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Anyone tried Szekeres MOSFET buffer after a tube?

post #1 of 9
4/12/10 at 10:59am
Thread Starter 
Has anyone tried this DC servoed Szekeres MOSFET after a SE tube stage?

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post #2 of 9
4/12/10 at 11:48am
A servoed Richard Murdey amp is the output stage for the Melos SHA using the tube heater as the source resistor. The tube stage of the melos is a long tail pair though.

At one time Rick mentioned that he was running something like you mention off of a SE 6sn7 and commented quite favorably on it. One of the Melos prototypes was rumored to be a 12au7 running SE with the mosfet output stage.

I have use a non-DC servoed simple source follower in a build. a 24V B+ using the millet hybrid tubes (12fm6) and the source follower DC coupled to the plate of the tube. I originally tried to use the tube heater as the source resistor but gave up in favor of separate source resistors to allow running the tubes at a scotching 16V on the plate. hindsight being 20/20 I could have used smaller resistors to drop 4V and kept the heaters set up the way I had them which would save some waste in my power supply. ooh well.
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post #3 of 9
4/12/10 at 12:41pm
I thought I read somewhere that the TTVJ/Millett FET-A had a DC coupled source follower output... so maybe that has a servo?
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post #4 of 9
4/18/10 at 6:49am
Thread Starter 
This is interesting too, another way to avoid caps. I'm surprised with no one has DIY'ed an ouput capless moffset hybrid like the famous Melos:

RJM Audio - Szekeres VE Headphone Buffer
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post #5 of 9
4/18/10 at 7:06am
ecp.cc
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post #6 of 9
4/18/10 at 9:31am
Quote:
Originally Posted by regal View Post
This is interesting too, another way to avoid caps. I'm surprised with no one has DIY'ed an ouput capless moffset hybrid like the famous Melos:

RJM Audio - Szekeres VE Headphone Buffer
I have been looking at that amp many times and think it would be great with Grados or other easy to drive headphones. Dsavitsk has build one and mentions that it sounds good at ecp.cc so check that too.

RJM did in fact build a version with a tube gainstage called the twilight I'm not sure if it's ideal as it seems to have a gain of 20 but it might be worth examining.
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post #7 of 9
4/18/10 at 11:41am
I made a DC Coupled Millet hybrid (original) by creating a +12V virtual ground and VERY carefully biasing the tubes to get the output within a few mV. It sounded quite nice although paranoia set in and I undid it in favor of the safety of having a cap between my headphones and 12V.

As a tangent and perhaps more interesting, you can make a "capless parafeed" style tube amp using a similar technique. You just have to make a good HV reference that can be adjusted to light up at EXACTLY the same voltage as the tube. (I make it sound soooo easy, and I cant say that a cap does worse to your signal than your voltage source, but it oculd be a fun project) Maybe use a gas regulator tube and adjust the tube buy adjustable cathode/grid bias, maybe a fancy discrete SS voltage reference.

Since there is a big fat transformer a tinny bit of DC offset wont blow up your headphones, you can get the transformer wound with a gap for like 5mA which is WAY better than full DC current.
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post #8 of 9
4/18/10 at 1:38pm
I've built this using a couple of $5 12 volt switching supplies tied together to get the +12 and -12 rails. The sound is very good and the entire amp can be built for about $40, less if you have a supply of spare parts. Just remember that the headphones do not run at ground so the headphone jack must be isolated from ground.

My friend used it to drive a pair of 300 ohm hd650s and decided that his $300 headphone amp didn't sound as good.

Quote:
Originally Posted by regal View Post
This is interesting too, another way to avoid caps. I'm surprised with no one has DIY'ed an ouput capless moffset hybrid like the famous Melos:

RJM Audio - Szekeres VE Headphone Buffer
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post #9 of 9
4/19/10 at 4:38am
Unless I'm wrong about the "Szekeres" thing, I remember someonee doing this with a yaha. I remember because I wanted to Try a cheap tube amp, and the diyaudioprojects page came up. The cool name "Szekeres" stuck out. I'll try to find it again.

Hears a headwize post I found with google right off:
HeadWize: View Post [DIY Workshop ยป The YAHA amp thread part 2]

Here we go:
DIY 12AU7 (ECC82) Tube - IRF612 MOSFET Hybrid Headphone Amplifier

There's been a really recent update: is it the same concept, just optimized? I have'nt had time to look at it, and I'm too tired to think about it anyway.
http://diyaudioprojects.com/Solid/12...LM317-Headamp/

I'll take my completely un-knowledgeable self back out of the topic now.
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