Simple ground question

[Hemmel]

Hi guys !

I've been asked to create an audio switch box for 2 stereo RCA inputs.
It's a sports bar where they show hockey games (mostly). They used to have a DJ who handled the console but now they're understaffed and the DJ is only available once or twice a week (most of the time, not during hockey nights). So it's up to the barman to fade out the sport show's sound during the commercials, and fade in music coming from a laptop.
The DJ used to do that with the sliders in entries 3 and 4 on the console. However that console is quite far away from the bar and the barman has to run at every commercial break to change the sound, and run back at the end of the break.

So I thought of providing them with a small box with a pot to emulate the fade-in/fade-out, like this :



I thought of putting the output in the console's #4 input, and use the TV's output and laptop's output as input 1 and 2 on the box.
Will this work ?
They don't want a switch as it wont "transition" between sound inputs.
If I connect all grounds together, wont this cause a ground loop ?
Is a B10K dual-gang pot enough (the only dual-gang I have in stock are B10K)

Thanks

[ashcat_lt]

The optimal size of the pot depends on the impedances involved around it and the acceptable level of bleed.  The taper be decided by the relative impedance on either side.

There's already a ground loop.  Your box doesn't really add one.  It does, however, add at least a bit of wire to the loop.  I would imagine the length of that wire is going to determine the extent of the problem.  Best (as in safest - most likely to succeed in more situations) would probably be transformer isolated balanced sends from the devices themselves.

[Hemmel]

The optimal size of the pot depends on the impedances involved around it and the acceptable level of bleed.  The taper be decided by the relative impedance on either side.

It's kind of hard to determine. As I said, both inputs will be stereo RCA coming from either the satellite TV receptor or a laptop (1/8" stereo jack to double-RCA adaptor).
I have no idea how to measure impedance in both these connections.

There's already a ground loop.  Your box doesn't really add one.  It does, however, add at least a bit of wire to the loop.  I would imagine the length of that wire is going to determine the extent of the problem.  Best (as in safest - most likely to succeed in more situations) would probably be transformer isolated balanced sends from the devices themselves.

I'm pretty sure they wont be "transformer isolated balanced sends". I'll have to try it out at home first, I guess.
I was mostly concerned about creating a ground loop if I connected the grounds from input 1 and input 2 together.

[ashcat_lt]

It's kind of hard to determine. As I said, both inputs will be stereo RCA coming from either the satellite TV receptor or a laptop (1/8" stereo jack to double-RCA adaptor).
I have no idea how to measure impedance in both these connections.

Look 'em up in their respective manuals and/or make an educated guess based on "typical" impedance of similar devices.

I'm pretty sure they wont be "transformer isolated balanced sends". I'll have to try it out at home first, I guess.
I was mostly concerned about creating a ground loop if I connected the grounds from input 1 and input 2 together.

Sure, the mixer already does that though, no?  Don't know that testing at home will help even if you do use cables of appropriate lengths because you won't have the same power connections, nor the same EM/RFI environment.  Ground loop buzz is pretty specific to the actual installation.

[Jdansti]

If the TV and computer grounds  are only connected at the new fader box, there shouldn't be a ground loop, right?  The only difference between this and what you had before is the fader. I would definitely use shielded cables between the various components.

The green lines below are the ground paths. Is this how you'll be connecting everything?

[Hemmel]

Is this how you'll be connecting everything?

It is. I'll give it a try.

Look 'em up in their respective manuals and/or make an educated guess based on "typical" impedance of similar devices.

I will, but TBH I have no idea what impedance is and how the size of the pot is affected by it. I'll have to read up on it.

Sure, the mixer already does that though, no?  Don't know that testing at home will help even if you do use cables of appropriate lengths because you won't have the same power connections, nor the same EM/RFI environment.  Ground loop buzz is pretty specific to the actual installation.

Ok. My main concern was about connecting grounds from the TV and the computer outputs, as I have no idea how ground is managed inside the mixer.

[ashcat_lt]

I think that it's safe to assume that on all three devices the shield of the cable is connected to the chassis and is tied one way or another to whatever it's AC wall power thinks is "true" earth ground.  The problem comes mostly when they disagree on what true earth ground is.

The absolute best chance to make it work is to balance the things through a transformer before sending them to your box.  You could then unbalance them before your fader since wont probably want to source a for gang pot.  Do you really need stereo for this, though?  Anyway, this would require building or buying 3 boxes - essentially two DIs (preferably 1:1, though) and your mixer.

