Thứ Năm, 9 tháng 9, 2010

General PA FAQ

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General PA FAQ
(Reference on
Yes. In fact there is a way to connect them, which gives you most of the hum cancelling youd get if both units were balanced. Heres how it goes:
If the mixer has XLR outputs and the amp has ¼" inputs, use a balanced patch cable with a female XLR at one end and an TRS (stereo) ¼" male jack at the other. Modify it as follows:
1.   Remove the ¼" TRS plug,
2.   Solder on a standard mono ¼" plug as follows; - tip wire to tip tab, ring and sleeve wire (shield) to sleeve (ground) tab (alternately you can just modify the wiring inside the stereo plug,
3.   Mark that cable with some tape to indicate that its the one for this application.
If the mixer has XLR outputs and amp has screw terminals or phono (RCA) connectors, use a balanced female XLR - to - ¼" male TRS cable and remove the ¼" plug. If you need to install an RCA plug, solder the tip wire to the tip of the RCA and solder the other wire plus the shield wire to the RCAs jacket (ground). If the amp has screw terminals, attach the tip wire to the positive (+) screw and the other two to the negative (-) screw. If it doesnt work, the most likely problem would be how the mixers XLR output pins are wired so be prepared to unsolder and re-solder a connection or two at the XLR end of the cable.
{Note: The standard balanced XLR wiring (here at Yorkville and with most companies) is pin 1=ground, pin 2 = positive (+), pin 3 = negative (-) }
If both the mixer and the amp have ¼" connectors, its best to use a balanced TRS-TRS patch cord modified as follows; (1) Remove one of the ends, (2) Replace it with a standard mono ¼" plug soldering the tip wire to the tip tab and the other two to the ground tab (the longer of the two). (3) Mark the mono-plug end with some tape so that in future youll know this is the cable for this application and that the mono end goes into the unbalanced jack.
{Note: The standard balanced ¼" wiring (here at Yorkville and with most companies) goes tip = positive (+), ring = negative (-), sleeve = ground }
{Additional notes: (1) This form of "trick" balancing works very well with one small exception; if you switch off the mixer but leave the amp on, you may hear some hum through the speakers. If so, it is because the balancing in the mixer, which is generally an active circuit these days, has also been switched off (naturally), ergo theres no more hum cancelling. Just remember to switch off the amp too. Oh, and one other thing - if you hadnt already figured it out, these balancing tricks work for EVERYTHING; unbalanced mixer to balanced amp, balanced mixer to unbalanced EQ, or crossover, or compressor/limiter, effects units, etc., etc. }
You will need to put the guitar through an amp or guitar preamp. If you plug it directly into the PA head the sound will be wrong, the tone controls wont work right and you wont be able to get any sustain, distortion, etc.
If you decide to mic the guitar amp, use something like an Apex770 or Shure SM57 and place it near the centre of one of the amps speakers.
The sound is usually changed at least slightly when you put an amp through the PA. But when you Mic the amp it may not be necessary to change the EQ much.
For sending a direct line to the PA, use the guitar amps "line out" jack. DO NOT use a "speaker" or "headphone" output as they can distort or damage a mixers input circuitry. Plug the other end into one of the mixer-channel ¼" "Line" inputs. Its also a good idea to use a balanced shielded patch cord. The guitar amp tone controls should be set up to make the guitar sound right through the amp - worry about the PA sound later. If necessary, use the PA channel EQ, not the main EQ (no sense lousing up the vocals etc., to make the guitar sound better). You may need to turn down the PA channel treble a bit, but try to keep the controls as close to centre as possible.
Many companies, ourselves included, make both 4-Ohm and 2-Ohm power amps. Why not have anything but 2-Ohm amps? The problem with a 2-Ohm amp is, you HAVE to run it into 2 Ohms in order to get full power. But in most cases, especially club systems where there is only one or two main cabinets on each side of the stage, because nobody makes 2-Ohm cabinets, you need the ability to deliver full power to a 4-Ohm load. eg. Two 8R cabs or one 4R cab per side. Actually, all Yorkville power amps can either run into 2 Ohms, or have twins, which can. For example, the AP800 and AP2020 both have switches that maximize output into 2 or 4 Ohms. The AP4020 and AP4040 cover the 2400-Watt power range, and the AP6020 and AP6040 cover the 4000-watt range (we cant do the impedance switch trick with those big amps so we make dual products instead).
