What type of circuit is in the FeQ-50?

The FeQ-50 utilizes passive “LC Resonant” circuits (L-inductor and C-capacitor). 'Resonant' means that the impedance stays the same throughout the frequency spectrum until the target frequency is reached, then the circuit resonates, causing a change in impedance and gain- FREQUENCY SELECTIVE IMPEDANCE ADJUSTMENT. The circuits are tuned for specific frequencies.
The FeQ-50 has switched Q (bandwidth) settings to avoid putting a resistor before the LC circuit which can create artifacts, and heat issues which can cause gain changes. This switch selects separate inductance and capacitance paths. This allows the filter to maintain predictable with a repeatable response, and no change in volume.
There is no transformer in the signal path and the makeup gain for the insertion loss is a quad op amp pushing no current with a discrete transistor buffer for current gain.
I/O includes: Inputs combo XLR/ ¼”, Outputs- FOUR outputs, +4 and –10 vacuum tube and solid state.