F2J Input impedance

[drjbf1]

I am building an F2J, and am not sure what the input impedance is. Firstwatt website has 25k in the table of amplifier models, but NP says 10k on the F2J page with the updated schematic. I was going to change R6 from 8.2k to 9.1k (because I have 'em) and was wondering what the impedance will end up being with the extra 1k resistance?

Thanks

 

[Zen Mod]

I'm aware that Pa sez this :

The F2's that I have updated with the JFETs need the
following changes:

R6 = 47k

C1 = 10 uF

Z1 removed

The potentiometer must be re-adjusted for minimum
distortion at 4.5 watts (symmetric clipping) and readjusted
after an hour.

The distortion results are typically a reduction from about 1%
at 1 watt to .25%.

http://www.diyaudio.com/forums/pass-labs/91168-diy-f2-clone-17.html#post2094956


is there later info ?

 

[drjbf1]

That was the R6 from the F2 clone thread, but I am working from this (from Firstwatt site):

 

[Zen Mod]

I slipped that one 😉

then it's as Pa sez


with 1K more you'll have roughly 1K greater input impedance

 

[drjbf1]

Thanks for that.

Actually I got it round the wrong way, the html F2J text page by NP says 25k, but the products page table of models says 10k. I am too dumb to work out the Zin from the schematic; ~1k added to 10k vs 25k is a large difference and I would like to try to calculate the size of my input cap.

Thanks again, and cheers

 

[Zen Mod]

it's more like R6 + what's left of P1 down + R5 ........ in parallel with input impedance of mosfet (+gate res) itself ............in parallel with 1M

 

[drjbf1]

Thanks ZM.

Ok, so it looks like 10k with R6 at 8.2k to me.... 1M high, and jfet high, so, normal volltage divider With R4 2.21k and R5 2.21k (plus 5k pot), in parallel, plus 8.2k. 😕

 

[Zen Mod]

Jfet high , but variable

that's why R's are low