Looking for Preamp...Found This !

[petemoore]

  
  http://www.rason.org/Projects/jfetamp/jfetamp.htm
  Built it.
  Took just over an hour to complete the circuit, because I remembered to locate the source to the middle of the JFet Sockets, and it wasn't hard to wire up.
  I used trimpots, but with this design I think they're unnecessary, if you stay close to the drain R values and don't go under them.
  So it fired right up, and seemed kinda loud for 1/2 volume, so I turned it up, and to my amazement and very pleasant surprize, it kept increasing in percieved level, past where the little-er speakers than I'm planning to use with this rig [Jfet pre into 1/2 of Dyna ST70]...it had room to go even at that point. Up to very loud, no wierdness, just the sound of Jfets pumping up the amp, a great tone emitted. Above a certain point a hardness occurred with the speakers or something.
  I've tried 12v 12ax7, HV12ax7'S with lots of promise and some surmounted problems, Opamps...I'm sure there are other ways.
  For the output clarity and character, level, and ease of build, this one is topper of all I've attempted to get great tone and volume from the Dyna...I'm glad I looked through the links and found that page !!!
 

[brett]

Hi Pete.
Yeah, these are SO cool.  Substantial boost, with just the tiniest amount of sweet distortion.
Welcome to the FET fan club!!
The Stratoblaster (http://www.generalguitargadgets.com/diagrams/asb_sc_i.gif) is very similar to these, and is my favourite booster.
cheers

[mathflan]

very interesting!!

I'am searching some JFET preamp scheamtics for Microphones so with LOW impedance..

[Aurin]

Nice article and explanation - looks like it might be fun to build... and tinker with.

[petemoore]

  For this purpose, I've built a Stratoblaster to compare. Took a while debugging before trying a different Jfet...the first one was around 3.3v-3.5v...all pins...
  [I used to have these around...built a batch sent two SB's with DPDT/100k/push pull bypass to my brother for in guitar use. He wanted just the sound of it, not much of the boost [so that with the volume all the way up it's just above unity, makes sense so you don't "Blast yourself do death trying to get it turned on around unity. I don't remember whappened to the other one... 
  Anyway, the Jfet preamp has a little bit too 'hi-fi' distortion, I'm finding the Stratoblaster to sound super good, I forgot how much this thing boosts...I suppose the headroom of the amp matters on how that gain knob works...kinda 'airy' sounding on clean, super Jfet sounding with FF in front.
  I'm getting it to my standard now of "Hot Dog' setup, really different amazing sounding amp with just these things driving it.
  I can't quite crank this rig yet because of the speakers, I'll try it with the bigger speakers...it sounds like it's waiting for that...

[d@vide]

I cannot reach that page... 
the link seems boken...

[Gripp]

Link works for me.