The Solid State Logic X-Rack VHD Microphone Preamp is an input module that has a front end combining classic SSL character with Solid State Logic's new Variable Harmonic Drive circuit. You get a time machine of overdrive characteristics.What is VHD?It may look like just another knob, but SSL's VHD (Variable Harmonic Drive) preamp is far from a traditional linear gain control - it's more like a time machine of overdrive characteristics.When VHD is used in conjunction with the input gain control, a huge variety of preamp sounds can be created, from gentle warming to serious distortion. At relatively low input gain settings the VHD Pre preamps deliver transparent signals with all of the sonic fidelity you'd expect from SSL. As the input gain setting is increased, more of the signal is passed to the Variable Harmonic Drive circuit, increasing the intensity of the effect from subtle to extreme. By adjusting the VHD control the distortion characteristics can be shifted from '50s valve-style overdrive (if the VHD selection control is turned fully anti-clockwise for 2nd harmonic distortion) to '70s transistor-style grit (if the VHD selection control is turned fully clockwise for 3rd harmonic distortion)… or a blend of the two.As a feature of Duality, XLogic E Signature Channel and XLogic Alpha Channel, VHD is used in studios around the world and is rapidly establishing itself as the 'go-to' tool that allows modern producers to achieve a sound that evokes the sonic heritage of SSL while adding a strong contemporary edge.