Stax Mafia Makes Unrefusable Offer and I Begin an Electrostatic Headphone Measurement Program

Alright, I'm going to knock it off with the Stax Mafia jokes because this is actually a very cool, and very generous thing. Birgir (Spritzer on Head-Fi) and Kevin Gilmore (who hardly needs an introduction) have done InnerFidelity and the community at large a huge service. They've built and donated to me a Special Reviewer's Edition of the KGSS (Kevin Gilmore Solid State) electro-static headphone amplifier.

I. Am. Humbled.

The KGSS is a staple among advanced headphone amp DIYers, it would be hard to go to a big headphone meet without finding one there. Though the case-work is often plain, a well-built DIY example is without question in the same league as the world's most expensive commercially available amplifiers.

But, Birgir and Kevin had another goal in mind with this amp: It needed to allow me to listen to and measure any electrostatic headphone. To accomplish that it would need multiple output jacks for the various pin-out and bias voltages for the three common standards: Stax Pro Bias (for current Stax headphones); Stax Regular Bias (for older models); HE90 (Sennheiser); and one with an adjustable bias for any other electrostatic headphone that might not use standard biasing. This amp is a high-voltage Swiss Army knife.


Here you can see the adjustable bias jack on the front panel, and the adjustment and metering test-points for the adjustable bias on the side panel.

To be truthful, I'm not up to speed on the particulars of the topology and inner workings of electrostatic headphone amplifiers, so I've got a bit of learning to do about this amp. But I do have Birgir's words describing some of the special aspects of this amp:

Since the KGSS is the most common after market Stax amp out there I thought it was a good fit for you as a baseline model. It's also a slightly leaner sounding amp that either the KGSSHV or the BHSE so the bass is not as prominent and perhaps a bit more "honest", whatever that might mean... This one was a bit more advanced as it uses the same output transistors as the KGSSHV and runs on +/-415V rails instead of the stock KGSS +/-350V. The power supply is a simplified version of the KGSSHV unit but superior to the ones found in any normal KGSS.

This is a reviewers special with four outputs all connected to the same amplifier so only the connector and bias voltage vary. On the right is a bog standard Stax Pro bias output fed from the same regulated bias supply as in the KGSSHV and next to it is a HE90/Orpheus connector. That one is fed through a voltage divider off the Stax Pro bias line to generate +520VDC—which is what most HEV90's measure at—and a 10M ballast resistor on the output, which is necessary for the Sennheiser 'stats.

On the other side we have a normal bias output with +230VDC bias line powered via another voltage divider off the main B+ (small perfboard next to the amp board, photo below). Finally we have the variable bias supply. This is a Stax pro bias socket so it would be easy to make adapters but the actual supply is a Tangent Thread (or Steps... can't remember ) feeding an EMCO 12V->1000V module. On the side are jacks for multimeter probes and a multi turn pot to adjust the PSU so it can be done externally. Internally I fitted a 5M ballast resistor but it can be easily swapped out for something else as it is screwed in place. This is to make the amp future proof and make it easier to review some future models which some oddball bias voltage/ballast resistor.

Other than that is a pretty standard build with XLR inputs and a loopout on the back and a switchable 117/230V IEC input which also houses the power switch. Four gang Alpha volume pot and the power on indicators are underneath the amp as I felt the front panel was too busy as it is. Call it Nordic minimalism but we can't have clutter.

This one is truly unique though, nothing like it in the world. It uses the same amplifier PCB as Justin (Wilson of HeadAmp) used in his KGSS amps but this one has better output and gain transistors, larger heatsinks to accommodate the extra standing power and every component is hand selected/matched. The power supply is a play off the T2DIY and the KGSSHV design which followed it. It has only been used in this amp and contains five supplies, all of them regulated. +/-15V, +/-415V and the bias supply of +580V.


Inside the KGSS Special Reviewer's Edition

Compatability With Previous Measurements
One thing that worried me was whether or not the new amp would perform similarly to the HeadAmp Blue Hawaii used for previous measurements...both are designed by Kevin Gilmore so I had good reason to hope there wouldn't be any problems. I happen to have an SR-009 here at the moment so I set-up the new amp in the measurement system and measured them.

Stax SR-009 on KGSSSRE

Click on graphs image to download .pdf for closer inspection.

Stax SR-009 on Blue Hawaii

Click on graphs image to download .pdf for closer inspection.

You can see the two sets of measurements are very, very similar. Surprisingly similar. Probably speaks more about Stax quality assurance than it does about the amps really. Anyway, it's obvious to me that we won't need to remeasure models that already exist in the database.

A Huge Thank You!
Thank you so very much Birgir and Kevin, this is a wonderfully generous gift. I will put it to good use. I understand Justin Wilson of HeadAmp provisioned the killer teflon sockets. Thanks mate!

On that note.....

Electrostatic Headphone Measurement Program
Many of you have requested measurements of electrostatic headphones in the "Measurement Request" thread managed by Ultrabike here in the InnerFidelity forums. I've started a "Electrostatic Headphone Measurement Program" here in the forums in which readers can self-organize the process of getting electrostatic headphones to me for measurement. Ultrabike has graciously accepted my request to manage the thread. Thanks, man.

Those of you who have electrostatic headphones of any brand or model and would be interested in participating in the project, please make your way over to the thread. PLease allow time for Ultrabike to reserve the second and third post prior to posting. (I'll have to delete any posts that jump in too quickly, sorry.)

For the rest of you, kick back and relax, a couple of months from now you'll get to start enjoying new e-stat headphone measurements in the InnerFidelity Update posts.

We're having some fun now!

Jazz Casual's picture

A generous gesture indeed from the Stax Mafia even with the strings attached. So when do you get made? Looking at the impressive measurements for the SR009 reminds me that I must hear one someday.

ultrabike's picture
And hope to see interest in the program. Electrostats are unique in some ways and IMO deserve the attention and consideration of phone enthusiasts... and audio companies.
Claritas's picture

The nicest mafia in the world--helping us all.

neo's picture

Awesome job by the Mafia. Bring it on Tyll

bobusn's picture

ESP 950s on this amp? Thanks!

spritzer's picture

With an ESP950 to Stax adapter then the amp will have no problem running them via the adjustable bias supply. In fact the amp can run pretty much every electrostatic ever made though I'm not certain about the Floats. They use speaker drivers (tweeters actually) so they need much higher drive voltages than the Stax and similar need. Off the top of my head these will work perfectly given the use of proper adapters and correct bias supply: Beyer ET-1000, Sennheiser HE60, all Stax OEM sets (Magnavox, Realistic, Marantz etc. though they will need a much larger ballast resistor), any of the King Sound units (no adapter needed), any of the HE Audio units and the Audiotec CES.

Now if you want to get truly adventurous then the Koss ESP6,7,8,9 and 10 can all be rewired for the Stax standard and then measured. Most electrets can also be tested though some like the Sony ECR units run at silly high drive voltages and are thus incompatible.

markus's picture

Will you be able to measure their hunger for current and voltage?

Bob Katz's picture

Dear Tyll: That's a fabulous offer. I hope you enjoy the sound of this KGSS amp as well. It's a killer. Anyway, watch out... if you open it up, stay away from anything with any probe. If you slip you'll make a spark you will regret. This is serious stuff. As far as measuring the voltage on any of the stuff that's ballassed with a build-out resistor, forget about any voltmeter you might have. I haven't found a voltmeter or oscilloscope with a high enough impedance not to load down the voltage and give an incorrect reading. So if you are setting the voltage, make sure you are doing it on a low impedance output, and be VERY VERY careful. This is very serious voltage.