Unity gain simply means that the gain stage is neither boosting nor attenuating the signal. It is, in effect, multiplying by one, unity. As a concept, unity gain represents the right philosophy for making the best use of dynamic range (avoiding noise at low levels, but avoiding distortion at high levels), but the equipment designers have to share this philosophy and layout the equipment and label all the faders and pots and sliders accordingly.
The key is to read the manual and figure out the settings necessary for your particular piece of equipment to be at unity gain. Then, if a 0.775 volt signal goes in, it will be 0.775 volts going out.
Here is a brief run through, of how to go about achieving unity gain on an inline mixing desk.
1) Select mic or line level.
2) Solo the channel.
3) Set unity gain by adjusting the gain pot to;
-4dB to 0dB, if you are using Volume Unit Meters (VU).
0dB to +3dB, if you are using Peak Program Meters (PPMs).
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