Tube Amps / Music Electronics
For current discussions, please visit Music Electronics Forum. The sunn still shines online!

ampage archive

Vintage threads from the first ten years

Search for:  Mode:  

Re: Origins of the JFET mu-amp circuit /Source resistor

6/2/2000 6:21 PM
Jay Doyle
Re: Origins of the JFET mu-amp circuit /Source resistor
I think you got the right idea there, sounds right to me. The only limitation that I can forsee would be a lowering of the overall output volume, as you would be limiting the available headroom of the bottom JFET (the actual amplifier), thereby limiting the overall output voltage. [And at a point I think you would start messing with the bias of the bottom FET.] Of course you could try another one of Doug's "pot mods" and as you turn up the resistance of Rak, thereby lowering the overall volume, it also turns up the gain in an output buffer/booster accordingly.  
I'd say I'll let you know soon but fact is, I am playing so much that it is seriously limiting my shop time. Not a terrible thing to complain about, but all this is making me salivate!  
Antique Electronic Supply, I got Boscorelli's book [search for "Stompbox Cookbook"] from them with a 5 day turnaround. Incredible book, I can not reccomend it highly enough!  
Jay Doyle
6/2/2000 6:22 PM
Jay Doyle

Sorry Aron that was  
6/2/2000 6:49 PM
""so just increase Rak and get more distortion?""
I actually think what you get is a different *kind* of distortion. Single ended JFET with a current source load is a high single device gain with cutoff distortion on one side and saturation distortion on the other - inherently asymmetrical. Truly push-pull circuits, as the SRPP is advertised to be would distort symmetrically in saturation on both sides. Modulating between the two would cause a transition between the two types, something like what's-his-name's "dancing harmonics".  
Just a thought...
6/2/2000 6:33 PM

The lower the current through the amplifying fet then the higher the gain up to the limits of the device. Anytime you increase gain you will decrease the amount that the input signal can vary before hitting the limits of the gain stage (therefore less clean headroom). This is true of jfet, bipolar, mosfets, opamps, or tubes.  
A source resistor between the stacked fets will lower the current and therefore increase gain to some extent.  
The output impedance also increases as the current is reduced through the fets.  
6/2/2000 6:35 PM
Jay Doyle

I forgot about the output impedance problem, but if you use an output buffer/booster to "reamplify" the signal, it wouldn't be a problem. Interesting.
6/2/2000 6:45 PM
"Which makes me think I should be a trim in between the 2 FETs and try it."
... or use an envelope controller or LFO to modulate a variable resistor...
6/4/2000 11:59 PM
Ed Rembold
Re: Origins of the JFET mu-amp circuit as used in the "Booster"
Thanks, this is a great read, My head is still  
spinning. However, I just can't say "Minibooster"  
without saying "Jack Orman". That's just the way it's gonna be. Ed R.

<<First Page<PrevPage 2 of 3 Next> Last Page>>