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Old 23rd May 2012, 06:19 AM   #2171
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Hi,

there must be a few schematics in this thread using single and double pairs of fets.

I´ve used a single pair, had to change the bias, the resistor after the fet (from 592R to something around 900R), the McMillan resistors.

DC offset can be set with a pot 10R or 20R that also fuction as a pair of source resistors. This makes matching a bit easier.

Amp has been like this for a few years now. Works fine.

Here´s the thread:

AlephJ-X

William
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Old 23rd May 2012, 05:01 PM   #2172
gl is offline gl  United States
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Here is the most recently posted schematic for the AX100J's that I built. Note that this project pre-dates the Aleph J and the X'ing of that circuit and slightly post-dates Williams work.

AX100 100W Aleph-X Monoblocks

The JFET's each run 5ma of bias to duplicate the 10ma through the original MOSFET's. This means no tinkering with other circuit values. Tinkering is to be avoided with the Aleph X. The transconductance with the JFET's is lower than with the MOSFET's. However there is still enough OL gain to keep things good.

I eventually removed the 15 ohm JFET source resistors.

Here is a comment from Nelson on the topic.

AX100 100W Aleph-X Monoblocks

Graeme
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Old 23rd May 2012, 09:24 PM   #2173
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Thanks for that! Can I use just one pair of the 2sj74 (matched) in this case? And I assume I still want to make sure they are thermally coupled? Do they require heat sinking as the 9610s do? I think I read about people gluing them together and some fancy custom sinks for a pair.
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Old 23rd May 2012, 10:11 PM   #2174
gl is offline gl  United States
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The 2JS74's will work fine. If you use my schematic you will need 4 per channel to replace the two 2SJ109's I used.

I did not match the 2SJ109's to each other in each channel. I just used one half of each on each side of the diff pair and relied on the cascodes to keep any current hogging from getting bad. It seems to be working. Maybe I just got lucky pulling the parts out of the bag. If I did this over again I would use matched 2SJ74's instead. Popular opinion says a 10% match is good enough.

Some people thermally connect the transistors together. It can't hurt and I do it when it's easy to do. For four TO-92's running 5ma each with 8V across them I probably wouldn't bother. The Aleph-X has a lot more DC offset stuff going on so you probably wouldn't see the benefit of thermal tracking in actual use.

Graeme

Last edited by gl; 23rd May 2012 at 10:13 PM.
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Old 23rd May 2012, 10:28 PM   #2175
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My pcb only allows for a pair. Is it too much to ask one pair of the jfets to handle their role on their own?
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Old 23rd May 2012, 10:36 PM   #2176
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you can piggyback them

or you can solder them together

use your imagination
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Old 23rd May 2012, 10:44 PM   #2177
gl is offline gl  United States
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Your question is simple but the answer is long and full of if-then-elses.

If you have a PCB and are limited to what you can mount on it then I would recommend that you stick with MOSFET's unless there's a solution in one of the threads on X'ing the the Aleph-J.

Graeme
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Old 23rd May 2012, 10:55 PM   #2178
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Agreed! I'm a pretty good board stuffer. But that's about it. Appreciate the guidance.
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Old 24th May 2012, 05:11 AM   #2179
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Hrm... I'm still thinking about the jfet upgrade. I realize that it would be easier to have the conversation with the schematic but didn't know if it was bad form to ask for help while using boards from Jim's audio. I bought them from a fellow diyaudio member who is supportive of this website. I myself have also donated and am a proud "aficionado."

Sorry for the off-topic question... perhaps a moderator can clear this up without having too much back and forth on what has likely sucked up too much of peoples' time and energy previously.
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Old 24th May 2012, 06:56 AM   #2180
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Hi,

how many fets do you have in your Aleph-X? With 12 using two jfets can be done but 4 is better. With 8 I think it is possible (that´s how mine run at the moment)

William
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