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PostPosted: Sat Feb 24, 2007 6:49 am 
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Quince wrote:
I know that, of course. My question is, why not have N-channel type in both stages.


Because it would require some pretty fancy footwork to retain the dc coupling between the two stages, not to mention to the load.


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PostPosted: Sat Feb 24, 2007 6:50 am 
Well, can always use common base BJT as level shifter.


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PostPosted: Sat Feb 24, 2007 6:59 am 
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Quince wrote:
Well, can always use common base BJT as level shifter.


Or 0A2's but why the Procrustean approach ? If it is a hybrid you want, then go for something intended to be a hybrid.


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PostPosted: Sat Feb 24, 2007 7:03 am 
What do you mean "intended to be"?


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 Post subject:
PostPosted: Sat Feb 24, 2007 7:07 am 
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Quince wrote:
What do you mean "intended to be"?


Something designed from the ground up to be a hybrid.


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PostPosted: Sat Feb 24, 2007 7:16 am 
And how do you design for a hybrid?


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 Post subject:
PostPosted: Sat Feb 24, 2007 7:42 am 
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Quince wrote:
And how do you design for a hybrid?


I imagine you decide how much power you need and fashion an output stage to suit. Then depending on the valves and the associated topology you choose or whether you prefer BJT outputs to mosfets you may opt to add driver transistors. As they say all roads lead to Rome. Just pick one. Come to think of it the are a few desighs floating out there already.


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PostPosted: Sat Feb 24, 2007 9:59 am 
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keeping caps in circ is certainly simpler;
due to x-topology caps are somewhat less influent than usual

you can start with this

just re-arrange feedback and solve mosfet biasing :snakeman: :finga: :Hangman:


You do not have the required permissions to view the files attached to this post.


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PostPosted: Sat Feb 24, 2007 10:19 am 
What about loading input stage plates with CCS instead?


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PostPosted: Sat Feb 24, 2007 10:38 am 
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Location: Viborg
Quince wrote:
Magura, it's not. Check out the curves Pass found on the 9240:
Fig.13 in http://www.passdiy.com/pdf/mos.pdf
Bob Cordell confirmed this with his own measurements. Seems other IRF P-FETs have the same problem.
Quote:
The transconductance dips down as you approach the midrange. Then it shelves off flat for a bit, and proceeds on its ordinary decline at high frequencies.


Yes, but that's just the IR parts, Fairchild dosn't suffer the same issues, or at least not as bad.

Magura :-)


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PostPosted: Sat Feb 24, 2007 11:03 am 
I was thinking of something like this sketch; if voltage gain is enough, could even have source followers on the output instead of the Aleph-X's common source. If it looks totally stupid, you'll have to excuse me as it's late and I'm not thinking straight... not sure if the tube will deal with enough current for reasonable output MOSFETs gate capacitance, since the common-base has no current gain. This configuration I got the idea about from the electrostatic headphone amp I've discussed elsewhere.

Image


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 Post subject:
PostPosted: Sat Feb 24, 2007 2:41 pm 
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on first look-good enough
but-even if I'm toob man,I really can't see any benefit from that,comparing it to (say) Babelfish X ..........or real XA
make toob susy preamp,and I assume that approach is best of both world;
minimalistic enough ......and clean


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PostPosted: Sun Feb 25, 2007 12:19 am 
I didn't know I needed a preamp. The volume control in my system is on the DAC output, before the buffer.

What's Babelfish X?

BTW sorry for the terrible drawing--some guy stole my nice Wacom tablet so I'm stuck drawing with a mouse.


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 Post subject:
PostPosted: Sun Feb 25, 2007 12:41 am 
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Quince wrote:
I was thinking of something like this sketch;


And the rationale behind this topology is ?


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 Post subject:
PostPosted: Sun Feb 25, 2007 12:45 am 
DC coupling. I'd rather insert one more transistor than a capacitor in the signal path.


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