Preamp / Crossover / Tweeter-Midrange Amplifier

                      Bradley Berg    March 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 circuitry that is not needed 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.

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 Cz set to .22 uf and Rz is 1 ohm; which seems low.

         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

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


         For:  C1 = C2 = C3 = .01uf --> R1 = 15_915_494 / F

               F2 = F / 2 --> R2 = R1 * 2

               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

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

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


         For:  C1 = C2 = C3 = .01uf --> R1 = 15_915_494 / F

                                   R2 = R3 = R1 * 2


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)