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Old 18th February 2010, 12:53 AM   #11
DougL is offline DougL  United States
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Just for kicks, could one put a CCS in the R14 position? Using a simple one like the one used as a plate load for the 5842, assuming you had the parts laying about?
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In principle, the source follower is most linear with a very large source resistor (R14), or a current source in that position.
Yes, you could in theory substitute a current source.
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Last edited by DougL; 18th February 2010 at 12:54 AM. Reason: Clarity
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Old 18th February 2010, 02:59 AM   #12
jrenkin is offline jrenkin  United States
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"in theory" makes me nervous. Maybe i will build with the resistor, measure the current then try swapping in a CCS for fun and listening.
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Old 18th February 2010, 05:11 AM   #13
Sheldon is offline Sheldon  United States
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Originally Posted by jrenkin View Post
"in theory" makes me nervous. Maybe i will build with the resistor, measure the current then try swapping in a CCS for fun and listening.
Don't worry about the theory part. I don't think Doug was questioning the validity. It will work as long as your current source can handle the voltage. From a precision driven engineering perspective, it's the preferred approach. Will it sound better to you? Not necessarily. You'll have to try. No need to measure the current. You can estimate it from the expected voltage over R14 (grid voltage - bias supply voltage). For -40V on the grid, you'd see about 5mA. That's a good starting point.

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Old 18th February 2010, 09:34 PM   #14
DougL is offline DougL  United States
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I was questioning if the current source would be an improvement in that application.
Since the 20K resistor is much larger than the 2 ohm output Z, I would guess that the difference would be very small.
That is in stark contrast to the CSS plate load, which helps a great deal.
I would trust that George tried it, and he judged it was not worth the extra complexity.
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Old 19th February 2010, 12:06 AM   #15
Sheldon is offline Sheldon  United States
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Originally Posted by DougL View Post
I was questioning if the current source would be an improvement in that application.
Since the 20K resistor is much larger than the 2 ohm output Z, I would guess that the difference would be very small.
That is in stark contrast to the CSS plate load, which helps a great deal.
I would trust that George tried it, and he judged it was not worth the extra complexity.
I agree, and don't think it would be an audible improvement. But part of the fun of DIY is to try stuff as a learning experience.

There is one possible modest advantage of the current source in this application, and that is that the quiescent current through the FET remains the same, regardless of the output tube. With a resistor, the current can vary significantly between tube types. Does it matter? Unlikely, but for those obsessive types...

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