And what do the pros do about 2-Ohm loads? It depends on the definition of "pro". The big touring rigs tend to favor 8-Ohm loads, 4 Ohms minimum. This is because delivered power losses over long cable runs are lower when the speaker impedances are 8 Ohms. They can become very high - 60% percent or worse (!) - into 2-Ohm speaker loads.
Feeling confused? Check out the power loss chart in our on-line Audio Guide in the Speaker section). Club pros, on the other hand, can contend with whatever loads they encounter principally because the speaker cables dont have to be so long hence the cumulative resistance doesnt get high enough to rob large amounts of power.
As of 1970, we switched to a 7-digit serial number system which contains the manufacturing date. The first digit is the last digit of the year. Eg, "0" would be 1970, 1980, 1990 or 2000. The next two digits are the month. Eg, "04" would be April. So how can you tell in which decade your units were made? Phone or email us and ask. Generally, we either discontinue or re-name products after 10 years to avoid confusion.
Meters are calibrated in decibels because thats the common language of the audio industry. The fact of the matter is that everything actually occurs in volts - eg. 0dBV = 750mV. But since sound pressure is measured in dB (A), so are other audio levels. In practice, all meters have some sort of overload indication and thats their main saving grace. Visit any recording studio and youll see the meters all hitting on the "red" which means theyve been calibrated to give that indication at a voltage level equal to or slightly below tape preamp saturation levels. Signal-to-noise is thus optimized.
In PA applications, its a matter of avoiding clipping in the mixer and connected devices - amps, processing, etc. - while optimizing signal to noise. The rule of thumb is to run the channel levels and meters "hot" and the masters "cool". This explains why channel meters have been replaced by Clip LEDs in so many live soundboards - its MUCH easier to spot input clipping when theres a flashing light. So the place youre most likely to find meters on a PA board is in the masters. How the main meters in a PA board relate to the power amps is relevant only if you know what they mean. To figure that out, during a gig look at the meters or clip lights on the power amp(s) and note at which mixer-meter reading the tend to flash. That level then becomes your peak or clip reference. Just make sure that your power amp volume or gain controls are at max or some setting which will not be changed. If you do change them, youll need to repeat this process.
Yes, to a certain degree. To start off, here is a chart showing the way sound pressure dissipates over distance.
SOURCE SPL (dB at 1 meter) @ DISTANCE(meters) = RESULTING SPL(dB) [Note: 64 meters = apx. 210 feet]
  • 130dB @1m=124dB, @4m=118dB, @8m=112dB, @16m=106dB, @32m=100dB, @64m=94dB
  • 125dB @1m=119dB, @4m=113dB, @8m=107dB, @16m=101dB, @ 32m=95dB, @ 64m=89dB
  • 120dB @ 1m=114dB, @4m=108dB, @ 8m=102dB, @16m=96dB, @ 32m=90dB, @ 64m = 84dB
Using this chart, if you wanted to fill a hall 32 meters (105 feet) deep and you wanted to generate a sound pressure level of 100dB at that distance, you would need to generate 130 dB at the stage. This could be achieved using a pair of 1000-watt speakers with an efficiency rating of 100dB at one watt at one meter each driven by 1000 watts. Or you could use four equally efficient (100dB@1W@1m) 500 watt speakers arrayed in close-coupled pairs driven with 500 watts each.
Now, dont be surprised if you go to someplace like a gymnasium with your sound-pressure meter and your 130dB PA, pace off 32 meters (105 feet) from the PA and measure an spl thats higher than 100dB. The rule of thumb (spl dissipates -6dB as you double your distance from the source) is taken to be in anechoic conditions - eg., outdoors with no wind in a flat area like a football field. Indoors this rule can vary considerably with the acoustics of the place. In "live" places like gyms it can be less than -6dB and in some clubs the acoustic attenuation can be considerably more than -6dB so that you get much less than 100dB at 32 meters (or whatever).
Maybe now you see why there arent any really valid rules of thumb - too many acoustic variables. The standard one is, however, sufficiently worst-case to work for the average clubs architecture.
