KOSS ESP950 + STAX SRM212
Jun 10, 2015 at 12:20 AM Thread Starter Post #1 of 36

luvmusik

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Has anyone tried this combination - KOSS ESP950 + STAX SRM212 specifically ?
 
How did that combination compare to changing back to matched-system amp as - KOSS ESP950 + KOSS E/90 for sound impressions ?
 
Jun 10, 2015 at 11:26 PM Post #2 of 36

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The srm-212, to my ears, has some kind of mid range glare thing going on, irritates the crap out of me, especially when using with 003's. It is not as bad driving the 303, and a little more listenable still with the 950's, but the result is still not great. Some resistor upgrade at the input stage can make the irritation mostly go away. Only "mostly" though.
 
The 950 requires quite a bit of power to run well, I don't think the 212 has enough juice for it. A transformer box might be a better option.
 
I have never used a E90 so don't know how it sounds. (if you send one over I can compare it for ya...
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Jun 11, 2015 at 12:54 AM Post #3 of 36

luvmusik

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Great info, thanks. The SRM-212 drives my SR-202 (Basic System II, SRS-2020), and it made me curious about running the KOSS ESP950 with it thru an adaptor.
 
I can always grab a better STAX amp to run the KOSS with too, as many suggest it really opens them up to full potential.
 
Maybe on the big ESP950 thread here, there is probably some posts as to which STAX amp matches the 950 best, and I'd like to go solid-state (preference).
 
Great point about RMS output voltage - the SRM-212 puts out 280V. The E/90 output is 600V (differential) & ( ? 2300V (push-pull with soft limiting) ?=?meaning? )
 
Would like to keep the cost down (< $500) if going that route; there are some STAX amps offering really great value (superb sound per dollar) -  I think, such as - primarily  interested in SRM-1 MK2 Pro at 370V first choice, or SRM-313 at 350V, (nice as along with Pro bias socket they also both have a normal bias socket to run Lambdas on for versatility) and then maybe SRM-323 at 400V or SRM-3 (only at 300V), but not sure which models match the ESP950 well sonically. If the SRM-1 MK2 Pro matched the KOSS well, I might grab that model.
 
Do you think output voltage RMS of 350 or 370 or 400 could drive the KOSS ESP-950 ? Is that enuff voltage ?
 
For SRM-1 MK2 Pro it was made in the versions A, B or C and wonder if specs were same, or if any 1 of the 3 matches ESP950 better.
 
Together with your advice it gives some options to look forward to.
 
Jun 11, 2015 at 1:36 AM Post #4 of 36

luvmusik

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Any transformer box models you can recommend ?
 
Thinking back at the 8 or 9 or so makers of electrostatic amps in the past, with my <$500 limit as used, I may only be able to get those STAX models, if they can drive the 950, or would have to stay with the E/90. Not sure if there is any brand's model electrostatic amp used <$500 with output RMS voltage near 600 volts ? Maybe would have to step up into tubes rather than SS.
 
Jun 11, 2015 at 9:37 AM Post #5 of 36

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when I said "juice" I didn't mean the voltage swing, I was talking about the potentially available current -- the 212's output stage runs at only 1.2mA. E90 is probably about the same, though.
 
Jun 11, 2015 at 3:41 PM Post #6 of 36

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I ran the 950's quite nicely with the SRM3 which had  300 voltage swing or whatever they call it these days.   I believe that is typical of Stax tube amps.  The transistor models go higher, 340, 360 and the 717 (and I believe 727) does 450 volts.  I believe that is the most for any Stax amp. 
 
I believe that voltage swing matters particularly if the phones are inefficient.  Eg  My 007A's sound somewhat lacking in dynamics/prat on the SRM1Mk2 but better on the 717. On the other hand, with some phones eg. the SR003, 280 is probably sufficient. Some of the high-end aftermarket amps have much higher voltage swings.  At some point you run into possible damage to the drivers.
 
Jun 11, 2015 at 7:34 PM Post #7 of 36

luvmusik

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Oh, OK, I think the E/90 output stage is 1.0 mA but not positive on that. So, maybe I'm OK trying the SRM-212 with the ESP-950. I may get the exact mA figure in a day or so, as I asked a tech who knows.
 
Jun 11, 2015 at 7:46 PM Post #8 of 36

luvmusik

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  I ran the 950's quite nicely with the SRM3 which had  300 voltage swing or whatever they call it these days.   I believe that is typical of Stax tube amps.  The transistor models go higher, 340, 360 and the 717 (and I believe 727) does 450 volts.  I believe that is the most for any Stax amp.
 
I believe that voltage swing matters particularly if the phones are inefficient.  Eg  My 007A's sound somewhat lacking in dynamics/prat on the SRM1Mk2 but better on the 717. On the other hand, with some phones eg. the SR003, 280 is probably sufficient. Some of the high-end aftermarket amps have much higher voltage swings.  At some point you run into possible damage to the drivers.

Got an adaptor cable on the way now, got a used one shipped for $84; a little pricey but half of new price and saves me the time & parts search (if could find the male pin plug) to make one, so I'm OK with buying it used in real good shape.
 
Good info on the swing voltage. I never knew that STAX tube amps had lower voltage swing than their solid state amps, always learning something here. With after-market amps that swing really high up, that potential damage is scary; I'd be afraid to get into it all that deep and also outlay the mega-bucks.
 
Jun 11, 2015 at 8:55 PM Post #10 of 36

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  Any transformer box models you can recommend ?
 
.....

 
WOO WEE can be had for $300~$350 used. I don't think the Stax SRD's can push the 950 well enough.
 
The 950 have large diaphragm drivers therefore requires quite a bit of current (signal-slew-rate * phone-capacitance) to control the diaphragm. Transformer boxes can output a lot of current since the power is mainly determined by the speaker amp feeding it.  
 
A direct drive electrostatic amp, at least in theory, is more detailed than "speaker amp + transformer box", but the 950 itself is not super detailed so the extra fine resolution might be wasted anyway.
 
Jun 11, 2015 at 9:02 PM Post #11 of 36

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  Got an adaptor cable on the way now, got a used one shipped for $84; a little pricey but half of new price and saves me the time & parts search (if could find the male pin plug) to make one, so I'm OK with buying it used in real good shape.
 
.......

 
considering you will likely move up to something else (say, 007's), buying that used "commercially made" adaptor was a great idea. Easier to buy, and later on easier to sell. You probably get to use it for free....
 
DIY is not that great an idea unless absolutely necessary (or you are bored). And this comes from somebody who tinkers a lot...
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Jun 11, 2015 at 9:22 PM Post #12 of 36

luvmusik

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I have an SRD-7SB and (2) SRD-4, one is an extra...maybe that's another experiment to try on the extra box. If I try either transformer model, is there any risk of damage to either the ESP950, the transformer box, or the speaker-hi-fi-amp ? I would watch the volume on hifi amp.
 
Also, always wondered about the electrostatic headphone amp vs transformer box theory and have a question. Realize the amp is finely crafted and especially with increasing price, some great units are around with super sound.
 
Question is...let's say the hifi amp has a very pure (hi-end) output for audio; then, wouldn't the next in line unit (headphone amp or transformer box) convey that signal best if it has the least internal componentry, considering internals are on an equal quality level for electronics inside the amp & transformer boxes equally ?  The transformer box probably has less circuitry, but maybe the amplifier is crafted more finely to convey that signal. I always wondered about this -what's the scoop ?
 
I guess golden-ear audiophiles like to have the least in-line as possible to maintain the purest audio, and the dedicated headphone amp almost seems like "amping-twice" (double), which would seemingly enter more problems into the sound equation.  But the amps seem to outperform ( ? maybe ? ) the transformer boxes, and the hi-ender's mostly all go with the amps -how is it so ? Is it sorta like the headphone amp tailors the sound like a hifi PREamp would ? I need to learn.
 
Jun 11, 2015 at 10:19 PM Post #13 of 36

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you can try the 950 with SRD-7SB, but make sure you don't play it loud..... the SB is a low bias box so the 950 output will be lower than normal, if you increase the phone volume by cranking up the speaker amp power, by the time the sound is loud enough the actual signal swing might be too high.
 
higher grade transformer boxes can sound quite good ( some one who tried my Lundahl box said it is about the level of  717/kgss), but we are talking about $700 ~$1k of parts plus you still have to build it then add a good speaker amp, meanwhile a used 717 can be had for around $1k ready to play. And if you later want to sell and move on.... selling 717 is easy, selling transformer box.... probably not so much. (So here it comes again... DIY is not that great an idea unless absolutely necessary (or you are bored). And again this comes from somebody who tinkers a lot...)
 
Jun 11, 2015 at 11:08 PM Post #14 of 36

luvmusik

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Probably best I keep the SRD-7SB solely for my SR-Lambda Normal Bias. The SRD-4 matches electrets - my SR-30 Pro & SR-80 Juniors, and I have an extra SRD-4 - what would happen if that type transormer energizer (SRD-4) was used with the ESP950 ? Is it too lo-quality sonically to put with ESP950, and will it not match electrically ? Also don't want to damage anything. Just wondering.
 
Jun 11, 2015 at 11:21 PM Post #15 of 36

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srd-4 has no bias output to energize the diaphragm --> the 950 won't make any sound.
 
I think a used WEE is the best bang for the buck as far as 950 driving units go, whether amp or trans box.
 
(Now the best bang for the buck driving unit for Lambda of any kind is probably a used T1....sell all the SRD's and use the $ toward a T1? )
 

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