ej_whyte said:
What would you estimate as a decent noise figure to aim for in such a setup?
With 4 injection resistors connected to the bus, the summing amp's noise gain is about 15dB.
With the 990C, which boasts -133dBu EIN, it would compute to -118dBu
With the 2520, which is close to a 5534 in terms of noise, that would be about -110
In these conditions, it seems the dominant source of noise is the THAT 1200's, which are rated at -107; four of them would worsen this by 6dB, resulting in -101.
This is valid only if the bus-injection resistors are of low enough value to make the noise current term negligible.
For the 2520 anything below 10k is suitable.
For the 990c, the optimum source impedance is about 1100 ohms. That would suggest the use of injection resistors of less than 4.7k (unbalanced bus) or 2.2k (balanced bus). Using 10k res in an unbalanced bus configuration would make the contribution of noise current dominant, with noise from the summing amp measuring at about -110, which, again is sufficiently below the 1200's noise to be negligible.