How much power is too much? Thats simple. If a speaker is rated at 100-Watts "continuous program" or "long-term program" (pgm) power, apply NO MORE than 100 Watts, period! And what if the speaker is rated in "RMS" Watts or "peak"? Then youve got a problem. Better contact the speaker manufacturer and ask them for the maximum continuous applied program power. And if somebody says, "You dont have to know that, just multiply the RMS by" - - - two or three or whatever - - - or "just divide peak by" - - two or three or whatever, dont do it. Phone the manufacturer. Those old rules-of-thumb are inaccurate if not downright dangerous.
And what about applying less than the maximum? This is where there seems to be some controversy. One of the various applied-power philosophies seems to say that you must apply no less than the maximum power specified by the speaker manufacturer because if you dont the speaker will be destroyed by amplifier distortion. But this presumes that the user is in the habit of ignoring the amplifier clip lights while wringing every last decibel out of the system. Of course some people actually do things like this and can expect some blown drivers no matter how much power they apply (remember, 200 clipped watts will always kill a speaker faster than 100 clipped watts).
Lets put it this way, a good speaker system these days will have an efficiency rating of around 100 dB @ 1 Watt @ 1 meter, maybe even a few dB more in some cases. That being the case, you can get 120dB out with 100 Watts applied and 120 dB is often enough for small to medium clubs. And if the speaker is rated at 200, 500, 1000 Watts or whatever and youre only applying 100 Watts, would it make any difference? Nope. If 120dB is all you need, 100 Watts is all you need to apply. However, you will get more sound pressure by applying more power; for example doubling applied power gives you +3dB spl or about a 30-40%increase in average perceived loudness. For that reason it can be worthwhile to drive a 200 Watt speaker with a 200 Watt amp, but its not a "must" as some people seem to think. What you should NOT do is drive a 200-Watt pgm. speaker with MORE than 200 Watts, at least not without checking with the manufacturer first.
Sounds like youre trying to EQ the music instead of the system response. When it comes to minimizing feedback potential, the manual is right. The RTA itself is just one third of what you need to level the system response. The other two parts are a calibrated (very flat) microphone, and a pink noise generator, one of which may be built into your RTA. Before the audience arrives, you plug the mic directly into the RTA and set it up out in the middle of the audience area, then plug the pink noise into a mixer channel with the channel EQ set flat (main EQ too) and effects shut off. This way the mic picks up the main system as the pink noise goes through it and what you see on the RTA is the systems actual frequency response. NOW, with the pink noise and RTA still running, you can do some EQing to smooth it out.
The only remaining question is, how "flat" should you try to EQ it. The answer is, concentrate on the mid and upper- mid frequency ranges. Dont try to make the lows go down below the systems natural low-frequency limit and ditto for EQing the highs. Remember that all an EQ can do is alter a speakers response, it cant really extend it.
That done, you can just disconnect the RTA & mic and pack it away for the next hall. Why not repeat the process when the hall fills up with people? It would be nice if you could do this because peoples bodies absorb sound at various frequencies and reflect it at others, but you KNOW they will not be amused by all that Niagara Falls racket.
And what if you just keep doing what youre doing? Expect continuing problems. With the RTA on the output of the EQ, all its seeing is what the stage mics are picking up plus all the channel EQ, effects , etc., etc. It makes for a cute light show but tells you zero about system response. If you do it the right way you should minimize feedback potential while smoothing out the sound.
That "separate vocals" effect might be a little pronounced if youre putting some reverb or echo on the vocals while the instruments & amps dont have any. If thats the case, try doing a job with the effect shut off. The solution could be as simple as that although it depends on the room acoustics. Generally, small rooms with low ceilings tend to have a very tight, dry sound which is perfect for what youre doing - minus PA reverb
There are pros and cons to putting everything through the PA. On the positive side, you can achieve a more polished sound. To a degree, it depends on what you do onstage. If the drummer plays like King Kong, you might not benefit from micing the drums, but If your drummer is able to play softly, we can look at mixing everything through your existing system.
A standard drum set needs at least six mics - one for bass drum, one for snare, one for hi-hat, one for the tenor toms, one overhead for the ride and crash cymbals and one for the floor tom. Everything except the bass drum mic should be condensers, although neodymium dynamics such as Shures Beta series or the APEX models 350 and 380 may suffice. There are a few mics especially good for bass drum, one of them being the Apex 120. Good condenser mics for drums (everything but bass drum) include the Apex165 with miniature clip-on gooseneck for drums and the Apex170 for cymbals.
