The FAIRCHILD 670 mkII is equipped with a Mix Control which allows you to set a perfect balance between the compressed signal and the original non-compressed input signal.
Some people call it the Wet / Dry control.

To avoid confusion, and to keep maximum flexibility, the FAIRCHILD 670 mkII has two sets of stereo Outputs :
1. The Classic 670 out, straight from the Transformer Output
2. A Mix Out which is the blended Output from the MIX control.
The Mix control goes gradually from input (equivalent to being in a Bypass situation) to Output (equivalent to the standard classic 670mkII Out)

See also Back Panel page for more details
IMPORTANT :
The MIX Out is fully protected against bad unbalancing, momentary shorts or unwanted 48V, not the normal OUTPUT, which is the classic unprotected 670 OUT.