Quartet: Is the long thumb screw on the back an earth ground connection? Chris February 23, 2021 21:23 Updated Answer: No ,The thumb screw on the back panel is designed to be used as a cable clamp, it is used to secure the USB, and or the AC power cables in a loop so they are not easily disconnected. Related articles Grounding Noise Quartet's speaker icon is missing, and the speaker touchpad is inactive Is Quartet compatible with Mac OS X Mavericks (10.9)? Can a USB mouse, hub or other device be connected to Quartet's USB MIDI Port? Comments 0 comments Article is closed for comments.