Yes, conjugate networks perforce should be tuned for every loudspeaker. They also must be placed at the transducer end, otherwise the driver will get the same signal as before because most amplifiers [1] are voltage sources, and one would miss the point of nulling out impedance swings at the end of a non-ideal cable.
[1] except for a few of Papa Nelson's more amusing offerings
Agreed on 30 degrees typically being where the load is highest for subwoofers. However, having a 60 degree load phase angle means there will be a 30 degree angle at a lower impedance magnitude; if the maximum phase is 30 degrees then the worst-case EDPR will occur at a 15 degree load phase, more or less, at less stress on the amplifier. Large phase angles are an indication problems may or may not occur elsewhere.
Outside SOA considerations [2] a plate amp/subwoofer system won't really need a conjugate network because the cable length is so short.
[2] which don't apply to class-D
[1] except for a few of Papa Nelson's more amusing offerings
Agreed on 30 degrees typically being where the load is highest for subwoofers. However, having a 60 degree load phase angle means there will be a 30 degree angle at a lower impedance magnitude; if the maximum phase is 30 degrees then the worst-case EDPR will occur at a 15 degree load phase, more or less, at less stress on the amplifier. Large phase angles are an indication problems may or may not occur elsewhere.
Outside SOA considerations [2] a plate amp/subwoofer system won't really need a conjugate network because the cable length is so short.
[2] which don't apply to class-D
Class D subwoofer amplifiers often don’t have any filtering AT ALL, running the PWM straight into the voice coil.
If you want to use a conjugate impedance network inside the loudspeaker box to improve the stability of the amplifier (per the subject of this thread), you will either need to build the amplifier into the loudspeaker box or use a loudspeaker cable with a high-frequency characteristic impedance close to the loudspeaker box impedance. Loudspeakers are usually around 8 ohm, it might be difficult to get the cable characteristic impedance that low. With a coaxial construction, you would need a thick inner conductor with a thin insulation around it.
