bushwick wrote on Thu, 06 January 2011 13:29 |
Do any of y'all have S/N specs on large format boards with all channels summed to the L/R buss. It came up in conversation today and I am very curious. Something north of 40 channels and along the lines of a 72 ch SSL, or 72 ch of 8078, or a legacy.
Thanks, josh
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It's been a while so I don't have any numbers off the top of my head (the most I ever sent to a L/R bus in a console design was well over 100 (36 inputs, and 3 sends to L/R bus from each of the 24 monitor strips for another 72, + a handful of efx returns with assignment capability 4 or 8 more pairs).
The question suggests to me some discussion might be in order about the sundry noise sources in a console and how they combine.
In a console summing bus we are dealing with coherent and incoherent noise contributions. The coherent sources generally follow the number of inputs linearly (actually n+1 for the popular virtual earth sum bus). Incoherent noise sources like the mic preamps and input strip build as the square root of the sum of the signals squared.
So pulling some hypothetical numbers out of my butt, a 72 input virtual earth bus with a good quality low noise bus amp (say -120 dbu ein) will contribute 73x that noise, or roughly -83 dBu.... But that is ignoring the rest of the console.
Lets say the input strips (ignoring the mic preamp) have a -100 dbu noise floor with fader at unity and pans centered. 72 of them squared and then square rooted will add another -82.5 dBu of noise. Since these are incoherent with each other they will sum to about -80dBu... So that's your noise floor before one mic preamp is cracked open.
The theoretical ein for a low z mic preamp input is around -130dBu so one perfect hypothetical mic preamp running 50dB of gain will come up to another -80dBu all by itself... and this is ASSuming the room noise is lower than the theoretical mic preamp noise. Of course you will likely have more than one input turned up if 72 channels are punched up to the 2 mix. Of course not every input will get 50dB of mic gain, and not every fader will mix in to the L/R bus at unity gain fader.
I don't doubt there are differences between different console spec sheets, and there may even be slight audible differences listening to WFO bus noise floor with all inputs muted, but in practice most competent master bus designs should be well below the noise floor of the sundry input sources being combined.
While this noise analysis suggests to me there is no real noise concern between competent designs another consequence of the high noise gain virtual earth topology (73= 37dB noise gain) is that the pole caused by the compensation cap to stabilize the sum amp can lead to significant phase error in the top octave if total loop gain margin is not adequately high. This should be well known to (big) console designers so managed using one of several strategies.
Distortion will also rise with falling loop gain margin just like the phase shift if the design is marginal.
My numbers are crude, but my point is that noise is not the only or even the dominant problem in a large bus structure. In my judgement.
I hope this makes some sense. In my experience people read too much into bus noise floors and that number is not completely representative of final mix sound quality.
JR
PS: This is the short simplified answer and I rounded off some of my numbers for a neat result.