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Old 5th January 2010, 12:22 PM   #1
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Default JFET question - Idss values

JFET question - Idss values

What do they actually effect?

I understand that if I'm building a stereo amp of some sort I match the Idss values of the opposite parts, but what would be the effect of using a high Idss instead of a low Idss?

To put it int context I am going to build the following circuit using 2SK170 JFETS
They are availible in three varients
GR: 2.6~6.5 mA,
BL: 6.0~12 mA,
V: 10~20 mA
The schmatic dosent specify which.

Click the image to open in full size.

I am intending on getting the BL because their in the middle but I thought I'd ask first if there is any time when you would use one over another or are they just grouped like this for easier matching.
And advice apriciated.
J
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Old 5th January 2010, 01:49 PM   #2
AndrewT is offline AndrewT  Scotland
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have a look at the RIAA pre by Salas.
thousands of posts in there to guide you.

Simplistic NJFET RIAA
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regards Andrew T.

Last edited by AndrewT; 5th January 2010 at 01:51 PM.
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Old 5th January 2010, 01:56 PM   #3
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Quote:
Originally Posted by JammyBStard View Post
The schematic dosent specify which.
Seen the 2K4 drain resistor above the JFET ?
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Old 5th January 2010, 02:36 PM   #4
SQLGuy is offline SQLGuy  United States
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Quote:
Originally Posted by jacco vermeulen View Post
Seen the 2K4 drain resistor above the JFET ?
Sounds like you're saying GR, since 5mA would leave the circuit most centered wrt the 24V rail?
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Old 5th January 2010, 02:59 PM   #5
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Quote:
Originally Posted by jacco vermeulen View Post
Seen the 2K4 drain resistor above the JFET ?
I notice MAD-K's Paciffic specifies Idss = 4mA to 6mA matched!

I'll take a guess working back from that:
From the data sheet VDS = 10V
Idds = V-VDS/R = 24-10/2400 = 0.005A = 5mA?
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Old 5th January 2010, 04:51 PM   #6
h_a is offline h_a  Europe
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So that is the le Pacific that I see mentioned now and then.

Honestly I don't like it much; as you already realized it is pretty picky about IDSS. Being a phono stage you would love to use the high IDSS parts as these have the nicely high transconductance.

The missing source resistors already show that the designer badly wants gain and does not care so much about distortion.

But that does not work out so well, as the low rails do not leave much drain-source voltage for the jfet if it is from the V-class.

That leaves only average IDSS BL-parts or use higher rails, at least 0.5W resistors and higher IDSS.*

Have fun, Hannes

PS: be sure to use a good power supply, PSRR is poor...

*EDIT: or high rails and cascode the jfets, that would also increase bandwidth. Maybe this is an educational circuit? Many changes feasible.

Last edited by h_a; 5th January 2010 at 04:56 PM.
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Old 5th January 2010, 06:45 PM   #7
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I have already made one with bog standard parts and I think it sounds excellent. I want to build one with higher quality parts and then look at modding it; I was thinking of using batteries for a super quite power supply. eight 9v aranged as 36v and theb regulated down, they should last for ages! and the rail voltage can be turned up very easily for mods.

Is my assumption above about the Idss correct?
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Old 5th January 2010, 06:59 PM   #8
juma is offline juma  Germany
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Quote:
Originally Posted by h_a View Post
....
The missing source resistors already show that the designer badly wants ....
Less noise
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Old 6th January 2010, 08:38 AM   #9
h_a is offline h_a  Europe
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With 100k at the gate of the first stage
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Old 6th January 2010, 10:36 AM   #10
EUVL is offline EUVL  Europe
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Go and have a look at the Pass Lab Pearl Phono (Version 1).
It is essentially the same circuit, a bit more refined, and with lots of detailed information you might need.

http://www.passdiy.com/pdf/pearlphono.pdf


Patrick

Last edited by EUVL; 6th January 2010 at 10:40 AM.
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