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In Reply to: RE: The money is already spend :) posted by Al Noakes on October 16, 2014 at 11:18:14
Hi Al,Thanks for your opinion Al :)
So, to be sure... diodes will not raise the Zout of the stage like an Rk does?
I use a VCCS... Lotsa sand, but no idea about what is and what's not in the input current loop.
Another problem I see with diodes.... It looks difficult to get the exact amount of bias you need. Especially with high mu tubes.
NC
Edits: 10/16/14Follow Ups:
You can pad down the diodes with small value resistors to get your required bias point .
Why worry about Rk ? Are you driving something that needs a low impedence such as an LCR filter ?
Al
But thats only in the first stage. The second stage drives an IT, which drives an TVC.
NC
I'd say : try it ! Should be easy to revert back if you don't like what it does . I'd suggest on the input stage first
Al
Did you see my RIAA stage?
Its quite a bit of work, but I will try it once... Thanks for the tip!
(still curious about the theory of the diodes influencing Zout though)
NC
It takes an appreciable amount of resistance in the cathode to affect OP impedance significantly.
The equiv circuit of diode is a voltage source (by definition zero impedance) with a series resistor, which is typically only a few ohms at the milliamps of current typical of a gain stage.
Seriously, try Schottky, they sound great and are dead quiet. SiC will give you around 0.8V and standard Schottky runs about 0.2V in these applications so a combination will give you flexibility up to a few volts.
Naz
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