Which FETs for JC-2 preamplifer?

[citationfan]

I want to try two chinese mono pcbs for building a JC-2 clone. This pcbs where very cheap

and seems of quite well quality.

The FETs has to be K170 / J74.
What IDSS I need? BL, GR or V?
Can anyone help?

 

[lineup]

Best is 2SK170/SJ74 V.
But they are very hard to find.
Then comes BL. They are also very good.

I would not buy GR.
Not if I had another option.

 

[BesPav]

What IDSS I need? BL, GR or V?

The core of the scheme - input differential stage, which named “four-quadrant self-biased”, works at ~0,8-1mA.
So you can pick any grade tight-matched quad.

 

[kannan_s]

Read JC 2 threads here - couple of them explain in detail.

 

[Mark Johnson]

John Curl's ORIGINAL JC2 schematic, which he published in The Audio Amateur, used JFETs part number E175 and E212. Today you can buy the 175 in thru hole Mouser link #1 but you can only buy the 212 in surface mount Mouser link #2

 

[john curl]

This is a modified JC-2 circuit. The input devices could be 2sk170, 2sj74 Yellow as the input stage runs at such low current, but matching is going to be tough for an amateur. An easier choice is a matched set of American jfets like the j113, j174 complements, but they will be tough to get also. Darn, I don't know what to recommend.

 

[scott wurcer]

I would assume the LSK parts at the DIY store would work.

 

[john curl]

I meant to say, 2sk170, 2sj74 Green, not yellow.

 

[PH104]

MMBF175 as J113 complement perhaps? It's a SOT-23 part however.

 

[EUVL]

You can (still) get 2SK170GR / 2SJ74GR matched pairs, genuine, if you are willing to pay.
http://www.diyaudio.com/forums/pass-labs/317563-replacement-toshiba-2sk170-2sj74-11.html#post5429153

40 USD for 2 channels.
Just ask him even if he does not have them listed currently.
He has stock.


Cheers,
Patrick

 

[citationfan]

Thanks very much for all the replies! Very helpfull!!!

Yes I will go with two quads from Israel... or from our DIYstore.
@JohnCurl:
What is the meaning of green, yellow? Green=GR? Yellow=BL?
How is it possible to get a higher current from these boards?

 

[citationfan]

I have laying back since long a quad Toshiba 170/74 BL from a Harman Kardon HK6800 from 1992.
Still worth to try these?

I have no idea how to measure to check if they matched correctly.

 

[adason]

Figure 9
Biasing JFETS

 

[BesPav]

How is it possible to get a higher current from these boards?

You need to provide Vbe of the second stage drop on the R2 and R4.
Changing them together with VR1 setting will allow you to tune idle current of the input stage. But keep in ming higher dissipation, noise and bias.

Also you can alter second stage to a common base like this:

Doing this would free second stage from Cob influence from relatively high VAS output voltage.