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Origins of the JFET mu-amp circuit as used in the "Booster"

6/1/2000 2:44 PM
R.G. Origins of the JFET mu-amp circuit as used in the "Booster"
If you're interested in the origins of this circuit, take a look at:  
perhaps especially what happens when you put in a source resistor.
6/1/2000 3:42 PM
Eric H

Thanks, R.G.  
It'll take me a while to work through that one :)  
There are lots of good tube-oriented articles at that site --if anyone's interested.  
There's another good article on the SRPP/Mu-amp here:  
6/1/2000 4:39 PM

>If you're interested in the origins of this circuit  
I understood the origins to be from this 1970 app note by National Semiconductor:  
6/1/2000 6:52 PM
R.G. Origins of Origins
Yeah, that's the one Jack was going from. I meant the origins of the National JFET circuit - it was a JFET version of an even older triode tube circuit, as shown in the SRPP explanation. I liked the in-depth discussion of other things the circuit could do that the National app note didn't go into.
6/2/2000 5:46 PM
Re: Origins of the JFET mu-amp circuit /Source resistor
You mean source resistor from the source of the TOP FET to the drain of the bottom right? This is what the article is calling Rak??  
Please correct me if I am wrong but what they are saying is that if you increase the resistance between the top FET source and bottom FET drain, you will get more gain with the expense of clean "headroom". In other words, the circuit will distort faster and have more gain.  
At least that's what I thought I read before I fell asleep :-)  
Which makes me think I should be a trim in between the 2 FETs and try it.  
6/2/2000 5:58 PM
Jay Doyle

Funny thing is, I was reading this last night before I fell asleep and I thought the same thing, "so just increase Rak and get more distortion?" Thing is that I don't think it will improve the performance of the JFET type circuits that we are thinking about. The distortion that we like so much (I believe) in these circuit has to do with the "squashing" that occurs in a JFET at the headroom limit, similar to a tube, just not as drastic, beyond that squashing limit you get an output that resembles diode clipping (this is coming from a small bit of experience and the discussion of it in Boscorelli's Stompbox Cookbook - BUY THIS BOOK). Therefore I think that reducing the "clean headroom" will only limit the "squashing zone" (which is dependant on supply voltage) that rests at the headroom limit, of course I don't know what it sounds like with a higher Rak...  
Jay Doyle
6/2/2000 6:05 PM

You are probably right - OK, you did read it the same way I did.  
But imagine this..... You put a pot to control the Rak and hmmmm...... If it's on the 2nd booster in the Booster 2.5/Sweet Thing..... would we be able to get horrid square waves as well as nice sweet distortion?  
Worth a try I guess.  
I will check out that book.  

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