Professional audio equipment includes: monitor mixing consoles; powered mixing consoles; portable mixing consoles; power amplifiers; dynamic microphones; condenser microphones; wireless microphones; speakers; monitor speakers; powered speakers; subwoofers; equalizers; reverb units; effect processors; delay units; compressors; limiters; crossovers; noise gates; CD players; cassette recorders; video disc players; projectors; pitch shifters; song request (karaoke) machines; headphones, and many other devices.
The sound engineer is the professional responsible for all the audio equipment on a performance stage. First, every sound source must be gathered into the mixing console; then they are divided into the various sub-channels, grouped and adjusted; and finally the master output is sent, through the main system equalizer, to the power amplifiers and speakers to produce sound, so that it suits the acoustic characteristics of the venue.
The sound engineer needs to know the musical characteristics of every link in the chain, the frequencies most prone to feedback and the resonant points of the venue, in order to control the volume of each song and apply equalization to it. He must also know how to use effect processors to improve different voices and instruments, and be familiar with the functions and limitations of every piece of equipment so that no distortion or other undesirable effects arise. The equipment chosen must be guaranteed for quality and reliability; preparations must be made in advance for possible breakdowns, and effective remedies applied promptly. Sometimes the preparation has to be finished several hours before the performance; tight as the schedule is, for the sake of safety every signal cable has to be connected by his own hands before he can rest easy.
This article may serve as reading and reference material for beginners, amateur audio enthusiasts and nightclub sound engineers. Readers are asked to point out any shortcomings.
I. Power Amplifier (POWER AMPLIFIER)
In large events a professional amplifier has to work continuously for long periods, and must also withstand vibration and impact during transport. Compared with amplifiers for ordinary audio use, professional amplifiers therefore place more emphasis in their design on durability over long use and on structural reliability. The amplifier has a major influence on the sound quality of a sound-reinforcement system, accounting for roughly 30% of the whole audio setup. So in order to give full play to the performance and function of the audio equipment, attention must be paid to the quality of the amplifier. Otherwise even a high-quality sound-reinforcement system cannot perform as it should.
There are three kinds of power amplifier: 1. standalone power amplifiers; 2. mixing console and power amplifier combined; 3. speaker and power amplifier combined.
Standalone amplifier: this kind of power amplifier is a separate unit, so you can build up an audio system yourself according to your own plan. Generally one amplifier consists of two channels.
Mixing console and power amplifier combined: this kind of amplifier is simple to connect and convenient to operate, and is widely used in small and medium sound-reinforcement systems.
Power amplifier and speaker combined: because the matching of amplifier and speaker has been taken into account, it is simple and convenient to use, and is mostly found in monitor speakers and keyboard instrument speakers.
When connecting one amplifier to another, or an amplifier to a loudspeaker, the impedance matching between them must be taken into account (impedance is measured in ohms). Impedance matching means that the rated output impedance of the power amplifier should equal the rated impedance of the speaker, at which point the speaker absorbs the greatest power. If the rated impedance of the speaker is much lower than the rated output impedance of the amplifier, the operating current will rise sharply, which in turn damages both the loudspeaker and the amplifier. When one channel of an amplifier drives two speakers, the combined impedance of the speakers falls, so the load impedance seen by the amplifier falls, and the amplifier will overdrive under what is nearly a short-circuit condition. Therefore when matching an amplifier to speakers, be sure that the input impedance of the speakers lies within the load impedance range of the amplifier.
Power matching: in principle the rated output power of a power amplifier should equal the rated power of the speaker, but because the output transistors will produce severe non-linear distortion once overloaded, the rated output power of the amplifier is usually deliberately raised so that it exceeds the rated power of the loudspeaker. The correct connection is: the output power of the amplifier should be 30% greater than the nominal power of the speaker. If the speaker's power is much smaller than the amplifier's power, take special care when using the amplifier: the volume should be turned up gradually from low to high and must not be set too loud, or the speaker will be damaged. In practical work a relatively large amplifier output power is favourable for improving sound quality. In addition, the dynamic range of the source material is very wide, so pay close attention to the risk that an instantaneous overload of the amplifier may damage the speakers.
