Idiotic JFET question from the usual suspect...

Can this circuit be powered from say 40V, if both JFETs have a max. VDS of 25V?

http://www.schmitzbits.de/micpre.html

I wouldn't

JR

Could you use a dc to dc module/converter to get to the voltage you need? Or something along those lines? That's a big drop though so I am not sure how that works...

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> http://www.schmitzbits.de/micpre.html  gain of about 100 to 1000 .... a low output impedance...

This struck me as wrong. In fact the output impedance is hundreds of K, which is not "low" in audio. Cable capacitance will bite the highs. Unless you load it with modern-typical dozen-K loads, where gain is far less than the range stated.

Thanks alot!
I have a zener/bd139 series regulator for 24V. I just had the silly idea it wouldn´t be necessary with two devices stacked :

I have also a directly coupled emitter follower tacked at the output.

> two devices stacked

If you give it a 40V supply, and put in a big signal, the output will swing nearly from zero to 40V. Each device feels about a 2V to 38V swing. This exceeds the stated 25V rating G-D.

I actually think you might get away with it. For a while. The "25V" is really a 40V-target diode, generously de-rated because "nobody" puts big voltages on JFETs. As such there is a 90+% chance your actual diodes won't break-down before 36V (the safe limit for opamps from the 40V process). And breakdown may not be instantly fatal. Depends on the current which flows. With only 1K of explicit resistance, the current may be not-small.

BTW, "...gain of about 100 to 1000, depending on the supply voltage..." is also dubious. With a "hard" JFET (high drain dynamic resistance) the gain drops 1% for supply at half voltage. It may drop a bit more with "soft" JFETs. But these days it is hard to know what type a JFET is.

For a one-off DIY, with little risk of burning down the house, or ruining gear, try it and see what happens. Back when I was designing gear for mass production I respected component specifications, and saw my share of component failures that were within specs.

I also recall one power amp engineer who played fast and loose with capacitor high line voltage specs to save a dollar... that bit him in the butt.

JR

JohnRoberts said:

For a one-off DIY, with little risk of burning down the house, or ruining gear, try it and see what happens. Back when I was designing gear for mass production I respected component specifications, and saw my share of component failures that were within specs.

I also recall one power amp engineer who played fast and loose with capacitor high line voltage specs to save a dollar... that bit him in the butt.

JR

Click to expand...

And don't assume the same part from different manufacturers will perform thesame.

Cheers

Ian