Mic Splitter?

[tbryanh]

Here are the things the manufactuer claims about the Mic Splitter:

Say you are running two mixers at once – perhaps one for the
monitor system and one for the main PA (or one for a broadcast
feed and one for the PA). If a performer wants his/her mic split
into a powered monitor and the PA, you will need the ability to
split the signal from the source so that the signal can go both
ways. It is also important that the output of the source encounters
the right impedance. For example, if you simply used a “Y”
adapter, signal strength and quality would be lost. For this reason,
the SplitCOM professional splitter has a built-in transformer.
Since you may be splitting the balanced output of an instrument
amp or preamp, another consideration is “hum” caused by a
ground loop. The SplitCOM splitter box also features a ground
lift switch to reduce that problem.

I want to split the mic from my isolation cabinet into 2 PA mixers. One mixer will drive a reverb tank, and the other mixer will mix the wet and dry signals together.

I plan to use the Mic Splitter, but I am trying to understand why I need it and how it works.

It seems that the load impedance the mic sees is less no matter what. Assuming the mic has an impedance of 600R, with a Y-adapter, the mic would see 300R impedance, an obvious mismatch, but how does the transformer prevent this from happening?

What are the impedances of the primary and secondary of the transformer?

I understand the lift switch can prevent a ground loop, but is the Output still balanced with the ground lifted? If so, how?

[tubeswell]

One signal goes straight through, and the 'output' is tapped off the 'through' signal line via the tranny, so maybe the tranny is wound in such a way as to alleviate the impedance split when both the 'through' and 'output' plugs are in? One way to find out would be to measure the impedances when you've got the Output plugged in and not the 'through' one. That way when they ain't both plugged in it doesn't matter. But if so, then when you've got the 'output' plugged in and not the 'through' one, you might notice a change in signal (if that's the case).

I would imagine that the shielding to the output cable is linked up at the other end of the cable to the mixer/amp/whatever if you are getting a ground loop hum. Therefore when you lift the switch in such circumstances, then the output cable is still operating in 'balanced'/shielded mode.

That's my tupence worth.

[bbartilson]

I want to split the mic from my isolation cabinet into 2 PA mixers. One mixer will drive a reverb tank, and the other mixer will mix the wet and dry signals together.

That's a reasonable use for the device.

I plan to use the Mic Splitter, but I am trying to understand why I need it and how it works.

Sure. No problem.

It seems that the load impedance the mic sees is less no matter what. Assuming the mic has an impedance of 600R, with a Y-adapter, the mic would see 300R impedance, an obvious mismatch, but how does the transformer prevent this from happening?

Here is where you are starting to make presumptions, and where the trouble starts. Most of the desks you'll find in operation today do not have low impedances at the input. Most are rather high, say, 10K ohms or so. Some extend well up into the hundreds of kohms. The microphone, however, sets this impedance rather low when it is connected, usually somewhere in the 600 ohm range. But the high impedance that the desk represents isn't any appreciable load for the microphone. One could presumably split the signal to ten different mixers without adversely affecting the output or the frequency response of the microphone. There are exceptions, both at the mixer end as well as the microphone end, but this general rule works quite well in the realm of what is common these days - Mackie mixers and SM57's and the like. So you certainly could employ a simple Y cable and get away with it. In that case, what's the splitter for?

1. Electrical isolation of the two systems being connected. Each system has it's own power supply and connection to AC (meaning line power) ground. Each system's power is a finite distance from a real Earth ground, and those distances are usually different. In the case of a concert system, there are hundreds of feet between the house mixer and the monitor console. They may not be fed from the same power, or even the same phase of power. But they have to work together in that they need to be fed the same audio signals - one for the stage mix, and one for the house sound. Enter the multichannel snake with a transformer split. (essentially a whole bunch of the box you're asking about)

Remove that electrical isolation, and you are asking for trouble. Since each system has a different reference (and distance from) AC ground, the difference can impress itself as a signal on one or both systems. The larger the difference, the larger the signal. This comes through the system as a nasty hum or buzz.

2. Electrical isolation beyond the potential for ground loops and the associated noise problems: Early on, once we concert engineers figured out we could split signals with Y cables, we did so. Who needs transformers anyway? Surprise!!! Middle of the gig when the front of house guy turns the gain up on channel 14, all of a sudden that channel is howling onstage through the monitors.

Yes, we found out connecting the two systems together allows not only the ground (and attendant signal) to interact, but the two CIRCUITS can interact. Anything you do on the one mixer MAY or MAY NOT change what is happening with the other. Usually this doesn't go beyond the trim controls, but it's awfully annoying during the first three songs of a big act.

What are the impedances of the primary and secondary of the transformer?

