The Lindell Audio Vintage OPA - Melcor 1731 Style Opamp is a pin-compatible plug-and-play operational amplifier designed to work with the Lindell Audio 500 series 6X-500 Microphone Preamplifier, 7X-500 Compressor/Limiter, and PEX-500 Rev.2 Pultec-inspired passive equalizer. It is also designed to work with the Lindell Audio 18XS MKII single-channel microphone preamplifier, DI, line amplifier, and passive Pultec equalizer. The opamp provides a smooth and punchy sound when installed into your compatible Lindell Audio lunchbox or rack equipment.