Marchand Electronics manufactures and sells hand built and custom designed audio equipment. Phil Marchand produces active, passive, solid state and tube crossovers, power amps for audio or laboratory usage, tube and solid state pre-amps, bass eq, discrete op amps, and many more items.
It’s worth noting he has three free
software tools for the audio enthusiast. The coolest is simple component calculator for capacitor and resistor values for a variety of crossover models with crossover frequency and slope variables. I know this can probably be accomplished easily in excel these days but I’m personally a fan of dedicated tools that do one thing well.