Summing bus overdrive occurs before the master fader — pulling master fader down does not fix it

In a mixing console, channel signals are summed at a mixing bus through summing resistors into a summing amplifier, before reaching the master fader. If too many channels contribute too much level to the bus, the summing amplifier clips — this clipping occurs upstream of the master fader. Reducing the master fader level reduces the output of the already-clipped signal without removing the distortion. The fix is to reduce the level of individual channel faders or aux sends contributing to the offending bus, which lowers the signal level at the summing amplifier input. Alternatively, a bus trim control (if available) sets the summing amp’s operating level. This is a critical gain-staging principle: distribute gain reduction across the signal chain rather than only at the master output.

Examples

32 channels at 0 dB fader saturate the program bus even with the master fader at –10. Pulling the master to –20 makes it quieter but still distorted. Solution: reduce all channel faders by 6–10 dB, then compensate by raising the master.

Assessment

Why does pulling the master fader not fix bus overdrive distortion? At what point in the console’s signal flow does the distortion occur?