Introduction to Live SoundInputs and Gain

Structure

• Gain structure

• Balanced vs Unbalanced

• Mic or line level?

A front of house operator in live audio controls the Left and Right outputs of the console to the speakers that the audience hears.

Some of the time, a band's foldback or monitor mix is controlled from the same console as well.

FOH- "Front of House"

This is done by taking the various inputs from the stage (a DI or mic input) and connecting them to the console with XLR connectors, usually using a multicore.

Multicores replace a large number of XLR leads running to the FOH, which would be very messy.

XLR Trademarked name for circular 3-pin connectors developed by Cannon (now owned by ITT). "XLR" was originally nothing more than Cannon's part designation for the connector, which meant "X Series," "Latch," and "Rubber." In fact, you'll also sometimes see these connectors referred to as "Cannon" connectors. XLR has since evolved into a generic industry term, and many manufacturers now make this style connector. In audio work, XLR connectors are normally used for transmitting balanced mic and line level signals.

What does XLR stand for?

Gain StructureTypically the first thing an engineer will do once everything is set up and plugged in onstage is do a line check, and set gain levels.

They start with the initial point that signal can be be controlled, at the gain pot at the top of each channel.

The PFL button can be engaged at this point to give a more detailed reading on a large meter.

Gain StructureHow much an electronic circuit amplifies a signal is called its "gain".In most specs or references you will see gain expressed as a decibel value (i.e. 6 dB of gain).

Occasionally, you may sometimes see gain expressed as a straight numeric ratio (a voltage gain of 4 or a power gain of 2).

Gain StructurePre-Fade Listen (PFL)

In a console, pre-fade listen is a very useful tool for listening to signals individually without disturbing the general mix.

PFL generally sends a signal to monitor outputs regardless of the setting of that channel's fader, and simultaneously mutes the other channels.

In other words, PFL allows you to solo a channel even if the fader is pulled all the way down. Note that on most consoles, this affects monitors (meaning your headphone send) only, and does not interfere with main(FOH), tape, or auxiliary outs.

Gain StructureSo when setting gain levels, a good procedure to follow is to

•Hit PFL

•wind up the gain as the musician is playing until you have a good solid 0 average on the main meter

•Disengage PFL and slowly push the fader up until it sounds right/is loud enough

Do it this way until it becomes automatic - guarantees you never give the speakers and yourself a nasty shock.

Unity GainA device or setting which does not change signal level (amplify or attenuate the signal) is said to be at "unity gain."

• Amplify: to increase the level of a signal

• Attenuate: to decrease the level of a signal

In practice, unity gain is often a desired setting for maintaining gain staging, and for optimizing operating levels and signal to noise ratios. Correct gain settings at all points in the signal chain is very important to a good mix. Mixers call it good "gain structure".

Can't see any signal?With PFL engaged and the fader down:

•Check if the input needs phantom power,

•Check if the pad switch is in,

•Check you are looking at the right channel,

•Check that the mic/DI is actually plugged in,

•Check that the multicore is plugged in correctly at the back of the console.

Still nothing? Follow the chain and eliminate one thing at a time: swap the mic, then the lead, then change channels on the stage box, then change channels on the desk. Do it in logical sequence so it's easy to remember what you've tried. Sometimes this is crucial when the pressure is on and people are screaming at you and possibly worrying about other things; the location of their rider, why the guitar amp isn't switching on, how they haven't got enough vocal in the drumfill.

If you can see the signal on PFL but can't hear it, that means you check a different end of the chain, ie: post channel.

• Is the channel bussed to LR?

•Are the outputs of the console connected?

•Do they run to the graphics?

•Are the graphics switched on?

•Are the outputs of the graphics connected to the amps and are the amps on and up?

Signal leads carry voltage

XLR leads carry voltage which then hits the preamp in the desk, which we control with the gain pot. They carry AC voltage, or alternating current which flows two ways/ has two polarities, and we call them positive and negative. The XLR lead also has an Earth or ground conductor, which carries interference and passes it down to Ground.

Balanced vs. Unbalanced

So, the term balanced refers AC electrical signal having two "legs" independent of ground. One leg is generally considered positive (+) and the other negative (-) in voltage and current flow with respect to ground. Unlike unbalanced audio lines there is no "signal" carried in the shield or ground connection (unless there is a fault.)The main benefit is that any noise that gets induced into the line will be common to both the positive and negative sides and thus canceled when it arrives at its destination, assuming the destination is balanced.

Balanced vs. Unbalanced

This phenomenon is called "Common Mode Rejection" and basically just means that any signals common to both the positive and negative legs of balanced lines get canceled. This happens because when the receiving device looks at the signal the common noise actually shows up as out of phase with itself, and gets cancelled.

Balanced lines are generally much better for long cable runs due to their ability to reject induced noises. XLR and TRS type cables are designed to transmit balanced audio from one balanced device to another. A standard 1/4-inch guitar cable is an example of an unbalanced cable.

These 6.5 jack plugs come in unbalanced and balanced.

Mic Level vs Line Level

In order to set the correct gain coming in to your console, you might want to think about whether your input source is at mic level or "line level".

Mic Level

The level (or voltage) of signal generated by a microphone. Typically around 2 millivolts. Compare this with the two normal line levels (1.23 volts and .316 volts), and it becomes apparent just how much amplification is going on in a microphone preamp, and why it is essential that preamps be of as high quality as possible!

Line LevelLiterally, the average voltage of an electronic audio signal. While technically any voltage over 25 millivolts RMS is considered line level, in the modern audio world we narrow the scope a bit to the two line level references in use today:

Balanced "pro" gear runs at around +4 dBm (1.23 volts), while unbalanced "semi-pro" gear operates at approximately .316 volts (-10 dBV). "Pro" and "semi-pro" may be almost meaningless terms anymore, but the two operating levels must still be dealt with.

The important thing is to match the levels of the gear you are using so that -10 equipment isn't directly feeding +4 equipment, and vice versa. If you use gear of both levels, there are various level matching devices on the market to properly interface the item.

+4dBm

-10dBm