Pedja Rogic's J-fet buffer for dummies.

[Nordic]

After my bad luck with the Les Sarge circuit, I decided to give Pedja Rogic's J-fet buffer a try....

Well it worked straight away, and is sounding very mature... It seems to hang with the music relentlessly, and there is none of the hash in transients, I could hear with my other preamp attempts, except for maybe the ECC88 buffer this is replaceing.



So, I thought I will add it here for the future, and other noobs struggleing to get a good preamp going.







http://www.pedjarogic.com/gc/index.html

Thanks PEDJA, and NUUK, for pointing me in this direction with your site and layout...mine is a little diffirent, trying to reduce wire jumpers, putting power connection on output side, and accommodating diffirent size caps I had... you can see that it is easy to adapt the outut filter caps to whatever size you have...

[mlloyd1]

for folks looking into understanding the jfet parts of this kind of buffer design, this paper is pretty good reading:
http://www.analogzone.com/col_01302004.pdf
or see article and more info about the author here:
http://www.analogzone.com/col_012004d.htm

mlloyd1

[Steve Eddy]

Why not use a dual matched JFET like the LSK/2SK-389? Also, a source resistor on the top JFET would let you null out the DC offset and you could eliminate the coupling capacitor.

For the cascode MOSFET, how 'bout something like the Supertex DN-2540, which is a depletion mode device which would make it self-biasing which would eliminate the need for the voltage divider?

For that matter, you could also use one on the current source and end up with no more parts than you're using now.

se

[Nordic]

Sorry Steve you are getting ahead of me there... offset is minimal about 6mv... allthough there is a 2V DC spike at power on.

From what I understood, the lowpass was a neccesary element to get the circuit to work well with a chipamp which is internaly frequency limited...


What I would like to know is, if one should add decoupling caps to the power rails?

Will try to understand what you said later when I'm awake...

[Nordic]

How do I know when I have a groundloop?

I suspect so, as the circuit is quiet when using earphones but have a slight hum with my My_ref C.

Could this be because of the lack of bypass caps...? once again, didn't see any in the schematic.

WHat would be the easiest tests for groundloop problem?

Input and output sockets are isolated from chassis.. have exactly the same earth decoupling network in the preamp as in the main amp... both connected to mains ground... no problem with my valve buffer, using same arrangement.

[Nordic]

Bump!

You guys are so fast to look smarter than each other, so how about a little help then?

[Steve Eddy]

Originally posted by Nordic
You guys are so fast to look smarter than each other, so how about a little help then?

Oops. Sorry. Got sidetracked. I'm just calling it a night but I'll make a note to reply in the morning.

se

[Nordic]

Baaah, it is the morning over here... next time I'll give you guys helpfullness pills before I go to bed, so you can solve my problems while I sleep...

But, in the meantime I'l be waiting open mouthed in the heat.

[Steve Eddy]

Originally posted by Nordic
Baaah, it is the morning over here...

Not my fault. Move to a better time zone.

Anyway, don't think your hum is for lack of bypass caps. They're not really doing anything for you at power line frequencies anyway.

What happens if you remove the buffer's ground from the mains safety ground?

se

[Nordic]

I was waiting for someone to give me the go-ahead... gotta wait for the wife to finish watching TV now...

Thanks for comming back ...