The MFJ-640 combines or selects audio from two different receivers -- Great for operating "split" for HF contests and DXing, or cross-band for VHF/UHF satellite. Just connect the audio output of two receivers to the two 3.5mm audio connectors on back of the unit. Then plug in your stereo headphones and you are ready to go. MFJ-640 provides both 1/4in and 3.5mm stereo headphone jacks -- no need for adapters! From the front panel you can select the audio output desired - Mix, Radio 1, Stereo, or Radio 2. In the Mix mode both radios are combined in both the left and right earphones, and the audio mixing levels between the two radios are controllable. In the Stereo mode, Radio 1 is heard in the left ear and Radio 2 is heard in the right ear.
