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FET vs BJT input phono preamp
FET vs BJT input phono preamp
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Old 21st July 2020, 03:29 PM   #1
dreamth is offline dreamth  Romania
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Default FET vs BJT input phono preamp

Is there any solid proof that a real world moving magnet cartridge behaves better with a fet input than with a BJT input?
 
Old 21st July 2020, 04:16 PM   #2
analog_sa is offline analog_sa  Europe
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FET vs BJT input phono preamp
In what sense? Less noise with the FET, avoidance of input caps, or bias current ?

You probably mean the effects of the bias current, right? Wasn't there an identical thread running recently?
 
Old 21st July 2020, 04:46 PM   #3
Mark Johnson is offline Mark Johnson  United States
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FET vs BJT input phono preamp
If you've got JFETs, or if you can get JFETs, use them! They are obsolete and they are disappearing fast. Buy em now and use em now, the window is closing.

Later, if you feel like it, you can build a BJT circuit to do the same function, and compare the two. But do the JFET version first, and do it right away. If you sit around in Analysis Paralysis, thinking and worrying rather than building, you'll find it harder and harder to obtain the devices.
 
Old 21st July 2020, 04:51 PM   #4
PMA is offline PMA  Europe
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Quote:
Originally Posted by dreamth View Post
Is there any solid proof that a real world moving magnet cartridge behaves better with a fet input than with a BJT input?
Yes. Vogel, The Sound of Silence, Springer Verlag.
 
Old 21st July 2020, 04:55 PM   #5
dreamth is offline dreamth  Romania
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I have them all, built fet preamps, listened used or mod commercial fet preaps of all qualities , but they don't even come close to a good valve preamp while some bjt phono preamps just blew me away with their results for their cost and simplicity and overall sound was always better with BJT than fet in my experience...That is all i can say from my part.
 
Old 24th July 2020, 08:59 PM   #6
dreamth is offline dreamth  Romania
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Yes. Vogel, The Sound of Silence, Springer Verlag.
I saw its first edition a long time ago .
There's a problem with the real world measurements oposed to that utterly convincing theoretical exhibition.I have a feeling that the guy never tried to use a 2sk170 at 10ma supplied at 7.5v DC.I did that and experienced some of the most noisy transistor circuit i ever heard! Actually 2sk170 works best at 4...5v DC DS and about 0.5...2ma
 
Old 24th July 2020, 10:17 PM   #7
MarcelvdG is online now MarcelvdG  Netherlands
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The IDSS can be anything from 2.6 mA to 20 mA, depending on exemplaric spread. Maybe you had a unit with a low IDSS and the gate-source junction got forward biased when you tried to get 10 mA through it?
 
Old 24th July 2020, 10:32 PM   #8
dreamth is offline dreamth  Romania
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You might be right, i used the GR version, didn't have the BL or V one, but the minimum noise for 2sk170 is defined at 1ma in the datasheet .It may be like that just to include the GR specs....I did also try 2sk369BL too which is almost the same noise at 5x higher current(5ma)

Last edited by dreamth; 24th July 2020 at 10:35 PM.
 
Old 25th July 2020, 01:05 PM   #9
MarcelvdG is online now MarcelvdG  Netherlands
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I don't see anything in the Toshiba 2SK170 datasheet that implies that a 1 mA drain current would result in minimal noise. The equivalent input noise of a JFET usually drops with increasing current as long as the gate-source junction doesn't go in forward conduction and as long as the drain-source voltage is not so high that you get excessive gate leakage due to impact ionization. The plots in the datasheet seem to be consistent with that. I guess that 1 mA is just a current that lies comfortably below the 2.6 mA minimum IDSS, but is still high enough to get reasonably close to the minimal equivalent input noise voltage density.
Attached Images
File Type: png noiseID2SK170.png (43.6 KB, 911 views)
File Type: png noise2SK170_2.png (50.6 KB, 909 views)
File Type: png noise2SK170_3.png (47.5 KB, 902 views)
 
Old 25th July 2020, 01:36 PM   #10
PMA is offline PMA  Europe
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Quote:
Originally Posted by dreamth View Post
I saw its first edition a long time ago .
There's a problem with the real world measurements oposed to that utterly convincing theoretical exhibition.I have a feeling that the guy never tried to use a 2sk170 at 10ma supplied at 7.5v DC.I did that and experienced some of the most noisy transistor circuit i ever heard! Actually 2sk170 works best at 4...5v DC DS and about 0.5...2ma
You should not teach me about real world measurements. I designed and built enough of JFET line stages including MM preamps and my main MM preamp uses quad JFET input.

http://pmacura.cz/phono_modul.jpg

Once, about 10 years ago, I purchased a batch of "2SK170/2SJ74" from a Hong-Kong supplier, and they were all fakes. And they created "one of the most noisy transistor circuits", as you are saying.
 

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