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juma 22nd June 2009 08:12 AM

BF862 Preamp
2 Attachment(s)
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:

juma 22nd June 2009 08:15 AM

2 Attachment(s)
The JFET on the adapter board:

juma 22nd June 2009 08:17 AM

2 Attachment(s)
Preamp - pic 1

juma 22nd June 2009 08:18 AM

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Preamp - pic 2

spavleski 22nd June 2009 06:11 PM

Great work, Juma :D

AndrewT 22nd June 2009 06:28 PM

I think C8, C9 & C18 and D6 to D9 are inverted.

scott wurcer 22nd June 2009 06:42 PM

Glad you're happy with the result.

juma 22nd June 2009 06:44 PM

2 Attachment(s)

Originally posted by AndrewT
I think C8, C9 & C18 and D6 to D9 are inverted.
Yes, you are right, that's what happens when copy&paste are used without proper checking - but that's when you come into play - thanks Andrew ;)
I paid more attention when drawing PCB although modern elkos don't pop that loud :D

Corrected schematic:

juma 22nd June 2009 07:01 PM


Originally posted by scott wurcer
Glad you're happy with the result.

Thanks for the great tip on BF862 - you've been my hero since I tried AD797 for the first time. :)

juma 22nd June 2009 07:47 PM

2 Attachment(s)
This is how adapter PCbs look prior to soldering an cutting (DIL8 OpAmp is in pic for size comparison) :

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