STAX and Bottlehead

[tdogzthmn]

I have been interested in getting myself some STAX earspeakers and I would really like to be able to use them with my bottlehead.  Has anyone here modified their amp to drive STAX?

[ironbut]

If you're talking about electrostatics, you may be able to use one of the old Stax transformer boxes but even with that, I don't think that any of the stock Bottlehead amps will have the voltage swing to drive them. I'm not familiar with the old Stax electrics so I don't know what kind of requirements they have but all the electrostatics need lots of voltage.
Electrostatic drivers also need a bias voltage and the signal input needs to be push-pull.

Here's a good primer on what electrostatic drivers are all about.

http://gilmore2.chem.northwestern.edu/projects/showfile.php?file=cmoy3_prj.htm

[tdogzthmn]

Are electrostatics similar in amping requirements to orthodynamics?  I was also interested in the HE-r orthos which have an impedance of 38ohm and Efficiency of 86 DB.  The orthos would certainly be simpler to use with the Crack.

[ironbut]

No, electrostatics have very different requirements from orthos or dynamic headphones. An electrostatic requires a 5 or 6 wire cable. A positive and negative (or phase and antiphase) for each channel plus a bias voltage conductor to charge the diaphragm that's between the positive and negative stators.

[Grainger49]

The classic Koss and Stax electrostatics came with a polarizing voltage supply and a step up transformer that gave them the voltage swing that electrostatics need.  A dynamic speaker is a current device.  It has a coil that wants current.  Electrostatics are capacitive and want a large voltage swing.

That makes it a whole 'nother animal.

[tdogzthmn]

I have always been intrigued by other driver types and their manner of operation.  I have only experience with Dynamic systems which are great but I have always been curious to try another system but it seems like Bottlehead's gear is aimed at driving dynamics. (which makes lots of fiscal sense)

[Grainger49]

With the proper step up transformer, manufacturer supplied, and the bias supply I mention Bottlehead amps should drive them.  The SEX amp and Stereomour might not have the power that the big electrostatics need. 

However, in my experience tubed amps (usually push pull) are better at driving electrostatic speakers than transistor amps.  Tube amps typically run from higher internal voltages than a transistor amp.  The step down transformer (output) on Bottlehead amps, except the Crack, changes the voltage on the output.  But they swing a lot of voltage.  Electrostatics like that!

[tdogzthmn]

I wish I had the knowledge to build a transformer for electrostatics but alas I do not.  It would be great to have a modded stereomour with a built in STAX plug, but that might just be wishful thinking.

[Paul Joppa]

I wish I had the knowledge to build a transformer for electrostatics but alas I do not.  It would be great to have a modded stereomour with a built in STAX plug, but that might just be wishful thinking.

I'm afraid it would. There are a few big problems:

* There are two STAX standards for the bias voltage, 340v and 580v if I recall correctly. No bottlehead amp has 580v available, so it would require an extra power supply section.

* The STAX is designed for push-pull signal voltages, and all our amps are single-ended. So the STAX cannot be powered directly - you need a transformer.

* You can't wind a transformer for high voltage and for low voltage at the same time; it's like asking for a car that can use 10" or 40" wheels equally well, and with a choice of 2WD or 4WD at the same time. In both cases it is not impossible but it is totally impractical.

[tdogzthmn]

I was also highly curious if there is a possible way to design an amp that can automatically adjust itself to be optimized to the load it's driving.  I would think an amp with self-adjusting gain would be a desirable feature to have in an amp if it would result in a discernible difference.  An amp that will sound its best no matter which headphones you use is appealing to me.