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Key Facets Of Interfacing Microphones With Sound Systems

A look at phantom power and other output and impedance issues with microphones, in addition to a discussion of best mic cabling, connector and accessory practices

By Tim Vear November 20, 2017

A key area of microphone use is the interface of the microphone with the sound system. This primarily involves electrical considerations, so here are a few simple rules for proper interface based on the electrical characteristics of the microphone output and the sound system input, and on the requirements for cables and connectors to achieve maximum reliability.

All condenser type microphones require power for their operation. This is provided by an internal battery in some models, or by phantom power in others.

If a condenser is selected, care must be taken to assure that the appropriate power source (battery or phantom) is available. A battery-powered condenser is fine for applications such as portable recording, but phantom power should be used for any permanent micro phone installation.

Phantom power, sometimes called “simplex,” is provided through the microphone cable itself. It is a DC (direct current) voltage that may range from 9 to 48 volts, depending on the microphone requirement and the phantom power source rating.

This voltage is applied equally to the two conductors of a balanced microphone cable, that is pin 2 and pin 3 of an XLR-type connector. The voltage source may be either in the mixer itself or in a separate phantom power supply connected in line with the microphone cable.

Most recent mixers have phantom power built in, and the actual voltage will be stated on the mixer or in the operating manual.

The voltage requirement for a phantom-powered condenser microphone will also generally be stated on the microphone or in the manufacturer’s literature.

Some types, particularly those that are externally charged, may require a full 48-volt supply. Electret types, which have a permanent charge, will typically operate over the entire range from 12 to 48 volts.

Unless specifically stated otherwise by the manufacturer, these microphones will deliver their full performance at any voltage in this range, and further, they will not be damaged by a full 48-volt supply. Supplying less than the recommended voltage to either type may result in lower dynamic range, higher distortion, or increased noise, but this also will not damage the microphone.

Phantom power schematic. (click to enlarge)

Dynamic microphones, of course, do not require phantom power. However, many mixers have only a single switch that supplies phantom power to all microphone inputs, which may include some used by dynamic microphones.

The presence of phantom power has no effect on a balanced, low-impedance dynamic microphone. It is not possible to damage or impair the performance of a balanced microphone correctly hooked up to any standard phantom supply.

If a balanced microphone is incorrectly wired or if an unbalanced, high-impedance microphone is used, there may be a loud “pop” or other noise produced when the microphone is plugged in or switched on. In addition, the sound of the microphone may be distorted or reduced in level.

Even in these cases, the microphone will still not be damaged and will work normally when the wiring is corrected or the phantom power is turned off. If an unbalanced microphone must be used with a phantom-powered input, an isolating transformer should be inserted.

By the same token, it is also not possible to damage any standard phantom power source by improper microphone connection.

Good phantom power practices:
• Check that phantom voltage is sufficient for the selected condenser microphone(s);
• Turn system levels down when connecting or disconnecting phantom-powered microphones, when turning phantom power on or off, or when turning certain phantom-powered microphones on or off;
• Check that microphones and cables are properly wired.

Following these practices will make condenser microphone use almost as simple as that of dynamics.

Not Necessary Or Even Desirable
For the expected sound level, microphone sensitivity should be high enough to give a sufficient signal to the mixer input. In practice, most mixers are capable of handling a very wide range of microphone signal levels.

Occasionally, for extremely high sound levels, an “attenuator” may be necessary to lower the output of the microphone. These are built into some microphones and mixers. Otherwise, accessory attenuators are available that may be inserted in line with the microphone cable.

It has already been mentioned that balanced, low-impedance microphones are recommended for the majority of worship facility sound applications. This will allow the use of long microphone cables, and result in the least pickup of electrical noise.

In any case, the microphone impedance should be similar to the rated impedance of the microphone input of the mixer or other equipment. It is not necessary or even desirable to match impedances precisely. It is only necessary that the actual input impedance be greater than the microphone output impedance.

In fact, the actual impedance of a typical microphone input is normally five to ten times higher than the actual output impedance of the microphone.

The microphone input impedance of most mixers ranges from 1000 ohms to 3000 ohms, which is suitable for microphones of 150 ohms to 600 ohms.

In-line transformers.

When it is necessary to match a balanced, low-impedance microphone to an unbalanced, high-impedance input, or vice versa, transformers with the appropriate input and output connectors are readily available.

Transformers provide an impedance matching function and can also change the configuration from balanced to unbalanced as needed.

Ideally, transformers should be connected so that the bulk of the cable run is balanced, low-impedance, for maximum allowable length and minimum noise pickup. This would normally place the transformer at the connector of the unbalanced, high-impedance device.


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About Tim

Tim Vear
Tim Vear

Senior Application Engineer, Shure
Tim is a senior application engineer with Shure. He is noted for his teaching ability, presenting seminars about sound to a wide range of audiences and is the author of the Audio Systems Guide For Houses Of Worship.
http://www.shure.com

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Eric Miller says

The same thought went through my head.  However, most people reading this will not have ribbon mics.  Or if they do, they will be aware of proper care and feeding.

Lee Buckalew says

You really need to clarify the statement, “The presence of phantom power has no effect on a balanced, low-impedance dynamic microphone. It is not possible to damage or impair the performance of a balanced microphone correctly hooked up to any standard phantom supply.” as this is misleading.  Ribbon microphones are, in fact, dynamic (those which do not have an active section) and can be damaged by phantom power, particularly when being plugged and unplugged when the phantom power is on.

Brent says

Most modern ribbons that people will be able to afford in the local church WILL NOT have a problem with phantom power, as long as they are not plugging and unplgging the ribbon while phantom is on.  That will be a disaster.  There are a few videos on YouTube demonstrating this is fact.

Many budget mixers with global phantom do not even offer a full 48V, or a full 48V on every channel.  So the danger is minimized when condensers on the lecturn, choir, piano and active DIs are being used already.  Check it out.

Lee Buckalew says

“...as long as they are not plugging and unplgging the ribbon while phantom is on.” (typo in original)
I stand by my comment.  It should be clarified that plugging and unplugging standard dynamic ribbon microphones while phantom is engaged can lead to problems with the ribbon mics.  The danger comes in the fact that, unless both sides of the XLR make and/or break simultaneously then the momentary voltage imbalance that this can create can cause the ribbon to be forced into a condition of over-excursion and be damaged. 
Most church sound folks don’t turn off phantom when swapping mics the way that many recording engineers do.  Just trying to alleviate potential problems.

ali503 says

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