Using a external digital delay dd7

Yep, you are (making a school boy error) 😀

I have to go sound engineer on you here for a minute.

Since it’s a relatively simple mixer, it only has an aux out (often labelled as FX Send/Return) and no aux return, but it is stereo. In bigger (studio) mixers aux-buses are typically mono, but will – generally speaking – have aux send/return combined in one 6.3mm jack, meaning it comes back through the same “channel” as it’s sent. Also typically aux sent is what is known as “pre-fader”, meaning that the position of the channel fader does not influence the amount of signal flowing to the aux bus. This is done so that level of return signal can be adjusted separately from the channel volume.
Thusfar the “normal” setup on a (studio)mixer.

This way of routing means that by controlling the aux level knob for an input channel, you can determine how much of that channel’s signal is sent to the FX unit. The return (FX-ed) signal is then sent back to the (often) separate aux return channel where you can control the overall FX level for all channels at once. And then the FX unit itself has some level settings (like dry/wet for example). All-in-all plenty of knobs to fiddle with to control the amount of FX per channel, per FX and for the total mix.

Your :32 though is, as said, a simple mixer. It has only one AUX out. So what to do with the return signal? You got this part right, routing it to line inputs of the third channel would make the most sense, although some other (more elaborate) setups could be possible, but are beyond the scope of this answer.

Concluding you have the physical hook-up done correctly, here is where you go wrong:

In order to get sound to the FX unit, you turn up the aux volume ON THE FX RETURN SIGNAL! What happens now is that you have created a sound loop which pretty much is identical to what happens when you hold a mic in front of a speaker with the same horrific (FX-ed) screech as a result. By opening your aux volume on the FX channel, you basically send the output of the FX back into the FX. It will quickly (almost instantaneous) amplify itself until max level screech is reached. Your channel meter will max out, your speakers and ears will get mad at you and end of fun.

So, what ARE you supposed to do?

Clearly you want to send music to the FX unit. So you use the MUSIC channel aux knobs to determine how much you are sending out. The aux knob on the return channels stays turned ALL the way down. What you have now created is that music from your playing track is sent to the FX unit and comes back as a regular line input signal, as if it were a third player (CDJ/Vinyl/whatever). With the return channel FADER you determine how much of the FX-ed signal you want to add to the mix (like a wet/dry knob).

One thing you will have to test if the aux channel is indeed pre-fader. To do this, start playback on a channel, leave it’s fader down and only turn up the aux button. Now open up the FX channel fader. You should hear the FX-ed sound come from the speakers, WITHOUT the original sound from the playback channel. This is what I expect it to be and what would make most sense. In this situation you can control the playback volumes as you normally would, determine the amount of FX you want for EACH channel by using each channel’s aux knob and how much you want to add to the complete mix by using the return channel fader.

Your cue button on the return channel is now a pre-listen option for the FX. You can listen in your headphones to what the FX will sound like, so you can set it to your liking, before adding it to the mix with the fader, just like you would any other sound input.

I hope that makes a bit of sense. The most important thing here is to keep the AUX knob on the return channel CLOSED to avoid that horrible feedback loop/noise. Also set the return channel to line (not phono) of course and the input level setting (gain/trim) on the back to 0dB average on the meter to avoid gain level issues.

My 3 cents as usual.