Audison SPM4 stereo passive mixer (4 channel).

Stereo passive mixer (unpowered) featuring 4 input channels and 2 output channels, designed for active OEM multi-way systems where each speaker is powered by one specific amplified channel.
SPM4 mixes audio through 4 audio transformers specifically developed to achieve very low distortion and highly linear acoustic response.
SPM4 provides the ability to mix the midrange/woofer and tweeter channels coming from the amplifier and supply one single mid-high channel.

The Ch1(L/R) channels feature two inputs, one features an inverting input compared to Ch2 (L/R) channels; the mixing process often gives better results when one of the two channels undergoes a 180° dephasing.
SPM4 is especially recommended to extend the number of inputs of the Audison digital processors or amplifiers with built-in DSP because they feature a De-equalizer (De-Eq).
In that case, any possible non-linear acoustic emissions due to the mixing of the pre-existing filtered channels are duly compensated.