You might be able to get away with pseudo-balanced connections from the devices.  This is easier in that it doesn't require a transformer, just some pretty simple adapter cables.  You'd still need either transformers or opamps at your mixer box.

Does it really need to be stereo?

On pot value - do you know what a voltage divider is?  Looking out from one of the sources, it sees a voltage divider where the top resistor equals its own out-Z plus whatever portion of the fader pot is between it and the wiper, and a bottom resistor equal to the rest of the pot plus the out-Z of the other source both in parallel with the in-Z at the mixer.  How big that ratio can get determines how much attenuation you'll get at the extremes - how much bleed will come through.  Honestly, 10K is probably plenty.

[Hemmel]

Do you really need stereo for this, though?  (...) Does it really need to be stereo?

I thought about that and the problem comes from the sources. Both sources put out stereo signals, so there's a chance that one channel might be lost if I use only left or right.
i.e.: the music coming out of the PC is in stereo and I can hear it since we hear everything from songs that use a lot of panning. However, there are so many speakers everywhere in the bar that the stereo effect is lost. But, if only one channel was used, let's say "left", then anything playing on the "right" channel wouldn't be heard at all.

The bar doesn't have any power outlet near where they want the fader since it will be above sinks and a lot of liquid  .

My thought was : plug the PC in input 1, the satellite TV tuner in input 2, then the output of the fader into the console. Then while the console's volume is at say 50%, set the fader on PC and adjust the PC's volume. After, set the fader to TV and set the TV's volume.
That way, they'll know never to touch the volumes on wither the TV or the PC and just change the volume on the console.

[ashcat_lt]

That way, they'll know never to touch the volumes on wither the TV or the PC and just change the volume on the console.

 

You could mix the L+R through resistors at some point before your box.  The impedances could be important here also.

If'n it was me... Take one of those paired RCA cables and cut it in half.  Get two XLRM plug ends from somewhere.  Hope that they're big enough to cram the wire and two resistors in.  Connect L tip to a resistor, R tip to another resistor, and the other end of the resistors to pin 2 on the XLR.  Connect one or both of the shields to pin 3.  Leave pin 1 N/C.  Twice.

Build the box with exactly the way you've drawn it, but with XLR ins and either XLR or TRS out.  I'd go XLR all the way, but I'm...    For both inputs and the output treat pin 2 (or TRS Tip) as the L in your diagram and pin 3 (TRS Ring) as R.  All the pin 1s connect to each other through the chassis. 

Now you put the adapter cable at each source and chain as many mic cables as you need to get from points A and B to C, and another mic cable from point C to point D, and nobody cares what anybody else thinks is "real" earth ground.

Except!!!

1)  One of the sources is a laptop and probably has an 1/8" TRS output.  You could either just the appropriate plug in place of the dual RCA above (hack apart the cable that took it to the RCA inputs before instead) or make that end female RCA, or deal with coupling adapters...  Probably best to just build it to suit the application.

B)  I've assumed that the "mixer" that this terminates at has some form of balanced input.  I would just jam it right into mic preamp, but there's that impedance thing again.  Most mic pres are pretty Hi-Z compared to most outputs nowadays...

Anyway, I honestly believe that this is your best bang-for-the-buck, most likely to succeed solution.

[Hemmel]

That way, they'll know never to touch the volumes on wither the TV or the PC and just change the volume on the console.

You could mix the L+R through resistors at some point before your box.  The impedances could be important here also.

If'n it was me... Take one of those paired RCA cables and cut it in half.  Get two XLRM plug ends from somewhere.  Hope that they're big enough to cram the wire and two resistors in.  Connect L tip to a resistor, R tip to another resistor, and the other end of the resistors to pin 2 on the XLR.  Connect one or both of the shields to pin 3.  Leave pin 1 N/C.  Twice.

Build the box with exactly the way you've drawn it, but with XLR ins and either XLR or TRS out.  I'd go XLR all the way, but I'm...    For both inputs and the output treat pin 2 (or TRS Tip) as the L in your diagram and pin 3 (TRS Ring) as R.  All the pin 1s connect to each other through the chassis. 

Now you put the adapter cable at each source and chain as many mic cables as you need to get from points A and B to C, and another mic cable from point C to point D, and nobody cares what anybody else thinks is "real" earth ground.

Except!!!

1)  One of the sources is a laptop and probably has an 1/8" TRS output.  You could either just the appropriate plug in place of the dual RCA above (hack apart the cable that took it to the RCA inputs before instead) or make that end female RCA, or deal with coupling adapters...  Probably best to just build it to suit the application.

