Intrigued by good reports (Scott Wurcer and EUVL) on BF862 JFET I also got some. The idea was to build a preamp based on JBOZ and B1 concepts with gain of 2 (enough to drive F5) and ability to drive 5K next stage impedance. Also, 6-7 V of undistorted output signal is needed.
This is what I ended with and it's all I hoped for, it sounds great, better than 2sk170 in same topology - I suppose it's due to much higher transconductance and smaller parasitic capacity. The only "problem" is that BF862 is an SMD part but that's easily cured with small adapter PCB.
This is the preamp and PSU schematic:
This is what I ended with and it's all I hoped for, it sounds great, better than 2sk170 in same topology - I suppose it's due to much higher transconductance and smaller parasitic capacity. The only "problem" is that BF862 is an SMD part but that's easily cured with small adapter PCB.
This is the preamp and PSU schematic:
Attachments
scott wurcer said:
Thanks for the great tip on BF862 - you've been my hero since I tried AD797 for the first time. 🙂
Take my word. Solder the JFET directly on your PCB without adaptor. And come back and tell us the difference.
😉
Patrick
PS One way to match without adaptor :
http://www.diyaudio.com/forums/showthread.php?postid=1828332#post1828332
😉
Patrick
PS One way to match without adaptor :
http://www.diyaudio.com/forums/showthread.php?postid=1828332#post1828332
EUVL said:
Curiosity got better of me so I gave it a try, with and without grid stopper and there was no difference worth mentioning (compared to JFET on adapter PCB).
The only thing I noticed is a slight change of Id - due to a larger cooling surface I suppose...
What's your experience?
juma said:
The J310 reqires some serious selection for noise. I have heard that FET's from this series, J305 on, are used in very low noise applications by some. In general I'd keep a small gate stopper on the BF862 since it has a 700MHz FOM, someone here was successful with SMT ferrite beads to preserve ultra-low noise.
It's good to know that for MC/MM/mic preamps - we are talking line level (CD/DAC out) here so noise level is not a primary concern
juma said:
I dreamt last nite that a copy of those boards appeared under my pillow!!!?? Did you make them??😉
It must have made quite an impression on you 😀
I'm responsible just for design of this simple micro-PCB, but the heavy burden of actually producing them fell on my friend's and DIY fellow's shoulders - he is the one with means and the will to manufacture such a miniscule PCB 😉
I'm responsible just for design of this simple micro-PCB, but the heavy burden of actually producing them fell on my friend's and DIY fellow's shoulders - he is the one with means and the will to manufacture such a miniscule PCB 😉