Feeding foh and monitor boards with the same sources b games unblocked

Here’s the basics. When you have a low-impedance output such as a microphone or DI box, it’s typically capable of driving a 600 ohm load. That’s from the good old days of Ma Bell, when telephone systems were all 600 ohms. But modern mixers don’t have anywhere near that low of input impedance. Most are in the 10K Ohm range. That implies that a single mic would be able to drive something like 16 mixer inputs (10,000 divided by 600 equals 16.67), but of course we don’t need to drive 16 different mixers, we only need to drive two mixers in your case, FOH and Monitor.

But there’s more than just letting the signal go down two paths… you also need to isolate phantom power and lift ground loops. Of course, phantom power is the 48 volt DC bias that’s applied on pins 2 & 3 of the mixer inputs, which then powers condenser mics and active DI boxes. If you use a simple Y-splitter cable from a mic to feed two mixers, any channel with phantom engaged will back-feed to the other mixer, reducing the phantom voltage and shutting down the mic. You "could" turn on the phantom power on the same channel from each mixer, but that’s not generally done. There’s a simple way to block DC voltage while allowing AC signal to flow, which is a capacitor. So in a simple splitter snake you will have one of the set of mixer inputs be direct wired to the mic output, and that mixer output is the one that supplies phantom power, while the other output is capacitor isolated from the phantom power. Typically the hard wired output with a phantom path is reserved for the monitor console since the operator is much closer to the stage and knows which mics/DI’ls need phantom or not.

But we’ve still not addressed the issue of ground loops, which can induce all sorts of hum into a PA system. Let’s not get into the details of how and why ground loops occur (I could write a book on that) but focus on how to get rid of them. While there are various power grounding techniques and XLR pin-1 lifts available, the gold standard is to use an audio transformer split to feed one of the mixing consoles (usually FOH) while the other output is the direct wired one feeding the monitor mixer. This allows the introduction of a true ground lift on the center-tap of the audio transformer, which eliminates the hard-copper bond in the shields between the mixing consoles. The result is that ground loop currents are eliminated, along with that pesky hum.

These splitters are also available in active DA (distribution amps) that have powered outputs, most useful when you need even more splits for the broadcast board and whatever else you like. Plus an active split is better at feeding very long cable runs (over 250 ft total) that will start to sound dull using passive splits simply due to the capacitance of the twisted pair cables in the snake(s) shorting out the high frequencies of the audio signal.

So you can get a snake with a built in hard-wire or transformer isolated split. Or you can get a splitter box with 16 to 32 inputs and 32 to 64 outputs. You can even get active splitters with 4 or more mic level outputs per each mic input. But, of course, the price goes up with each level of technology.

Last things first: It’s not that phantom voltage adds or subtracts, it’s that there are 10K build-out resistors which feed the mic. If you using a simple Y-cable split, then the 10K resistor in the board with phantom on is bonded to the input pin(s) of the second board. If that board’s phantom is off, then your mics are being powered from the center-tap of two 10K resistors in series, and likely only receiving 24 volts. Some mikes don’t care and will run off 24 volts, but old-school mics need the full 48 volts to operate. Yes, you can turn on the phantom in both consoles, and it will work, but we don’t like that much for concert work since that’s one more thing to remember during setup. It won’t hurt anything… we just don’t like to do it due to more error possibilities.

As far as ground loops, I’ve been doing ground loop experiments in my studio lab for the last 2 years, and dealing with them on stage for the last 40 years. A properly designed snake with transformer isolated audio splits will never pull any ground loop current, and thus can’t hum. Pin-1 XLR lifts will work in many circumstances, but their success is predicated on the CMRR (Common Mode Rejection Ratio) of the balanced input stage of your console(s). If your console manufacturer used first grade laser trimmed resistors for the input stage, then it will work most of the time. If your console manufacturer used throw-away parts from Radio Shack, then Pin-1 lifts won’t work at all. Ground loops are really make-up currents that the shield in your XLR cables are forced to endure because the power outlet wiring was incorrect to begin with.

And below is a graphic from my No~Shock~Zone seminar where I demonstrate how to test for ground loops with a standard clamp meter. Note that since the actual currents are only flowing in the shield of the XLR cable(s) or snake channels, not the twisted pair(s), there’s no need to split out the twisted pair from the shield to test with a clamp meter. You just place a clamp meter around the outside of the cable. Many ground loop voltage gradients will be up to a volt or two, but even 1/10 of a volt will make some powered monitors hum. This is largely due to a design issue known for the last 20+ years called the pin-1 problem where the XLR shield connection is routed through the circuit board first on its way to the chassis ground. Too much ground loop current can even cause these circuit board traces to overheat and melt.

Note that you’ll likely see about 1 amp of ground loop current per volt of ground differential, so any clamp meter that will resolve down to 0.1 amps will work. You can even clamp around a group of cables to eliminate all of them as ground loop hum candidates.

Yes And No… While I agree with the idea of using transformer isolation to rid ground loop hums, it is not always necessary. The only time I’ve observed problems with a lack of common mode rejection was when the inputs were not balanced and the hum was caused by another high voltage inductive force. The ground loop currents that you speak of are hardly near 1 amp at 1 volt, it happens, but, I’d think its generally half or less. Ground loops can happen even though the electrician has performed with diligence. A ground loop is caused a difference in potential…. that is all. This is the reason most foh power is provided from the same electrical feed as the monitor and system gear…. to try and keep this potential at a minimum. Some of these ground loop problems can be solved with inspecting the electrical connections to ensure a tight connection.

And below is a graphic from my No~Shock~Zone seminar where I demonstrate how to test for ground loops with a standard clamp meter. Note that since the actual currents are only flowing in the shield of the XLR cable(s) or snake channels, not the twisted pair(s), there’s no need to split out the twisted pair from the shield to test with a clamp meter. You just place a clamp meter around the outside of the cable. Many ground loop voltage gradients will be up to a volt or two, but even 1/10 of a volt will make some powered monitors hum. This is largely due to a design issue known for the last 20+ years called the pin-1 problem where the XLR shield connection is routed through the circuit board first on its way to the chassis ground. Too much ground loop current can even cause these circuit board traces to overheat and melt.

Note that you’ll likely see about 1 amp of ground loop current per volt of ground differential, so any clamp meter that will resolve down to 0.1 amps will work. You can even clamp around a group of cables to eliminate all of them as ground loop hum candidates.