STAX headphone amplifier "Pluto 2A3/SE"

After a many positive STAX reviews I decided to try them out. Wish i got this idea much sooner!  

Soundwise nothing compares to micro detailed, wide stereo spaced sound. STAX are very low distortion electrostatic transducers (here even expensive speakers have troubles).    

I saw on internet original STAX amplifiers schematics, both transistor and hybrid ones, was not satisfied with them. I wanted to go single ended, with some good "iron" instead 

Lundahl Transformers were chosen, because of exceptional quality and endless possibilities of their products. Windings are split into many symmetrical sections, way better than just winding taps.
You can wire them in many ways, to get desired impedance / voltage ratios.
What was used:
Amorph core input transformer LL1544A (10Hz - 70kHz +/- 0.5 dB @ 0,775VRMS):
Interstage standard core transformer LL1660/18mA (better option-amorph core, twice expensive):
I spent about two weeks, calculating various working points of 2A3 output stage. Everything had to be considered, from Stax headphone capacitive load behavior, optimal tube distortion profile, and comparing each transformer parameters (parasitic and primary inductance, optimal transformer airgap).
Wanted to have this amplifier much versatile as possible, with minimum compromises. Reason why 2A3 tube, and why not 300B is simple - better distortion profile at low currents!

Calculated a few transformer examples, LL2745 in stepdown mode 5,6:1 could be quite good for my old Beyerdynamic DT990pro 250R. With some relay contacts (many of them needed; reason why i abandoned this), it could be possible to use this transformer for Stax also. (2,8:4)  Similar examples: LL1671-4:4        LL1692A--3,5:4.
Low impedance, stepdown type: LL1689 - this could be ideal for dynamic headphones or orthodynamics! But not enough for STAX. (18:1   20-50R     18:2  60-200R     18:4  250-800R)

Days later I decided to go with LL1660 / 18mA; this model is made with higher impedances/voltages on mind. It suits better Stax usage. Wired "backward" 2,25:4 voltage ratio, measured primary inductance is ~33H, parasitic inductance ~20mH (very good for such hi-impedance transformer). Parasitic capacitance ~120pF.
This is good for range 15Hz....50kHz (-3dB) !
As you can see, no global feedbacks used. Distortion is low, compared to typical speaker
amplifiers.  This shows how good quality the components are.

Endstage parameters during calculated 100dB output: (skewed loadline due to STAX capacitance load at HF)


Endstage parameters during calculated 250V RMS max output: (earbleeding volume here)

Powersupply and amplifier schematic:
Mercury vapor thyratrons makes powersupply harder than vacuum rectifiers, and got them cheap from ebay. Choke loaded filter is required. Measured hum is negligible.

During autumn 2015, Lundahl announced new interstage transformers. Those are mostly for 1:1 usage, and could be nice with righ rp directly heated triodes, like 10Y, 801A and others. I can only imagine delicate sound coming from such setup with thoriated DHT´s, matched to silver wires-amorph core transformers!   

STAX headphone features 
Electrostatic transducers are ultra low distortion, superior even against most expensive speakers known. Very thin diaphragm moves between two stators by electrostatic force.
No magnets here - like on typical dynamic transducers, or heavy voice coils glued to thick membranes (movement inertia, and resonances).  Micron thick diaphragm does not have own resonance point, even if it had some, surrounding air would dampen it. Diaphragm mass is small, compared to surrounding air.
Therefore STAX has no problem reproducing smallest music details, stereo space is very wide. Turning volume to max, will not change sound character or introduce resonances / distortion or boomy sound.
Standard headphones/speakers will sound harsh if played too loud; this is not happening with Stax..  It gives you desire to increase volume, very easy to loose self control (and damage ears).

My biggest surprise was listen to mp3, it was not bad as i thought it will be. Good quality mp3 has some hidden potential, and Stax was able to recover it. However, lossless formats or HD quality tracks are different monsters, amount of detail is huge. I am rediscovering music albums again, which i was very familiar for years.

Compared to Beyerdynamic DT990pro, the STAX SR207Lambda sound more lifelike, upper range bass does not sound boomy (subbass is rock hard).
STAX Lambda plays "outside of your head"; amount of detail retrieved from music is huge. You can feel the recording room size, also fading reverberation-for example each hit of the drum.
Midrange on Beyers sounds attenuated, acoustic guitar lacks fullness of sound. Treble is quieter too, like under blanket, top end treble range sounds a bit louder (harsh?) --it´s not sounding "all equal" like STAX does. Beyer´s midrange dip is noticeable, also the stereo size is small- music plays "in head".
Comparison to STAX SR007mk2. If entry level STAX Lambda sounds so good, what to expect from ex-flagship? Wear comfort is very good, even if listening to it whole day. (headphones feels lighter than cable, which is very wide)
Soundwise, it is unbelievably good. Price difference is big, but it is worth it. Enjoyable sound without peaks or dips, feels even flatter than SR207. Probably bigger circular diaphragms responsible for sound improvements. Bass is great, no resonances, just deep growl. Very different than standard headphones.

Sound source is a USB DAC, XmosU8 + PCM5102A DAC. It is a assembled unit from ebay. With unlocked drivers capable of 32b/384kHz rates.

Prototype status, new chassis is incoming:

Update 11.1.2016
Recalculated ECC803 input stage again, small change to working point to improve harmonic distortion. Reverb of instruments stereo space feels a tiny bit better, feels more improvement in firm bass/drums sound.
And more gain is just a bonus.
After (left), before (right)

Update 14.1.2016
Changed from JJ ECC803 to Sovtek 12AX7WB. This has a boxy anodes, it should be bit more linear (it has ~+10% gain). Feels like more details there. Voltages are different, at anode and cathode.

Update 19.4.2016
Modified circuit with MOSFET, now the 12AX7WB sings! Really good amount of detail can be heard. Circuit has smal output impedance, drives 2A3 very well. Impedance for driver tube is high, several hundreds kiloohms (lower distortion).

(I think, for treble roll off is responsible Miller capacitance of end tube, which is ~90pF . This represents additional load of 130k, which lowers gain at treble freq.)
Gyrator third revision, another step up in sound quality: