From: peterca@ento.csiro.au
Newsgroups: rec.audio.high-end
Subject: valve gain stages (Re: CD tweaks)
Date: Tue, 22 Dec 92 11:52:59 EST
Organization: University of Wisconsin - Milwaukee
NNTP-Posting-Host: 129.89.7.4
Originator: tjk@csd4.csd.uwm.edu

>From: Thomas Krueger <tjk@csd4.csd.uwm.edu>
>Peter: Post the SRPP circuit schematic, please. Looks interesting.

Our esteemed moderator suggests that I post the SRPP circuit schematic. 
I've never tried to draw pictures with this so here goes:

                +~220V
                 |
               plate 
         ______grid 
        |     cathode
        |        |
        |      2Kohm
        |________|____________||___________ out
                 |       |         |
               plate     |         |
in__||_________grid      2Mohm     1Mohm
   |1uF       cathode    |         |
   |             |       |         |
   |_____________|_______|_________|___signal earth


The dual triode can be a 12AX7 or 12AT7.  This is what I have for the CD
stage.  My phono stage has a similar circuit using a 12AX7 then passive
RIAA then 12AT7.  

                +~220V
                 |
               plate 
         ______grid 
        |     cathode
        |        |
        |      1Kohm
        |________|________||_______________ out
                 |                |
               plate              |
in_____________grid             1Mohm
   |          cathode             |
   |          ___|__ ____         |
  47Kohm      |     |    |        |
 +30pF        =     =   1Kohm     |
   |__________|_____|____|________|___earth
            470uF 0.47uF

The 47K can be what ever load resistor your cartridge likes (ditto the
30pF).  The coupling cap is 0.22uF for the 12AX7 and is part of the RIAA
(which goes in where the 1Mohm is indicted.  For the second valve 12AT7 the
coupling cap is .47uF but I will change it for 1uF for less bass roll off
into ~80K load of vol. control.  The gain of these stages is about 22db I
believe.  The upper triode acts as a constant current source for the lower
amplifying triode or more intuitively; as the lower triode is turned on its
plate voltage falls which tends to turn off the upper triode which reduces
the current and vice-versa.  Each stage of the circuit is phase inverting. 
It is claimed to have <0.01% distortion for 1V out.  Also distortion is
supposed to fall with increasing frequency.  The phono design was in
Elector in 1986 in an article by Guls and the variant above was copied from
the circuit of an amp by Laurie Cohen.  
A similar circuit was in Glass Audio recently (drawn from unreliable memory
below) and has cap coupling of the lower plate to the upper grid, a larger
plate resistor inserted.  The output is taken from the upper cathode.  The
advantage of this is supposed to be lower output impedance but requires an
extra coupoling cap.  The above circuit with 1,5Kohm cathode resistors and
a 12AU7 is the line stage of the Guls amp and has an output impedance of
2k4 rather than ~800ohm for the Glass Audio design.  The phono stage was
only given as "somewhat higher" than the line stage in the Guls article.  
  
                   +V
                    |
                  plate 
            ______grid 
           |     cathode
           |        |________||_______________ out
           |      1Kohm              |
           |_1Mohm__|                |
           |        |                |
           =      80Kohm?            |
           |________|                |
                    |                |
                  plate              |
   in_____________grid             1Mohm
                 cathode             |
                 ___|__              |
                 |     |             |
                 =    1Kohm          |
      ___________|_____|____ ________|___ signal earth
               
I hope these diagrams stay together on their way through the ether and
prove useful.  

----------------------------------------
Peter Campbell
C/O Div. Entomology, C.S.I.R.O.
Box 1700 Canberra A.C.T. 2601
Australia  (peterca@ento.csiro.au)
Ph.61-6-2464158 (w), 61-6-2516213 (h),
61-6-2464173 (fax)
----------------------------------------