To configure your Allen & Heath desk for use with an external FX processor carry out the following steps.

1. Connect an Aux* output to the mono or left input of your external FX processor.
2. Connect the stereo LR returns from your FX processor to a stereo channel on your desk, alternatively use two mono channels (panned hard left/right).
3. Use the channel send pots for the Aux in step 1 to feed dry signals to the FX processor, the processed signal will be returned to the inputs in step 2.
4. Make sure any Aux master pots and channel input faders are turned up to allow the signal to pass.

* Where possible use a post-fade Aux. This will ensure the channel send stays relative to the fader position.

This process is called send and return and is typically used for time based FX such as Reverb and Delay where you want to blend the FX with the original channel. If you want the FX processor to affect 100% of the audio then you might consider using your FX processor as an insert. This is more preferable for FX like flange and chorus.