JFET guitar Pre-amp

[JammyBStard]

I've been looking for simple guitar pre-amp circuit build; Don Tillman's JFET circuit seems to be raved about: Don Tillman's discrete FET guitar preamp - Telecaster Guitar Forum

It uses a J201 N-Channel FET which I'm sure I could get hold of, but I have a load of 2SK170 JFETS that need a home and I could do with learning a bit about circuit design. So I was wondering if I might be able to use one in this circuit? Is there a name for this type of circuit? I'd like to find some info on it and possibly modify for the 2SK170.

Any help getting started would be apriciated.

[moschfet]

Hi Jammy,

this is a sourcecircuit and the 2SK170 should also work, (but with different values for the components)

[mjf]

hello.
if you use the search button,you can find threads like this..........

http://www.diyaudio.com/forums/music...g-problem.html

greetings

[tinitus]

ah, I remember now that Tillman wrote he chose the J201 because of its lower voltage demands

[JammyBStard]

Cheers guys "Common source" great that's what i was after.

from Wikipedia; Voltage gain =

What's "gm"?

[tinitus]

related to transconductance
but thats all I know

[mjf]

there are 8 pages - and more -

FET Preamplifier, Part 1

greets

[JammyBStard]

Excellent; thanks, that's ideal.

Just as you were posting that i found in a textbook:
There is a special parameter of FETs called transconductance factor (gm) and it is the slope of the transfer characteristics at the point of operation; it is represented in siemens [S] which is equivalent to inverse R.
gm = 2xIDss/VP or {1-(VGS/VP)}

What would VP be?

Thanks again.

J

[Arnulf]

Pinch-off voltage. You really should read some introductionary text on the subject of JFETs, Google is your friend.

There is no point in calculating gain for your schematic though because it will vary bewteen transistors of same type with such a low supply voltage and arrangement where Vds is even lower due to self-biasing. Gain will be low either way (less than 10x, probably around 3x).

[JammyBStard]

Originally Posted by Arnulf

Pinch-off voltage.

So it would be the same as VGS (off) on the data sheet.

Originally Posted by Arnulf

There is no point in calculating gain for your schematic though because it will vary bewteen transistors of same type with such a low supply voltage and arrangement where Vds is even lower due to self-biasing. Gain will be low either way (less than 10x, probably around 3x).

If you take the middleish values for Idss and Vp of the J201 it does indeed have a gain of 3.
I think a guitar has an output of up-to about 2v, so for a clean signal you wouldn't really want a gain of more than 3 on a nine volt battery. I guess that's why on Tillmans site he warns it might not work with all J201's