Why A crystal lattice filter is a bandpass filter built from quartz crystals wired in a lattice (bridge) configuration, where pairs of crystals are offset slightly in frequency to set the passband. The very high Q of quartz gives a narrow passband with steep skirts, which is why these filters are the standard IF filter in SSB and CW receivers and transmitters. Quartz elements handle only tiny amounts of power, so the filter is placed in low-level signal stages, never in a high-current path.
Watch out The power supply choice is the trap: quartz crystals are RF resonators that would be destroyed by supply currents, and supply filtering is done with capacitors, inductors and regulators. The audio filter description with crystals spaced 1 kHz apart is invented wording, not a real device.
Quartz is delicate: lattice filters sit in the low-level IF chain, not in the power supply.