[JimL11]

I just published an electrostatic headphone amplifier design in AudioXpress magazine November and December, 2015 issues.  It is based on the classic Stax SRX DIY design from the 1970s, with the additions of cascoded MOSFET current sources and a shunt regulated power supply.  The articles include an analysis of the original SRX circuit and a rationale for the use of current sources in electrostatic headphone amplifiers. 
 
The modified design uses two 12AT7 tubes per channel, and a 6SN7GTA/B output tube running at 7 mA per section (14 mA per channel) with +/- 325 volt regulated supply, conservatively designed so the tubes should run for 5000-10,000 hours before needing replacement.  The output current loads can be driven to clipping with less than 5 microamps current, leaving 99.9% of the output stage standing current to drive the headphones.  The frequency response into a headphone load is flat across the audio spectrum, and 3 dB down at around 45 kHz at 100 volts RMS output.  It is inexpensive to build, uses currently available parts, and is suitable for all Stax headphones as well as most other electrostatic headphones.  It can be built point-to-point, although Dr. Kevin Gilmore has kindly designed a PCB for the amplifier part.  The gerber files can be found on his website under srx6.
 
In case anyone is worried about tube reliability, let me point out that the 12AT7 and 6SN7GTA and B were specifically designed for the largest tube post-war tube market in the 1950s – tube televisions.  These were bought by millions of people as APPLIANCES.  That is, they were used day in and day out, year in and year out without maintenance, and were expected to be reliable for years without tube replacement, which was usually done only when the set stopped working.  Since market competition was fierce, tubes made in those days were reliable, or else.  None of this modern audiophile BS of “we’re pushing the boundaries so you have to expect some breakdowns.”  You wouldn't tolerate that in your toaster, why would you tolerate it in your hi-fi? Although the 6SN7GTA and B had a maximum DC plate voltage of 450 volts, RCA in its data sheet showed tube curves running up to 650 volts, and also specified a peak plate voltage of 1500 volts, so no worries there.

 

[JimL11]

If you are not able to find the relevant issues of AudioXpress, you can also find a discussion on the internet.  Do a Google search for kevin gilmore srx revisited.

 

[JimL11]

For anyone interested, a group buy for circuit boards is running on the other website.

 

[trivium911]

How does this compare to the o2?....is there a link to the website?

 

[JimL11]

I'm not sure what you are asking.  The Stax Omega 2 is a headphone, the SRX Plus is a headphone amplifier.  I think the SRX Plus does a good job of driving my Omega 2 headphones but I can't compare it to other amps.  Do a Google search for SRX Plus circuit board Group Buy.

 

[tomb]

How does this compare to the o2?....is there a link to the website?

 
 

  I'm not sure what you are asking.  The Stax Omega 2 is a headphone, the SRX Plus is a headphone amplifier.  I think the SRX Plus does a good job of driving my Omega 2 headphones but I can't compare it to other amps.  Do a Google search for SRX Plus circuit board Group Buy.

 
My guess is that he's a noob trying to boost his post count.  He doesn't understand that your amp uses a power supply of about 300 more volts than the O2 (a solid-state dynamic headphone amp using 2x9V batteries with a walwart).

 

[JimL11]

Ah, OK.  I thought he was talking about the Omega 2 headphones, which are sometimes referred to as O2s.  Yeah, the O2 headphone amp is for dynamic headphones, the SRX Plus is for electrostatic headphones, the O2 is an commercial amp, the SRX Plus is DIY only.  Completely different categories.

 

[trivium911]

Ah, OK.  I thought he was talking about the Omega 2 headphones, which are sometimes referred to as O2s.  Yeah, the O2 headphone amp is for dynamic headphones, the SRX Plus is for electrostatic headphones, the O2 is an commercial amp, the SRX Plus is DIY only.  Completely different categories.

Not trying to boost my post count as i wouldnt even know what benefit that would be to me. Dont need to be rude... i just like diy projects and was asking how the amp compares to the o2 amp as i am either buying one or building one. I will need to look up what eletrostatic headphones are since i only know what dynamic and balanced armeture are.

 

[JimL11]

Excuse me, but how am I being rude?  I said I didn't understand your question.  I didn't say anything about post count.

 

[tomb]

Ah, OK.  I thought he was talking about the Omega 2 headphones, which are sometimes referred to as O2s.  Yeah, the O2 headphone amp is for dynamic headphones, the SRX Plus is for electrostatic headphones, the O2 is an commercial amp, the SRX Plus is DIY only.  Completely different categories.

Not trying to boost my post count as i wouldnt even know what benefit that would be to me. Dont need to be rude... i just like diy projects and was asking how the amp compares to the o2 amp as i am either buying one or building one. I will need to look up what eletrostatic headphones are since i only know what dynamic and balanced armeture are.

No one was being rude. It was simply a guess as to why your comment couldn't have been further off-topic than it was.

Just realize that the DIY section doesn't get much traffic or attention compared to other sections of Head-Fi. Consequently, it's used by some relatively new members to boost their post count in order to meet the minimum requirements of the For Sale section.

 

[trivium911]

Excuse me, but how am I being rude?  I said I didn't understand your question.  I didn't say anything about post count.

Not you but i was offended at tombs comment automatically assuming and calling me a noob. Interesting....i was unaware there was a for sale section. At any rate i looked up what stax headphones are and they are very expensive and need a special amp, but i will have to look into it further as it looks like they have entry level models.

 

[Jaab]

Hi,
i'm wondering where we can find information about this amp project? schematics, pcb, group buy etc

Thanks

 

[JimL11]

The articles on this amp were published in AudioXpress in November and December 2015, as mentioned in the first post. The power supply is being revised, but Kevin Gilmore developed a PCB for the amp circuit and has Gerber files for it - you would have to look for them on another website which must remain nameless. There is also a build thread on that website. There is no current group buy. The amp is simple enough that it can be built point-to-point, which is what I did.

 

[Jaab]

Thanks

 

[NicMor]

Hello JimL11,

I have built the amp in this project and am pleased with the results, overall.
I thank you for sharing your knowledge both in print and through your posts.

I have a pair of Stax 007 Mk 1, and wish the amp had a bit more gain. It works satisfactorily with my Omega Pros, but the gain with the 007s seems to be lacking.

Any ideas would be appreciated.

NicMor