Using 2SK170/J74BL in Ebay JC-2 "Clone"

[ammel68]

I purchased one of the JC-2 "clone" kits off Ebay. It came with matched 2SK246/J103's for the input FET's.
Attached below is the schematic of the circuit.

I also have some matched 2SK170/J74BL devices that I would like to try in place of the K246/J103 combo.

Will I need to change the value(s) of any resistors in order to use them?

The instructions state to adjust the 1K ohm trimmers until you get a voltage drop of 2.5-3V across the 1.5K ohm resistors.
Would this also apply to using the K170/J74BL combination?
Two milliamps(3V across 1.5K ohm) doesn't sound like much current to me.😕

Thank you...

 

[ticknpop]

2ma was ued by Borbely in the jfet differential input of his preamp, Dac, and power amp. It's fine

 

[ammel68]

Thanks ticknpop.

That answers one of my questions, but I still need to know if any resistors need to be changed if I use the K170/J74's.

 

[ammel68]

165 reviews and no one knows if I can use K170/J74BL devices in the above circuit??

 

[mlloyd1]

one reason for silence might be there is at least one other thread where this circuit and your proposed change was commented on by the original designer.
borbely wrote a couple of articles on designing with jfets that would be very useful info to you also.

regards,
mlloyd1

 

[ammel68]

I read over an older thread here where someone was trying to duplicate the original Levinson JC-2 modules.

It's my understanding that the Ebay "clone" is not the same circuit as the module circuit below. They don't look the same to me.

 

[john curl]

The two schematics are VERY similar, but values rescaled for different input devices.

 

[ammel68]

I just came across this variation that someone posted here back in 2008.
Drain and source resistor values are quite a bit lower than the version I first posted.

 

[bear]

Best to use the Linear Systems devices sold in the DiyAudio store.
Alleged Toshibas are usually forgeries and do not meet specs.

fyi.

_-_-bear

ps. hi John...

 

[ammel68]

Thanks Bear for the advice.
The K170/J74BL's I have are indeed genuine and were verified by another member who tested them with curve a tracer.
Some I purchased from fetaudio.com.

 

[dodo]

Ammel68
Try to locate PDF with EB-604\410 Borbely starter kits part 1
Read it and you will fine the answer to your question

 

[flg]

I believe the filename is 305borbely.pdf 😀
I can send it to you if you PM me your e-mail 😉

 

[ammel68]

Ammel68
Try to locate PDF with EB-604\410 Borbely starter kits part 1
Read it and you will fine the answer to your question

Is this what you're referring to?
http://www.pearl-hifi.com/06_Lit_Archive/14_Books_Tech_Papers/Borbely_Erno/Borbely_on_Line_Amps^.pdf

 

[ammel68]

First off, Borbely is using 24V supplies where I'm using 15V supplies. Not sure how much that will change things.

I see where he states running his first stage at 2mA just as ticknpop stated in post 2.
So I'm assuming 2mA is going to work using K170/J74's, though I read on another JC-2 thread where John Curl stated that running them at 6mA will sound better.

None of the circuits from the Borbely PDF below look identical to the circuit I posted initially, though the first half of figure 5 looks very similar.

 

[john curl]

The actual input stage current is not very important. What is important is the Idss of the input jfets. IF they are 10ma or more, then 6ma or so is better. If they are 5ma (Idss) or less, then 2 ma is better. This design is over 40 years old and was first used at +/-24V as a line driver for the Greatful Dead 'Wall of sound' starting in 1973. Later, in 1974, a 15V version was made for the Levinson JC-2 line stage.

 

[ammel68]

Thank you for clarifying that point, Mr. Curl.

Most of the matched K170/J74BL JFET's I have are around 7-7.5mA Idss.
From what I understand, trying to find K170/J74BL in 10mA or higher Idss is literally like "trying to find a needle in a haystack".🙂

 

[john curl]

You could move the Id to 4ma without any problem. Just halve the value of the input stage load resistor.

 

[ammel68]

So, is this correct?

I made the changes in red.

Do the 100 ohm resistors need to be changed to 50 ohms as well?

 

[john curl]

Not necessarily, but the bias pot will have to be reduced to 200 ohms or less.

 

[ammel68]

I'll try the changes you suggested, Mr. Curl.

Thank you for the help!🙂