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Old 9th January 2011, 06:57 PM   #1
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Default JFET guitar Pre-amp

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

Click the image to open in full size.

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.
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Old 9th January 2011, 07:23 PM   #2
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Hi Jammy,

this is a sourcecircuit and the 2SK170 should also work, (but with different values for the components)
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Old 9th January 2011, 09:26 PM   #3
mjf is offline mjf  Austria
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hello.
if you use the search button,you can find threads like this..........

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

greetings

Last edited by mjf; 9th January 2011 at 09:29 PM.
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Old 9th January 2011, 10:12 PM   #4
tinitus is offline tinitus  Europe
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ah, I remember now that Tillman wrote he chose the J201 because of its lower voltage demands
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Old 9th January 2011, 10:25 PM   #5
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Cheers guys "Common source" great that's what i was after.

from Wikipedia; Voltage gain = Click the image to open in full size.

What's "gm"?
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Old 10th January 2011, 12:37 AM   #6
tinitus is offline tinitus  Europe
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related to transconductance
but thats all I know
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Old 10th January 2011, 07:31 AM   #7
mjf is offline mjf  Austria
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there are 8 pages - and more -

FET Preamplifier, Part 1

greets
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Old 10th January 2011, 09:06 AM   #8
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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
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Old 10th January 2011, 10:55 AM   #9
Arnulf is offline Arnulf  Europe
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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).
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Old 10th January 2011, 04:35 PM   #10
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Quote:
Originally Posted by Arnulf View Post
Pinch-off voltage.
So it would be the same as VGS (off) on the data sheet.

Quote:
Originally Posted by Arnulf View Post
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
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