Never combine signals using a simple Y-cable. Use a source switch or a line mixer to choose which mixer is feeding the speakers. Theoretically always having one off might work, but sooner or later someone's going to turn them both on. Even then, it might not be a major issue, but it's definitely bad practice to combine signals with a passive Y-cable. Splitting the various sources shouldn't be a problem.

If it were up to me I'd just disconnect one mixer and patch up the other one, but I do understand that churches often have people of limited skills performing tech duties.

Just this weekend I did a contract job for a chorale concert in a church. They have a digital mixer which has the ability to recall different configurations. So as long as you connect the mics etc. to the same inputs you can easily recall a scene just the way you left it.

I would second what Boulder says - do not use Y-cabling for this job.

The output first: assuming you have un-powered mixers (i.e. the type that does not have power-amps built-in), then you need a balanced switch box to route the speaker outputs to come from the selected mixer. When I last installed a system of this sort, I had to make my own changeover box as there were not any commercially available. I think there are now. The one I made used DPDT relays with the NC (normally closed) contacts fed from the standard mixer, and the NO (normally open) contacts from the infrequently-used concert mixer. The relay coils were activated through a transistor switch fed from a phantom power connector on a spare channel of the concert mixer.

Now the inputs: if you have no condenser mics or other inputs requiring phantom power (PP), use a standard transformer-isolated mic splitter. You need one with enough ways to handle all the channels you currently use plus some spares. If you have just one or two mics that use PP, use the standard mic splitter as mentioned but add separate phantom power supplies on the mic side of the splitter. If many or all the channels require PP, you may have to connect the direct route (non-transformer) of the splitter to the standard mixer and have that switched on even when you are using the guest mixer.

Tell us a bit more about your mix of input channels, and we can be a bit more specific.

At a club i used to work at djs would bring their own mixers sometimes. I reserved two channels on the house mixer, with a line in cable connected. When the djs or soundperson came in they just plugged the line outs from their system into the house mixer.

This is perhaps not technically ideal for signal to noise, but generally worked pretty well.

Boswell, I routinely use passive splitters to connect mics to two mixers at once (live mixing + recording system) without any problems, even with phantom power. The normal practice is to have phantom power from only one mixer. One once have I had any problem and it was using an ancient Harrison mixer (which we called the Harrisaurus) where phantom from one mixer generated an irregular pop from the other. We switched which mixer was providing the phantom and the popping stopped. As I understand it, xfmr isolated splitters are mostly useful when the second mixer is on a separate power feed with potential for ground issues. I'd be interesting in any input you have on this.

Also, wouldn't a simple solution for the outputs be a line mixer?

My reading of the OP's situation is that he would like either mixer to be usable on its own with the same set of inputs and driving the same loudspeakers without any manual switching or re-plugging.

In my experience, you have to allow for a mixer that is connected to the mains but switched off to be a hum source, and that means that its output should not be present in the feed to the loudspeakers when it is not in use. The output relay box I mentioned achieves this. An output line mixer could also do this, but it requires extra knobs or sliders to be set correctly for each type of usage, giving scope for error.

The inputs to the main mixers are less of a problem, and there is more than one way of achieving an acceptable result. Phantom power complicates things, but not insolubly.

The mic splitters I use have a direct output and a transformer split output. The transformer output gives you the ground isolation that you mention, but it also serves as a protection against both destinations trying to provide phantom power to a single source. I meet a lot of mixers up to the medium level that have phantom power switching controlling multiple channels.

Tp3366, post: 462939, member: 51789 wrote: ...a rented group also uses our mixers etc often causing issues.

Hi, and welcome!

Could you be more specific, please? Do the renters bring their own microphones / instruments / other sources? Or are the sources shared as well?

The discussion being whether you're; A) trying to split common inputs to feed 2 mixers and then combining the outputs of the 2 mixers into one amp/speaker system, or B) just combining the 2 mixer outputs.

Any other details about the existing and potential new mixers and your crossover (if any) / amp / speaker configuration would be helpful. Number of mics, instruments, etc. - any of that would be useful information.

As Boswell said we would have two set of mixers but only one will be powered (we have means to ensure that). No need for phantom power. All mics will be in place. Renters only bring their guitar to be plugged to the snake end. They will have access to one mixer. They cannot power the other.
With this scenario will not a simple y connector be enough to split the inputs from the snake to the mixers? And will a simple y be enough to combine the mixer outputs to the speakers assuming that only one mixer can be powered at a time. Is complicated switches or relays or transformers really needed. The renter will use one mixer when they come in. And when we come we will power and use the other mixer ensuring that the other is switched OFF.
The mixer output goes to a dbx PA processor before going to the speakers.
To answer dvdhawk: I will be using option A but only one mixer will be powered at any point of time.
My only concern is: though one mixer is always switched off, will the y cables at the input and output cause any impedance issues?

OK. We've established one guitar.

How many mics need splitting?
Do you plan to solder up the XLR Y-cables, or buy them?
Can we assume there aren't any open inputs on the snake?
Are there any unused Returns on the snake?
Are the snake Returns XLR or 1/4" TRS?
Are you using any processing at FOH between the Main Outs of your exisiting mixer and Main Returns on your snake? (EQ, Compressor/Limiter)
Is the speaker system running is stereo or mono?
Is the objection to the renters using the system simply that it messes up your Sunday settings?
Are there other reasons?
What kind of mixer are you considering buying?

