Help wanted: Jfet front-end for Aleph 2

[Audio4Fun]

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 I’m 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/esantane/movies/JFET-input.jpg. The second one is from Graeme published on diyaudio thread http://www.diyaudio.com/forums/pass-labs/73543-ax100-100w-aleph-x-monoblocks-46.html#post1468122.

I have read many posts on diyaudio and other places about people asking for Jfet front-end for Alephs, but I didn’t 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 http://www.diyaudio.com/forums/pass-labs/146767-j-fet-front-aleph-5-a.html#post1869842.

Questions:

- How the schema looks like, I’m crazy or it’s 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.

 

[Dennis Hui]

The Mighty Zen Mod has written a note on changing the Aleph front end to
Jfets:

http://www.diyaudio.com/forums/pass-labs/75281-babbelfish-j-pcbs-38.html#post1112763

Hope this helps.

Dennis

 

[Zen Mod]

for Aleph 2 , cascodes for JFets are must

something like this

 

[KatieandDad]

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.

 

[Zen Mod]

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

 

[Audio4Fun]

Thank you to all, now is my turn I will make a new schema with changes suggested.

Regards,
Alejandro

 

[Audio4Fun]

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

 

[Zen Mod]

most left one

you can make R71 and R72 as just one

 

[wayne]

Yes the left one but stick some resistors in the base leg of cascodes 220 Ohms - 1K.

 

[Audio4Fun]

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

 

[Zen Mod]

sorry - I wasn't clear enough

you must make them as one , exactly as I drew earlier

like you drew (2 resistors ) , input stage is loosing LTP function ......... big nono

so , either as one resistor , or without it , then having some more dissipation in CCS above

 

[Audio4Fun]

Yes the left one but stick some resistors in the base leg of cascodes 220 Ohms - 1K.

Thanks for the tip, as I could understand I will make a new schema with a pair of resistors in between the base legs of Q75 & Q76.

What value should I use for those resistors? 220Ω - 1KΩ is a wide range.


Regards,
Alejandro

 

[Zen Mod]

anything between 220R to 1K

they're base stoppers , so not critical value

 

[Audio4Fun]

New Schema

New schema with modifications suggested, R72 was dropped and added R78 & R79, the 390Ω value is just because I have some qty of them.

Also I want to ask for capacitor type recomended for C71 (10µF).


Thanks again.
Alejandro

 

[gl]

I would strongly recommend matching all the JFETs in each channel as closely as possible.

I only used the 15 ohm source degeneration resistors in the AX100J because that was the value I had on hand. I also tried no resistors and that worked fine too - but the JFETs were all tightly matched.

I think your Aleph 2J is going to sound really, really nice!

Graeme

 

[Zen Mod]

C71 - 10uF/25V electrolytic is fine

 

[moe29]

I would love to try this - i've just been looking at my long dormant Aleph 2s, to re-do the wiring and fix a few things.

A chance to refresh them with a new front end would really rock!

I wish i could do more than just offer moral support, but it's over my head technically

 

[Audio4Fun]

I would strongly recommend matching all the JFETs in each channel as closely as possible.

I only used the 15 ohm source degeneration resistors in the AX100J because that was the value I had on hand. I also tried no resistors and that worked fine too - but the JFETs were all tightly matched.

I think your Aleph 2J is going to sound really, really nice!

Graeme

Thank you for your advice, fortunately today is still possible to buy the Toshiba 2SJ74BL in matched quads. I have some LSJ74 but I haven't luck with match, I will need to continue buying more units to get a good matched subset.

Best regards,
Alejandro

 

[Audio4Fun]

C71 - 10uF/25V electrolytic is fine

OK thanks, I need to change the symbol in the schema, I was thinking in a non polarized capacitor.


Thanks,
Alejandro

 

[Audio4Fun]

Daughterboard design finished

I have finished the Aleph 2 Jfet daughterboard design (I hope ), there are 4 attached images, from left to right, (1) original Aleph 2 input stage, (2) the revised Aleph 2 Jfet input stage, (3) the actual mainboard IRF9610 diff pair and (4) the Jfet daughterboard PCB.

J1 & J2 are the connections from daughterboard to Q1 & Q2 (IRF9610s) on the main Aleph 2 board respectively, the orientation of pins is exactly as they are on the mainboard. Also the distance between Q1 & Q2 is the same as in between J1 & J2 (30mm or 1.18in), the idea is to directly plug the daughterboard into Q1 & Q2 pin connections.

An important fact is that drain pin of Q2 (center pin) is directly connected to negative rail, another fact is “source” pin of both Q1 & Q2 are already direct connected at the mainboard level as the original design states, as a way of simplify daughterboard design only the “source” pin of Q2 was used.

The rest is in the previous posts.

I have changed some resistors designations in the schema because there was a repeated number.

If anyone has time to see the diagrams and the PCB, and post any comment I will be very gratefully.


Thanks in advance.
Alejandro