Splitter Snake and Phantom Power Concern

[Hoochie Coochie Man]

We just picked up a splitter snake that is directly wired, with no transformers, no diodes, no capacitors, and no resistors to in any way prevent phantom power that one mixing console might present to another console, while the splitter snake makes a direct, hard wired connection between inputs of the two consoles.

We will use this to record, (where we will not be operating the house console). We will use a Presonus 24.4.2, and a Behringer XR 18 Air to do our recording.

I am concerned that inadvertent application of phantom power from one piece of equipment may be applied to another piece of equipment, (while that input may, or may not also be supplying phantom power.

I have considered modifying the splitter snake by adding switches to each input that lifts pin 1 on one set of the output legs.

Or, I wondered if I could put blocking diodes on both sets of output legs, so no phantom power could feed from one console to the other, (with the caveat that the phantom power from both could be feeding one input channel on the stage box).

Or, I could block DC to one leg by series capacitive coupling one leg.

Or, I wondered if I could install series resistors to limit the current on one leg of the snake to prevent damage to either console.

I will post this in Recording Gear and Equipment thread, as well to garner as much advice on this question as possible.

Thanks to all who may provide advice on this question!

 

[Geri O]

I don't see how you would be in the scenario where you are applying phantom power to a channel and it somehow makes it to another.

Otherwise, there are no worries with a passive split snake and applying phantom. Just make sure that you apply the phantom from ONE desk and ONE desk only.

(this is why I hate desks with phantom global power)

 

[Hoochie Coochie Man]

I don't see how you would be in the scenario where you are applying phantom power to a channel and it somehow makes it to another.

Otherwise, there are no worries with a passive split snake and applying phantom. Just make sure that you apply the phantom from ONE desk and ONE desk only.

(this is why I hate desks with phantom global power)

You don't understand. Behringer has advised us that impressing phantom power from an external source on the input to an XR18 Air WILL MOST LIKELY RESULT IN DAMAGE TO THE UNIT. Likewise, we don't want to cause damage to anyone else's mixer.

We will be out in the field, where we are connecting our rig to other people's equipment, (which we won't be controlling).

That is why I would like to hear other people's ideas on how I will modify our splitter snake to prevent the problem. I can think of a two low-cost solutions. Installing pin one lifting switches on one send trunk, or installing blocking diodes on both send trunks.

 

[Geri O]

You don't understand. Behringer has advised us that impressing phantom power from an external source on the input to an XR18 Air WILL MOST LIKELY RESULT IN DAMAGE TO THE UNIT. Likewise, we don't want to cause damage to anyone else's mixer.

We will be out in the field, where we are connecting our rig to other people's equipment, (which we won't be controlling).

That is why I would like to hear other people's ideas on how I will modify our splitter snake to prevent the problem. I can think of a two low-cost solutions. Installing pin one lifting switches on one send trunk, or installing blocking diodes on both send trunks.

It would be nice to have understood that up front. I know what you are talking about now and I know what to do, but these posts wind up frustrating me to no end and I'm sure you will get suggestions elsewhere.

 

[agedhorse]

Ooooh, this is a very interesting question with (yet another) "it depends" answer.

First off, let's look back a little bit into history to understand a bit about how commonly used standards came into being with phantom power and splitting.

Originally, many consoles utilized transformer balanced inputs, and in some of these consoles, the primary (microphone side) was center tapped and phantom power was fed via the CT (through a resistor). Since no DC can cross between the primary and secondary, and the primary was designed so the core wouldn't saturate with the DC current, no blocking caps were required for the head amp circuitry (other than what was necessary for the circuit to function normally). The advantage of this method was common mode rejection that was purely a function of the matching of the 2 primary windings. The disadvantage is a larger core, and what happened to the input impedance of the circuit when phantom power was switched off from that channel. If it was left open, the input impedance would rise, if terminated with a resistor to ground then DC would flow the other direction when connected to another console providing phantom power (likely causing noise and reducing the available current to the mic)

Other consoles used transformers without CT's, and phantom power was fed simultaneously to each end of the primary of the transformer. Now the common mode rejection is dependent on the balance between the two resistors, and since the two resistors are not ground referenced (and are effectively in series), they impact the input impedance less if left in place (simplifying switching). In this instance, since there is no path to ground, voltage impressed on both ends of the transformer simultaneously cancel out and no current flows through the primary. Since there is no DC flowing through the primary, the core can be smaller which can lead to a higher quality audio signal (more complicated than this, but I will leave the details of the tradeoffs out). Now with this circuit, if either pin 2 or pin 3 become grounded or connected to pin 1 through a resistor (via an adapter or via a DI that does not adhere to the traditional standards) some DC current will now flow through the primary of the transformer which does not have sufficient core to support it and this may affect the audio quality.

