current bias j-fet input 2sk170 ,2sj74 at input stage - diyAudio
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Old 30th May 2005, 11:12 AM   #1
YUTK is offline YUTK  Thailand
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Default current bias j-fet input 2sk170 ,2sj74 at input stage

I design poweramp use j-fet input 2sk170 ,2sj74 cascode and current mirrow with 2sc2856 ,2sa1191 but idon't sure about current bias at input stage ,how much of current bias at input stage for high stability and performance if anybody know pls help
me
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Old 30th May 2005, 05:07 PM   #2
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I don't know the parameters of these, but it 'usually' is 2-7mA (tail current) I think.
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Old 30th May 2005, 05:34 PM   #3
aht is offline aht  Thailand
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Smile Same circuit

I'll be diesign 2Sk170 & 2SJ74 (2SK389 & 2SJ109) same you.
I think it's use about 2.5 mA with supply +/- 12V (after cascode)
You can use 2SK389 & 2SJ109 same me (I have a lot of 2SK389 & 2SJ109)

Mail me if you want some prototype

Regard
AHT :-)
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Old 30th May 2005, 07:22 PM   #4
boholm is offline boholm  Denmark
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My humble advice:

There are 3 types of 2sj74/2sk147: GR, BL and V. This tells you how big the Idss will approx. be. Look in the data sheet:

http://www.semicon.toshiba.co.jp/doc...=2SJ74&lang=en

On page 2 you can see a curve called ID - VGS. From this I have concluded that one must have the signal as close to the top as possible to have minimal distorsion. And the smaller the signal is, the lower distorsion.

Unfortunately there are big differenties between the same types. One 2sj74 can measure Idss to 5 mA and the next 2sj74 can measure 10 mA. Therefore you must measure out all your fets and make sure they measure equal.

Onhttp://www.borbelyaudio.com/special_articles.asp you can read more about fets. In my opinion one should do this if one is not used to working with fets. Especially "The new Frontier" tells it all about bias and everything else.
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Old 31st May 2005, 05:20 AM   #5
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Hi Yutk,

The 300F power amplifier I designed for Magnet used K170/J74 in the front, complementary cascode followed by LED feedback biassed complementary cascode Vas. From memory (not very good) it was about 2 mA each so 4 mA tail current. BL range devices, but we hand matched them.
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Old 31st May 2005, 06:25 PM   #6
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I have taken them up to 10 mA (each) and they still worked very
well, provided you are staying within the dissipation specs.
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Old 1st June 2005, 01:02 AM   #7
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In fact I also used them in the PR9R folded cascode preamp (released Jan 1995 Bangkok Hi Fi show) which used the FET complementary diff'l pairs, a bjt folded cascode and EF output, once again at about 2 mA each FET. It had a beautiful smooth open and detailed sound AND a (volume) remote and, I believe is still current in their range. Sold like hot cakes.
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Old 1st June 2005, 04:32 AM   #8
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Any Complementary or Non-Complementary High Voltage JFET available to be implemented in amps with rails at + - 120VDC.......
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Old 1st June 2005, 05:04 AM   #9
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Hi Kanwar,

I don't think so, You would best use the K170/J74 with your 2N5551/5401 cascode to take it up to such heights.
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Old 1st June 2005, 02:04 PM   #10
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Quote:
Originally posted by amplifierguru
Hi Kanwar,

I don't think so, You would best use the K170/J74 with your 2N5551/5401 cascode to take it up to such heights.

Thanks Greg,

But I have seen somewhere in one of the professional amps using 2 series connected Cascodes i.e. total 3 JFET pairs in series ..... does it suffice or only adds distortion or add something else......
Secondly how do you came to know about that we are using 2N5551/5401 pair in our designs...
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