The Stax Thread III

Discussion in 'High-end Audio Forum' started by currawong, Aug 20, 2013.
  1. JimL11
    There is actually a pretty straightforward reason why those three amps sound progressively better. The 252S circuit is reportedly similar to the 323/353 circuit, but runs at a much lower output current, so if you have a tendency to crank it, it begins to run out of power. The SRM-1 Mk2 Pro circuit is somewhat different, with a similar output stage standing current to the 323/353, but uses resistors for the output load rather than the current sources used in the 252 and 323/353. This means a significant portion of the output current is wasted driving the resistors and is not available to drive the headphones, so it current limits sooner than the 323/353, and also generates more distortion as the output devices have to work harder.

    The 727 in stock form suffers from its non-global feedback output stage, resulting in uncontrolled bass, among other problems - see the review in InnerFidelity for more details. Modifying the amp to add the global feedback improves the sound significantly and is easy to do but does void the warranty. I haven't heard the 323/353 so can't comment compared to the 727. I personally prefer my refurbed and modified SRM-T1 with constant current output loads, but that's just me.
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  2. hpeter
    after a almost 2 weeks without stax on my head (holidays) today i´m back...
    like living the dream once again LOL
    (that time being away was very hard, like drug addict) :skull:
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  3. jcn3

    my stax rig is l700s driven by an srm-007t. i'm finding the set-up a bit too laid back and would like to get more punch out of it. can this be done with tubes? any recommendations?
  4. rpeebles
    Hello there !
    I did finally get the King Sound KS-04 and portable M3. Sounds awesome ! Very neutral, not bright but not very high volume.

    But as always...I am now looking into a STAX combo. Looking into the SRD - X Pro portable amplifier - from ebay or the SRM T1 - from ebay. On the earspeaker side the L-700 or SR-007A.

    Would it work ? How good ?

    It seems I could also use the KS M3 with the L-700 and/or the SR-007. How would that work ? Any experience.

    Or if not, what would be the most appropriate and not too expensive amp for the L-700 - 353x?

    Sorry many doubts !

    I would like a relatively Bright and Detailed sound, as a complement to my KingSound arrangement.

    Please your help - recommendations much appreciated !
  5. JimL11
    The M3 definitely doesn't have enough output to drive the SR-007. It's not a question of voltage swing as lack of current output in a portable amp. The SRM252 is in the same class. The 353X has enough voltage and current output to drive either the L700 or the SR-007, although driving the latter also depends on how loudly you listen. The SRM-T1 in stock form is good for the L700, somewhat marginal for the SR-007. Modified, the SRM-T1 can drive either. HTH.
  6. DJ The Rocket
    At 580V, is there really any current to speak of in the first place?
  7. JimL11
    Well, not a lot, but....

    So, the SRM-252M has a reported quiescent current of around 1.3 mA/device, with 2 output devices/channel. The SRM-T1 has a quiescent current of about 4.8 mA/tube plate (2 plates/channel). Since these amps are push-pull, this means the maximum output current of the 252 is 2.6 mA peak, the SRM-T1 is 9.6 mA peak. It's actually less because there is some output current which is burned up in the feedback resistors. Now, Kevin Gilmore has reported measuring the SR-007 as using up to one watt peak - not sure if that is both channels together or one channel only - we will assume both channels together to be more conservative, so assume 1/2 watt/channel. Of course, the wattage burned in the headphones depends on how loud you listen, but at least this gives us an idea.

    if we assume a voltage peak of, say 200 volts, this means that there is 2.5 mA peak current. This is as high as the 252 can deliver, even without feedback resistors. The T1 has 2 sets of 153 kilohm feedback resistors for each channel, plus two sets of 66 kilohm plate resistors for each channel, which all together uses up about 8.6 mA peak, which leaves 1 mA/channel to drive the headphones. Now, I don't know about you, but 1 mA is less than 2.5 mA where I come from. So, yeah, there is some current to speak of.
    Last edited: Jan 5, 2018
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  8. rpeebles
    Dear Jim...Thank You Very Much for your very clarifying answer !! Much appreciated.

