Multiple JFET Stages

[Rayman]

Newbie here -

Can anyone direct me to information regarding how to design common-source JFET (or BJT) gain stages in series to create distortion (like those on ROG and Joe Davisson's site)?  This forum has been very informative regarding biasing individual stages, but I haven't found much with respect the relationship between one stage to the next to create distortion.

Any help is appreciated.

[petemoore]

  Once you get enough gain going into a stage to make it clip your'e into distortion.
  Check out all the JFet designs that use the technique. 2 Mu Amps [BSIAB's have 'em] get plenty of that.
  The more stages you series, and the higher the staged gains get...next thing to think of [or what you'll end up wanting to have to do] is voicing...
   I was messing with 'voltage starved Mu Amp's...Pretty Cool too.
   Jfets are fun to tweek into distortion.

[Rayman]

thanks, I'll take a look at those circuits

[petemoore]

There are lots of them, some are easier to find than others, all the ones I'm typing of are Jfet, and are biased for different gains, but within 'good operating standards' for the device.
  Many of the multi transistor [Od/Fuzz] Jfets bias setups may resemble those of 1 stage Jfet circuits like
  Stratoblaster
  Fetzer Valve
  Sparkleboost
  It's not uncommon to see four or five of these in series, setup so that stages after the first one or two clip, so that all the clipping is not taking place in just one position.

[Rayman]

Is there an advantage to clipping over multiple stages as opposed to one stage?  Am I correct in that the Rd and Rs values in the common-source JFET circuit set the gain?

[WGTP]

See Mark Hammer's response to the Clipping Threshold post.  It addresses multiple clipping stages.  IMHO it sounds way better.   

[Mark Hammer]

Both RG Keen (www.geofex.com) and Jack Orman (www.muzique.com) have some excellent little documents on designing JFET gain stages.

[cd]

Is there an advantage to clipping over multiple stages as opposed to one stage?

Nice explanation with tons of examples here:

http://www.harmony-central.com/Guitar/guitar-amp-evolution.html

Specifically applies to tube amps, but you should get the idea.  Note that a JFET stage it at best vaguely similar to a 12AX7 stage.

Am I correct in that the Rd and Rs values in the common-source JFET circuit set the gain?

Assuming Rg is kept the same, yes.  Higher Rd/Rs ratio = higher gain... BUT then there are biasing issues as well.  If you have a breadboard, play around with different Rd and Rs values, keeping on eye on the drain voltage, and listen to what happens to your signal.

[Rayman]

many thanks for responses!