B)  I've assumed that the "mixer" that this terminates at has some form of balanced input.  I would just jam it right into mic preamp, but there's that impedance thing again.  Most mic pres are pretty Hi-Z compared to most outputs nowadays...

Anyway, I honestly believe that this is your best bang-for-the-buck, most likely to succeed solution.

That sounds a helluvalot more complicated ! In all honesty...
Plus, the console where everything's plugged into the bar's speakers is this very simple Radio-Shack-bought 4-inputs and only one of them has an XLR. All 4 have stereo RCA.
The only XLR connection is used by the DJ's mic. You should the connector, I'm afraid of touching it !

So I guess I'll try it out with my initial concept.

[ashcat_lt]

You can't really hurt anything!  It might buzz.  If it does, you may be able to get away with just lifting the shield on one or more devices.

[armdnrdy]

The bar that I frequent has the same type of setup....but instead of hockey (Southern California) it would be basketball or football on the screen.

Now pay attention to this next part.....

During commercials, the bartender...from behind the bar...turns down the satellite volume and turns up the music.

He uses these super magical devises....referred to locally as......REMOTES!

Why can't he use remotes from behind the bar?   

[Jdansti]

It might be simpler for you to run stereo to your fader box and then use a few resistors inside the box to convert stereo to mono (as shown in the link below) before going through your fader pots.

Stereo to mono: http://www.diystompboxes.com/smfforum/index.php?topic=100317.msg882361#msg882361

Note that the resistor values probably depend on the values of your pots, but I would start with what's shown in the link.

[armdnrdy]

My suggestion may have seemed like I was joking...but I wasn't.

You can leave the sliders up on the console and control the volume from the satellite receiver remote and the laptop remote.

http://www.amazon.com/BestDealUSA-Laptop-Remote-Control-Controller/dp/B005KP0UIM

No wires, no boxes, no switches, done deal!

Every modern sports bar that I've been in controls the music/TV in this manner.

[Jdansti]

I agree. Remotes (assuming you have one for both units) require no changes to the system.

I should have said that my "simpler" idea was placing the stereo to mono feature in the box instead of making special cables, which would work, but the OP said he would rather not go that route.

[merlinb]

So I thought of providing them with a small box with a pot to emulate the fade-in/fade-out, like this :

Your idea should work; 100k pots would probably be appropriate (plug into the mixer hi-Z or line input). Either use sheielded cable inside the box, or use a metal box (connected to audio ground). I would think you're more likely to get buzz from the laptop switching PSU than from conventional ground loop hum. But then, bars are not exactly high-fidelity performance spaces anyway!

It's such a simple thing that it may be quicker simply to build it and try it out, than to hang around here getting embroiled in transformer balancing discussions  

[Hemmel]

Remotes are the first thing I suggested, actually. They had one for the satellite TV, and they lost it, and they don't want to spend to buy and then lose another (the manager is kind of an a**hole when it comes to spending).
Also, remotes wouldn't allow for a quick and smooth transition between audio channels (yes, they want a real quick transition). The barman hasn't got the time to stop, find the remotes, turn up music, turn down TV, then set the remotes down to go back to customers. They are understaffed and they have more customers now that the hockey season has started.

Jdansti, I don't understand what would be the benefit of transforming the stereo signals to mono ? Is it simply to avoid using a dual-gang pot ?
Merlin, the box would be aluminum and I have shielded cable.

[Jdansti]

>Jdansti, I don't understand what would be the benefit of transforming the stereo signals to mono ? Is it simply to avoid using a dual-gang pot ?

If you want to preserve the audio from left and right channels and have mono output, you'll need to combine them. You'll still need the dual ganged pot to fade from one signal to the other.

Here's what I am thinking:



If all of the circuitry is inside the metal box, you only need shielded cable outside of the box. Just make sure you connect the circuit ground to the box.

[Hemmel]

Thanks for the schematic, Jdansti, but I still don't understand why I should have a mono output.
The console has 4 stereo inputs, all in RCA connectors.
It's not like RCA connectors are expensive ($0.17 at Tayda's, for example) so I don't get why I couldn't use a stereo output on the fader...

[Jdansti]

Ok. If you want stereo out and one knob fading, you'll need a four ganged pot assembly to preserve each channel and skip the mixing resistors. Alternately, you could use two-two ganged pots and have two knob fading (edit: as shown in your schematic).

But if you don't need stereo out because the speakers are all over the place, and you want one knob fadng, you can follow my schematic.  This assumes your console has a stereo/mono output switch. I can't guarantee the resistor values.