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GB for DC coupled B1 buffer with shunt PSUs

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#2,863
Yomanze said:
Thanks Tea-Bag just paid.
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#2,864
Salas said:
Don't. It will work but we don't want such impedance [ndr.50K instead of 20K-25K]. It will hinder the quality you can attain. In any case if you must, you should make the 220k resistor a 520k one.
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i apologize for quoting this old post, and probably resurrecting an old and well explained argument, but i'm not able to find these infos in this huge thread, nor in Pass-B1 original sheet..
(I was only able to find an AndrewT consideration on keeping a low Zin for a better RF noise rejection, which could be picked up by High-Zin inputs)

Is there any specific technical consideration about this choice, and where i can find it?
Or was this choice adopted accordingly to the above shareable-but-general consideration, and/or listening sessions?

i know that 20K-25K MUST be theoretically well-driveable by any decent source,
but i often read about sources giving better sonic results when loaded with higher impedances (50K-100K) and these high values are almost standard in preamps/buffers or integrated amps..😕
excepts for SRPP's, i can presume a general consideration "the more the output current, the more the output THD" or am i wrong (ok, 0.0....01% of added THD is not a trouble 😛 )
thanks in advance 🙂
 
#2,865
To save you searching, it has to do with the high capacitances of K170. You can use any pot value but the higher you go, the more it has an impact on bandwidth. The recommended 20-25K max, has to do with quality considerations, its not an engineering limit. Lightenig the load a bit to an already capable source does not worth the loss in transparency when using high value pots for DCB1 in my experience.
 
#2,866
Salas said:
To save you searching, it has to do with the high capacitances of K170.
Click to expand...
Are you talking about Cgs and/or Ciss?
ciss1.jpg

Programmi di simulazione e jfet - ElectroYou
so, i presume, a consideration very close to sizing lowpass input filter, used for grounding RF input interference, and strength IMD performance..

Thanks for the comprehensive answer Salas 🙂
 
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