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Mọi vấn đề khúc mắc, trợ giúp về kỹ thuật mình rất vui lòng được học hỏi và giải đáp cùng các đồng nghiệp
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GETTING STARTED
The following is a simplified set of instructions for general applications. The aim is to get a basic system up and running with as few problems as possible. [1] A BASIC STAGE SETUP GOES AS FOLLOWS:
(a) Place the front monitors near the edge of the stage, aiming back at the the performers.
(b) Place the mics and stands in front of their respective monitors at a distance of 1 to 3 feet, depending on the cabinet's up-facing angle - the speakers should be aiming directly at the backs of the mics. Other monitors should be located as closely as possible to the performers, also aiming at the backs of their mics. This is to reduce feedback potential.
(c) Place the main FOH (front-of-house) speakers at stage front at the far corners aiming straight out at the audience. Do not aim them in at the audience in front of centre stage unless the stage is deep enough for the mics to be set farther back to reduce feedback.{ TIP - If stage-front-centre audience coverage is a problem, perhaps because they are too close to the stage to hear the FOH speakers, try turning a spare monitor around to face them}.
[2] CONNECTIONS GO AS FOLLOWS:
(a) Connect all mics, line-level signal sources - i.e., tape decks, CD players, instrument amp line outputs, etc.- also processors and external effects units, to their respective channel inputs or send and return jacks on the mixer (for more information see INPUTS and OUTPUTS under THE MIXER).{ TIP - It is also a good idea to identify the various channel sources, perhaps with small stick-on labels at the bottoms of the channels, eg, "lead vocal", "guitar", "drum vocal", etc.}
(b) Do not connect speakers yet. Transients from things being plugged into the mixer and switched on can cause speaker damage eventually if not immediately. Connect speakers last.
(c) Similarly, when you are connecting one or more external power amplifiers and speakers, be sure to connect the speakers to the power amp after it has been connected to the mixer and the mixer has been powered up. The reason for this is because some mixers do not have turn-on transient suppression built in. Any power amp(s) which are connected to the mixer and running with their speakers connected when the mixer is switched on can amplify this large burst of signal voltage with a resounding "pop" or "boom" and the speaker system may be damaged. Even if the speakers survive this type of accident the first time, repeated accidents will eventually take their toll.{ TIP - If there is a power failure during the job, try to switch off the power amps immediately, before the power comes on again. Mixers with built-in power amps usually don't suffer from "pop" problems when switched on, but it's not a bad idea to turn the master levels off before powering them up.}
(d) Electronic crossovers or speaker processors should be connected between the mixer and power amplifiers. The term "between" means mixer output to unit's input and unit's output to power amp's input(s). There are four types of processors which you may encounter. First, there are simple processors which provide pre-equalization for specific speaker systems. They are connected between the mixer and power amp(s) in the same manner as an equalizer.Secondly there are adjustable active crossovers which are connected in a similar manner except their outputs must go to separate power amps or amp channels each driving the appropriate woofers, horns and tweeters, or subwoofers and full-range enclosures. Remember to find out the speaker manufacturer's recommended crossover frequencies for the various elements in order to set the crossover accurately. If this is not possible, be careful when doing this by "ear". What sounds right to you may be wrong for the components' long-term reliability. To be safe, set the output level controls all at maximum so that they are at the same output level (at lower settings, potentiometer tolerances could cause them to be different) then counter-adjust (lower) the crossover's input level to avoid over-driving the power amps.
The third type of processor is a combination crossover/processor. This would be connected and employed as if it were a crossover, although such units tend to be for specific speaker systems and therefore do not usually have variable crossover frequencies. Now a fourth type of unit has recently emerged from the depths of techno-wizardry to test our connecting skills. The "sensing" processor / crossover / compressor is designed for use with specific speakers in a touring system. In this case, everything gets hooked up as if it were a just a crossover - with one exception, additional speaker cables are run from the speaker outputs on the power amps to "sensing" inputs on the processors. Then, when the system is up and running with the amps set at full output, you push the processor's "calibrate" buttons. A set of tones is sent through the amplifiers and speakers while the "sense" circuitry samples some of the amplifier's output signal and programs the processor's variable parameters such as crossover frequency and gain.
