24 in that amplifier means 6 per bank. You can get by with
fewer, but more parallel devices lowers the basic distortion
and gets higher transconductance out of the stage.
The downside of device capacitance comes in at some point,
but we aren't past the point of diminished returns with 6. The
larger of the capacitances is Cgs, but because of the increased
transconductance, this effect does not increase proportionally
with the number of devices. The Cgd is always an issue, but with
balanced output stages, this is effectively halved for any given
output voltage.
fewer, but more parallel devices lowers the basic distortion
and gets higher transconductance out of the stage.
The downside of device capacitance comes in at some point,
but we aren't past the point of diminished returns with 6. The
larger of the capacitances is Cgs, but because of the increased
transconductance, this effect does not increase proportionally
with the number of devices. The Cgd is always an issue, but with
balanced output stages, this is effectively halved for any given
output voltage.
WillyWaff,
in case you get sick from fumbling with single JFETs, there's still some 2SJ109 to be had over : here
(not exactly cheap though, unless you go for a 100 or more but the posting costs are low. Upside, it's only about a 15 minute drive away from me. If you feel like a drive when visiting the family you're also welcome to drop by for beer and beach SOB)
in case you get sick from fumbling with single JFETs, there's still some 2SJ109 to be had over : here
(not exactly cheap though, unless you go for a 100 or more but the posting costs are low. Upside, it's only about a 15 minute drive away from me. If you feel like a drive when visiting the family you're also welcome to drop by for beer and beach SOB)
Hi,
at the moment I´m getting sick heaving my amp up- and downstairs 5 times a week. The next amp will be a gainclone (mono) wich should be a bit easier to carry around and I won´t cut my fingers on sharp heatsink edges.
I´m not shure the problems I´m encountering have anything to do with using 2SJ74 instead of 2SJ109.
I will return one amp to it´s original state tomorrow morning and then see wich problems are vanished and wich aren´t.
What kind of beer do you normally drink?
Cheers,
William
at the moment I´m getting sick heaving my amp up- and downstairs 5 times a week. The next amp will be a gainclone (mono) wich should be a bit easier to carry around and I won´t cut my fingers on sharp heatsink edges.
I´m not shure the problems I´m encountering have anything to do with using 2SJ74 instead of 2SJ109.
I will return one amp to it´s original state tomorrow morning and then see wich problems are vanished and wich aren´t.
What kind of beer do you normally drink?
Cheers,
William
William,
I thought about what you posted the last couple of days and can only come to the conclusion that you might have marginaly instability at high output amplitudes. The reasoning is that the amplitude you measured at the diff pair source is way too high to be normal if my sums are correct. Added to that also you see funny things at the rails at no load. And the phenomena disappears when loaded with 4 ohm.
I also don't think it has to do with 2SJ74 or 109, but a good 109 would allow you to get rid of the source trimmer, which might (or might not) help.
But I am really sick at the moment (hay fever) so I need to look at it again when I am not feeling dizzy all day.
Patrick
I thought about what you posted the last couple of days and can only come to the conclusion that you might have marginaly instability at high output amplitudes. The reasoning is that the amplitude you measured at the diff pair source is way too high to be normal if my sums are correct. Added to that also you see funny things at the rails at no load. And the phenomena disappears when loaded with 4 ohm.
I also don't think it has to do with 2SJ74 or 109, but a good 109 would allow you to get rid of the source trimmer, which might (or might not) help.
But I am really sick at the moment (hay fever) so I need to look at it again when I am not feeling dizzy all day.
Patrick
Sorry to kick a dead horse. I just stumbled onto this thread during and internet search. I was wondering about the specs for the XA .5 amps. They have some pretty high max current output. This would suggest that they have changed to push-pull follower type outputs. I was a little surprised by that. Can anyone confirm this?
Take a look in the Aleph-X thread from here and onwards:
http://www.diyaudio.com/forums/showthread.php?postid=1228467#post1228467
This is nice too:
http://www.electori.co.jp/PASS/XA100.5.pdf
http://www.diyaudio.com/forums/showthread.php?postid=1228467#post1228467
This is nice too:
http://www.electori.co.jp/PASS/XA100.5.pdf
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