As your sound system expands, it will eventually be necessary to provide additional mixes from locations other than the main mix position
June 29, 2011, by Al Keltz
If church sound system has evolved along a familiar path, what started out as a pretty simple, small group, “sound-on-a-stick” has gradually become more and more sophisticated.
Many have replaced that powered mixer with separate components and added a snake to allow for a mix position in the listening area.
Words like “direct box”, “balanced”, “low impedance”, “crossover”, etc. have become part of the sound team’s vocabulary as they strive to provide today’s expected level of sound quality and production – for both the listeners and the performers.
In this article we just might add some additional terms to your audio vocabulary as we discuss microphone splitters.
When To Split?
As your sound system expands, it will eventually be necessary to provide additional mixes from locations other than the main mix position.
Although it’s possible to provide a separate monitor mix from the main console, a person located nearer to the performance area can hear what the performers hear, see their cues more easily and just generally be able to provide a better monitor mix.
Or you may be called upon to provide a separate mix for recording or broadcasting your performances. That mix will be at its best if the person providing it is isolated from the confusion of hearing the live sound. In any event, you’ll most likely need to split your mic signals and feed more than one mixing console.
Proper design of signal flow in an audio system dictates that low impedance outputs (mics) feed high impedance inputs (mixers). When a signal is split to be sent to more than one mixing console, the input impedances of those consoles provide additional paths for the electrical current.
This actually increases the overall load presented to the mic signal and limits how many times it can be split without degrading tone or introducing distortion.
Microphones can usually be split to up to three, and in rare cases even four, destinations without the use of electronics. The number of splits that can be accomplished depends on the application, impedances present in the system, length of the cables and the quality of the components used in the splitter. This is called passive splitting – no power required.
Active electronic splitters will most likely be required when splitting microphones to four or more consoles.
There are two types of passive splitters: parallel and transformer isolated.
The simplest form of splitter is the parallel type split. This involves taking a mic cable and simply “Y” connecting the plus, minus and ground wires to two or three other cables.
Although this method successfully connects the mic to multiple mixing consoles, be aware that it connects the consoles directly to each other as well.
Most modern consoles will behave rather nicely configured this way but sometimes, mostly in older units, the consoles will interact with each other. This can happen when making adjustments at one console causes the electrical characteristics of its input circuitry to change.
These changes may reflect through a parallel split and possibly upset the input circuitry of the second console.
For example, if adjusting the gain on console A causes the DC voltages of A’s input to change, this will show up at the input of console B. In some cases, this might cause the gain to change on console B, definitely a bad situation.
If you are considering a parallel split, you can test for this by making up a microphone Y cord, attaching a mic to both consoles and trying it out.
Make adjustments on each console, especially to the trim or gain section, while listening to the other. Turn the phantom power on and off each console. If you don’t notice any changes in volume or quality of sound, then a parallel split should work OK. (Adding the actual lengths of cable to each leg of the split can present other issues, such as added capacitance and increased susceptibility to RF interference.)
However, if the consoles are interacting, or if you are traveling and will be splitting to unfamiliar consoles regularly, then it would be wise to accomplish the split by using transformers.
Transformer Isolated Splits
In a transformer splitter, the microphone is wired straight through to a “Direct Out” and also to the input of a splitting transformer. (See Figure 1 below.)
This transformer has a 1:1 turns ratio and its output side is connected to the second or “Isolated” split output. (Transformers with two or more secondaries are used for achieving more than one iso split.)
The transformer will pass the microphone’s AC audio signal but will block DC voltage in either direction. This helps prevent interaction between the consoles.
Figure 1: Schematic diagram for a 2-way isolated split with Whirlwind TRSP-1 transformer. (click to enlarge)
One of the outputs is usually wired as a direct connection because the transformer will also block phantom power (DC). Remember to plan on connecting this direct leg of the split to the console that will be providing the phantom power.
