Virtual earth summer G<1, adjustable

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Rogy

Well-known member
Joined
Jun 3, 2004
Messages
247
Location
Belgium
Hi,

Say I want to sum 48 input channels using a standard virtual earth summer.

I don't care about 0dB channel in being equal to 0dB summer out.

Assuming all 48 channels are feeding the bus, and containing somewhat uncorrelated signals, the output of the summing bus could be up to 16dB hotter than each individual channel's input level.

Is there any penalty in running the summer at a closed loop gain smaller than 1? Say I want to lose 12dB in the summer, make Rfeedback 2.5K with all Rinputs being 10K?

Assuming the above does not cause major concerns, what would be the most elegant way to make the summer's gain adjustable? Making Rfeedback a 7.5K pot in series with a fixed 2.5K to set the gain anywhere between -12dB and 0dB?

I do realise that designers tend to choose a fixed gain as to have fixed operating conditions all the time; still I'd like to find out what audible  influence the flexible gain would have.

My main concern is headroom in the summer stage without having to run the inputs at too low a level.

Many thanks,

Best Rogy
 
In fact that is a good place to run less than unity gain (it helps improve loop gain margin).  Mackie made a big marketing push about running their master bus down a few dB but that was mostly marketing whohah (their actual expertise).

I did a big console with a variable master bus gain trim....  I had the potential for over 100 sends to the L/R bus but only provided for -10dB gain trim.  The level build up is not as much as feared because many inputs are not summed full up, or active constantly.

Fixed gain will make it easier to maintain differential balance. in the sum bus.  In a large console you can reduce crosstalk by referencing both audio hot and audio 0V from each channel to + and - sums.  It is more than a little optimistic to ASSume master bus ground and channel send ground will be the same 0V.

JR

PS: I am sure we have discussed this and other variants on this here before.
 

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