RG's SRPP macro-booster questions

[squidsquad]

I just made it....as per schem at this page's bottom:  http://geofex.com/Article_Folders/modmuamp/modmuamp.htm
WOW...quite a powerful boost!  And a VERY easy build.
A few questions:  There's an unmarked cap on the left of the schem.  What should it be?  Why is it there?
I've popped a few different types into position & hear no difference in sound w/or without them.
Also...I hear no difference changing the cap between the JFETS either....a bit odd?
Additional note...I used a 10k trimmer instead of a 1k resistor to couple them.

[petemoore]

  Also see AMZ Minibooster..

[Jered]

  If I remember correctly (big if) .001, I think it helps shunt any noise, not sure.
  As far as the .1 cap connecting the upper FET gate and lower FET drain. You should hear a significant change in the sound. Put a .022 in there and see if you hear a difference.
  Jered

[R.G.]

I just made it....as per schem at this page's bottom:  http://geofex.com/Article_Folders/modmuamp/modmuamp.htm
WOW...quite a powerful boost!  And a VERY easy build.
A few questions:  There's an unmarked cap on the left of the schem.  What should it be?  Why is it there?
I've popped a few different types into position & hear no difference in sound w/or without them.

It's to shunt any thermal noise from the biasing resistors to ground to make it quieter. Read:
http://geofex.com/circuits/Biasnet.htm

Also...I hear no difference changing the cap between the JFETS either....a bit odd?
Additional note...I used a 10k trimmer instead of a 1k resistor to couple them.

In my version of the SRPP, the cap makes little difference. Here's why: The cap is coupling a high impedance node - the drain of the lower FET - to another high impedance node, the gate of the upper FET and the 4.7M biasing resistor. The loading on that cap is the 4.7M resistor and the input impedance of the JFET gate in the source-follower configuration, which is even higher. It's hard to calculate the actual loading on that point. But if the input of the JFET were only another 4.7M - and it's likely very much higher than that - then for the cap to couple a full guitar range of 82Hz on up, the cap would only need to be C = 1/(2*pi*82*2.85M)= 681pF.

So no, it doesn't matter much what you put in there if it's over say 1000pF. The're all massive overkill. That's one consequence of the 1K resistor in the source of the upper FET - the input impedance gets very big and the output impedance gets small. Put a 100-220pF cap in there and you should hear the bass thin out. 

The 1K to 10K trimmer won't make a whole lot of difference other than making the thing have less headroom.

Also see AMZ Minibooster..

Yeah, the article referred to the "minibooster", and its history in JFETs from National, as well as it's history in triodes.