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Old 11th July 2007, 01:42 AM   #1
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Default SWM Seeking JFET /w low noise + high Vp

I am looking for a FET that has a high enough pinch-off voltage and low enough noise to use in a discrete input stage preamp (with a bit of gain) that can handle a +4 dBu or roughly 1 volt peak with enough headroom to keep it linear if there are any transients. Any ideas? I usualy use 2sk170 as my de facto fet, but not for an input this hot.

I was thinking a 2n5462, or go through the bother of testing a bunch of 2sk170 to weed out a few with unusually high pinch off figures...
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Old 11th July 2007, 02:27 AM   #2
CBS240 is offline CBS240  United States
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Hi

2sk170 is N-channel, and 2N5462 is a P-channel device. I have used 2N5460 and 2N5461(actually the SOT-23 version), but not for an input stage. These are fairly low conductance devices, less than 6 mmohs for the 5462. Does this stage have a follower? J177 has a much higher conductance and Vgs, it is also P-channel. Some circuit detail might help your cause here, or better, a scematic.
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Old 11th July 2007, 05:12 AM   #3
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Quote:
Originally posted by CBS240
Hi

2sk170 is N-channel, and 2N5462 is a P-channel device. I have used 2N5460 and 2N5461(actually the SOT-23 version), but not for an input stage. These are fairly low conductance devices, less than 6 mmohs for the 5462. Does this stage have a follower? J177 has a much higher conductance and Vgs, it is also P-channel. Some circuit detail might help your cause here, or better, a scematic.

I am not to concerned about NPN or PNP as long as the parameters fit. I have not decided about a follower yet though since I am still brainstorming components. I think I may try a few BF245 to see how they work out... not super low noise, but with that big a signal it is not super important.

Thanx
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Old 11th July 2007, 01:18 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
1Vac is not a particularly big signal.
CD and similar send 2Vac.
+4dbu is 1.23Vac or about 1.74Vpk.

Most pre-amps based on opamps can send out a maximum of about 8Vac from +-12Vdc supplies.
Discrete versions often work upto 20Vac from +-30Vdc supplies.
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