abbey road d enfer said:
This is theoretically correct and certainly applies to the Neve desks I know, those using summing amps with input transformers.
even a perfect noiseless mic preamp will boost the self noise of a microphone, these combine incoherently in a perfect sum amp to establish an "ideal" noise floor, BUT unless you are recording inside your anechoic chamber you will have much higher ambient room noise...
I am repeating myself but the difficulty from overly large bus structures is phase shift and distortion caused by inadequate loop gain margin.
By using heavy audio reference bus bars, it made them quite impervious to longitudinal noise build-up.
All the mixers I know that use VE summing amps suffer from longitudinal noise, that is often much in excess over the simple N+1 noise.
So actually, reducing the overall summing noise is not a useless endeavour.
better is always better... :-*
Actually, designing a good analog summing for a large number of channels consists in two things;
-Optimizing the structure (differential bus being my fave) for noise
crazy not to use differential for large conventional VE buses.
-Optimizing the GBW of the summing amp; transconductance /hybrid being pproven solutions but some recent opamps seem pretty capable in this respect.
the transamp/cohen topology delivers superior GBW for that application.
For known large conventional VE bus structures you can under compensate the bus sum amp taking advantage of the free stability margin afforded by high noise gain.
What's more, operational bus noise can be greatly improved by choosing to disconnect the feed resistors of the unrouted channels.
1) you need to disconnect both the bus signal resistor and any local ground bus resistor or the differential bus math becomes unbalanced.
2) if using a decompensated or under compensated bus amp, you need to insure that the noise gain does not rise above stability criteria. You could back ground the deselected bus send resistor to maintain bus balance but would not enjoy any noise gain reduction. Alternately you could back ground the deselected bus send through a capacitor so it looks connected at HF for stability concerns but open circuit for noise build up (you'd want to cap couple local ground bus resistors too).
Except for the main bus (and the main reverb bus) the actual number of channels routed to a single bus is amazingly low, so the actual "bus" noise is comparable with the channel noise.
So only a few bus need exceptional massage.
my largest console had well over 100 feeds to L/R bus, I am pretty confident that is not a record. In that console I used current source summing for more than half the feeds, current sources and sum resistors can be used seamlessly together.
Voltage summing does not lend itself to this, though.
Let's not ignore console math... You don't want to undertake even simple changes 48 times.
I am repeating myself but in my judgement the simplest and most cost/time effective way to upgrade a conventional VE bus with some 48 stems is to drop in a transamp/cohen topology sum amp. One (or two) and done.... KISS
If you remain invested in staying with 990 DOAs at least tweak the compensation cap to take advantage of the elevated noise gain (I ASSume they still use dominant pole compensation inside). You can use an old op amp trick to stabilize decompensated op amps operated at lower gain using a series R and C from VE - input to ground or op amp + input. A little slicker if deselected bus send resistors are open circuit is to back ground through a tiny capacitance to maintain HF stability (but again too much per channel detail work for a simple bus amp upgrade.)
There may be modern uber opamps that are competitive with (maybe better than?) 990 for ein and open loop gain while I haven't seen an IC with inductors in the input LTP yet. 8)
JR