Amps can be direct-lined into the mixer via their line-level outputs and the mixers line-level channel inputs; use tip-ring-sleeve balanced patch cables to take advantage of any balancing there might be. The guitar amp may sound "brittle" direct-lined in which case youll either need to mic it up or insert a speaker simulator in the line out such as Hughes & Kettners Red Box Pro or one of the units by Sans amp, Line 6, Palmer or Groove Tubes.
All in all, youll be using eleven channels including vocals. You should also put the bass drum mic and the bass amp line output through a two-channel compressor to avoid input channel clipping and tighten-up their sound.
If you want to try this all out before committing to a major investment, rent the mics, stands, booms, cables and compressor for a week or two and see how it goes. The sound benefits can be quite appreciable, but it represents extra work as well as expense, and youll need a drummer who can lay back in small venues. Or you may opt not to mix at all. Thats understandable.

There must be a million different PA speakers on the market all with different shapes, sizes, weights, features and specifications. The first thing to do before choosing is to perform a little inventory of your needs. Actually this process works for other things too.
Things to consider:
(1) Where are the speakers likely to be used?
Do you expect to fill outdoor stadiums, small, medium or large clubs, churches, hockey arenas or school gyms? Remember if you say "yes" to all the above, you may have to buy a huge system and try to shoehorn it into the smaller venues. Tip - be realistic. If you dont expect to play huge venues, keep that in mind. Ditto if you do.
(2) What sources are you amplifying?
Do you only need to amplify voices and a few instruments not including bass or bass drum? If so, you probably dont need big woofers in your full-range cabinets or a subwoofer. In fact, you may not need them even if you are amplifying those things. There are fairly compact speakers now which can handle an entire band at medium-club-size volume levels. On the other hand, if you are a DJ or youre amplifying bass or bass drum at high volume levels, you will probably require one or more subwoofers.
(3) What kind of music are you amplifying?
Again the question is subwoofers. A group which plays predominantly blues, jazz, classical music or old rock & roll may not need subwoofers. Although subs are a nice-sounding addition to any type of music, they are more commonly used for rock, dance and country rock and more commonly still for DJ applications
(4) How many speakers do you need?
For small speaking engagements or sales presentations, demonstrations, etc. one speaker is quite often all you need. In music applications, four main full-range speakers are better than two if you expect to be playing in oddly shaped halls where another speaker or two would cover areas not covered by the front system. But two speakers will suffice for small to medium rectangular halls with the stage at one end. Of course multiple full-range speakers and subs are required in large concert systems
(5) What impedance should they be?
The answer to this one partly depends on the minimum load rating of the amplifier and partly on how many speakers you intend to use. Eg., if its a stereo amp with a minimum load rating of 4 Ohms/channel and you want to use four speakers, they should be 8 Ohms each. But if youre only using two speakers with that same amp and you want maximum power delivered to them, the speakers should be 4 Ohms each. If its a mono amp (remember, a PA amp with only two speaker jacks is usually mono, not stereo) and the minimum load rating is 4 Ohms, you will need to employ two 8-Ohm speakers. The formula for calculating parallel speaker loads is on our website in the Audio Guide under Speakers.
(6) What kind of power (sound pressure) do you need?
Although power is what everybody thinks of first, its actually less important than you might imagine. What you really need to know is the speakers maximum sound pressure level (spl). Why? Because power ratings only tell you how many watts the speaker can take. But if the speaker has low efficiency, even thousands of applied watts might not be sufficient to do the job. Fortunately max. spl is often specified in manufacturers literature today. Here is approximately what to look for:
  • 120dB to 126dB @ 1 metre = most small to medium clubs.
  • 126dB to 130dB @ 1 metre = medium to large clubs, arenas and small outdoor conceTRS.
  • 130dB to 135dB @ 1 metre = in the realm of indoor or outdoor concert systems.
These are per-speaker ratings. Remember, each time you double the number of similar, close-coupled speakers and (or) the applied power, you get +3dB.
