can a external mixer be connected to the 6400

From the Flex-6400 Hardware Reference:

"7.5.2 Line Level

There are a number of different definitions of “line level”. The audio input device in the FLEX-6400/FLEX-6600 series radios is designed for a “consumer line level” specification. This is defined as -10 dBV. If you plan to use “professional line level” devices to feed your radio, you will need to reduce the output level so that it does not exceed the -10 dBV level (-8 dBu). The hardware input will clip around 0 dBm (~-2.2dBV).

7.6 ACCESSORY CONNECTOR

The accessory connector is a high-density HD-15 female connector. There are a number of inputs and outputs on this connector.

! - This is the same type of connector used by VGA monitors. Do not connect to VGA monitors.

Δ – All Accessory inputs are 3.3VDC MAX Input

7.6.1 Pin 1: Line In

This audio line input can be used to feed consumer level (-10dBV) audio into the transmitter. Refer to the SmartSDR documentation for information describing how to enable this input, and what configurations are available."

In a word, yes you can feed in an external mixer. Just follow the Flex manuals, including SmartSDR manual for instructions on enabling the "line input".

73, Craig KØCF