The Craft_The FOH Mixer Pt. 1Presentation Transcript
Sound Engineering LESSON:The Front-of-House Mixer Pt. 1
LESSON: The Front-of-House Mixer• You must understand everything about your mixer!!• The better your understanding of the primary mixer functions, the better your chances of experiencing creative freedom.• The mixer is also called: A console A desk Board Front-of-house mixer FOH
LESSON: The Front-of-House Mixer• Signal Path is simply the route that a signal takes form point A to point B. For speed and efficiency in any situation, it’s essential that you’re completely familiar with the signal paths involved in your setup.• A good maintenance engineer know that the only surefire way to find a problem in a system is to follow the signal path deliberately from its point of origin (Point A, for example, the microphone) to its destination (point B, the speakers).
LESSON: The Front-of-House Mixer• There are several possible problem spots between point A and point B. A thorough knowledge and understanding of your signal path lets you deal with any of these problems as quickly as possible.
LESSON: The Front-of-House Mixer• Here are the concepts that we will cover that relate to the signal going to and coming out of the mixing board: • High and low impedance • Direct boxes and why they are needed • Phantom power • Line levels
LESSON: The Front-of-House Mixer• A mixer is used to combine, or mix, different sound sources. These sound sources might be: • On their way to the front-of-house mix • On their way to effects from instruments or microphones • On their way to the stereo or multitrack service recording device • On their way to the monitor system
LESSON: The Front-of-House Mixer• Let’s begin at the input stage, where the mics, instruments, playback, and presentation devices plug into the mixer. Mic inputs come in two types: high impedance and low impedance. There’s no real difference in sound quality between these two as long as each is used within its limitations.• In practical application, microphone connections are almost always low impedance, whereas instrument outputs are typically high impedance. In order to plug a guitar or keyboard into a microphone input, you must incorporate a line matching transformer.
LESSON: The Front-of-House Mixer• One of the first things your signal from the mic sees as it enters the mixer is the mic preamp (sometimes called the input preamp or simply the preamp). The preamp is actually a small amplifier circuit, and its controls are generally at the top of each channel. The preamp level controls how much a source is amplified and is sometimes labeled as the Trim or Gain.• Remember that mixers work at line level, so a mic signal needs to be amplified by the preamp to line level before it gets to the rest of the signal path.
LESSON: The Front-of-House Mixer• Best results are usually achieved when the preamp doesn’t need to be turned up all the way. A preamp circuit usually recirculates the signal back through itself to amplify. This process can add noise, then amplify that noise, then amplify that noise, and so on. So use as little preamplification as possible to achieve sufficient line level.• Some mixers have an LED (light-emitting diode, or red light) next to the preamp control. This is a peak level indicator and is used to indicate peak signal strength that either is (or is getting close to) overdriving the input. The proper way to adjust the preamp control is to turn it up until the peak LED is blinking occasionally, then decrease the preamp level slightly. It’s usually okay if the peak LED blinks a few times.
LESSON: The Front-of-House Mixer• It’s a fact that sometimes the signal that comes from a microphone or instrument into the board is too strong for the preamp stage of your mixer. This can happen when miking a very loud instrument, such as a drum or electric guitar amp, or when accepting the DI of a guitar, sound module, or bass with particularly strong output levels. Some microphones produce a stronger signal than others. This is a particular problem when miking drums or loud guitar and bass amplification systems. If the signal is too strong going into the preamp, then there will be unacceptable distortion. When this happens at the input, theres not fixing it later.
LESSON: The Front-of-House Mixer• This situation requires the use of an attenuator, also called a pad. This is almost always found at the top of each channel by the preamp level control. Sometimes, especially on condenser microphones, there is a pad between the mic capsule and the microphone circuitry. Try this attenuator before using the mixer attenuator.• An attenuator restricts the flow of signal into the preamp by a measured amount or, in some cases, by a variable amount. Most attenuators include 10-, 20-, r 30-dB pads, which are labeled -10dB, -20dB, or -30dB.
LESSON: The Front-of-House Mixer• It’s possible, acceptable, and standard procedure to use a direct box to match a high- impedance output to a low-impedance input or vice versa. A direct box is also called a line-matching transformer, impedance-matching transformer, or DI (direct injection).
LESSON: The Front-of-House MixerPassive versus Active DIs • There are two main types of direct boxes: passive and active. Passive direct boxes are the least expensive and generally do a fine job of matching one impedance in to another impedance out. Active direct boxes are usually more expensive and contain amplifying circuitry that requires power from a battery or other external power supply. These amplifying circuits are used to enhance bass and treble. An active direct box typically gives your signal more punch and clarity in the high frequencies and low frequencies.
LESSON: The Front-of-House Mixer The hi Z input• The hi Z input is a Y. One side of the Y sends the signal to the transformer; the other side of the Y sends the signal to the Out to Amp jack. This makes it possible for instruments to plug into the direct box then into the amplifier from the Out to Amp jack.• If you hear a loud ground hum after plugging into the direct box, it will usually go away if you press the Ground lift switch.• Most Dis have a pad to help keep strong signals from overdriving the console inputs.
LESSON: The Front-of-House Mixer We must use meters to tell how much signal is getting to theconsole, recorders, outboard gear, and so on. There are two different types of meters in common use today: the volume unit (VU) meter and the peak program meter (PPM).
LESSON: The Front-of-House Mixer VOLUME UNIT (VU)• In the modern sound reinforcement world, VU meters are the least common type of meter; however, they’re commonly used on analog tape recorders.• VU stands for volume unit. This meter, with an average rise and fall time of about .3seconds reads average signal levels, not peak levels or fast attacks.• A VU meter has a needle that moves across a scale from about -20 VU up to about +5 VU.
LESSON: The Front-of-House Mixer PEAK PROGRAM (PPM)• Peak program meters accurately meter fast attacks from percussive instruments. Nearly instantaneous, sharp attacks, such as those from any metal or hard wood instrument that is struck by a hard stick or mallet, are called transients.• Peak meters contain a series of small lights that turn on immediately in response to a predetermined voltage.
LESSON: The Front-of-House Mixer• A transient is the percussive attack present in all percussion instruments when one hard surface is struck with a hard stick, mallet, or beater (cymbals, tambourine, cowbell, claves, guiro, shakers, maracas, and so on). Transient attacks are also a consideration when reinforcing a piano or acoustic guitar—especially a steel string guitar played with a pick.• When metering instruments that contain transients with a standard VU meter, adjust levels so that the loudest sounds register between -9 VU and -7 VU. This approach results in a much more accurate and clean percussive sound. The transient is usually at least 9 VU hotter than the average level, so when the standard VU meter reads -9 VU, the input is probably seeing 0 VU. If you meter 0 VU on a transient, the input might see +9 VU!
LESSON: The Front-of-House Mixer• A peak LED is normally just one red light that comes on when the signal is about to oversaturate tape, overdrive a circuit, or exceed maximum digital level.• It’s usually okay for the peak LED that lives in one corner of a VU meter to blink occasionally but if it’s on continuously, back the input level off until it blinks less.
Sound Engineering LESSON:The Front-of-House Mixer Pt. 1