[3] THE SOUND CHECK
It is very important to do a sound check before the job starts. It is also important that the band is playing during the sound check. However if this is not possible, have someone test the mics while you adjust the mixer.
(a) With the master levels turned off, adjust the channel input gain or attenuator settings so that the input "clip" indicators flash. Now turn them down slightly. This ensures that adequate amounts of source signal are driving the channel circuitry to provide an optimum signal-to-noise ratio.
(b) If the mixer does not have input gain/atten. controls or input clip indicators, it is a good idea to turn up the channel level or volume controls around one-third before turning up the master levels.
(c) Now bring up the masters slowly and re-adjust the various channel level controls for the desired mix through the FOH PA.
(d) When the sound check is finished, remember to turn off channels which only get used once in a while - eg. harmonica mic channel, acoustic guitar channel, special percussion instument mic channel, etc. This is to prevent open mics from causing feedback and noise problems. Channel mute buttons are handy for this function (see >>MUTE under THE MIXER). Otherwise you can use a washable marker to mark the settings before turning these levels down.
(e) Generally speaking, channel EQ controls should be kept as close to flat (centre position) as possible, especially on mic channels. In any case, start the soundcheck with all channel EQ controls at centre. Adjustments should be made for specific purposes - remember, this is a PA system, not a home stereo (amoung other things, home stereos don't have to produce 110 to 130dB SPLs with zero feedback). Possible channel EQ settings could be as follows;
[ vocals - all flat ]
[ acoustic guitar - bass flat, mid -3dB, treble +2dB ]
[ bass drum - bass flat, mid -3dB, treble flat ]
[ snare drum - bass -6db, mid & treble - flat ]
[ floor toms - all flat ]
[ tenor toms - bass -3dB, mid & treble - flat ]
[ cymbals - bass -12dB, mid & treble - flat ]
[ trumpet, tenor & alto saxophone - bass -3dB, mid & treble flat ]
[ trombone, barritone saxophone- all flat ]
[ guitar amp - bass & mid flat, treble -3dB ]
[ bass amp - bass flat, mid -3db, treble flat ]
[ harmonica - bass -6dB, mid flat, treble -3dB ]
(f) Reverb or echo should be added principally to vocals and in small amounts. Lead guitar, keyboards and horns can sometimes also use small amounts of either effect, but drums and bass guitar should usually be kept "dry" to allow a firm-sounding foundation for the rhythm section.
(g) Final monitor or aux. send levels should be set according to the artists' needs. To get started however, assume that the vocal channels will need to be loudest through the monitors, acoustic guitar will also require a fairly high monitor send setting, but aside from that, other active channels should have lower monitor settings. Drum mic channel monitor sends may be left off at first, then turned up as required. A secondary monitor system, perhaps for keyboards or drums, or multiple monitor systems could be adjusted similarly at first then re-adjusted according to the various artists' wishes.
(h) If there are effects-to-monitor masters, remember that reverb or echo should be added to the monitors in smaller amounts than to the FOH system to avoid feedback.
(i) If you are using a stereo mixer, the channel pan controls should be set at center position. The only time you might set them differently would be if you decided to run a mono PA, i.e. with the main power amplifiers all inter-connected to a summed mono output on the mixer (see >>MAIN, SUM OR MONO under MIXER OUTPUTS). Then you could use the pans to send all the drum mic channels, for instance, to the left submaster, and the rest of the band to the right submaster thus creating two submixes to facilitate level adjustments (hopefully the mixer will have a MAIN, SUM or MONO buss and master to permit this).
(j) If you're playing a new hall, you might consider pre-equalizing the system against possible future feedback problems. The process is simply a matter of increasing the main levels until you get feedback then EQing it away (see >>EQ under THE MIXER (Master Section Controls)). Then you increase the levels some more and re-adjust the main EQ. You could do the same with the monitor master(s) and EQ, but the band is NOT likely to appreciate it. The downside to this practise is that the system's response is now EQ'd sufficiently to sound strange, and it may get worse because the house acoustics will change when the audience is in place. A better practise is to position the FOH speakers so that feedback even at high volume levels is minimized.
