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Old 6th January 2010, 02:07 PM   #3351
Magura is offline Magura  Denmark
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Quote:
Originally Posted by syn08 View Post
For a phono stage, it doesn't make sense to consider a 25K source impedance. For a realistic 50ohm @ 20KHz MC impedance, the distortion gap widens to about 3:1, but numbers are already low (0.01% to 0.03%). Bottom line, for MC preamps, paralleling JFETs has practically no impact on distortions and bandwidth (because of the low source impedance).

EDIT: for MM, the situation is significantly different, both because of larger input signal (2.5...5mV) and large equivalent source impedance @ 20KHz.


But where is the limit then?
When the drawer labeled 2sk369 is empty?


Magura
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Old 6th January 2010, 02:23 PM   #3352
syn08 is offline syn08  Canada
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But where is the limit then?
When the drawer labeled 2sk369 is empty?


Magura
Not sure about what "limit" you are talking here, but to me the 2SK369 (and the 2SJ109 counterpart) rush after syndrome doesn't make any sense. They are nothing but matched pairs. They have nothing special and there's absolutely no reason why they couldn't be replaced with sorted 2SK170 and 2SJ74, at least for audio purposes (instrumentation applications could be different). Actually, you can easily sort a single JFET population to closer matching than the dual part spec.
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Old 6th January 2010, 02:26 PM   #3353
AndrewT is offline AndrewT  Scotland
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the 2sk369><2sk389.

It's a single Nchannel jFET.
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Old 6th January 2010, 02:29 PM   #3354
Magura is offline Magura  Denmark
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Quote:
Originally Posted by syn08 View Post
Not sure about what "limit" you are talking here, but to me the 2SK369 (and the 2SJ109 counterpart) rush after syndrome doesn't make any sense. They are nothing but matched pairs. They have nothing special and there's absolutely no reason why they couldn't be replaced with sorted 2SK170 and 2SJ74, at least for audio purposes (instrumentation applications could be different). Actually, you can easily sort a single JFET population to closer matching than the dual part spec.
Syn, I think you got me wrong, I wrote 2sk369, not 2sk389.

As for "limit", I meant the number of fets in parallel.


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Old 6th January 2010, 02:29 PM   #3355
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But where is the limit then?
When the drawer labeled 2sk369 is empty?


Magura
Recycle, and think of all the burn-in time.

Seriously, do you keep every preamp in a drawer when you go on to the next one? There is a new reality show "Hoarders", you know some but they have found some stunners.
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Old 6th January 2010, 02:38 PM   #3356
syn08 is offline syn08  Canada
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Quote:
Originally Posted by Magura View Post
Syn, I think you got me wrong, I wrote 2sk369, not 2sk389.

As for "limit", I meant the number of fets in parallel.
Sorry, my tired eyes are playing tricks... I still have to get used with these progressive lenses.

Depends on application. For a MC preamp, there is no practical difference between one 2SK369 and e.g. 2 x 2SK170 in parallel. For a line preamp, I would choose one 2SK369 (if available).

Last edited by syn08; 6th January 2010 at 02:40 PM.
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Old 6th January 2010, 02:42 PM   #3357
Magura is offline Magura  Denmark
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Originally Posted by scott wurcer View Post
Recycle, and think of all the burn-in time.

Seriously, do you keep every preamp in a drawer when you go on to the next one? There is a new reality show "Hoarders", you know some but they have found some stunners.
Oh no, I think this came across the wrong way. I usually give them away

I meant the drawer with the jfet's. The reason for the question is that I'm currently building a phono pre (MC), and have just used 4 in parallel, as that seems to be the standard, but if there is any benefit of more fet's in parallel, then I might as well do that, as I have loads of the 2sk369bl.


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Old 6th January 2010, 02:46 PM   #3358
syn08 is offline syn08  Canada
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Oh no, I think this came across the wrong way. I usually give them away

I meant the drawer with the jfet's. The reason for the question is that I'm currently building a phono pre, and have just used 4 in parallel, as that seems to be the standard, but if there is any benefit of more fet's in parallel, then I might as well do that, as I have loads of the 2sk369bl.


Magura
There is another limit, and that's stability. Paralleling more than 4-6 x 2SK170BL with no degeneration is looking for trouble. Depending on the layout, you can get an 100MHz oscillator.
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Old 6th January 2010, 02:48 PM   #3359
Magura is offline Magura  Denmark
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There is another limit, and that's stability. Paralleling more than 4-6 x 2SK170BL with no degeneration is looking for trouble. Depending on the layout, you can get an 100MHz oscillator.
Oh, so I will have no doubt when the limit is reached.
That's kinda what I was trying to figure.

Thanks.


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Old 6th January 2010, 03:07 PM   #3360
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For line stages I would still do a search of whats out there, I'm not convinced that there are not more linear choices in non-short channel devices.
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