Walrus Audio Mira Optical Compressor

The Mira is a true bypass optical compressor offering studio-grade sound sculpting and sustain packed with controls you’d normally only find on studio units. Use the Blend knob to mix in your compressed sound with your clean tone to maintain note clarity while offering increased sustain.
Mira also offers in-depth control over Attack, Release, Ratio, and a Make-Up knob to help maintain unity levels after your signal has been compressed. For higher headroom, Mira runs internally at ±15VDC.  
Adding to the bonus features, Walrus decided to also provide a push-button High Pass Filter in the side-chain (the part of the circuit that controls the optical element in the compressor).
This is great for preventing bass frequencies 120Hz or below from engaging the compressor as much as higher frequencies. The keen result of this keeps the compressor from overworking on lower notes and is great for bass guitar or keeping low-frequency dynamics dramatic for guitar players.