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Questions about splitting signals for monitoring (48v / Transformer Isolation)
Old 18th January 2013
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Questions about splitting signals for monitoring (48v / Transformer Isolation)

This is a pretty long explanation - I like to err on the side of too much information. If I'm going about this the wrong way, please let me know!

Here is what I am trying to accomplish. I play in a 10 piece cover band that recently got a DL1608 for gigs where we run our own sound. We liked using the DL1608 and being able to control our monitoring on stage. We are tired of the monitor mixes we get being inconsistent at gigs where sound is provided (insufficient setup time / difficulty communicating with hand signals / magic disappearing sound guy). So, we would like to start using the DL1608 for monitoring at all our gigs, whether or not FOH sound is provided. We use more than 16 channels at times, but we only really need 16 channels for monitoring.

From my searching, it sounds like the best way to do this is with splitters that have transformer isolated outputs. We have a pretty small budget for this (< 650). I had selected the Art S8 splitter (x2). It seems to have gotten good reviews from people doing a similar setup. People seemed to disagree on who you should give the transformer isolated outputs, but the gist I got was that either way would work.

My main concern is with connecting line outputs to the splitter. In addition to all the mics, we connect 6 line level inputs including a balanced direct out from a bass amp (XLR), a balanced direct out from a keyboard amp (XLR), and 4 line level balanced mixer outputs (XLR or TRS) for additional sub mixes. The Art S8 has a 30db pad, which indicates that it's not a crazy idea to connect those line outputs to a mic splitter. But if I understand right, it is risky to connect line inputs through XLR this way. Since the FOH engineer would likely be going into an XLR input, there is a chance they could accidentally send phantom power to a line output and potentially do damage. Since the DL1608 has phantom across the board, I would have to send the direct outs to FOH and hope that they only switched phantom power on for the correct channels. I would rather not risk that. I couldn't find any indication that engaging the pad on the S8 would prevent phantom power from being passed (which seems like a logical thing to do). I asked Art and never got a response. If engaging that pad does in fact prevent phantom power being passed, then all of the stuff I talk about below may not be needed.

While I could avoid the splitter entirely for line level connections, it makes it easy to manage the connections in one place and run fewer cables. Basically, I can have the mixer permanently connected to the outputs of the S8 splitter and we can make all our connections to the labeled front of the rack and FOH can make all their connections in one place on the back (also labeled). I can tell all the band members that regardless of the gig, you just plug this cable in this hole, and they can help more with set up.

So all I would really need to do is prevent phantom power being passed to the line outputs. If I understand right, transformer isolators or ground lifts will disengage phantom power. So I could use either of those things somewhere between the line outputs and the mixer providing phantom power. I don't know any of the technical details, but from what I have read, using transformer isolators may also prevent one blown up piece of equipment from blowing up other equipment and may even reduce unwanted noise in some cases. These are a few options that I was looking at (direct boxes and transformer isolators):

Art T8
Art DualXDirect
Whirlwind LBS

I like that these all give XLR and TRS connections for input. That will probably mean less cables for me to buy. I am leaning towards the Art T8 for a number of reasons. It's probably the only one that's really in my budget, it has flexible connections, and it has a couple extra channels in case another need arises later. I have always been happy with the gear I bought from ART, including a T8 that I already use in my home studio to help clean up some noise between consumer gear with unbalanced connections.

Am I correct that simply connecting anything through the T8 will prevent phantom power being passed? I have not noticed a difference in the audio quality in my use of the T8. So in this case, putting it between would simply offer additional protection, possible noise removal, and greater flexibility in connection (fewer $ spent on cables). It seems like there has to be some reasons against doing it, but I can't think of any. I could be totally wrong about this. If so, please tell me!

So there are several different ways I could connect it:

1. Connect everything directly to the S8 (including the line level outputs), feed the transformer isolated outputs into the DL1608, feed the direct outs to FOH (routing the line output channels through the T8). This way the FOH guy can manage phantom power (which apparently they enjoy doing?) and the mics that don't need the protection aren't running through any transformers before going to FOH. (Some people said they noticed a negative effect on the sound of the transformer isolated output, some people said no noticeable difference - not sure who to believe)

2. Connect everything directly to the S8 (including the line level outputs), feed the transformer isolated outputs to FOH, feed the direct outputs to the DL1608 (routing the line output channels through the T8). This way the DL1608 can provide the phantom power (over the shorter distance in most cases, which some people said is good) and the FOH engineer doesn't have to worry about which mics need phantom power.

3. Connect the line inputs through the T8 into the S8, connect the transformer isolated outputs into the DL1608 and the direct outputs to FOH.

4. Connect the line inputs through the T8 into the S8, connect the transformer isolated outputs to FOH and feed the direct outputs to the DL1608.

In 3 and 4, if the signal is altered by the transformers in the T8, I suppose we would at least be hearing the same alteration in monitoring, so maybe that would be best for musical reasons. Do you have any reason to suggest connecting it one way or another?

I know I could also use direct boxes with ground lifts, but generally speaking direct boxes aren't as flexible on input and output connections, and it would probably be more expensive. Plus there is no real reason for me to alter the type of signal for all these balanced line level outputs. If I need to pad the line level devices for FOH's XLR inputs, I could always use the pad switch on the Art S8. Maybe I should use them any way to be safe, though it seems like a bad idea to me to drop the signal down and boost it back up again when you don't need to. But really I'm just guessing at all of this.

If anyone actually managed to get through that explanation and has any input to give, I would appreciate it!
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