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There are a few ways to do this in Logic. I'd 1. go to the Mixer window and select all the channels (rhythm track).
2. On one of the Channel strips send to an unused bus (e.g. Bus 10) and set the send level as desired.
All the selected channel strips should now be sending to Bus 10.
3. On one of the Channel strips Click on the Output 1-2 in the I/O section and select [i]No Output[/i]
4. Make sure the Aux 1 in your Mixer, (which is being fed the audio signal from Bus 10) is set to your main Output and adjust the level as desired and record your vocals.
You can also send to more than one Bus, thus creating different sub-mixes. Just remember to mute or select [i]No output[/i] on the Aux's that you don't need!
To disable a send Option click on it.
Another way which is simpler but less flexible:
1. In the Mixer select the desired rhythm channel strips.
2. In the I/O section assign them to a Bus of your choice e.g. Bus 10.
3. A new Aux track fed by Bus 10 is created.
4. After you've finished recording, select the channel strips and revert back to Output 1-2.