Average output power means the power delivered over long, continuous operation. Peak power means the maximum power that can be handled within a short period; it is much greater than the rated power. The output of a sound-reinforcement system is determined by the amplifier; a concert of a given scale needs a given amount of power, the standard being one watt per person. The power will vary according to the type of concert, the size of the venue, the reverberation and the number of speakers. Total power / output power of one amplifier = number of amplifiers required. Bridge output: bridge output is a method of using a stereo amplifier as a mono amplifier. It is a circuit configuration adopted in order to obtain a large power output, and is also called BTL mode. The principle of the bridge connection: channel A amplifies the positive half-cycle of the signal and channel B amplifies the negative half-cycle, so that the output obtains double the power.
The bridge connection and its steps in detail: when an amplifier's two channels work separately, the rated output power of each channel is 400W/4 ohms, and this is the ordinary stereo connection. When a larger rated output power is required (below 400W), the bridge connection can be used: 1. set the mode switch to the "BRIDGE" position; 2. feed the signal in through channel A; 3. take the power from the "+" terminals of both channels, with channel A as output "+" and channel B as output "-".
Amplifier output level display: the display is a ladder array of coloured light-emitting diodes, used to show the amplifier's level in real time. A normal level shows green; when the amplifier is called on to carry a sustained high-frequency signal, the level indication turns yellow; at peaks in the music signal or on drum hits the red light-emitting diode flashes (flashing intermittently). All of the above are normal. If the red light-emitting diode stays lit continuously, it means the amplifier may be overloaded. This often happens when one amplifier channel drives several loudspeakers; in that case the system should be reconfigured so as to eliminate the overload.
Amplifier peak display (PEAK): when the PEAK diode flashes, the gain control should be turned down. Amplifier protection display (PROTECTION): under certain operating errors, the amplifier's built-in protection circuit will trip automatically, and the protection indicator will flash. Once the operating error is cleared, the protection indicator will go out.
II. Mixing Console (MIXER)
Mixing consoles fall into two kinds: those for studio use and those for stage and ballroom use.
The functions of a mixing console are: 1. to pick up signals and amplify them; 2. to apply treble, midrange and bass tone equalization as required; 3. to send signals as required to the left and right buses or to carry out group control; 4. to apply artistic processing to the signals sent to the auxiliary buses; 5. to carry out output control as required.
A mixing console can be divided into input units and output units. (1) Input unit. The input unit is an important part of the mixing console. It is a multi-channel parallel circuit, and every channel is much the same. It can generally be divided into the following parts.
A. Input selection section. 1. TAPE: tape; 2. MIC: microphone; 3. LINE: line
B. Input attenuator (PAD). If the level of the microphone or line input signal is too high and the gain control cannot bring it down, switch on the attenuator; a 20 dB attenuator is then inserted between the preamplifier and the input socket, so as to avoid overload.
C. Input gain control (GAIN). The sound sources of a mixing console include microphones, instruments, tape, effect processors, sound-reinforcement equipment and so on. Because their output levels all differ, the gain control on the console is used to adjust the input sensitivity so that they can be matched. If the input signal is too large it produces clipping distortion; if on the other hand the input signal is too small, the noise becomes uncontrollable. The gain control exists to ensure that the console works within a fixed dynamic range. On the panel the gain level is marked with 0 dB = 775 mV as the reference, and it is set at different positions according to the output level of the source.
Input signals and gain levels are shown below.
Gain (dB) / input signal: -60 to -50 = low-level microphone; -35 = high-level microphone (condenser), electronic instruments; -20 = low-level line (ordinary audio)
D. Signal input sockets. These are divided into low-impedance balanced inputs (LO-Z XLR) and high-impedance unbalanced inputs (HI-Z two-conductor).
