A S.E.X. for Stax. Putting the E back in sEx :)

[johnsonad]

I've been needing an amp for my Stax headphones for some time.  I'm not happy with any of the current Stax headphone amp offerings and most of the other "high end" offerings are very expensive and variations on the same theme.  I got the wild idea of trying a Bottlehead amp for this project.  I ran it past PJ who thought it would work and some custom iron was ordered along with a SEX with C4S.

The overall goal is to replace the OPT of the SEX amp with the custom iron from Sowter, add a bias board courtesy of Spritzer (of HC and HF) and a Stax output jack. 

Here is a link to the iron p://www.sowter.co.uk/specs/8423.htm.  It's a phase splitting interstage transformer re-purposed for this project.  Brian Sowter was great to work with and made a custom pair with zero air gap to run PF.  As calculated I should get around 200v PP which is twice as much as the headphones need to reach ear splitting level.  Here is the Stax headphone load:

 Capacitance :110pF (including cable)
 Impedance :145k Ω (10kHz)
 Sensitivity :100dB / 100V r.m.s. 1 kHz
 Max Sound Pressure :118dB / 400Hz

I'm just beginning the build at the time.  The top plate and bell ends went off for powder coating yesterday and I am staining and finishing the base this weekend.  I plan to update this thread as things progress.  Worst case, if it fails to meet expectations, I end up with a sweet SEX amp to use with my speakers. Wish me luck

[xcortes]

Awesome! Love the project! Can you give a link to  the bias boards?

[johnsonad]

Thanks! No link but he is available at either HC or HF. If you are a member at either place, send him a PM and he usually has them available.  They are around $100 USD a board, fully built and tested. 

[tdogzthmn]

Interesting project! I have thought about doing this awhile back but decided not to invest in a STAX system just yet.  Which model will you be using with your modified amp? 

[johnsonad]

I have a pair of 507's.

[tdogzthmn]

I've been looking at getting a Sennheiser HE60 at some point in the future.  If your experiment works out I will definitely consider converting my S.E.X into one capable of driving electrostats. 

[johnsonad]

A short update, it's coming together nicely and should be finished by the weekend.

[HF9]

Very exciting! Would definitely like to see photos and hear your impressions when it's complete.

[johnsonad]

Will do!

[adamct]

I'm curious, too. Look forward to hearing how this plays out...

Best,
Adam

[johnsonad]

Working on it now and it should be finished tomorrow.  Hopefully it fires up and doesn't smoke my cans.

[grufti]

Be sure to measure a few things before you connect the cans. That should prevent smoke from coming out of your head, or not quite as bad, your cans.

Working on it now and it should be finished tomorrow.  Hopefully it fires up and doesn't smoke my cans.

[johnsonad]

Well let me just say, it was a success   I'm listening to my Stax SR-507's as I type this post  

First the build.  It went together without a hitch; all of the resistances and voltages were within limits on first start up and somehow it all fit under the hood.  The OPT's and PC's swapped positions.  The OPT's just hang over the edges of the top plate. I installed the C4S from the beginning and mounted the bias board on standoffs near the IEC jack running the bias wire above the rest of the build (the blue wire you see). The RCA jacks were moved to just in front of the tubes and have a fixed resistor input (47k, it's the largest I had in house, no volume control). I reamed out the headphone jack hole to fit the Stax plug. All but two grounds went to the star ground near the IEC jack.  As the Stax headphones are a capacitive, not a resistive load, I left the PF cap as is at 1.5uF for now and am pretty happy with the bass output.  With a 1 volt 1K sinewave input there is 245v output at each of the OPT legs.

Now the sound.  With no burn in at all it sounds pretty nice, nice enough to know this is a keeper. I'm listening at around -39dB on the BeePre and very happy.  There is just a little hum in one channel and this could be tube related but it doesn't distract from the music and is very low level.  Remember this are about as sensitive a pair of cans you can get and every noise is heard.  Bass response seems pretty good from the get go which means the PF cap is about right.  They sound smooth and none fatiguing from the beginning.  I'm excited to hear how they open up with time and will post later impressions.

I would like to thank a few people that helped make this happen; Paul Joppa for all of his guidance and for answering many many questions, Brian Sowter for the fantastic iron, Birgir (Spritzer) for building such an easy to use bias board and Justin of Head Amp for the great Teflon Stax plug.

Now for the eye candy

[Paul Joppa]

Great to hear (sic) - thanks for the report!

[johnsonad]

Day two and time to start tweaking

The PF value for best bass response doesn't follow the normal rules for this amp.  PJ thought I should start with the stock 1.5uF and play around with it.  Today I hooked up my trusty H&P signal generator and listened for roll off from 100Hz down.  Too keep this post short, I noticed no difference between the values and if anything, it may have had better bass response with a lower PF cap value (at least to my ears).  Here is what I tried:

6.8uF Solen
1.5uF Solen
1.0uF Auricap
0.47uF Audio Cap Theta

To my ears the Audio Caps sounded best (I also have these as a coupling cap) even with zero burn in on them.  Now, these are electrostatic headphones and headphones at that so bass isn't going to be earth shattering.  I had strong signal down to 40Hz with significant roll off down to 30Hz.  There was nothing below 25Hz.

I'm tempted to try an even lower PF value but for now I'm happy with the 0.47uF Audio Cap Theta's.  With such a low PF value I'll be able to afford to roll in some nice caps in the future

Second, the hum I wrote about yesterday.  I disconnected the signal from the preamp and the S.E.X. is dead quiet.  The hum is coming from my BeePre.  I've got one 300B that is going bad.  It crackles from time to time and will be replaced soon as I can find a better sounding tube.  Of interest, I powered the BeePre up and then the S.E.X about 10 seconds later.  I was able to hear the 300B's warm up and it sounded like they were singing to me!  The hum is very low level and non volume dependent. 

The S.E.X. sounds great and better than any Stax amp in my audio memory.  Now back to listening