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Obviously, by definition, a stereo source can be plugged into a stereo channel. However, because I've never had a reason do that, I can't see the need to plug in stereo sources as common enough to warrant the number of stereo channels on so many mixers.

So I assume that there are uses for these channels that I am just not seeing (or I'm too narrow sighted to see the usefulness of being able to plug a stereo source into a single channel of a mixer).

What are the most common uses for a mixer's stereo channels?

This question also applies to stereo effect returns (which I also can't see a common use for because I neither use stereo effects nor notice others often using them).

Edit: Some of the answers and comments seem to be taking umbrage at the second sentence of the first paragraph. The sentence was not meant to suggest that I think that no valid use cases exist outside of mine. I recognize that my usage is keeping me from seeing other use cases, which is a shortcoming of my own and the reason that I posted this question. I apologize for inadvertently wording the question so provocatively.

Edit 2: I titled this question poorly (I'm hesitant to change it now since it has got so many views), but the intent was to ask "What are the benefits of a stereo mixer channel over 2 mono mixer channels?" I seem to have thought that simply asking for a list of uses for stereo channels would make those benefits clear, but it's probably more helpful to receive answers to the underlying question directly.

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  • I'm not sure I'm understanding what you're asking, but English is not my native language so I might be lost in translation. What do you mean by "need to warrant the number of stereo channels"?
    – musicamante
    Oct 2, 2021 at 1:15
  • In other words, if the main function of stereo channels is not commonly used, then it seems like the channels themselves should not be so commonly found. I realize that my premise--stereo sources are not commonly used--might be incorrect.
    – cheaterpushups
    Oct 2, 2021 at 2:27
  • Edited the OP, hopefully clarified that sentence.
    – cheaterpushups
    Oct 2, 2021 at 2:30
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    As you said, you've "never had a reason do that". But there are lots of instruments that provide stereo output, and lots of effect pedals too. Many stand-alone electronic instruments have stereo outputs, such as synthesizers, drum sequencers, but also portable workstations (eg. Akai MPC). For all those cases, having 2 separate channels is not only pointless, but also a distraction, if not even a problem: not only you need to limit inputs for instruments that have stereo outputs if you don't have enough channels, but you also need two separate channels to consider for each stereo input.
    – musicamante
    Oct 2, 2021 at 2:41
  • Not necessarily pointless, the other side of that is one might prefer the flexibility of two mono channels with the ability to group them if needed. A stereo channel can only be used as that, two mono channels can be used independently.
    – cheaterpushups
    Oct 2, 2021 at 3:30

9 Answers 9

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It's a common misconception that the point of stereo speakers is mostly to be able to pan individual signals to have different strength in the two output channels and thus achieve “locations in the stereo spectrum”. Sure that can be done, and in the 60s it was sometimes done extremely, but it was neither the original idea nor is it considered good practice today to pan mono signals strongly to one side (except as a special effect).

The main point of stereo is to create a sense of immersion. Our ears are actually not very picky with differences in signal strength (3 dB more doesn't sound a lot louder, but is actually twice as much power!), but they are very sensitive to phase relations. In particular, if every part of the mix you're hearing has the same phase relation between both ears, the ears notice that and interpret it as: the entire ensemble is crammed together at a single spot in the room, or very far away. This is so even if there is some panning going on and there are actually multiple PA speakers broadcasting the signal.

Acoustic instruments spread in a concert hall without amplification will have lots of different reflections, timing differences etc. going on. There are also phase differences between the different parts of a single instrument. That's why many instruments, perhaps most notably piano, benefit so much from stereo microphoning: if you listen to both channels of a stereo piano recording, you may well wonder what's the point, because each individually actually sounds very similar to the other. But pan them to 100% L/R and listen to them together and it feels like the instrument is suddenly much bigger and/or closer, without needing to be any louder. Even for essentially “monopole” instruments like brass, the phase effects of the room come into play, which is why at least reverb should almost always be in stereo: mono reverb sounds more like the instrument has some kind of sympathetic strings on it, rather than an actual room where you're together with the musicians.

If the channels are mixed together, phase differences can actually cause problems (some frequencies may be cancelled). In a studio production, it can make sense to mix the stereo image to adjust width, but live there's little point to either having the channels anything but 100% L/R, or applying different EQ to them, so then a stereo channel works just as well as two mono channels, while being easier to control and taking up less space.

