Complementary JFET-JFET cascode input, BJT VAS-Drivers, Lateral Output - Page 3 - diyAudio
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Old 18th October 2011, 05:32 PM   #21
AndrewT is offline AndrewT  Scotland
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2.5mA @ 30Vds amounts to Pq=75mW. That's a warm running jFET.
By the time you ask for 3mA you are at almost 25% of the cold capability of the jFET. Then you need to de-rate it for elevated temperature >25degC.
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Old 18th October 2011, 05:39 PM   #22
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Not saying its a good idea, just making the point that they can go higher if it is worth it and other operating conditions are considered.
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Old 18th October 2011, 06:00 PM   #23
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assuming a fixed maximum voltage swing at the input fets, then with a high Vds that fixed input/output swing is a large portion of the total applied voltage.
Double or triple the applied voltage and the input/output swing becomes a much smaller part of the applied voltage.
That is what reduces distortion. The same logic applies to a VAS stage.
Ah yes, that makes sense.

Well, guess I'll run the 2SJ74 at 20 Vds. With only 4 mA through them it is well below of that they can handle.
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Old 18th October 2011, 06:01 PM   #24
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Nelson says that the jfets operate safely up to 30v if dissiaption is maintained
I would like to keep it running within spec, just to be sure.
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Old 18th October 2011, 06:04 PM   #25
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2.5mA @ 30Vds amounts to Pq=75mW. That's a warm running jFET.
By the time you ask for 3mA you are at almost 25% of the cold capability of the jFET. Then you need to de-rate it for elevated temperature >25degC.
You really sure that it would be a problem? The JFET is rated at 400 mW dissipation.

4 mA @ 20 Vds = 80 mW dissipation.

That is 1/5 of maximum. Should be within reason, even taking derating into account.
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Old 18th October 2011, 06:08 PM   #26
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Are you building a production amp that needs to last 20-30 years without fault or are you looking for best sound in a self designed amp? I am not saying to violate all paramaters, but there was an suggestion that higher vds may lead to less distortion. If you can move it up to 25 vs 20, achieve better numbers, and still maintain appropriate temperature control measues, why not try. Put appropriate pots in place for adjustment, build it, listen at different levels and decide based on sound and not datasheet
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Old 18th October 2011, 06:08 PM   #27
AndrewT is offline AndrewT  Scotland
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no great problem with 80mW of dissipation. But I would want to check what de-rated power the warm jFET can handle when in an ambient temperature >25degC.
I don't have enough experience to make an informed estimate of where Pq lies with respect to Pmax. That's why at this stage in my learning I have to rely on others and on the arithmetic available to me.
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Old 18th October 2011, 06:08 PM   #28
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Or I could do something different and go with the 2SK246/2SJ103 combo as input JFETs.

Not as high qualtiy and low noise as the 2SK170/2SJ74 pair, but the maximum rating of 50 V for the P-channel makes it a little mroe flexible with regards to cascode voltage.

But then again, there probably is good reason for people not using 2SK246/2SJ103 for that application.
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Old 18th October 2011, 06:09 PM   #29
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these alternatives make good cascodes for the 170/74 pair.
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Old 18th October 2011, 06:11 PM   #30
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Are you building a production amp that needs to last 20-30 years without fault or are you looking for best sound in a self designed amp?
It is only for personal use, but I still want to make sure that everything is run within spec.

Besides, if you get THD-20 of 0.007% or 0.008% doesnt make much of a difference.

I would rather take the safe route.
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