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Old 15th June 2013, 09:49 AM   #1
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Question Single-Stage Hybrid Cascode Power Amp

Found an interesting design over at Softone - it's a single-stage cascode amp, since the MOSFET is providing most of the gain, I am curious how it would sound? (I can't breadboard it now, as all my tools are in storage...) Comments and feedback are appreciated.

Original article in Japanese.
It's basically a single-ended version of this design.


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Old 15th June 2013, 10:19 AM   #2
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Output tube is grounded grid connected and driven by a MOSFET.
Output impedance is high, but reduced a bit due to GNFB.
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Old 15th June 2013, 04:10 PM   #3
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This trumps the main weakness of any mosfet: variable capacitance squirming back
and forth, from drain to source, as the bias changes. The cathode pins the drain to
a fixed bias, and stops multiplication of drain capacitance by Miller.
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Old 15th June 2013, 04:39 PM   #4
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Quote:
Originally Posted by jazbo8 View Post
Found an interesting design over at Softone - it's a single-stage cascode amp, since the MOSFET is providing most of the gain, I am curious how it would sound? (I can't breadboard it now, as all my tools are in storage...) Comments and feedback are appreciated.

Original article in Japanese.
It's basically a single-ended version of this design.


Jaz
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thanks for link, got that on my mind how to drive hi mu tubes..
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Old 15th June 2013, 04:46 PM   #5
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Quote:
Originally Posted by kenpeter View Post
This trumps the main weakness of any mosfet: variable capacitance squirming back
and forth, from drain to source, as the bias changes. The cathode pins the drain to
a fixed bias, and stops multiplication of drain capacitance by Miller.
seems this schema makes tubes as serial constant U reg..by resistor divider
mosfets "just" changing current

low damping factor, what basically is preferred with hieff boxes
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Old 15th June 2013, 11:32 PM   #6
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Thanks for the feedback, gents. I will breadboard it once I get my bench set up... the differential push-pull version is interesting as well. I also have a question on the gain calculation, in the write-up, the gain for the cascode stage is just gm x (ra//RL), where gm is the transconductance of the mosfet, ra the plate resistance of the tube and RL the primary impedance of the OPT, and yet he mention the ra should be > 50K even though none of the tube types used has ra anywhere near it, so why make a fuss about it?
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Old 16th June 2013, 05:53 AM   #7
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voltage seems changes up to few 10s, he could implement voltage fb circuit like in tube voltage regulators to have steady drain U.
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Old 16th June 2013, 07:18 AM   #8
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not optimal i know
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Old 16th June 2013, 06:24 PM   #9
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Power tubes in cascode for output section?
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Old 16th June 2013, 08:21 PM   #10
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Quote:
Originally Posted by Sergey A View Post
So how does it sound? Is it more of FET or triode characteristic?

Jaz
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