The SSL XRack VHD Input Module XR627 is a combination of the patented SSL preamp design & the classic SSL Listen Mic Compressor. The VHD Input Module XR627 features VHD circuity (Variable Harmonic Drive) which is a totally analog circuitry that means that you can add a controllable blend of 2nd or 3rd harmonic distortion. The versatile VHD Input Module produces a warm yet crisp sound that is normally reserved for vintage modules. The built in SSL Listen Mic Compressor is designed to prevent overloading the return feed with a fixed attack & release ideal to be used with ambient drum mics & more.