Sorry for the 10-point quiz, but there's a reason I'm asking each question.

To join the chorus on this and get right to your question; Passive Y-cables to split the inputs would be a second tier solution, but might be passable.
If the number of channels isn't too high there might be better isolated solutions that really don't cost much more.
Passive Y-cabling to combine the outputs of two separate mixers sounds like a disaster waiting to happen. There are simple alternatives.

If you want to pare this down to a minimum, then you may be able to get away with paralleling the inputs, given that you say you have no phantom power requirements. It's something you could try in the first instance, but you would waste the cost of the Y-cables if you subsequently find you have to go to a splitter solution.

The outputs are a different matter, however. It's unlikely you will do any actual damage by connecting the two line-level mixer outputs together, but you will almost certainly get distortion, as the unpowered outputs will effectively throw diode clamps across the signal.

To avoid this problem, here are some examples of output switch boxes. One has a manual switch and the others are relays that need a wall-wart supply to switch to the NO direction. The Radial is single channel (you would need two boxes), where the other two are 2-channel.

Radial XO relay box
ARX relay switcher
Sescom manual switcher

You can buy or make a box with a switch that will do the trick safely and simply.

Oddly, I've actually used the Y split method that is a poor way of doing things and it worked totally happily - but the reason being the output stages of the two mixers being compatible with each other. Even though it did work, it's a bodge and a 'for emergency' solution only in the real world. Not good for a proper install. These things are cheap enough to buy
https://www.amazon.co.uk/dp/B07R41LNKG/?tag=r06fa-20

kmetal, post: 462946, member: 37533 wrote: At a club i used to work at djs would bring their own mixers sometimes. I reserved two channels on the house mixer, with a line in cable connected. When the djs or soundperson came in they just plugged the line outs from their system into the house mixer.

For me this is a no no when bands come with their mixer to a big show.. (at least in Quebec, Canada) Going from a mixer outputs to inputs on a second mixer degrades the signal specially with digital mixers. Most live mix engineer will ask the direct wires that goes to the PA system. A few even ask to use their own crossovers..

But something I often see is using a split snake for sending the mic signals to 2 mixers (FOH + Monitoring) That's an option. but you'll need to be very carefull, only one can provide phantom power.

For an in-house system that doesn't move, I would build a box with physical switches 2 xlr in, 4 xlr out and 2 quality switches if only the outputs are needed to be switched.

This reminds me of my first show ever, I was drumming and we had borrowed a mixer for the night.. The setup was made and sounded ok then put to the side with a black cloth on it.. but the guitar player of another stepped on it before we got to play.. Doing a sound test in front of an audience isn't fun at all... Thanks for todays digital mixers and usb keys !!

I agree the method i used was not ideal, but it didn't break anything and had a few advantages of simplicity, low cost, a level of fail safe from blowing up the system, and people not disconnecting the mains each night. Again not ideal, and sorta "hack" but did get the job done. Especially useful when they would call me ten minutes before a show i wasn't working, wondering why there's "no sound". Lol "plug into the cable that says dj, turn up the volume knob".

They were mostly djs, so gain staging wasn't as critical as far as signal to noise, and the additional signal path wasn't degrading things to any degree that was noticeable. The mixers were analog, and did have line i/o.

When bands or more critical acts came in i would patch them into the main inputs on the driverack.

Live sound is often many compromises under high pressure. As exciting as it is, i prefer the studios control and quality.

Im unsure if a simple source select switch would be the fix for the OP.

Tp3366, post: 462939, member: 51789 wrote: In our church, apart from the main church singers , a rented group also uses our mixers etc often causing issues. We are planning to add a new mixer which we do not wish the renters to use.
So we were thinking of connecting all inputs to the mixer and the outputs from the mixer to speakers using Y connectors and locking the new mixer when we are not using it. Meaning only one mixer will be powered ON at a time. Will this work or any problems we might face?

Please comment.

Tp3366, post: 462951, member: 51789 wrote: As Boswell said we would have two set of mixers but only one will be powered (we have means to ensure that). No need for phantom power. All mics will be in place. Renters only bring their guitar to be plugged to the snake end. They will have access to one mixer. They cannot power the other.
With this scenario will not a simple y connector be enough to split the inputs from the snake to the mixers? And will a simple y be enough to combine the mixer outputs to the speakers assuming that only one mixer can be powered at a time. Is complicated switches or relays or transformers really needed. The renter will use one mixer when they come in. And when we come we will power and use the other mixer ensuring that the other is switched OFF.
The mixer output goes to a dbx PA processor before going to the speakers.
To answer dvdhawk: I will be using option A but only one mixer will be powered at any point of time.
My only concern is: though one mixer is always switched off, will the y cables at the input and output cause any impedance issues?

you have been offered very good advice. it is not good for outputs of different devices to be connected to each other even if one is not powered up as signal can "back up" into the outputs of the unused device and damage it. imo, you should use one of the solutions that have been mentioned or simply swap out the cables to your mixer when you aren't using it.