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-   -   Jfet input for Aleph 60? (http://www.diyaudio.com/forums/pass-labs/135787-jfet-input-aleph-60-a.html)

Dansk 2nd January 2009 10:32 AM

Jfet input for Aleph 60?
 
Hello everyone,
And a happy new year to all of you!

I've been reading my eyes out, trying to aviod bothering fed up DIY-experts with asking silly questions. It seems though, I'm going to have to do it anyway. :bawling:

My starting point being that I'm in possesion of two large toroids, each giving out 650 VA and +/- 34 V to the rails, I found that the Aleph 60 would suit both the specs of those and my need of amplification - but I would like to have a Jfet input. Has anybody successfully built that or cold somebody tell or show how it could be done? :scratch:
I would be very, very plaesed to hear about it!

carpenter 2nd January 2009 10:38 AM

Aleph J could give you some great hints.

Buhl 2nd January 2009 11:18 AM

JFET A60
 
Hej Hr. Dansk!

There has also been pretty detailed and well documented explorations on adding a Jfet inputstage to the A30 - which has an inputstage which is identical to the A60 if my memory serves me well. This could give you all you need - layed out and everything :)

However, as I also recall, the tricky thing about the Jfet A30 was that the supply voltage was very near to the max of what the small Jfets could handle - so building a A60 with a Jfet input stage might require a second, lover voltage supply for the front end - and if this is able to sving the output stage to full - that is up to the more qualified personel on this forum to comment.....

My advice - if you want Aleph J performance - build an Aleph J - transformators are not that expensive! (solely based on my own tiresome and expensive experience) :)

Kind regards & happyt new year!
Hans

Dansk 2nd January 2009 11:23 AM

I believe it could, but it seems throughout my reading that people have considered it problematic raising the rail voltage to +/- 34 V combined with the Jfets - and I must admit that I haven't got the experience and skills to be able to figure out the consequences by myself.:ashamed:
So I was hoping for a little advise... Please...

Zen Mod 2nd January 2009 11:34 AM

2 Attachment(s)
try like this

if you are not in mood for changing upper CCS , say and I'll sketch how to make original one adjustable

I presume that your 60 is made by standard schematic ?

Dansk 2nd January 2009 12:00 PM

Thank you very much!:D
You've got me jumping around in my livingroom for joy!

I assume the bc's can be swapped for the ztx450 and 550 without any problems, so the job for now is chasing down some 2sj74's...

Ooops! Yes, I'm building using the schematic from the service manual.

Nelson Pass 2nd January 2009 10:44 PM

You might allow for referencing the cascodes to ground. Try it
both ways and see what you like.

:cool:

Dansk 2nd January 2009 11:03 PM

I will do so. It might take some time before I'll be able to tell about my results, as I am just beginning to gather the parts - :blush: - but the 2sj74's should be on their way already!

- And thank you very much for both your original design and thoughts on the modification, mr. Pass! :worship:

Dansk 23rd January 2009 03:21 PM

Sound of different IRFP's?
 
Hi everybody,

I'm still working on my J-fet Aleph 60 - gathering the right parts.
- Well, I guess we cannot all be as fast building as "Lucky" Steenoe.

I'm writing again because I have the ability of buying some very cheap IRFP140N's to replace the usual 240's. But what about the sound quality? Off course it's nice to save money, only if it still gives a soundwise comparable result.

Nelson Pass 23rd January 2009 07:00 PM

They have a little more transconductance which is better, but are
a little more capacitance sensitive vs voltage which is not better.

If they're cheap, go for it.

:cool:


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