Choosing FET (preamp and other purpose)

Hi!

Last week I start playing with FET, I build a simple Tremolo for my guitar, with a 2n3819, which I bought a long time ago for another project.
It works quite well, and so I would like to learn more about fet design.
So I search on this forum, found a lot of interesting threads and links.
Now I would like to understand it more, and so I order some J201 from ebay to start plating with. I chosse those one because I had to buy some 1n4148 and the guy also sells j201.

I read borbelyaudio pdf, which is very easy to understand, and start to look at j201 datasheet and look for schematic with google.
I found this schematic :
http://www.till.com/articles/GuitarPreamp/index.html
But here there's something I don't understand :
the Rs resistor here is 2.2k, but when I read borbely pdf, for choosing Rs value, with the transfert characteristic of the j201, I get value between 100 and 200ohm... I don't understand why on the schematic above it's 2.2k?? is there something I miss here?

And my second question, maybe hard to understand : How to choose fet?

I will add a fet switching to my current tremolo, but it can be great if I can also add fet in audio signal (I use simple opamp buffer by now).

Well, I hope someone can help me
Thank you!
Pilo

[quote author="pilo"]¨
http://www.till.com/articles/GuitarPreamp/index.html
[/quote]
R2 is gain resistance. with ideal FET you will have
gain exactly R3/R2.
Also R2 do stabilization (optimal collector voltage for
good headroom). If you want gain higher, simply
put your new gain resistor (200 ohm) with series big capacitor
(100 microfarads) to paralel to R2.
You will have bigger gain without affecting stabilization.

xvlk

> I don't understand why on the schematic above it's 2.2k??

Large changes of source resistor tend to make small changes in bias point; the FET is a pretty sloppy device.

Borbely works for high gain and low noise.

The guitar amp does not need high gain or lowest noise.

Anyway: build it big with replacable parts and try different resistors. Probe the voltages, check the audio gain and maximum output, listen.

I like the J201. I used it in this simple guitar mini-amp design:
http://www.ionrecords.com/tapeop/fistofury.gif

JFETs can be a pain in the ass because of their wide spread of Idss. Here's a paper that suggests ways to deal with that through biasing methods.
http://www.vishay.com/docs/70595/70595.pdf

So is are JFET's just JFET's or are there particular one's you want to use for pre's, compressors, etc.? Or is noise/distortion the main thing you look for?

I've found that 3819s from different manufacturers require different values of source resistor.

In a pre-amp I made, I found that Siliconex 3819s needed a 470R resistor, while Fairchilds (Fairchildren? :grin: ) required more like the 2K2 mentioned above, so I use Siliconex now.

They sure do make great guitar pre-amps!

Steve

Actually, even if you buy a bunch of the same type number from the same mfr., the drain current can be all over the place. That's what sucks about JFETs.

The J201, 202, etc is really just a generic fet usually driven to saturation and used as a current source, with lets say, a 49.9 ohm resistor.
I have never tried them in audio, but give them a shot. They are fairly cheap.
cj