  We propose a postselecting parity-swap amplifier for Schr\"odinger cat states
that does not require the amplified state to be known a priori. The device is
based on a previously-implemented state comparison amplifier for coherent
states. It consumes only Gaussian resource states, which provides an advantage
over some cat state amplifiers. It requires simple Geiger-mode photodetectors
and works with high fidelity and approximately twofold gain.
