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Old 4th November 2008, 07:41 AM   #1981
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Quote:
Originally posted by Nelson Pass
Take away one each of the parallel 100 ohm resistors, and let it rip.

Done. Thanks!
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Old 4th November 2008, 06:32 PM   #1982
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why are the feedback resistors power resistors? Can normal 1/4 watt work here?
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Old 4th November 2008, 06:37 PM   #1983
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No. As it is the 3W resistors can get quite warm.
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Old 4th November 2008, 06:56 PM   #1984
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Quote:
Originally posted by Nelson Pass
No. As it is the 3W resistors can get quite warm.

OK but if we keep the same ratio and increase the valuses to kilohms then are there going to become hot? for example 15kohm and 2,5kohm ?
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Old 4th November 2008, 07:14 PM   #1985
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Quote:
Originally posted by kzeprf22



OK but if we keep the same ratio and increase the valuses to kilohms then are there going to become hot? for example 15kohm and 2,5kohm ?

if you increase source resistor for input Jfet(s) , you'll increasing something called "source degeneration" - fancy word for local feedback

that way you'll throw gain of amp through window .... or other favorite hole

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Old 4th November 2008, 07:17 PM   #1986
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Quote:
Originally posted by kzeprf22
OK but if we keep the same ratio and increase the valuses to kilohms then are there going to become hot? for example 15kohm and 2,5kohm ?
You can cool them down that way, but you will be dramatically altering the
open loop gain of the amp by increasing the impedances of the feedback loop
from 10 ohms to 2.5K ohms.

You can tolerate going from 10 ohms to maybe 20 or 30 ohms, but even
then you will see an increase in distortion for lack of feedback.

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Old 4th November 2008, 07:28 PM   #1987
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Ok if we are talking for the F5 distortion figures, yes we will increase distortion, by altering the feedback impedances. I wrote that because this is the way is done in most commercial amps (class ab-a). The feedback resistors are in kohms and provide high gain ratio (in most of the cases). Of coarse for commercial amps the distortion difference due to this does not contribute much in the total distortion figures. In F5 as you said it will make difference.
Good point, Thanks
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Old 4th November 2008, 09:44 PM   #1988
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The thing that tends to get lost in the shuffle is that the feedback on the F5 isn't 'normal' feedback. Well...it is, in the sense that it acts like (and is) negative feedback, but the fact that the feedback signal feeds into the Sources instead of the Gates is crucial. Tube folks do this sort of thing all the time, but for some reason it's uncommon in solid state.
Try thinking of it this way--pretend that there's one and only one JFET, not a complementary pair. (Remind me to tell you a story about how my love of puns got me in trouble with Nelson over the word 'complementary.' Very embarrassing. I embarrass myself regularly--keeps me humble. [No comments from the peanut gallery!]) The signal comes in at the Gate and exits via the Source. This is Common Source. No conceptual problems, right? Now look at the Source as an input--the node you will be routing the feedback into, in fact. Remember that the Source can run as either input or output. When used as an output, the configuration is known as a Source Follower. And what do you know about a Source Follower? That the Source is a low impedance node. Bear in mind that that does not change based on whether the node is used as an input or an output or not used at all...the Source is always a low impedance node! The idea that there are two JFETs back-to-back doesn't change that in the least.
And if you're trying to push a low impedance node, you're (generally) going to find that it's best to push it with another low impedance node. In this case that means the resistor network between the output and the input pair will need to be relatively low in impedance; lots less than a "normal" pair of feedback resistors, where the resistors would be many thousands of Ohms.

Grey
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Old 4th November 2008, 09:53 PM   #1989
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Quote:
Originally posted by GRollins
.......-keeps me humble. .........

are you sure that you don't have some serbian roots ........



edit :

more important than fun with peanut/humbliness is -

I always have impression that that sort of feedback arrangement , which Papa use in F5 , have greater amount of .......... speed ..... naah - not speed , but immediacy ;

that's probably just result of lack of my regular education in electronics
but some things I just feel , or visualize like that ......

taking in account capacitances or not ......

yup ; immediacy is a word
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Old 5th November 2008, 06:32 PM   #1990
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Yes, this form of feedback has been around since the late '20's or early 30's.

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