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-   -   what to Upgrade, Aleph-X (http://www.diyaudio.com/forums/pass-labs/141725-what-upgrade-aleph-x.html)

audiodan 4th April 2009 01:34 AM

what to Upgrade, Aleph-X
 
2 Attachment(s)
Hello All

Just the other day I got to compare my AX100 with an ex DIYer equipment which was a Krell amp.
Well the Aleph was not as bright as the krell but was missing something in the bottom end, with the soundstage not as good as the Krell.

After hooking up the AX100 the DC between speaker terminals was 1.5 V.:eek: not good for speakers, this lowered to about half this value- still to high.

So now im in the process of swapping my badly matched irfp240 with some nicely matched ones and arrange them differently on the heatsink. Since Im at it what else does one recommend for improvements, there must be something as this amp has been out for awhile.

Here is the schematic

EUVL 4th April 2009 06:16 AM

JFET frontend diff pair using 2SJ109.

There are a few threads, like the Aleph J thread, which have lots of details.

2SJ109 is still available here at USD8 each.
Search Market Place.


Patrick

audiodan 4th April 2009 07:49 AM

2 Attachment(s)
Thanks Patrick!:)

Seems like Im 10 steps behind everyone else.

Is this the schmatic for changing across to the J side.

Cheers

EUVL 4th April 2009 08:57 AM

Have a look also at the Aleph J schmatics from Nelson.
(Single 2SJ109)


Patrick

audiodan 5th April 2009 03:05 AM

hello!

I found it, Is there a balanced version of this schmatic somewhere

Cheers

gl 6th April 2009 06:11 PM

The double JFET diff pair was used to preserve the 20ma of total bias in the front end of the AX100 and increase the transconductance over one pair.

You'll note that the single JFET in the Aleph J is only driving a couple of output transistors. If you do the math you'll see that the AX100J-01 and Aleph J are quite consistent. I don't see anything in the Aleph J that you could adapt to improve your particular situation.

My recommendation is to stay with the AX100J schematic as a primary guideline.

Other people here on the forum have produced excellent results with matched 2SJ74's instead of the hard-to-find 2SJ109's.

If you have 1.5V of relative offset then something is very wrong with your amps. There is probably a miswire or a serious matching problem with the transistors. Do both channels have the same problem?

Cheers,
Graeme

audiodan 6th April 2009 08:14 PM

2 Attachment(s)
Hello Graeme

Thanks for the info.

As for the relative offset, cold start both amps are 1.5, both lower to about 200mA after a hour. My batch of 240s was not enough so matching is poor as well as my Fet layout is in need of change, newbie mistake.

So now im in the process of buying matched ones, then work out best method of laying these fets out on the heatsink. looking back i would change heatsink layout but construction is finished.

Im going to have 3 fets on top and 3 fet on the bottom of the same heat sink, so back heatsink will have no fets. Will offset fets back on the 350 heatsink.

Q: 1----------whats a good distance to seperate fets.
2---------- Should i stick with the same amount of output transistors

Cheers :)

here is my current setup fet wise:rolleyes:

audiodan 6th April 2009 08:34 PM

One more question

what does the changes done to the diff pair do to the sound of the amp!

gl 7th April 2009 11:26 PM

Hi Dan,

The layout of your MOSFET's looks OK but it's a really hard thing to judge from a distance and from a drawing. My preference is to run more output transistors with a lower dissipation on each. The AX100 thread states my choices and the reasons for making those choices.

Here are some posts from the AX100 thread regarding the sound of the JFET version compared to the original MOSFET front end version.

From Ian MacKenzie:

http://www.diyaudio.com/forums/showt...53#post1499953

From Nelson Pass:

http://www.diyaudio.com/forums/showt...05#post1404305

From me:

http://www.diyaudio.com/forums/showt...13#post1438413

There are additional comments in the thread or referenced in the thread.

Cheers,
Graeme

audiodan 8th April 2009 02:26 AM

Thanks Graeme!


here is a photo showing layout. Im sure its causing the problems with offset as the back heatsink heats up more than the rest.

Well with these comments of the changes of the X100 to XJ100 im surprised more people have not done the conversion!
now to get some 2sj74 or 27j109

Cheers


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