As in any audio circuit, the primary impedance is only reflected upon the primary by whatever load is on the secondary. In the case of modern consoles without transformers on the inputs, the situation is the same - split the signal as many times as you like - your 600 ohm microphone is most likely seeing 10K or more, which is no load at all. The turns ratio on those transformers is 1:1 in most cases. Excluding small real world losses, the input impedance of the mixer is all the load expressed on the primary, and therefore, the mixer that is directly connected is largely unaffected by the addition of the splitter. That said, there is a small signal loss. Hence the appearance of active splitters and DI's.

I understand the lift switch can prevent a ground loop, but is the Output still balanced with the ground lifted? If so, how?

Balanced simply means that the signal is carried on two wires that are out of phase with one another. The exceptional noise rejection comes from the fact that whatever noise is picked up by the wire is IN phase with itself as it reaches the mixer. The mixer takes the two signals, inverts one of them, and adds the two together. Whatever noise was picked up in the signal wire cancels itself out upon adding the out of phase signals together. And I'm not sure I follow that sentence either. It's easier drawn than said. If it makes no sense to you, I'm sure we can point you at a web reference with nice pictures.

That long winded sentence simply states that the ground wire doesn't carry any signal. Hence, it's loss at one end of the wire doesn't matter as far as the signal is concerned. All the audio is on the two conductors inside the shield. In fact, some folks wire their racks and drive snakes such that they are only grounded at one end, and thus eliminate the possibility of ground loops. There's a school of thought which believes this is the 'quietest' and therefore the only way to run a system. When you think about a large concert rig (100k watts or something), it's a big deal. 100mv of noise can become very audible watts in very short order.


I hope that clears it up for you. Splitters are a good thing. Now, splitting one microphone to go to two mixers that are side by side, and you aren't going to change the trims on the fly? You can probably get away with a Y cable. Makes noisy? Screwy problems? Save yourself time and get the splitter, and put the Y cable in your tool box for another day.

Best,
Bill

[Enzo]

There you go. I can attest to the fact that Bill knows this stuff.

Without knowing what mixers you are running how about this alternative: run the mic to the main mixer, then send an aux send from the mic channel over to the second mixer. SOme more involved mixers have buffered direct outs or insert points from the input stage as well. THat could be sent to the second mixer. That way you don't need spitters, Y cords or much else but one patch cord at the mix position.

[Paul P]

There you go. I can attest to the fact that Bill knows this stuff.

We noticed .

Hey Bill. I, and I'm sure a lot of other newbies like myself, really appreciate
your spending some time here. We can learn a lot from someone with your
experience and it's nice that you're willing to share it with us.

Of course this goes for you too Enzo and all you other experts around here.

Paul P

[tbryanh]

Yeah, thanks Bill.

[tbryanh]

So the ground is for shielding, it has nothing to do with the balance; i.e. it only takes 2 wires to have a balanced line, not 3 wires?

[Enzo]

That's correct. You don't need the shield except to reject common mode noise. To whatever extent the input being fed is sensitive to that.

Of course the shield is used as a ground return for phantom power, but that is a separate issue.

[bbartilson]

Hey Bill. I, and I'm sure a lot of other newbies like myself, really appreciate
your spending some time here. We can learn a lot from someone with your
experience and it's nice that you're willing to share it with us.

Hi Paul,

You, and anyone else that gets any use out of the info I can provide, are quite welcome. I don't have a lot of time to spend here, but I'll certainly continue to do what I can to lend a hand.

And quite frankly, this sort of forum is an outlet most technical types could and should make more use of. It's one thing to be very experienced and knowledgeable about a subject. It's quite still another to have someone to talk to who has any clue what you're talking about, much less any interest in understanding it.

I've found a pretty good outlet running audio services at the local community college. But I've been busy doing a fair amount of research this past year, and I've been starting to use and reuse a different set of chops. Being forced to codify one's thoughts in a readable form like this is therapeutic after a few hours at the bench. I'm glad you found it of some value as well.

Thanks again for noticing.

Regards,
Bill

[bbartilson]

Yeah, thanks Bill.

You're very welcome. Given the questions you asked afterward, it seems you understood quite well.

Feel free to let us know how your experiments work out.

-Bill

[Tucker]

a/b xlr to I think the same reasoning.

I am a newbie on here and a guitarist that hacks at engineering.

I use a boss rc-20 to loop guitar and vocals only problem is the mic input is a 1/4" so I built a splitter with a dpdt switch xlr i/o.

The issue is the phantom power "pop" that occurs when selecting between the board and my sampler.

I have read some things about rewiring the switch so that it toggles the ground between the two outputs and thereby solving the "pop" does this sound, sound?

Thanks,
Dave