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Old 21st June 2008, 09:02 PM   #421
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Quote:
Originally posted by Blues
Thanks so much, John. Something to play with on a long flight.

Be sure to show us what you come up with.
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Old 22nd June 2008, 04:04 AM   #422
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Sure will...if I can come up with something
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Old 22nd June 2008, 04:15 AM   #423
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I'm playing with an Aleph J with LU1014D cascodes on the lower half. I've got the three volt spread that the LU1014D wants, but I'm stuck with six amps regardless of what value I use in its source resistor. It wants about half that amount......

Hard at it!
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Old 22nd June 2008, 04:57 AM   #424
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Try setting the bias current at the CCS on top. Check Zv9 too for the bottom part.
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Old 22nd June 2008, 05:58 AM   #425
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> I'm playing with an Aleph J with LU1014D cascodes on the lower half. I've got the three volt spread that the LU1014D wants, but I'm stuck with six amps regardless of what value I use in its source resistor. It wants about half that amount......

It sounds like the LU1014 is overbiased.

The LU1014 Cascode wants to see 0V Vgs at the gate of the power JFET (i.e. the gate is at the same voltage as the negative rail for the lower devices), whereas the standard Aleph J circuit will give you about 4V.

The simplest solution is to use a current source (2SK170BL with the same Idss as your diff pair bias) to drain off the diff pair current to a more negative rail (at say 10V lower than the negative rail). That means one extra power supply (for 20mA or so) and 2 JFETs.

It has been discussed elsewhere. Nelson had the same proposal.

http://www.diyaudio.com/forums/showt...18#post1361918

Or you have already implemented this ? Then please post your schematics.



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Old 22nd June 2008, 01:58 PM   #426
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Thanks for pointing that out, Patrick. I remember you discussing it at length with ZenMod too.
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Old 22nd June 2008, 03:02 PM   #427
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Quote:
Originally posted by Blues
Try setting the bias current at the CCS on top. Check Zv9 too for the bottom part.

Thanks Allan and Patrick!

Just before bedtime last-night I discover that the upper current had to be lowered to assist controlling the LU1014D current value. So , thanks for adding the tip Allan--and showing interest.

I have time later today to play. I'm not quite there, but will post what I've accomplished so far.

Patrick, I've always valued your posts and will follow the link you offered. You suggestion is insightful. I'll take time to digest the contents.

Best regards.....
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Old 23rd June 2008, 05:25 AM   #428
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Arrived back in town late--rather weary. Here's some of what I'm doing. My apologies if I'm way off the mark, I don't wish to insult anyone's intelligence.
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Old 23rd June 2008, 05:44 AM   #429
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Patrick, I followed your link--perhaps a day late and a dollar short. You and the Master have given me wonderful insight. Thanks. I'll play with your ideas tomorrow.
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Old 23rd June 2008, 08:07 AM   #430
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Hi Carpenter,

I played with such a thing here,

The AlephJ-LU

I only had a short glance at your schematic, but it seems you're running at stock 1.3A and in this case the source resistors of the LU1014 are too high and won't give loadline cancelation.

For loadline cancelation you need about 600mV per amp if I remember correctly (you can check that value in the Zen#9 article).

Then it needs to run self-biased! Check the graph Patrick posted once.

Next I don't see how you got rid of the DC at the gates (no cap, no additional rails), with the stock 3.5V UGS you're just grilling the jfet.

Last, you will get funny frequency response peaks that you can tame as Patrick suggested with either a Zobel or by finding an elegant solution for the cascode-divider impedance (which I didn't so far).

If you simulate damping factor you will find that it performs on the same level as the original J (about 20), which I would like to get a bit higher. Only way would be by increasing NFB, but then you open just another can of worms

Have fun, Hannes
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