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Understanding Aux Sends for Mix Minus

  • By Mike Phillips
  • Wednesday January 16, 2013
Filed Under:
Audio Broadcasting

Internet broadcasters and podcasters often use Skype to bring in audio from guests on their shows. While Skype does provide echo cancellation that does a fair job of preventing an echo of the host’s audio, the best solution is to use a mix-minus feed. In short, a mix-minus feed is an audio feed from a mixer, usually sent from an auxiliary or “Aux” output, that excludes the audio from the Skype caller. The objective is to keep the audio from the Skype caller from being routed back to the Skype caller such that the caller hears everything but himself.

Some of the most popular mixers with auxiliary sends suitable for creating a mix-minus feed include Mackie, Behringer, Presonus, Alesis, Yamaha, and Allen & Heath. All too often, when people spend several hundred to several thousand dollars to buy a mixer, they don’t pay enough attention to the configuration of the aux sends.

Case in point: The Mackie ProFX12 appears to be the perfect mixer for an Internet broadcaster. There are four mono inputs, four stereo inputs, two aux sends, and some other interesting features. Mackie makes great products. However, purchasers of this mixer should be aware that one of the aux sends is pre-fader, and the other aux send is post-fader. Why does it matter?

Assume that the pre-fader aux send is used to create a mix-minus feed for a Skype computer. A cable is connected from the aux out (labeled “mon” for “monitor” on the ProFX12) to the Skype computer sound card input (left channel only). The audio mix fed through the aux send is created by adjusting the “mon” knobs for each channel. Remember to keep the “mon” knob for the Skype computer playback audio at zero.

Since the “mon” or aux feed is pre-fader, audio through the channel will be delivered to the aux send even if you turn your microphone off by moving the fader all the way down. The Skype caller can still hear every word you say. If your guest is not doing a good job on your show, some comments you make thinking the microphone is off may be sent to the caller. If the mix-minus feed is created with a post-fader aux send (labeled “FX” on the ProFX12), if the microphone fader is all the way down, the caller cannot hear the host’s microphone. Now you can say all you want about your guest without him hearing you.

Since the configuration of the “mon” aux send cannot be changed, that is, since it is not configurable to post-fader, the ProFX12 has one usable aux send as far as Internet broadcasters are concerned. It’s also important to note that using the FX send for a mix-minus feed determines the audio you send to the onboard effects (FX) processor – not that broadcasters should be using effects.

Contrast the ProFX12 mixer with the Behringer X2442USB mixer. The 2442 has four aux sends per channel: Two are pre/post-fader (meaning you can switch them to be either one – we want post-fader) and two are post-fader. Like the ProFX12, using the aux send labeled “FX” on the 2442 may create a routing issue for the audio you want to send to the internal effects processor.

The Presonus StudioLive 16.4.2 has six aux sends per channel. Each send can be configured as pre-fader or post-fader. As a bonus, the 16.4.2 has four subgroup outputs that allow you to create four mono or two stereo mix-minus feeds. That means you can have as many as TEN mix-minus feeds! Leo Laporte doesn’t use that many, as far as I know.

When you’re deciding which mixer to buy, pay particular attention to the configuration of the aux sends. If you think you will only need one, don’t be surprised when you discover on day two that you need two or more. All three of the mixers here are excellent products, but make sure you get one that meets your needs.

It’s almost always a good idea to buy a larger mixer than you think you’ll need. It’s better to have a few channels left over than it is to have one less than you need.

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Mike Phillips is formerly a radio broadcast engineer, as well as a disk jockey, sales manager, and general manager. His technical experiences include building and maintaining radio stations and transmitter sites. A production console he built for a radio station in the early eighties is still in use on a daily basis. He also has been an electrical design engineer for a manufacturing company developing analog and digital industrial transducers. His undergraduate degree is in Electrical Engineering.


  • Christopher Woods

    Christopher Woods posted 01/16/2013 at 03:21am UTC

    Great article Mike, I like the mixer comparison. I wholeheartedly subscribe to the “always buy to have spare” mantra, although my wallet constantly disapproves. wink

  • Tom Sinclair

    Tom Sinclair posted 01/16/2013 at 04:57am UTC

    Nice work, Mike!

  • Geoff Moller

    Geoff Moller posted 02/24/2013 at 05:06pm UTC

    Like the comparison, thanks. I have a Behringer 1204USB with pre and post fad, but the mix-minus configuration seems to elude me, especially trying to use one PC (windows 7). If you elaborate more on setting up a mix-minus configuration on the same or similar mixer that would be helpful.  Thanks.

  • TeacherCast posted 01/23/2014 at 12:07am UTC

    Great article Mike!  Thanks for all the help with my setup this past year.

  • Sean

    Sean posted 02/17/2014 at 03:13am UTC

    Very insightful piece! Would the Presonus 16.0.2 be suitable for a mix-minus? And how many mix minus’?


  • Phil meyer

    Phil meyer posted 02/21/2015 at 04:37pm UTC

    Does this work when using a group Skype call?

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