Another benefit of using a transformer split is that it increases each leg’s ability to reject interference by improving the “balanced” characteristic of the line (called “Common Mode Rejection” or CMR).
A disadvantage of this type of split is the added expense of the transformers. High quality transformers are essential for providing proper shielding and for preserving the frequency response of the mic signal - don’t cut corners here!
Not all grounds are created equal. In fact any time two pieces of audio gear are plugged into grounded outlets, their actual resistance to earth ground can vary quite a bit - even when the outlets are on the same circuit.
This can be due to the designs of the power supplies, the length of the cable from the outlet to the service box, poor or oxidized connections within the outlet boxes and service panels - anything that can affect the resistance in the electrical path to ground.
Even when using a transformer split, a problem can arise when the consoles’ grounds are connected directly to each other and here’s why: If console A “sees” a lower resistance to ground through its connection through the splitter to console B, then part of its AC ground return current will take that path of least resistance and AC current flows in the shields of the cable, through the splitter, and over to console B. This is called a ground loop.
Now, instead of the shields providing a defense against unwanted interference, they are carrying 60 Hz AC and radiating it directly into the signal conductors that they are supposed to be protecting! This is why ground loops produce hum.
Although it might solve the hum problem, you should NEVER use a three-prong ground lifter on the AC power cable of either console! This is not safe and can present an electrical shock danger to the people using the system.
A better solution to this problem is to break the ground connection of one or more channels between the consoles. This is accomplished by disconnecting each offending ground connection at one end (usually the splitter) and leaving it connected at the opposite end. The shield for that channel will continue to work because it is still grounded at one end.
Some technicians will clip all of the split grounds, leaving them permanently disconnected but it’s better to install ground lift switches for each channel or use lift adapters when necessary.
This way, the ground can normally be left connected but lifted if there’s a problem. Also, if the main console is unplugged or disconnected, the grounds can be left connected to the split console making it usable.
This is sometimes needed when churches or schools with large format main consoles decide to keep some of the mics split to a smaller, less complicated mixing console. This allows a less technically proficient person to operate the system when basic sound is needed for a small service or assembly.
Remember that a direct out or passive split will not pass phantom power with the ground lifted at either end and a transformer isolated split will not pass phantom power even with the ground connected at both ends
In the above examples of parallel and transformer isolated splitters, the signals are split without using any powered electronic circuitry.
As discussed, there are issues involved regarding impedances, frequency response and console interaction. These problems can essentially be eliminated by splitting the signals with active electronics.
In an active splitter, the mic signal is applied to an amplifier circuit. This circuit is actually a mic preamplifier similar to the input of a mixing console.
It can be designed to provide a wide and flat frequency response and present an optimum and constant impedance to the microphone.
Gain adjustment can also be provided at the splitter to boost weak signals before they have to make that long trip down the snake. This improves signal to noise ratios.
Phantom power can also be provided for at the splitter which eliminates the issue of deciding which console has to provide it when designing passive splits.
Sometimes an active splitter is used to feed line level signals to several destinations such as a bank of amplifiers, tape deck duplicators, headphone monitors, etc.
In this case, it is usually referred to as a “Distribution Amplifier” or “DA”.
The output of this mic-pre is fed to several more amplifiers that feed the various split outputs required. These separate amplifier circuits prevent interaction between the input and outputs (called “buffering”) and can be fine tuned to produce the best possible results.
Separate, buffered amplifier outputs also eliminate problems associated with the consoles interacting with each other.
Also, all outputs can be separately transformer coupled which greatly improves their balancing quality or Common Mode Rejection (CMR) of the output line. This makes them less susceptible to the effects of outside interference.
Electronic drive to each transformer can be designed to be extremely low impedance in nature which further improves noise rejection and response, especially bass frequencies.
These advantages over a passive circuit allow a signal to be split to more outputs and with better overall frequency response.
Al Keltz is a technical writer who works with Whirlwind USA.