Preamp / Crossover / Tweeter-Midrange Amplifier

                      Bradley Berg    January 2025


I. Combine components for a biamp stereo within a single unit.

    * Costs are reduced using:

          A low power IC amplifier for the Tweeter and Midrange.

          The amp and preamp/crossover share a common power supply.

          Interconecting cables, plugs and jacks are eliminated.

    * Short soldered signal paths keep distortion and noise low.

    * The critical high frequency path is contained within one unit.

    * The circuit is configurable to match a specific set of speakers.


II. Crossover Circuit

   If the speaker has a midgrange driver then it will need to use wa passive
   crossover between it and the tweeter.


                Op1H
                                             LM1875
                 |\
        +--------|+\             C2            |\
        |        |  >---+--------|(----+-------|+\
        |   +----|-/    |              |       |  >---+-----------O  Tweeter
        |   |    |/     |  G--/\/\/\/--+  +----|-/    |              Midrange
        |   |           |        R2       |    |/     |
        |   +--/\/\/\/--+                 |           |
    In  |   |    Rgh                      +--/\/\/\/--+---|(---/\/\/\/---G
     O--+   |                             |    20k        Cz     Rz
        |   +--/\/\/\/---|(---G           |
        |        R1      C1               +--/\/\/\/---|(---G
        |                                       1k    22uf
        |
        |       Op1L
        |                                     Op2W
        |        |\
        +--------|+\             R3            |\
                 |  >---+-----/\/\/\/--+-------|+\        50k
            +----|-/    |              |       |  >--+--/\/\/\/---G
            |    |/     |    G---)|----+  +----|-/   |     ^
            |           |        C3       |    |/    |     |
            +--/\/\/\/--+                 |          |     +------o  Woofer
            |    R1     |                 +----------+
            |           |
            +----|(-----+
            |    C1
            |
            +---/\/\/\/---G
                 Rgw


  Note that C2 and R2 have different values than in the LM1875 data sheet.
  The data sheet uses a 3Hz high pass filter; which is now set higher.
  This way if forms part of the second order filter.

  Rz and Cz form a Zobel network tuned to the speaker.
  It makes the speaker impedance appear as a resistive load to prevent ringing.
  The data sheet shows Rz set to 1 ohm and Cz is .22 uf.

         Rz = Speaker voice coil impedance (Re)

         Cz = Speaker inductance (Le) / (Rz^2)


  Rgh and Rgw set gains for the high and low signal paths.

         Rgh = (Gc - 1) / R1     Gh = 1 + Rgh / R1

         Rgw = R1 / (Gl - 1)     Gl = 1 + R1 / Rgw


  The crossover can be configured with first or second order filters.
  A progressive crossover can also be configured.  This is a hybrid design
  that combines features of first and second order filters.

  Note that the R2 resistor before the amp chip is in parallel with the
  input resistance of the amp (22k ohms for the LM1875).

  A 2 way progressive crossover has a first order crossover point
      and a second filter an octave away from the first.

        High:  high pass @ F   high pass @ F / 2 = F2

         Low:   low pass @ F    low pass @ F * 2 = F3


         For:  C1 = C3 = .01uf --> R1 = 15_915_494 / F

               C2 = .1uf

               F2 = F / 2 --> R2 || 22k = R1 / 5
                          --> R2 = 1 / (F / 3_183_098 - 0.0000454545)

               F3 = F * 2 --> R3 = R1 / 2



III. Volume and balance controls can be added using a fourth operational amplifier.


                             +---------------------o Full range output
                             |
    Input A  o        |\     |
             ----o----|+\    |      50K
    Input B  o        |  >---+---/\/\/\/\---G
                  +---|-/    |      ^
                  |   |/     |      |     50K
                  |          |      +---/\/\/\/\---G
                  +----------+             ^
                                           |
                                           o-------o  To crossover input