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  • I think this is leaving out the important fact that many stereo sources that would be plugged into a stereo mixer channel are not hard panned at the source and/or have internal panning controls. Effect returns are the primary example (normally I would expect keyboards to come through a DI box into separate adjacent XLR inputs). Another thing is that mono sources can be enhanced with stereo effects processing, which means wider penning on the effect returns can often complement a centrally panned mono source.
    – Todd Wilcox
    Oct 2, 2021 at 15:22
  • @ToddWilcox what is “hard-panned at the source” supposed to mean? In case of synths, the two channels will typically be two different channels of the signal processing chain, e.g. one with the chorus' variable-delay on it, the other not. In case of sampled instruments, they will correspond exactly to the channels of whatever was sampled – in case of piano, the mics probably will have been hard-panned there. In case of a reverb FX-return, they will also correspond to the hard-panned virtual mics of the IR. In many cases, anything but 100% L/R panning for such sources will cause phase issues.
    – leftaroundabout
    Oct 2, 2021 at 19:53
  • What I mean is that the output of a stereo reverberator is going to have content all across the sound field. Same with sampled instruments. Like a piano sample won’t have the highest key 100% right, and middle C will be in the middle. Whatever. Edit your answer or don’t. I just wanted to contribute my experience with why using stereo inputs on mixers is viable for most line level stereo sources.
    – Todd Wilcox
    Oct 3, 2021 at 1:21
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    To add to this great answer, it is almost impossible to make two (analogue) filters exactly the same, and hence the phase might be shifted differently if the stereo channel is EQ'ed. That's why the EQ sections are often rudimentary for stereo channels, even in high-end mixers. Avoid using the parametric EQ if the stereo signal is routed through two fully panned mono channels (unless you have to...).
    – user2821
    Oct 4, 2021 at 9:47
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    @user1079505 I was mostly talking about sampled instruments and effects. The stereo microphoning would then have been done in the studio (and in the studio you typically record into an audio interface rather than a mixer, and possibly use outboard mic preamps which can just as well plug into a stereo channel). IMO it seldom makes sense to set up proper stereo pairs for live amplification, but when you do it then yes, you'd generally use two mono inputs. (Which can with a digital mixer anyway be linked and thus used as a single stereo channel.)
    – leftaroundabout
    Oct 6, 2021 at 21:12
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Even though you don't use them, there are plenty of 'stereo' sources (I'll mention the difference between most of these and a true stereo image later) and plenty of effects with 'stereo' outputs, often derived from a mono input. A sampled piano may pan low notes to the left, high notes to the right. A guitar effects pedal might send a different delay to each side of its output. And there are many more ways in which 'stereo' outputs are used to give an effect of spaciousness.

Maybe you deal mostly with PA systems, filling large areas with the sound from microphones? In many spaces it's arguable that 'stereo' is unnecessary, ever counter-productive. It's enough of a job to achieve full coverage, let alone worrying about giving everyone in the audience a well-balanced 'stereo' experience. Even in the simple case of a pair of speakers, one each side of the stave, only a small middle section of the audience will hear both speakers equally. Most will get more from one side than the other.

Why am I fussing over what 'stereo' actually is? Well, listen to a quality broadcast of a classical concert. The aim will be to create a 3-dimensional 'solid' (and that's what 'stereo' means) image of the entire orchestra. Surprisingly, perhaps, this IS achievable with just two channels. It's a wonderful experience, but not a part of most of today's multi-tracked music production.

Then there's all the clever stuff that can be done with binaural recordings, specifically designed for listening to through headphones.

https://en.wikipedia.org/wiki/Stereophonic_sound

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  • Yes! In fact I would go a bit further and say that even in today's multi-tracked music production, and even with live PA systems, the kind of effects that classical concert recordings adjust so carefully are still important and the main reason for stereo.
    – leftaroundabout
    Oct 2, 2021 at 14:17
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There are many stereo sources that can benefit from stereo channels on a mixing board. Among them are keyboards with a stereo out, computers/software samplers/synths, any instrument going through an effect or multi effect with a stereo out and also any stereo audio source that might be incorporated into a performance, such as pre-recorded backing tracks.

That being said, providing stereo channels is also a way for a manufacturer to beef up the number of available inputs without raising the number of channel preamps provided. For example, a 12 input board may have 4 mono inputs with preamps and 4 pairs of stereo inputs which usually don’t have preamps. You end up with 12 inputs but only 8 channels.

Bottom line, I think it’s a combination of convenience and economics.

