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2N3823 as a Source Follower for Buffer?
2N3823 as a Source Follower for Buffer?
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Old 6th January 2019, 07:31 PM   #1
volleynbike is offline volleynbike  United States
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Join Date: Mar 2006
Location: So Calif
Default 2N3823 as a Source Follower for Buffer?

I'm hoping someone out there who's has a lot more experience than i (hardly any) with JFETs could answer what i hope is a simple question.

Years ago i built a Boozhound JFET Phono Preamp that is working well:
https://cdn.shopify.com/s/files/1/02...Preamp.pdf?108
This one has no buffer on its output, but i run some long cables to my amp with it anyway, and it does fine. Also, where i used to run it from batteries, recently i put in a Lamda PS that i had in the bottom of my parts stash. In many ways the Lamda sounds better than the batteries, except for noise.

So i had the idea of adding a JFET buffer to the output of the Boozhound. Boozhound uses a simple JFET source follower with a CCS on their latest Phono Preamp: https://cdn.shopify.com/s/files/1/02...58660262325290
but i thought of using John Broskie's Aikido-esk JFET Follower as show on the output of this MC preamp because of it's PS noise cancellation circuit:
Click the image to open in full size.

Like Boozhound, Broskie uses 2SK170's, long considered excellent in phono preamps. But alas, now unobtainium. As i rummaged through my parts bin, i found a stash of 2N3823's left over from the 1980's. I know they're not the same class -- the 2N3823 have about 3-4x the noise and, if i understand the data sheets correctly, a bit higher admittance. But i have so many of them.

My question is: In this usage, as a unity gain buffer, would it work Ok? Are there any gotchas? Would i notice much difference compared to the 2SK170? I'm also assuming that in this circuit i could pretty much just drop it in with the same surrounding resistor values (100 ohms all around)? Any comments? Thanks =]

Last edited by volleynbike; 6th January 2019 at 09:10 PM.
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Old 15th January 2019, 02:02 PM   #2
gvidmar is offline gvidmar
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Join Date: Dec 2014
>>>> In this usage, as a unity gain buffer, would it work Ok?
Electrically - YES. Sonically no, they could sound better, they could sound worse. You have to try and listen.
>>>> Are there any gotchas
They always are, but in my opinion you have pretty good chances to do it well.
>>>> Would i notice much difference compared to the 2SK170?
It depends on the rest of your set-up. If your audio components, I mean amplifier, loudspeakers,
headphones, ... are sensitive enough, you always notice the differences after
small changes or modifications.
>>>> surrounding resistor values (100 ohms all around)
100 ohm is a good starting point. Try to adjust the resistance for Id of 5-8mA .
>>>> 2SK170's ... now unobtainium
They are obtainable here:
Toshiba 2SK170 LAB MATCHED OCTET to 0.03mA / 4mV (8 matched FETs -- 7mA range) | eBay

There are also substitutes benig produced by Linear Systems
DMOS Switches, JFET Amplifiers, BIFET Amplifiers, MOSFETS

LSK170 used to be sold by DIY Audio Store here:
Linear Systems LSK170 Ultra Low Noise Single N-Channel JFET (8 Pieces) – diyAudio Store
but unfortunatelly they are currently sold out.
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Old 24th January 2019, 06:19 AM   #3
volleynbike is offline volleynbike  United States
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Join Date: Mar 2006
Location: So Calif
Thank you gvidmar. I'll breadboard it and see what happens...
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Old 24th January 2019, 10:43 PM   #4
gvidmar is offline gvidmar
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Join Date: Dec 2014
Please let us know about your findings.
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