Preamp / Crossover / Tweeter-Midrange Amplifier

                      Bradley Berg    September 2020


Combine components for a biamp or triamp stereo within a single unit.

    * Costs are reduced by using a common power supply and fewer interconnects.

    * 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.


                Op1h
                                             LM1875
                 |\
        +--------|+\            C2             |\
        |        |  >---+-------|(-----+-------|+\
        |   +----|-/    |              |       |  >---+-----------O  Tweeter
        |   |    |/     |  G--/\/\/\/--+  +----|-/    |             [Midrange]
        |   |           |       R2        |    |/     |
        |   +--/\/\/\/--+                 |           |
     In |   |    Rgh                      +--/\/\/\/--+---|(---/\/\/\/---G
     O--+   |                             |    20k        Cz     Rz
        |   +--/\/\/\/---|(---G           |
        |        R1      C1               +--/\/\/\/---|(---G
        |                                       1k    22uf
        |       Op1l
        |                                     Op2c
        |        |\
        +--------|+\            C3             |\
                 |  >---O-------|(-----+-------|+\
            +----|-/    |              |       |  >---+
            |    |/     |  G--/\/\/\/--+  +----|-/    |
            |           |       R3        |    |/     |           O  Coupler
            +--/\/\/\/--+                 |           | Triamp    | [Woofer]
            |    R1     |                 +-----------+--O        V
            |           |                                ---O--/\/\/\/---G
            +----|(-----+--+-----------------------------O       50k
            |    C1        |                            Biamp
            |              |                  Op2w
            +--/\/\/\/--G  |                                      O Woofer or
                 Rgw       |    R3             |\                 | Subwoofer
                           +--/\/\/\/--+-------|+\                V
                                       |       |  >---+-----O--/\/\/\/---G
                           G----)|-----+  +----|-/    |          50K
                                C3        |    |/     |
                                          |           |
                                          +-----------+


  For simple configurations some circuitry is not needed and can be eliminated.
  The Biamp / Triamp switch can also be eliminated when not applicable.

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

  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.

  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


  A 3 way progressive crossover adds a line out for a bass coupler with
    a first order band pass filter covering one octave.

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

     Coupler:   low pass @ F   high pass @ F / 2

         Low:   low pass @ F    low pass @ F / 2


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

               R3 = R1 * 2

               C2 = .1uf

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



  You can also put a coupler and woofer in the same cabinet with a passive
  crossover.  With this variation only one operational amplifier (Op1) is used.


      +-------|(-------O<   Coupler   High pass @ F/2     C = 0.159 /(Rc*f)
      |
   O--+
      |
      +-----UUUUUU-----O<   Woofer     Low pass @ F/2     L = Rw/(6.28*f)