JFET aleph- design comments please

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Hello,
I am planning on building the attached circuit, and I would like some feedback on the design. Please point out anything you see wrong, or that could be better. Thanks.
 

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Official Court Jester
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nobody special said:
Hello,
I am planning on building the attached circuit, and I would like some feedback on the design. Please point out anything you see wrong, or that could be better. Thanks.


current clumping circ in upper output fet's gate is missing but-who cares;

what I care is that 2,7mA through driving half of input diff stage is somewhat much less than I use (or I dare to use).
all in all - I can't see anything more to comment
go for it
 
two key concerns are:
1. how much current the jfets can handle safely/linearly/etc. and
2. how much current is needed to drive the output power mosfets to meet/exceed your "desired performance" (e.g. distortion, frequency response, etc.)

need to consult the datasheets and characterize your particular devices to bias at "optimum" points.

lots of good hints in nelson pass articles on http://passdiy.com/

good luck,
mlloyd1

nobody special said:
Thank you for the critique.
What is a typical bias current for a differential pair of 389's? Where is the sweet spot?
 
Nelson Pass said:
If you are using IR parts, I would consider P channel JFETs
in the input and drive the irfp240 parts, which have better
performance in CS than the P channel IRFP9240.

Thank you, Nelson. It's always exciting to get a direct response from you.
Is the problem with the N-channel Jfets due to the kink in the response of the IRF P-channels that you mentioned in your article?
What would you recommend for a dual p-channel jfet, and where could I buy them?
I had considered this, but I have the LSK389's laying around. I don't mind buying something else if it will improve the performance, and I do have a bigger selection of 240's than 9240's, so that would work out.
 
mlloyd1 said:
two key concerns are:
1. how much current the jfets can handle safely/linearly/etc. and
2. how much current is needed to drive the output power mosfets to meet/exceed your "desired performance" (e.g. distortion, frequency response, etc.)

need to consult the datasheets and characterize your particular devices to bias at "optimum" points.

lots of good hints in nelson pass articles on http://passdiy.com/

good luck,
mlloyd1


mlloyd1,
Thanks for the summary- good advice. I was just being lazy. I figured someone who had used them before could comment on where they found them to sing the best.
 
The one and only
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nobody special said:
Is the problem with the N-channel Jfets due to the kink in the response of the IRF P-channels that you mentioned in your article?
What would you recommend for a dual p-channel jfet, and where could I buy them?

You can avoid the "kink" by using Harris or Fairchild, not that it's
a big problem, and for P channel JFETs you can try 2SJ74 or
(matched pair) 2SJ109.

:cool:
 
Formerly "jh6you". R.I.P.
Joined 2006
nobody special said:
I am planning on building the attached circuit, and . . .


Interested while I see that the concept is the same with FIG8 of DIY Op Amps written by Nelson Pass. I hope you will go ahead with your experiment for us. BTW, any special reason of preference with P-Ch MOSFET?

-------------------------------------------------
PS
In the air of 35 deg C, catching cold is a real pain . . . I didn't know . . . :boggled:
 
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Re: Re: Re: Re: JFET aleph- design comments please

Babowana said:



Yes.

I'd have had to listen to wife's advice.
hehe . . . i'm clean out of her control . . . don't know how long could be . . .

O, let's go back to the topic . . . JFET, MOSFET (N or P?) . . .


hehe
Babothanwife :clown:

I must admit,before I scavenged few 2SJ109 ,I have same idea for flipping Aleph topology........problem was-I was too lazy (??!!!????-not anymore) to match tiny fets (if I go to use single ones) and I don't have any of 2SK389 also...........

in a meantime I was given with few 109 ,and that save me trouble of purchasing P mosfets,which I didn't have in my drawer in that moment.......

refresh my memory-it's 0549 at my place now and I'm too tired for searching (dac output IV stage proposal posts) -your fave jfet is....?
 
I hadn't noticed the similarity to the opamp article. It has been months since I have read it.
The reason for flip-flopping the Aleph was mainly because I have in hand LSK389 dual n-channel jfets for the front end.
On another note... I have an idea, but it might be thought of as total blasphemy:
How about an active load on the front end to raise the open loop gain (and lower closed loop distortion), and then adding a servo to the output to control the dc offset. Something like this (see attached).
It would still be a 2-stage amplifier, but the simulation shows 60db open loop gain, which knocks the distortion down quite a bit.
 

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