Help wanted: Jfet front-end for Aleph 2 - diyAudio
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Old 4th March 2014, 01:51 AM   #1
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Default Help wanted: Jfet front-end for Aleph 2

I had successfully build an Aleph 2 and now I want to go to a next step ahead changing the front-end to Jfet (2SJ74/LSJ74) building a small daughterboard that connect to IRF9610 diff pair mounting holes. My knowledge of electronics is very limited, I try to find something similar but only found Aleph-J or Aleph-X references that have a considerable lower rail voltage, so Iím asking for help.

The first thumbnail image represents the actual IRF9160 diff pair (simplified) on the Aleph 2 circuit.

There are two published approach that I found that are near to the requirements, but they are for a lower rail voltage, the first is from Eric's Aleph-X construction notes, the complete article section can be found on Aleph-X 100w Amplifier Construction Notes, and a sample diagram of JFet replacement at http://www.facstaff.bucknell.edu/esa...JFET-input.jpg. The second one is from Graeme published on diyaudio thread AX100 100W Aleph-X Monoblocks.

I have read many posts on diyaudio and other places about people asking for Jfet front-end for Alephs, but I didnít find a finished schema, so I made my own, please see second thumbnail image.

First all, J1 and J2 represent the actual IRF9610 (Q1 & Q2) on the board and J3 will be a direct connection to the star-ground point. The drain pin of Q2 on Aleph 2 schema is directly connected to negative rail, so I can take from here the -45. I choose to use the Toshiba 2SJ74BL because the 2SJ109BL are really impossible to get from a trusted source, and also the 2SJ74BL could be find matched quads. R61 to R64 are the Jfet source resistors that Graeme used is his schema. R65 represent a drain resistor to negative rail for Q2 like Graeme used on the AX100J and also Nelson Pass recommend on his following post j.fet front aleph 5.

Questions:

- How the schema looks like, Iím crazy or itís possible to do that?
- The values of R61 to R64 (15R) are ok for Aleph 2?
- The 2SJ74 Q61-Q62 and Q63-Q64 will require some type of heatsink, also they should be thermally coupled?

Any comment will be appreciated, thanks in advance.
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File Type: jpg actual-diff-pair.jpg (30.8 KB, 501 views)
File Type: jpg circuit.jpg (60.8 KB, 496 views)
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Old 4th March 2014, 03:40 AM   #2
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The Mighty Zen Mod has written a note on changing the Aleph front end to
Jfets:

Babbelfish J PCBs

Hope this helps.

Dennis
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Old 4th March 2014, 08:08 AM   #3
Zen Mod is offline Zen Mod  Serbia
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for Aleph 2 , cascodes for JFets are must

something like this
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File Type: jpg Aleph60-J.jpg (116.1 KB, 472 views)
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Old 4th March 2014, 08:16 AM   #4
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Zen, why did you state that the Aleph sounded lousy? I particularly like the sound of my Aleph 4 above anything else I have heard.

I'm referring to the Post #2 link.
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Old 4th March 2014, 08:33 AM   #5
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when I like something , I really don't care what others are saying about it

it must be bad color marker , fading with time

there was written joke all over it , in red
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Old 4th March 2014, 01:45 PM   #6
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Thank you to all, now is my turn I will make a new schema with changes suggested.

Regards,
Alejandro
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Old 5th March 2014, 04:21 PM   #7
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Default Which configuration recommend to try?

I've done my homework, at least up to my limited knowledge allow. I got 3 different configurations that may work, can someone review the images and let know what think about?

Thanks again to all that share their valuable knowledge and experience.

Regards,
Alejandro
Attached Images
File Type: jpg A2-Jfet-A.jpg (69.8 KB, 371 views)
File Type: jpg A2-Jfet-B.jpg (69.6 KB, 362 views)
File Type: jpg A2-Jfet-C.jpg (65.7 KB, 124 views)
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Old 5th March 2014, 04:56 PM   #8
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most left one

you can make R71 and R72 as just one
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Old 5th March 2014, 05:35 PM   #9
wayne is offline wayne  United States
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Yes the left one but stick some resistors in the base leg of cascodes 220 Ohms - 1K.
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Old 5th March 2014, 06:33 PM   #10
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Quote:
Originally Posted by Zen Mod View Post
most left one

you can make R71 and R72 as just one

Thanks for your reply, I will start collecting the parts to build a proto-board of the left schema.

I split the resistor in R71 & R72, because it will simplifly to me the final design of the daughterboard.

Regards,
Alejandro
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