    Would the M3 be able to power the L-700 ? Soundwise any good ?

    In terms of sound how do the L-700 & SR-007 compare ? I would like a relatively Bright and Detailed sound, as a complement to my KingSound arrangement.

    Or should I simply go for the STAX SRS-5100 - L-500 + SRM-353X ?

    Thanks once again for your help !
  9. JimL11
    The M3 should be able to power the L700. I have no idea how it sounds as I have never heard one. In fact, I've never heard any of Kingsound's stuff. However, based on Kevin Gilmore's published schematics from reverse engineering, the M10 is a copy of the Stax SRA10 from 1974 except for using FETS in place of BJTs, and M20 is based on a Stax DIY design from 1968, so I am not optimistic that it compares with Stax's current amps. BTW, both the M10 and M20 lack the output safety resistors that are standard in all the current Stax amps. The M03 as I understand it has transformer outputs which provide protection in case of amp failure. Perhaps it is based on the Stax SD-X, which I believe used a transformer output driven by a chip amp, since they appear to ripped off their other designs from Stax. Not that there's anything wrong with that, if you're going to rip off, do it from the best. Though I suppose the reason for using 40-50 year old schematics is to avoid infringement issues rather than advancing the state of the art. However given the size limitations of a portable setup the frequency response particularly at bass frequencies may be restricted.

    I haven't heard the L700, but most people seem to feel that the 007 is in a higher category, although there are some that prefer the L700 to the 007. However, the 007 is significantly harder to drive well than the L700, and tends to be on the dark side if under driven, so it may not be to your taste. Note that Bob Katz, the well known recording engineer, considers a port-modded 007 Mk II to be very close to neutral except for a mild bass lack. My modded 007 sounds neutral to me, driven by a DIY SRX Plus.
    Last edited: Jan 6, 2018
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  10. kevin gilmore
    The 580v is the bias, thru a 5 meg resistor. Maximum current is 10ua, even when the diaphragm shorts to a stator.

    All stax amps can be considered to be bridged amplifiers. So the voltage doubles, not the current.
    The headphones are pure capacitors, so voltage and current are out of phase resulting in virtually no real power.
    However the amplifier definitely sees the power as real. At 20khz, the headphones can pull the equivalent of more than 1 watt per channel.
    jgazal likes this.
  11. JimL11
    Well, I would disagree with that statement. It is true that electrostatic headphones resemble capacitors, except that pure capacitors do not produce sound. Headphones do produce sound (duh), and since sound is a form of energy, that means that by the law of conservation of energy, they must drain real power from the amplifier. That in turn means that there must be a component of voltage and current that is IN phase, otherwise, no real power would be consumed.
    Last edited: Jan 6, 2018
  12. rpeebles
    Thank You Jim...That is Very Kind and instructive from your part !!

    In the meantime I have been offered a STAX SRM T1, in supposed to be as new condition, and for the sum of USD 650. Anything good there ? Or would you just go for the New STAX SRM-353x ?

    By the same person - ebay- I am being offered the STAX 404 Signature. Combination of STAX SRM T1 + STAX 404 Signature USD 1.099. Anything good here ?

    Thank you in advance !
  13. rpeebles
    One more question...sorry !....has the STAX 404 Signature have the same number of pins as the L-700. Thinking of compatibility with the KS M3.
  14. wuwhere Contributor
  15. JimL11
    I like the T1 quite a bit, but it is over 20 years old, so at a minimum should have all the electrolytic capacitors replaced as routine maintenance - that's less than $100 in parts. Electrolytic caps can deteriorate sitting on the shelf, so that's a precautionary measure. In stock form the T1 is well liked for driving any Lambda series headphone, and modified with constant current loads it is capable of driving the 007.

    All the Pro bias headphones have the same jack.

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