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WTB - JFETs for F5 or F6

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jackinnj

jackinnj

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Joined 2002
tech-diy.com
Metropolis said:
Hi All - I want to build either an F5 or F6 (prefer to do an F6) but the store is out of JFETs - anyone got a matched set they are willing to sell?
Click to expand...

For one thing, they don't have to be matched...you'll get very good results with un-matched.

You can pair up 2N5457 and 2N5460 to approach the transconductance of a single K170/J74 pair -- mind you that the pinout is different. Their transconductance is around 5,000mS vs 20,000mS or more for the K170/J74, noise figure is two orders of magnitude worse but input capacitance much better.
 
jackinnj

jackinnj

Member
Joined 2002
tech-diy.com
Metropolis said:
Ok, so it sounds like you are saying I can buy the unmatched packages of K170 and J74 and install randomly pick one of each and it won’t cause any problems? I honestly have no idea
Click to expand...

It certainly won't blow up! In later versions of the F5 a 200 ohm trim-pot was used in an attempt to balance the gain of the P and N-channel devices.

The Linear Products variety are available from Trendsetter Electronics.

If you get a package of random devices from a third party, you can match for Idss quite simply -- short the gate and source to ground and measure the current which flows through the device using a 9V battery. This is Idss and matching Idss will start to get you close to ideal matching.

You would need to place in parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74, but this isn't an insurmountable task.
 

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kinku

Member
Joined 2013
That was the most specific and useful info I seen in such a long time from this forum.
Two questions remains how will the noise figures add up if you parralel them?
Jack , thank you!
How many of those linear system devices you need to have a matched pair ( typically)
 
EUVL

EUVL

Member
Joined 2003
xen-audio.com
> parallel about 4 each of 2N5457/5460 to get close to the gain of a single 2SJ74

2N5457/5460 have a transconductance of about 3mS each and Idss of ~ 3mA.
4x in parallel gives 12mS at 12mA.
4x 2N5460 in parallel has an eq input noise of 30nV/sqrtHz

2SK170 has a Yfs of 45mS at 12mA, 2SJ74 close to 50mS.
Noise is about 1nV/sqrtHz for both.

I guess not quite close ??


Patrick
 
cwtim01

cwtim01

Member
Joined 2012
I have a set (2 pairs) of LSK170/LSJ74 that I purchased from the DIYA store before. Never been used, apart from measuring them on a breadboard for noise and curve. Happy to sell them to you at the same price of the store (plus shipping and PayPal fees).

PM me if interested.

I’m selling them as I’ve now got a stash of Toshibas, and I prefer them to the LSK/LSJ.
 
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