Effectrode PC-2A Photo-Optical Tube Compressor All Tube: 100% pure analog vacuum tube audio path operating at amp plate voltages ensures the PC-2A compressor has huge headroom and very quiet, natural sound reproduction. The result is a compressor that performs exceptionally well with electric, active bass, 12-string and acoustic guitars. Limit/Compress Switch: Select between two completely different compression curves. Compress mode is a light, subtle compression that can be used to enhance tone and even out string dynamics  the tube and photo-cell yields an extremely natural sounding compression. Limit mode is heavier for a punchier, squished country tone. Internal Attack control: To allow bass players with high output/active pick-ups to adjust the compression response characterstic to tailor it to their needs. True Bypass Switching: With Effectrodes unique anti-pop or thump footswitching circuitry utilising sealed, gold-contact relay to eliminate the possibility of dirty contacts degrading the sound and minimial internal audio path.