The CL-6 Input Expander from Sound Devices adds 6 balanced, line-level inputs to the 664 Production Mixer. The Input Expander connects to the top or bottom of the 664 and includes 6 full-sized fader controls, PFL control and highpass control. It also features dedicated L and R routing buttons for Inputs 7-12 on the 664 Mixer.

The large, backlit Record and Stop buttons provide convenient access to transport functions. Other features include large, daylight-viewable LED meters (with track arm indicators) for tracks L, R, X1, and X2. When the CL-6 is connected, the TA3 connections for Direct Outputs 1-6 on the 664 convert to balanced, line-level inputs. These Inputs are numbered 7-12 respectively. Routing, ISO Track arming, and Fader control all function identically to the 664's main inputs.