My first Hamptone JFP (JFet Preamp).

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skipwave

Well-known member
Joined
Jul 8, 2005
Messages
1,416
Location
Los Angeles
And only my second preamp built from scratch. Finally got round to finishing this. Built as detailed in the TapeOp article. 2 JFP stages on a tiny pad-per-hole perfboard in a little box. Everything was recycled or from the piles of stuff I grabbed at my local RatShack store that was closing. Overall, super cheap and cheerful.

jfp_top_sm.jpg

Input transformer is a UTC O-16, the switch to the left is pad, the knob to the right is Gain. The jack on the left, with the beige cable is the power input. Box is recycled, I traded a burrito for it, and it already had a nice 5k pot in it.
jfp_inside_sm.jpg

Big white caps are 10uF mylar. Output is unbalanced, I tried to fit an Edcor WSM600:600 in there, but no space.
jfp_ps_sm.jpg

Power Supply. Now I just need to add a seperate little regulator board in there for phantom. RatShack project box cost a dollar, the tranny was six dollars. Could I be any cheaper? :green:

Must thank everybody who discussed this project here. Especially, Matta, ChrisSugar, buttachunk, Bauman, and (of course) PRR.

FETBoy:
http://groupdiy.com/index.php?topic=16907

JFP:
http://groupdiy.com/index.php?topic=1504
http://groupdiy.com/index.php?topic=340
http://www.groupdiy.com/index.php?topic=1184

Liniac:
http://groupdiy.com/index.php?topic=8905
 
Holy sh*t, this thing has gain to spare! It's really awe inspiring. Very quiet and high output. :shock:

EDIT: Problem solved with a 22k carbon comp resistor in front of the second Gate.

Oh, and I still have to squeeze in the DI jack and add a little phantom regulator in the PS box. This was a fun project, and should be a very useful tool. :grin:
 
If I understood corectly, u have this setup:
from 24v Vcc to one end of trimmer, other end of trimmer to 47k, other
end of 47 k to base of ztx653. If Ve of ztx653 reads 0v that means no
current trough 100 ohm resistor in emiter, thus ztx is not conducting.
Since u have voltage drop across trimmer, it means u have path to
the ground so that should imply that u have something shorted to
that ground in ztx base biasing network. My guess, junction of 47k
resistor and base of ztx.
If ztx is not conducting, u have no DC path to gtound for darlington.
Darlington is in cutoff with very close to 0v across base-emmiter junctions.
If you have 23v at darlington emmiter that means that you have 23v
at drain of input jfet. Under normal conditions darlington (and anything
otput related) cant cause this. Thus u have badly off biased jfet, which
is related to source circuit, so you should check that too.

my two cents
ypow
 

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