Aleph-X with 2SJ109 as input pair

[h_a]

Hi folks!

Being thrilled with the great Aleph-X design (thanks Grey!!) I want to combine it with another thing that has my attention at the moment: JFet inputs.

After reading wuffwaffs thread on it (see here if you missed it ) I think the dual package 2sj109bl with the same transconductance as the 2sa74 should fit perfectly, no?

Using a dual package should be the best in terms of matching, no?

Up to which rail voltage I don't need to cascode the jfets? I refer mainly to EUVL's post here , where he says 20V would be too close to VDS - but up to what would it be ok?

I want to build a stock 38W Aleph-X (+-15V rails), but I don't get matched IRFP044, just IRFP244 so I read that I've to use higher rails (> 20V) to lower distortion of these IRFP244.

If you wonder, what the main point in my post is, then..well I want to collect all design recommendations and maybe some time in the future I can stick them all together and get a nicely working Jfet input stage (dreaming).

Or what changes would be needed when I simply replace the IRF9619 pair with the 2SJ109? Maybe I'm lucky and somebody has too much spare time and cares to help me ?

If I need to cascode, what parts would fit perfectly?

Of course if somebody of you gentlemen drops a schematic, I'll gladly take that!

Yes I know many questions!

Thanks a lot for helping! Hannes

PS: info will be added to the Aleph-X wiki, so don't desparate, nothing will be lost and other people can look it up without bothering you again!

[Zen Mod]

few tips:

http://www.diyaudio.com/forums/showt...63#post1112763

this is still untested,but who cares :
http://www.diyaudio.com/forums/showt...12#post1078112

[h_a]

You're plain GREEEAAAAT !!

I'll read that stuff and bug you again, hehe.

Cheers, Hannes

[lgabco]

Very nice schemetics for JFET input is in service manual for Passlabs ONO. I have built one (bit simplified) and it was working perfectly. So I decided to build up also power amp on the same principple - and it is also working perfectly. It is difficult for me to say how it sounds because I have never heard original Aleph. I was looking for less analytical and more musical sound and this my requirement was fulfilled.

[h_a]

You mean the 2SK389 input that also the pearl has?

Interesting suggestion - thanks a lot!

However I do think this is a bit far from what I can use without doing actual measurements, since the Ono and the pearl as well use the 2SK389 only cascoded (the pearl uses the ztx450 if I remember correctly and the Ono the 2sc1844).

What about the current through the 2SJ109? What do you guys think is best?

Cheers, Hannes

PS: hehe I know never ask what's best...then you get dozens of replies - just what I want ;-))

[EUVL]

If you wish to build the Grey Rollins circuit with 4 FETs, then I would recommend you to use IRFP044. It has more transconductance then the 244, and you need that with 4 FETs. If you really have problems getting IRFP044s, you may conatct me for matched sets of 4's. But you should have no problem getting them from Conrad, Reichelt, RS, Farnell, etc.

Voltage would still be +/-15V, so 2SK109 is fine without cascode. If you use 1k resistor at the drain of the JFETs, you would need Idss of about 8 to 10mA. But you may find 2J109BLs very difficult to get.

If not in a rush, I suggest you wait until Nelson publishes his Aleph-J service manual.


Patrick

[h_a]

Hi Patrick!

Thanks for your reply!

Indeed getting the IRFP044 is not the problem, just getting matched ones are! Originally I wanted to build a stock Aleph 30, so I ordered nicely matched IRFP244 (unfortunately this source does not offer matched IRFP044 ); however since I just learned developing pcbs I can build now whatever I want and not only amps for which pcbs are offered!

So Aleph-X I'm coming

And yes, I'm not in a rush (actually I've to finish 3 other projects first, hehe), so indeed I want to have a look at Nelson's JFET input in the Aleph J service manual for which I'm daily visiting his site to see wether it's already online

I just love collecting information to build a great amp!

Thanks for all the input! Cheers, Hannes

[h_a]

Well I'm currently wondering about 2 things:

1. gain: the transconductance of the 2SJ109 is a factor of 4-5 lower than of the IRF9610...so what to do about that? Just live with lower gain? C'mon!

2. I started reading the initial Aleph-X thread and already on page 2 I found

Originally posted by rtirion
The current source above Q5 and Q7 provides +/- 20 mA. This means +/- 10 mA and 150 to 170 mW dissipation per IRF9610

300 to 350mW for a 2sk389 is a little to much. :-)

(2sj109 datasheet says 200mW per package (?) max, anyway constant current can be lowered)

and even more important

Lowering the current in this pair is an option but will wreak havoc on the DC setting. Using a 2sk389 will need a little more work than just a replacement I think.

It would be great if somebody could comment on this!

Thanks a lot for your reply!

Cheers, Hannes

[Zen Mod]

Originally posted by h_a
Well I'm currently wondering about 2 things:

1. gain: the transconductance of the 2SJ109 is a factor of 4-5 lower than of the IRF9610...so what to do about that? Just live with lower gain? C'mon!

2. I started reading the initial Aleph-X thread and already on page 2 I found



(2sj109 datasheet says 200mW per package (?) max, anyway constant current can be lowered)

and even more important



It would be great if somebody could comment on this!

Thanks a lot for your reply!

Cheers, Hannes

read my pdf

[h_a]

Dir Sir,

I read your enlightening pdf a multiple times, but I don't have a clue, how such a difference in transconductance does not lead to a different gain.

My humble greetings, Hannes

EDIT: is this because the current through input pair is also reduced to about the half?

The relative change in current (with a signal on gate) is then not the same, but at least only 2 times smaller.