[L0rdGwyn]

Hey AS - still a relatively new member, haven't had much to post about, but I built another amplifier, so I figured I'd show it off!

Unlike my last 6A5G SET that I posted here a few months back, this one is for high-end headphones specifically and sits on my desk.  It is a MOSFET cascdoe CCS loaded 6J5 input, MOSFET cascode CCS loaded 45 output parafeed design for high-end headphones. Mains transformer is a custom job by Sowter to my spec, OPT are Sowter 8983. Filament regulators are Rod Coleman's, coupling and parafeed caps are Rike Audio S-Cap 2 (paper, poly, aluminum, oil). Power supply is a single rail CLCRC.



Here is the schematic, although it is not comprehensive. Parafeed capacitance was determined experimentally by taking FR sweeps of various capacitances (courtesy of cheap Solen caps), along with subjective listening impressions. Ultimately decided on 6.6uF made up of two 3.3uF in parallel.



And the power supply schematic.



Bias points:

6J5 B+: 380V
6J5 plate to cathode: 200V
6J5 cathode: 6.2V
6J5 plate current: 8.5mA

45 B+: 398V
45 plate to cathode: 206V
45 cathode: 35.5V
45 plate current: 35mA

Here are some shots of the interior. Bottom plate is perforated aluminum sheet for ventilation.









Here is a FFT of the left channel at 350mV into 300ohms.



And the frequency response.



I pair the amp with my Sennheiser HD650 and ZMF Headphones Auteur in cocobolo wood.





Thanks for checking out my build

[FullRangeMan]

First class job, clean layout, I really envy you.
I think you will prefer XLR4 balanced outs, great sound.
Congratulations
Sowter made customs transformers at no extra cost.
What is the OPT Pri Z?

[L0rdGwyn]

Thanks!  Yes, Sowter has a service for custom mains transformers, you send them your specifications and they will provide a quote.  The OPT is the Sowter 8983 5K:8ohm.

[FullRangeMan]

Thanks. It can run Hifiman planars 30 ohms 90dB?

[L0rdGwyn]

No problem, and yes sir, should be able to run those planars no problem 

[poseidonsvoice]

LordGwyn,

Awesome layout and build! Thanks for the schematic. Feel free to post some scope shots too!

I am currently building a Spud amp actually...it will have Edcor 5K:6 ohm outputs. Sowter rocks! I was just on their website last night!

What chassis is that? Landfall?

Best,
Anand.

[L0rdGwyn]

Thank you, Anand.  Very cool, hope you will post some photos of your build when it is complete.  Yes, the chassis is Landfall, they do a great job, high quality work, so I keep going back!  Although I am thinking of asking them about powder coating rather than an anodized finish for the next amplifier I am planning.

[poseidonsvoice]

LordGwyn,

I think a Landfall enclosure with a wooden encasing will suit. Or at least wooden corners that turn into
a stand.

Please post pictures of the back of the amp!

Best,
Anand.

[FullRangeMan]

Thanks L0rdGwyn 

[Jon L]

Love it!
What would be the power output into 32 Ohms, 150 Ohms?

[L0rdGwyn]

Good question, I haven't driven it to clipping with various dummy loads, but I have all of the necessary equipment to take those measurements.  If you are very interested, I'm willing to do it!  Given this headphone amp uses full-sized speaker output transformers with a resistor paralleled on the output (a somewhat common way of converting to a headphone amp), the power output isn't going to be quite as high as a dedicated headphone OPT, which tend to have a lower turns ratio.  But the advantage with the higher turns ratio of the speaker OPT is you get a lower noise floor.  The paralleled resistor keeps the impedance curve consistent across the frequency spectrum for the OPT, so it seems a very steady and consistent load.  Since headphone only need mW, the 45 still has tons of power to drive headphones, I used this setup with the switchable headphone output on my other DIY amp and was very happy with the results