Thursday, October 27, 2011

More Things I've learned reading Head Case blog on Koss ESP 950's

I've been reading this blog at Head Case full of good information (and also much OT and flaming unfortunately).

1) Koss is generous with warranty.  If you have recabled it is only recommended that you remove cable before sending in, otherwise you'll never see your cable again.  Sending in with no cable is fine. You must include energizer and phones.

2) Lots of people use Stax, KGSS, and many other custom amps.  There's a whole cottage industry of electrostatic headphone amp makers, including some guy in Germany.  Most such amps are actually made for Stax Pro, but work fine with Koss because specs are virtually identical. Some amps have adjustable bias.  People have little good to say about E90 energizer, except it's included with phones. It also can run on batteries, and you can carry a player and everything around in the case Koss conveniently provides for that purpose, but since your D cells only last 3 hours, nobody is much interested.  Koss and/or Stax connectors are available from Amphenol and other sources such as Allied Electronics.  The Koss has more in-head sound than Stax, but when Stax amp is used much of that goes away, and the image is then similar to Stax, more in front and outside of head.  Some people also think the larger ear offset in Stax Omega phones, or the driver angling, has a further beneficial effect on the image.  [since writing this, I've learned out-of-head is a coloration, see later post]

3) Recabling is easy, except for removing the screws on headset which are very tight.

4) There was a long problem with squealing noise.  Also present but to much lesser degree with Stax phones and/or Koss using Stax amps. Nobody seems to have figured it out.  Koss will only fix if you send in both headset and energizer, then sometimes it appeared that they adjusted energizer more than doing anything with phones.  Many people suspected that the reduced bias, to as low as 300V, but that could be measurement error (these internet guys are not engineers).  Many thought it was dust or hair in the driver, but another theory was arcing around the headset terminals.  Koss finally redesigned the terminals (they now point away from each other) in 2008 and it seems less of a problem (or maybe not, and maybe squealing results from too high bias, the Koss 600V spec may be misleading).

5) One of the favorite stats is the Stax Omega MkI.  People hated the MkII, and thought the Koss was similar or better than that.  Koss has more forward midrange, Stax more laid back.  Which has better bass depends on who is talking, some think Stax bass is exaggerated and less defined.  Koss phones are obviously much more cheaply made.  But people say "good enough," especially with lifetime warranty.  Sennheiser made an astronomically priced Orpheus system, which may have been the best ever, but people were mortified about the potential repair cost.

7) They had picture of energizer main board, but on edge mainly to show very simple bias network (basically just resistors and capacitor, with DC taken from HV rail that amp uses, which comes from power board). That's the best I can do until next week.  You can see the rest of board has lots of small transistors and other parts.  Definitely not a purist design and consistent with my PWM guess, as are some findings above.

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I think I won't remove foam, especially at the risk of letting dust or hair into the driver.

1 comment:

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