🚀 go-pugleaf

In article <9723@thorin.cs.unc.edu> rohlf@pooh.cs.unc.edu (John Rohlf) writes:
>
>    I need the transfer function for the basic 3-band tone controls 
>diagrammed below:

Here's an input "deck" for a circuit simulator (such as SPICE) that'll give
you the transfer curve from 1Hz to 1MHz.

  john rolf's tone control, case2  (input node 10, output node 20)
  *  for case 1 just set rout to a small value like 1 ohm
  *
  *
  .ac   dec   20    1.0   1e6
  *
  vin   10    0     ac    1.0
  *
  * rout emulates the output impedance of the preceding tube stage
  rout  10    1           (rout value)
  *
  c1    1     2           (c1 value)
  r1    1     3           (r1 value)
  c2    3     4           (c2 value)
  c3    3     6           (c3 value)
  * treble control Pt.  Be sure (r2+r3)==Pt
  r2    2     20          (r2 value)
  r3    20    4           (r3 value)
  * bass control Pb.   Notice that (0 < r4 < Pb)
  r4    4     6           (r4 value)
  * midrange control Pm.   Notice that (0 < r5 < Pm)
  r5    6     0           (r5 value)
  *
  .print  ac  vdb(20)  vp(20)
  .plot   ac  vdb(20)  (-37.0, 3.0)
  *
  .end


If you want to do it analytically, just draw the ckt again but instead
of capacitors temporarily put dummy resistors RD1, RD2, RD3,   and for
the potentiometers put k*Pt and (1-k)Pt, xPb, and yPm   (where k, x, y
are between 0 and 1; they represent the % rotation of the pot shafts).

Now, using KVL and KCL, write the expression for Vout/Vin.  When you
have that, replace RD1 by (C1/s), replace RD2 by (C2/s), and replace RD3
by (C3/s).  Voila, you've got the transfer function.
-- 
 -- Mark Johnson	
 	MIPS Computer Systems, 930 E. Arques, Sunnyvale, CA 94086
	(408) 991-0208    mark@mips.com  {or ...!decwrl!mips!mark}