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Old 24th September 2014, 04:08 PM   #1
janim is offline janim  Slovenia
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Default High-voltage hybrid headphone amp

Hy to all!

As my first DIY audio project I decided to create a headphone amp. I found lots of info about power supplies and first (gain) stages. But I can't find any info on how to design an output stage. So I find myself with a 250V power supply (we don't want to let the tube hungry ) an IRF820 acting as a cathode follower (also doesn't complain) and no idea how to continue.
I tried something like this only with mosfet replacing the V2. It works, but there is no gain - with P1 on max the only amplified thing is the noise.

Soo... What do you think? I'll be very thankful for any help...
Thanks
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Old 11th October 2014, 12:28 PM   #2
janim is offline janim  Slovenia
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So I was looking for more info on the web. The main reason why I went with a mosfet cathode follower was because of this site: mosfet follies. But I see now that it won't be so easy. So I have several option:
- I can stick to the original schematic and buy some tubes
- I can modify a similar design I found on this forum (Hybrid tube headphone amp design) to work on 250V
- or add a 12V source to power a Seekers amp as shown on this project: link.

So what do you think? Any pros/cons with this designs?
And before you say it: Yes, it is tubes for tubes sake But I'm having so much fun!
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Old 11th October 2014, 06:17 PM   #3
Stixx is offline Stixx  Germany
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Have a good read around here --> Cavalli Audio

Even Mr. Cavalli hasn't succeeded in buidling a high voltage hybrid (the "Stacker"),
so abandon the plan or change your design to use low voltage tubes.
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Old 11th October 2014, 06:17 PM   #4
00940 is offline 00940  Belgium
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For high impedances headphones, I'm very happy with this: -follower hybrid headphones amplifier

It kind of fits your goals.
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Old 12th October 2014, 02:43 PM   #5
janim is offline janim  Slovenia
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It does actually. I have read about the mu-follower amp HERE, and it mentioned also low impedance outputs (well, actually with srpp design, but it's very similar). I never found a complete schematic that's been tested though. I'll check it out. Thanks!
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Old 12th October 2014, 05:05 PM   #6
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Quote:
Originally Posted by Stixx View Post
Have a good read around here --> Cavalli Audio

Even Mr. Cavalli hasn't succeeded in buidling a high voltage hybrid (the "Stacker"),
so abandon the plan or change your design to use low voltage tubes.
It seems that if you are willing to employ AC coupling and voltage clamps to protect the solid state devices, you could configure anything you want. You could even apply AC coupled global feedback for any popular configuration.

I know that a lot of you guys hate coupling caps, but properly applied they do not present a big challenge. Ad-hoc selection of low frequency poles can result in greatly inferior bass response; this much is true. But the trick is to make the input pole dominant, and separate the rest of the poles as best you can. The most important aspect is to have as large a gap as possible between the input pole and the feedback pole. If you can "bury" the feedback pole, then distortion from the feedback capacitor becomes a non issue.

I recently built a single ended, AC coupled headphone amp employing the TI 49600 buffer. Its dominant low frequency pole is 2.7 Hz. It has slow start (real slow; 2 second startup) and diode clamps on all the big audio caps to mitigate start up thump and eliminate shutoff noise and potential latch up at shutoff. It runs off of 12-30 volts DC. It is completely fool proof and convenient to use. From a practical point of view it leaves nothing on the table; it is super clear, provides more clean power than you should ever use, drives 32 ohm headphones super loud and clear with a 12 volt supply, and theoretically should provide ample power for high impedance headphones with a 30 volt supply. I don't see why a tube front end and a high voltage discrete solid state buffer couldn't be configured using these same principles, but then I have never tried either.
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Old 23rd November 2014, 05:30 PM   #7
janim is offline janim  Slovenia
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So I spent quite some time searching the net and came to the conclusion that will be much easier, safer and cheaper to get another transformer and add a low voltage output stage. The plan goes like this:
- a typical tube gain stage with 250V HT because I already have everything I need,
- a EM84 'magic eye' volume indicator (did I mention I'm scavenging an old radio receiver )
- a crossed circuit (just because I never had a chance to try it)
- then a pot for volume control
- and the 'low voltage' buffer
And now the questions
I haven't decided yet between an opamp buffer Cavalli style or a mosfet buffer Szekeres style, but I'm leaning toward the first option because of simplicity, dc coupling (price) and because Mr Cavalli used it in his hybrid amp design. What I don't like is that it is clipping in the spice simulation. But this could very much be my fault because this are my first simulations and also the mosfet version is clipping.
I also haven't decided yet on the crossfeeed circuit - Jan Maier, Ingvar Ohman, Linkwitz - but I can try them all out.
What do you think? Any comment will be most welcome.
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