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First watt F2j with SJDP120R085
First watt F2j with SJDP120R085
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Old 12th April 2017, 07:02 AM   #1
kinku is offline kinku  United States
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First watt F2j with SJDP120R085
Default First watt F2j with SJDP120R085

Hi,
Is it possible to use SJDP120R085 in Firstwatt F2j?
http://www.firstwatt.com/f2j.html
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Old 12th April 2017, 09:20 AM   #2
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First watt F2j with SJDP120R085
that would be completely another amp , considering changes needed

however , there are few M. Rothacher's amps with VFets where you can easily implement your SJDP
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Old 12th April 2017, 11:05 AM   #3
kinku is offline kinku  United States
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Can you please elaborate the changes and why it will be different?
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Old 12th April 2017, 12:25 PM   #4
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first - amp is made for enhanced type of devices , which means biasing voltage at gate of mosfet is positive ref. to source , while D in SJDP means that part is depletion type , demanding exactly oposite polarity of biasing voltage - negative at gate ref. to source

for that you'll need additional voltage source , based either on separate winding on xformer or on few other Papa's tricks (pulled from RDH era)

as I said - look for M. Rothacher's amps , there is at least one fulfilling your desires

principally very close to F2 , but optimized exactly for depletion devices

to conclude - that is exactly answer you needed for your question

sorry - I'm lazy to search and look for exact thread ........ just find M. Rothacher in member list and look for threads he started

or go to his site : AudioMaker | DIY Audio Projects & More
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Old 12th April 2017, 02:08 PM   #5
kinku is offline kinku  United States
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First watt F2j with SJDP120R085
Can I use a Baliga cascode to work around the biasing voltage issue?
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Old 12th April 2017, 02:15 PM   #6
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of course that you can ....... but in this case SJDP is taking role of invisible part , while mosfet is still governing one .......... and entire circuit would remain (almost) same in sound character

practically - same as if you cascode mosfet with same type of mosfet

btw. you can always try to swap your SJDP for someone's SJEP , then no more complications
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Old 12th April 2017, 02:28 PM   #7
kinku is offline kinku  United States
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I am not acting as an expert but I was reading that when baliga cascode is used ,MOSFET is practically transparent in switching because of low resistance and capacitance.
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Old 12th April 2017, 02:51 PM   #8
Michael Rothacher is offline Michael Rothacher  United States
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First watt F2j with SJDP120R085
If you're lucky enough to have some of the Semisouth depletion parts you might want to save them for building Nelson's DEF amp.

I imagine there are plenty of obtainable enhancement parts for making an F2 sort of amp.
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Old 12th April 2017, 02:56 PM   #9
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First watt F2j with SJDP120R085
Quote:
Originally Posted by kinku View Post
I am not acting as an expert but I was reading that when baliga cascode is used ,MOSFET is practically transparent in switching because of low resistance and capacitance.
in switching ...... yes ...... just because switching is sole purpose of cascoded stage

in amplification use , part which gate is directly driven with signal is dominating while cascoding device is pretty much irrelevant , without adding much of his own to stage character

go to firstwatt.com , search for Cascode article
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Old 12th April 2017, 02:58 PM   #10
kinku is offline kinku  United States
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Zenmode it is hard to source those SJEP jfets. Do you know any other drop in replacements ?
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