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  • I've noticed home use mixers with, for example, 4 stereo channels and 2 mono channels. Is it fairly common to be using more stereo than mono sources?
    – cheaterpushups
    Oct 2, 2021 at 3:34
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    @cheaterpushups yes. Home usage, especially nowadays, means using electronic instruments that already provide stereo outputs. Most electronic musicians have lots of instruments in their setups that have stereo outputs, and if you're also mixing on your own having half of the controls means that you can better focus on music making than about mixing. Consider the simple case of panning: if you've 2 controls for the "stereo image", you usually need two hands, even for a simple side-by-side panning. Also, having 2 separate chans from a stereo input is not the same as a stereo channel.
    – musicamante
    Oct 2, 2021 at 4:00
  • It probably depends on the type of music you make, the instruments you play, how many of you there are, whether you're recording or rehearsing or performing… — but I suspect stereo sources are pretty common. For example, my home ‘studio’ has several keyboards, synth modules, a guitar FX unit, a bass FX unit… all stereo. The only mono source is a mic (which doesn't go through my mixer anyway).
    – gidds
    Oct 2, 2021 at 17:34
  • This should be the accepted answer as it actually answers the question!
    – Steve Mansfield
    Oct 29, 2022 at 10:53
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    @SteveMansfield Thanks, I think that’s the idea!
    – John Belzaguy
    Oct 29, 2022 at 16:48
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Well, assuming you have a stereo PA arrangement, obvious stereo sources are keyboards (which have both stereo sampled sounds and stereo effects) and other virtual instruments. The most obvious stereo effect return is reverb since a spatial impression of reverb really warrants separate channels. Another effect with an obvious need for stereo effect returns is a Leslie emulator since Leslie speakers have a complex spatial response.

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It is quite common for presenters to use stereo sources when the mixer is used for presentations in schools, halls, and businesses. Often presenters will play pre-recorded songs or videos which are in stereo. And while I recognize that it is unlikely they'd need 4 channels for this, it's equally unlikely that they'd need 12 channels for mics.

Many smaller systems like the Fender PD250 output sound in stereo, as well.

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Common uses are naturally stereo sources such as: synthesizers, effect returns, music players ("mp3 players").

You seem to ask "why do other people have these uses even though I and my friends don't". Maybe it's due to the kind of music you make.

But there's another side to the question: what's the point of having a single stereo channel instead of 2 x mono, one panned to each side? With a combined stereo channel, you can control the volume level, EQ, sends etc. exactly equally for both sides at the same time. You only turn ONE single knob for each adjustment, ONE single fader for level, ONE single mute button, etc. and the exact same thing is applied to both sides of the stereo channel. This is both handy and saves components, but in addition to that, exactly equal processsing is required for retaining the stereo image. And since these are for line-level sources, mic preamps can safely be left out from those channels without affecting general usability.

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To an extent, it will be dependent on what the mixer will be used for. Mixing, obviously, but into a P.A. will often have different requirements from mixing into a recording desk - its other main use.

Often, the sound balance from P.A. speakers will come out as mono - at least that way, wherever the audience is in the room, all will be receiving the same mix. With that in mind, having stereo inputs seems superfluous. But, if the output from whatever instrument is being mixed is stereo, and there are two inputs for that channel, it makes sense that left and right both get fed into that channel.

In other P.A. situations, stereo can be used - as stereo. When I have the chance to use my own gear at a gig, I'll set up so the sound from keys is stereo. Leslie panning can then be heard, as can other effects. The audience may not notice (or care!), but I love it ! Without the facility, those effects aren't possible.

Mics are generally mono - one mouth producing one voice into one channel. O.k., that can still be panned left or right, but, as you say, there's no need for stereo channels for that. But, from a convenience and economic point of view, sometimes having another channel 'free' for another mic or mono instrument is a bonus.

For inputting to a recording desk, stereo has many advantages over mono. An awful lot of effects are now stereo, even when, say, input is a mono guitar. Here, a mono input robs us of the opportunities to pan. In my studio, there are at least half a dozen keyboards up (let alone modules) and ready at any time, all stereo, all stereo inputted, and without all the extra stereo inputs on mixers, I'd be setting up a heck of a lot more mono mixers, which would then mean a lot more knob twiddling, too.

So, since adding some extra channels to an already stereo mixer is fairly inexpensive, why not?

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Depending on the application, there are many reasons for stereo inputs. One example is stereo sources like iPods, radio feeds, etc. It can also be used to input stereo instruments, keyboards being the best example. It could also include mono instruments that are running through stereo effects.

In addition, stereo channels are a cheap way to add additional channels because they don't require an additional faders or other controls.

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For keyboardists, almost everything is stereo. A pair of two-tier stacks eats up 4 stereo channels without breaking a sweat. (How's that for a mixed metaphor?)

Also, I get the impression that in many cases it allows a mixer manufacturer to claim lots of input channels without having to build in a lot of (costly and breakable) faders. Most low-end mixers I've seen with stereo inputs omit the EQ, and effects controls on those channels.

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