the choke.  Thus, both resistor and choke loads are very inefficient in supplying current to the headphone, which is the point of the exercise.  By comparison, a good constant current load requires essentially none of the signal current, hence allowing all the signal current (minus that needed to drive the feedback resistors) to drive the headphones.  Not only is this more efficient, but it decreases distortion in the output device because it is not as heavily loaded.
    Michgelsen, joseph69 and Sko0byDoo like this.
  8. kevin gilmore
    So there are a few other transformer electrostatics
    eddie current electra is a dual tapped choke, push pull
    malvalve has a transformer as the load, electrostatics are driven off the plates, and dynamic headphones driven off the low impedance taps
    audiovalve rkv uses single ended pentode driving a transformer for electrostatic loads and either cap coupled or transformer coupled for low impedance
    demograf is single ended with large top plate tubes and even bigger transformers
    joseph69 and Sko0byDoo like this.
  9. 100VoltTube
    I'm not totally sure this is the right thread for this kind of post (the last thing you want to see a post begin with), but with all of the discussion of estat amp topology, I though it might be acceptable to explain a probably-not-new idea I've been mulling over recently:
    Sorry for the bad looking LTspice screenshot. [​IMG]
    In short, It's an opamp that uses a MOSFET to do its bidding. In more technical terms, the input opamp's inverting input sees the input signal, and the noninverting input sees the output voltage after it has been dropped with a resistor divider. the opamp's output is cap coupled to a MOSFET used in a CCSed common source amplifier. The gain is defined by the ratio of R4 to R5, as it comprises the negative-feedback loop. If I were to make it, I would use a push-pull arrangement with FETs on the output and a BDT (beam deflection tube) in place of the opamp, but still the same basic idea.
    EDIT: P.S. A tube could be used in place of the MOSFET
  10. paradoxper
    And they all suck.
  11. JimL11

    Helpful info as always.  Not sure the Electra is still being sold - it's not listed on their website.
  12. Ojisan
    Hi, I'm a new and happy SRS-3100 user (and still catching up with this thread).
    I have been intrigued by the DIY amp discussion in the last few pages. Are there starter kits out there or do most of you source your components? I could start from the schematics but if someone has published results and recommended components, I thought that would save some time :) Any pointers appreciated.
  13. JimL11

    No kits, unfortunately, you'll have to get your own circuit boards, parts and build your own chassis.  Lots of info over on another website for all the Kevin Gilmore designs as well as the SRX Plus.  The SRX Plus design was also published in AudioXpress in Nov and Dec 2015.
  14. joseph69
    Thank you for the very thorough reply. It did take me some time to (google) comprehend your post to better understanding it for myself, and it's much appreciated to learn and understand about my gear as much as enjoying listening to it. Thanks again.
  15. mulveling Contributor
    Lots of impressions out there, including mine lol. I prefer both the 009 and L700's "brighter" personality over the 007 Mk I. The 009 is more accurate and a superior performer to the L700; this is most apparent in soundstage (009 much bigger) and bass (009 is cleaner / firmer, L700 bass has a little bit of "bloom"). I love the midrange on both; the 009 is more detailed. They both can have a bit of "bite" on top, but not in the same way nor at the same points, so a person that has a problem with 1 may not necessarily have an issue with the other. Also the L700 errs in ways that aren't necessarily unmusical, for all music -- it can bring a fun perspective to things, even with having a 009 readily available. 
    I prefer pairing the L700 with tube amps, even more strongly than I prefer to pair the 009 with tubes. The Carbon w/ L700 tends to run a little bright...not to where I dislike it, but 007 lovers beware. The L700 is best out of the BHSE (well, likely best from a DIY T2 but I haven't heard that), and (at a lower level) pairs well with KGST. 
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