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What To Do With Those 2SJ28's
What To Do With Those 2SJ28's
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Old 1st May 2017, 01:36 PM   #121
jamahamvui1978 is offline jamahamvui1978  Viet Nam
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Originally Posted by Michael Rothacher View Post
If the extra voltage references don't bother you , then yes, or the V-FETRON with all the polarities flipped.
I've heard V-FETRON with 2sj28 many times and we all love it so much, that is one reason I fall in love with your schm(6) at first sight.
The other reasons, I dream one day I could understand and feel free to use auto bias methol for Vfet and I saw you used auto bias in schm(6).

Last edited by jamahamvui1978; 1st May 2017 at 01:45 PM.
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Old 1st May 2017, 01:56 PM   #122
Michael Rothacher is offline Michael Rothacher  United States
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What To Do With Those 2SJ28's
Your parts in the low 7's might be in the high 4's at 1.5-2A, possibly workable. Take the supplies up 20-22V perhaps. We could investigate dropping the last couple of volts with some nice SiC diodes if we really need to. Stay tuned.
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Old 1st May 2017, 02:26 PM   #123
jamahamvui1978 is offline jamahamvui1978  Viet Nam
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I already have some Cree in hand, If we use V drop in it to bias Vfet I think hight vol cree diode like C2D10120 is the good choice.
Could you give me the name and show me the way to investigate Vfet, Cree v.v ... I'll find and test them and report to you.

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Jamahamvui.
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Old 1st May 2017, 02:56 PM   #124
Michael Rothacher is offline Michael Rothacher  United States
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What To Do With Those 2SJ28's
That diode will drop about a volt(ish) at 1.5-2A.

If you want to move forward on your own, here's what you could do: build the circuit with 22-25 volt supplies, your lowest Vgs pair, using 4-5 25W ohm rheostats and start by balancing the current loops at 1.5A and low DC offset. I wouldn't bring the diodes into play until you've tried that. If you find you can't get the current below 2A, then you can pursue the diodes.

Don't forget to have fun
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Old 1st May 2017, 04:11 PM   #125
jamahamvui1978 is offline jamahamvui1978  Viet Nam
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Quote:
Originally Posted by Michael Rothacher View Post
That diode will drop about a volt(ish) at 1.5-2A.

If you want to move forward on your own, here's what you could do: build the circuit with 22-25 volt supplies, your lowest Vgs pair, using 4-5 25W ohm rheostats and start by balancing the current loops at 1.5A and low DC offset. I wouldn't bring the diodes into play until you've tried that. If you find you can't get the current below 2A, then you can pursue the diodes.

Don't forget to have fun
Thank you, Mr. MR! It is kind of you to answers all my silly questions

One more question:
Now I don't have low value wirewound potentionmeter Rheostat, but I find some 25ohm wirewound variable Rheostat which could be easy divided into two parallel 12.5ohm rheostat by cut resitance wire(about 50W). Could we use them to adjust bias and DC offset in schm(6)?
If not, I find 4-5 25W ohm rheostats.

Best Regards
Jamahamvui.
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Old 1st May 2017, 04:45 PM   #126
Michael Rothacher is offline Michael Rothacher  United States
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What To Do With Those 2SJ28's
Well, conceivably you could just use fixed resistors and trim them, but the convenience of being able to adjust under power is what makes the rheostats attractive.
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Old 1st May 2017, 04:55 PM   #127
Michael Rothacher is offline Michael Rothacher  United States
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What To Do With Those 2SJ28's
Quote:
Originally Posted by claudio52 View Post
Hi, a simple DEF.
I think it looks very nice
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Old 1st May 2017, 05:09 PM   #128
Soundhappy is offline Soundhappy  France
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Originally Posted by Circlomanen View Post
Attachment 613528

4 parts and a power supply constitutes a feed forward error correcting (SUSY-ish) power amp with very low levels of distortion. 2 X voltage gain.
It has a very good damping factor, due to the feed forward EC and the NFB.

If the 2SJ28 needs a gate resistor then the amp needs 5 components per channel plus a power supply.

L1 and L2 are two windings (bifilar) on an aircored choke. Since they are very close magnetically coupled together they will force the two outputs to closely follow each other in anti-phase (hence the SUSY-"ish" feed forward error correction).

Cheers
Johannes
Johannes amp are interesting too.
Hope he find one pair of 2SJ28 on the flea market for experimental builds, innovate creations.
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Old 1st May 2017, 05:57 PM   #129
Soundhappy is offline Soundhappy  France
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Originally Posted by Circlomanen View Post
Attachment 613946

Attachment 613947

A simple test with a battery biased SRPP dynamic current source and a 2SJ28 as a source-follower.

10+10 volt P-P input gives 7+7 volt P-P output. The 2SJ28 does not have a lot of transconductance. I am used to the IRFP7430 and IXTH140P05T so the 2SJ28 seems very weak and "soft".
Find more transconductance in Tokin 2SK180 if you like...well that other thread story
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Old 2nd May 2017, 06:22 PM   #130
Nelson Pass is offline Nelson Pass  United States
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What To Do With Those 2SJ28's
Something worth contemplating when you design with SITs:

The transconductance is a function of current and voltage, so that
it increases both with current Ids and voltage Vds. This is true
of all gain devices, but in SITs (and Triodes) the variation due to
voltage is much greater. We would say that they have a lower Drain
(or Plate) impedance.

In amplifying we see that the voltage Vds decreases as current Ids
increases and vice versa, so there is the opportunity for some
cancellation, giving a more constant transconductance figure, lowering
distortion.

At the "sweet spot" they cancel 2nd harmonic, giving the lowest
distortion numbers. Off the sweet spot, there is a 2nd harmonic,
variable in amplitude and phase.

There is a curve of such "spots" depending on the load, and what we
see is that the idle dissipation of an SIT in or near the sweet spot tends
to remain the same.

As a single-ended Class A example, the SIT-1 transistor does this
around 25 watts. Into an 8 ohm load minimal distortion occurs
around 15 volts and 1.7 amps. The output for this is around 25 watts
peak, and the dissipation is about 25 watts.

For a 16 ohm load, the voltage goes up to 21 V and the current down
to 1.2 A. Same dissipation and peak output.

A similar proportional relationship applies to push-pull operation, as well
as bias through mu followers. I leave you to figure the numbers out for
these cases (hint: they alter the apparent load).

Once you figure out what this number is, you can estimate the probable
ratio between supply voltage and bias current for a given SIT topology,
and if you work with these it may save you some time and effort.

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