If the manufacturer doesnt supply maximum sound pressure level specs, heres an applied power-to-spl listing for speakers with an efficiency rating of at least 99dB @ 1Watt @ 1 metre :
  • 100 Watts = 119dB max.spl (at least) @ 1 metre
  • 200 Watts = 122dB max.spl (at least) @ 1 metre
  • 400 Watts = 125dB max.spl (at least) @ 1 metre
  • 800 Watts = 128dB max.spl (at least) @ 1 metre
  • 1,000 Watts = 130dB max.spl (at least) @ 1 metre
  • 2,000 Watts = 133dB max.spl (at least) @ 1 metre
Now you can just refer to the first chart.
(7) What dispersion angle do you need? More realistically, do you need wide dispersion or narrow dispersion? A wide-angle box will generally be rated around 90 degrees and the narrow variety around 60 degrees. For a two or four-speaker system where you want as many people as possible to hear everything, look for a 90 degree rating or greater. The narrower-angle speakers are principally for large arrays where multiple speakers are close together and you need to reduce the effects of overlapping highs.
(8) Is there overload protection for the horn/tweeter and the woofer?
This is often overlooked and may or may not be something you consider important. Lets put it this way, if you have to lay out five dollars for a fuse or bulb (or nothing if its a circuit breaker) its likely to be preferable to paying many times more than that for a new horn diaphragm or woofer.
(9) Is there a speaker stand mounting built in?
Thats important on compact speakers. Most have them these days, but check to make sure.
(10) What is the warranty and what exactly does it cover? Also, what are the terms?
Speakers can be subjected to all kinds of nasty surprises. Somebody accidentally kicks out a mic cable then hastens to plug it back in. "SNAP, CLUNK, SQUEEL" - there goes a horn and/or woofer. Somebody squirts beer into the horn and kills the diaphragm. Feedback kills a couple of tweeters. A roadie drops the cabinet and breaks it. What does the warranty cover? Consider the following:
  • Many speakers have a "limited" or "manufacturers defect" warranty which basically means if damage occurs to the cabinet or any of its components, youre not covered. Does this products warranty cover damage?
  • How long is the warranty on "everything". The warranty period may be five or ten years on the cabinet and crossover, but only six months on the drivers. Check it out.
  • Is there a minimum charge for parts &/or labour? How much is it?
  • Will other dealers honour the warranty when youre on the road?
Be picky about warranties. If you can get the basic type of speaker you want from two or more companies, at roughly the right price, go for the one with the good warranty!
(Subwoofers - passive & powered) Most of the reasons for picking a subwoofer are the same as for a full-range cabinet - for example the max.spl should be roughly the same. However subwoofers have additional considerations.
Is there a built-in low-pass filter?
You probably dont want your low-mid frequencies reproduced by both the full-range cabinets and the subwoofers as it will tend to create a "hump" in the overall frequency response in that range (although some people like that sound). The ideal solution is to employ an active (i.e. electronic) crossover and power the subs with a separate amplifier. But that can be expensive hence some subs come with a built-in passive filter which prevents them from reproducing anything above the low frequency range. This is called a low-pass filter, not to be confused with a crossover which some subs may feature but not many. In either case the feature lets you power both subs and full-range cabinets with the same amplifier and without the need for active filtering. This passive filtering is a great feature for small to medium club systems, but dont expect to find it in high-powered subwoofers. The high-power copper coil inductors would have to be huge causing the sub to be even more heavy and expensive. As well, subs really should be bi-amped in a high-powered system because system response needs to be smoother to prevent feedback.
Is there a pole-stand adapter on the top?
Why buy tripod stands for your full-range cabinets if the subwoofers have an adapter on top to take a pole stand? Pole stands are smaller, lighter and less expensive than tripods so this adapter makes good sense.
Powered subwoofers usually have a low-pass filter. Some also feature a low-pass frequency control. Thats worth looking for. Here are some additional things to look for:
  • A volume or level control on powered PA subs is a "must" so that you can balance the lows with the full-range sound. Dont expect to find one on passive subs, however. That would only let you turn the sub down (and nobody wants to do that). 
  • A Speaker/Line input level selector is very convenient. Basically it lets you connect the powered sub to one of the full-range speakers as if it were an extension speaker, but without affecting the overall speaker impedance of the system. In "line" mode you can drive it with a line-level mixer output.

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