Ordinary instruments and audio equipment are usually connected in unbalanced mode, in which one end of the signal "+" and "-" is shared with the shield of the signal cable. For example, with single-core shielded cable the core carries signal "+", while the shield carries signal "-" and ground. This picks up less induced noise than unshielded parallel cable, and counts as a simple, incomplete form of shielding.
The inputs and outputs of professional audio equipment are all balanced: the signal is carried on "+" and "-" conductors, with a separate shield connection, and "+" and "-" use independent ground lines. The plugs used are XLR connectors.
E. Overload (CLIP). The overload indicator warns of instantaneous overload of the input signal. The indicator lights up 3 dB below the peak level (the level at which the signal is so large that distortion occurs), which helps in setting the position of the gain switch.
F. Input equalization section. The input channel equalizer is used to correct the tone of the input signal and bring it to a standard result. Because control is per channel, the console can apply equalization to each channel without the channels interfering with one another. The equalization is divided into: high frequency (HIGH), mid frequency (MID) and low frequency (LOW). The 0 position is flat; the + direction is boost, +15 dB (a fivefold increase); the - direction is cut, -15 dB (a fivefold reduction). It is continuously adjustable. Equalizers generally use a three-band design: treble (10 kHz), midrange (the centre frequency of the equalizer can be set freely between 350 Hz and 5 kHz) and bass (100 Hz).
Because each frequency band has its own independent control, the input signal can be adjusted in fine detail, bolder experiments in tone shaping become possible, and unwanted elements such as howling and noise can be effectively removed.
1. High frequency: 10 kHz ±15 dB / shelf
Affected range: the higher harmonics in the treble register of instruments. Boost effect: the metallic quality increases and the tone becomes rather sharp; boost too much and noise becomes clearly audible. Cut effect: hiss can be effectively removed, but cut too much and the transparency of the treble register is lost.
2. Mid frequency: 3 kHz ±15 dB / peak
Affected range: the upper register of instruments and voices. Boost effect: the tone becomes bright and the texture harder; boost too much and the ear tires easily. Cut effect: the musical balance leans toward the bass, and voices feel the same way.
Mid frequency: 1 kHz ±15 dB / peak
Affected range: the middle register of instruments and voices. Boost effect: the tonal outline becomes clear, the stereo image projects forward, and the attack of the drums is strengthened. Cut effect: the stereo image recedes.
Mid frequency: 500 Hz ±15 dB / peak
Affected range: the lower-middle register of instruments and voices. Boost effect: the tone becomes full-bodied and forceful; boost too much and a telephone-like tone appears. Cut effect: the attack becomes harder and the balance leans toward the treble; cut too much and the texture becomes thin.
3. Low frequency: 100 Hz ±15 dB / shelf
Affected range: the bass register of instruments. Boost effect: the tone becomes deep and rich; boost too much and sibilance becomes unclear. Cut effect: the sound becomes lighter and sibilance is good, while background noise and hum can be effectively removed.
G. Pan. The pan knob is used to adjust the left-right balance of the signal, and its position in the signal path is after the channel fader level control. The pan positioning of each input channel's signal between groups 1-2 and groups 3-4 is also determined by this knob. If the knob is in the middle, the pan position is in the middle as well. Turn the knob to the left and the position is assigned to group 1 or 3. Turn the knob to the right and the position is assigned to group 2 or 4.
H. Monitor send (MON/SEND). The monitor send is used to control the level of the input signal on the monitor bus. Apart from the gain control, this control is not governed by any control switch on the channel (including the channel volume control). The send signal is therefore relatively independent of the main bus signal.
I. Effect send (EFX/SEND). This covers all peripheral equipment and is used to determine how much of the internal effect or the external effect is added to the input signal. It is affected by the equalizer and the volume fader. Because every channel has its own effect send, adjustment allows some channels to carry the effect while others do not. Note, however, that the internal effect and the external effect share a single send control, so they should have the same sound source.
J. Pre-fade listen switch (PFL/CUE). When this switch is at "ON", the signal of each input channel can be monitored in the headphones and confirmed on the level meter. The priority order of the listen switches must be firmly remembered