The way I've always understood it, Unity
gain is the
point at which no
gain is being added or subtracted. On the head amp
potentiometer this means that there is no attenuation or amplification and on the
Fader this means that it's sitting right inbetween the rails at 0 back when the faders had actual positive and negative
voltage rails at the top and bottom.
On the output side unity is more or less a lie because all the amplification happens on the input
stage, so when shouting
throw the output
fader at unity, we really just mean
throw it to 0 and are using a colloquial term for the zero because to some mixers hear Zero and equate that 0 = off and to others 0 = center of the
fader's range.
Am I getting this right? In a decade of professional mixing I've never had anyone tell me that Unity is a lie, but I've also never questioned it too deeply.
Faders generally do (and did) not
fade a signal between the positive and negative
rail. All the signals in a
mixer are centered around signal
ground (midway between the rails), though of course varying above and below that with each cycle of the audio. Put another way, the signals are AC-coupled, not DC coupled. A
fader, in its simplest form, would have the input signal at one end of the
track, and signal
ground at the other, and then the wiper would have a variably attenuated
level of the signal; there is, of course, nearly always one or more amplifiers associated with it to boost the output signal, reduce loading on the input side,
etc., and the markings on the front panel of the
mixer show the
gain and output levels of the composite
system. If you have a
VCA system, the faders (or at least the
VCA groups) wouldn't be directly altering the audio, but rather affecting a control
voltage to the voltage-controlled
amplifier, and that control
voltage range could be most anything internally but likely not the full
rail to
rail voltage of the equipment.
A
decibel is a relative measurement, expressing the ratio of
power between two things. One of those things sometimes is an accepted reference
level, as is the case of
dBm or dBu or dBV. Other times, it's comparing the input of something to its output, giving an amount of attenuation or
gain. It's a
bit of an unfortunate thing that mixers often have both marked in different places, and it may be a
bit confusing to keep
track of which is which. Faders (and similar controls, like aux sends) are most often scaled with their
gain, so the 0 dB setting of a
fader means that the internal signal
level is unchanged, as could for instance be verified by comparing pre-fader soloing with post-fader soloing. The
level meters are usually calibrated in terms of output levels.
For a
microphone input, the
gain control would (generally) not need to go so low as 0 dB, so any marking of "unity"
gain is just an arrow that the manufacturer thought was useful to print on the case, I guess on the assumption that your microphones are
SM58's or some such and have certain sensitivities. Such markings should, to put it diplomatically, "be given the attention that they deserve." The input
gain gets set so that the maximum (loudest) input to the
microphone produces appropriate internal signal levels in the
mixer, such as by adjusting it whilst soloing the
channel until the
level meter is a reasonable margin below
clipping/overloading, as determined by experience.
(In digital audio applications, 0 dB is very frequently taken to be the absolute maximum levels possible, where the range of sampled values in the audio is the full range of the digital word size being used. In that situation, the nominal signal levels must needs be well below 0 dB to avoid
clipping.)
Yes the
mixer does have
line level inputs. Just did the quick and dirty plugged into the mic inputs backstage forgeting that it was a
line level source from
FOH. Just need to adapt to 1/4"
line level input at the
mixer. My bad. The main
mixer is a 24
channel but they only wired the
house for 2 inputs from the
stage. I asked for more when they revamped the
system but was ignored. We are also running 3
Shure MX202 overheads much further upstage plus
MP3 and CD inputs to the board.
Hmmm...so you have 14 wireless microphones, plus 3 PCC160's, plus 3 MX202s, plus a (stereo)
MP3, plus a (stereo) CD...that works out to 24 inputs, and your 24
channel mixer could accommodate them all directly if you only had an eight
channel snake, doing away with the submixer and its associated
gain staging headaches. In your shoes, I'd definitely be giving it the old college try to buy, beg, or steal something like a Hosa Little Bro:
Hosa SH6X2-50 50' Little Bro' Stage Box Snake, 6 XLR Sends, 4 1/4" TRS Returns | Full Compass Systems