Currently, most consoles no longer use transformers balanced inputs. The standard differential amplifier type circuit (pick one of hundreds of variations on essentially the same thing) is easy to protect against phantom power by using blocking capacitors to block phantom power that's impressed upon the 2 signal lines from entering the diff amp circuitry. The advantages of this are that there is no way for DC current to flow into the console from ANY source, 2 consoles can be connected together without damage or signal degradation (at least from this cause). The only remaining challenges come from maintaining close balancing of the impedances between the two lines, which is a little less critical these days with high rejection circuits.

So, with modern consoles using the common input amp architecture, there should be no damage problems connecting the two consoles together. The major issues that will remain, however, will be maintaining the balance, and noise caused by circulating currents between the two console's grounds. This can be corrected by lifting all of the pin 1 connections on one of the consoles (should be the one that does not intentionally supply phantom to the mics) but it should be noted that lifting pin 1 in this instance will not remove phantom power because both consoles are ground referenced through the power source's protective grounding system.

Diodes won't work, because you would block 1/2 of your audio (positive going) too!!!

Where one major issue remains, is interconnecting equipment that uses different input topologies (such as transformerless with transformer) as currents can flow in unexpected ways causing noise . It doesn't take much DC (modulated by God knows what noise) flowing in the primary of a mic input circuit, multiplied by the gain of the head amp, to be a real world problem.

The one way that's guaranteed to isolate is by using a splitter snake, each channel has one direct path (console that supplies phantom connects to this) and one or more transformer isolated split paths that do not use CT's. Often they will use additional shielding to prevent high frequency noise such as RF from crossing the core isolation, but these are generally designed to be quite robust and can solve problems that can not be solved any other way. This is how many of the real pro applications are configured, and one that I would recommend where you will be connecting to different gear every night. My last splitter snake (32x8) cost me close to $5000 including the rack. It's not a cheap solution, but a very effective one that my clients gladly paid for.

Since I am not familiar with the input circuits used by every console manufacturer, I suggest that you take this as a general informational description of how things work as there may still be cases where it doesn't apply 100%.

Hope this helps.

 

[Hoochie Coochie Man]

agedhorse,

Let me post here the response I posted to your answer in my duplicate post in the Live Sound forum.

Thank you very much for the very erudite response!

It looks like our best solution is to be very careful, lift pin one on all inputs to the console not supplying phantom power, and to communicate effectively with any house sound engineer we work with in this scenario.

 

[seamonkey]

What's the house console?

 

[Hoochie Coochie Man]

What's the house console?

The "house console" would be whatever mixing board any venue will have on hand to provide front of house sound.

In that situation our mixing board would be considered the "recording console"

We patch the band into both boards, each serving a different function.

 

[seamonkey]

The "house console" would be whatever mixing board any venue will have on hand to provide front of house sound.

In that situation our mixing board would be considered the "recording console"

We patch the band into both boards, each serving a different function.

If it's digital there are other options.

 

[Hoochie Coochie Man]

seamonkey,

Either board that we may take to a show are digital.

The Presonous is a large, 24 channel, heavy board, in a heavy road case. It takes two of us to set it n the stand.

The Behringer is a very small, lightweight, 18 channel, that fits in a small duffel bag.

I'm not a fan of Behringer, but I have to admit, it is really convenient, (if 18 channels is enough), (and did not cost much either). So far, we've used it four, or five times, and it has worked well. If it breaks out of warranty, we will throw it in the electronics recycling bin. The Presonous is nice, but it's a two man job for us old guys to set up.

The house boards are whatever they are. Sometimes we work with professional hired sound companies, who know what they are doing, and have high quality equipment. No telling ahead of time what some venues might have in-house. The owner might have his drunken brother-in-law running just about anything.

The best situation is when we do FOH ourselves. Then we need only one board to provide FOH, and record, with no need to split the inputs. This is more often what we do. But then, we also have to haul more heavy stuff.

It's a lot less work for us to leave the amps, mains, subs, and monitors at home, and just bring video cameras, a mixer, a computer, and an iPad, and get paid to record, and do audio, and video post-production.

We are a couple of old geezers, and are happy to get paid for our skills, rather than our ability to haul heavy gear.


Please suggest any other options you think might suit our purpose.

 

[seamonkey]

I get it. I would also expect some house engineers to say "go away" with your simple splitter. They own the house sound and inserting something in the delicate mic signal stream might be frowned on. I hope this all goes well for you.

My assumption was a single venue and the house had a digital board, then you can split the digital between them. The lower cost digital mixers don't have these options, except for they also work as full USB or Firewire interfaces. Where you can plug both into a PC and use a digital patch bay on the PC site. As you know, once a signal gets into the digital domain it best to keep it in digital as you pass it around.