Last month we discussed how to set gain trims, channel levels and master levels from Microphone to mixer output. This month we will talk about what to do after the mixer on through to your power amplifiers. This will include graphic equalizers, crossover levels, power amplifiers and digital versus analog levels, as they are dramatically different.
FROM MIXER LINE LEVEL OUT
Most people who are somewhat a novice in the field of audio, (and let’s face the facts, most gospel groups know very little about audio) and see very little difference conceptually between microphone level and line level. And at times it is confusing. Just a quick recap of last month; microphones are considered very low level signals and can be very wide ranging from the very low to almost full line level. However mixers such as the Mackie, Allen and Heath, and other popular brands, require a set range of level going through the mixers that is quite narrow in scope. Therefore we use the input channel trims to compensate for the input level variations from our many input sources, so that the mixer is “seeing” individual channel levels that are within it’s range of good noise and headroom performance. (Please refer to last months article for a bit more in depth view.)
Therefore, once we have our live mix, and the proper levels emitting from our mixer of choice, we must now pass those signals on to the other items in the chain. These could be graphic equalizers, electronic crossovers, power amplifiers and self powered speaker cabinets. All of these devices can be designed with a different concept of what “line level” truly is. Are you confused yet? Don’t be, its not that hard once you get into the particulars.
Basically there are two standards for analog line level, the consumer standard which is generally considered to be –10 dbv (about 0.3 volts) equals 0.0 db on its inherent vu meter. In other words, the level that makes the meters read zero on their own scale. Devices with such levels are consumer CD players, Mini Disc players and virtually all consumer hi-fi equipment. There are exceptions to the rule, but generally this will be accurate. The other standard is called the “Professional Standard” which is generally considered to be +4dbv, (2.2 volts) which will be the “zero” for these devices. Devices with this type standard, among countless others, are Mackie and Allen and Heath mixers, 360 Systems Instant Replay Machines and most professional rack equipment such as graphic equalizers, electronic crossovers, etc. Marrying these two standards together within a confined sound system can be a daunting task for someone not familiar with the terms. However, there are ways of determining how it should be done. Most equipment will have a specification sheet, which will state these numbers in plain English for all to read. We men tend to not read instructions until it is our last and final hope, so we get in trouble. And too, some audio devices have switchable levels for the input and output levels. Typically you will see these on the back panel of the device and it will give a choice of +4 or –10 levels. Please note, there is a whopping difference of 14 db between these two standards. Has anyone ever tried to connect a Instant Replay machine to an audio mixer? Because the levels on the digital side have been mastered to be as hot as a CD will allow, the input from one of these devices is hotter than a pistol as the old saying goes. Output levels typically +10 db are very common.
You must set the gain through the system to maintain unity gain from mixer output to power amplifier input. If you do not, one of two things will happen. Either you will have a signal that is too low in level and noisy, or you will have a signal too high in level and distorted. It is imperative that your gain structure be uniform and correct.
ANALOG LEVEL vs DIGITAL LEVEL
This is where many lose their way. Analog processing, such as mixers, equalizers and analog crossovers, have a designed headroom of some specified db above the stated zero level, For instance, if you run your Mackie mixer well into the red of the meter, to +10 db above zero, the mixer will sound quite fine for it has a headroom factor of about 15 db or better above zero operating level. The same is true of virtually all line level analog devices, but not power amplifiers, more on that later. Digital devices on the other hand have a totally different method for stating their operating level. Zero level on most digital devices is within mere micro-volts of hard clipping. If you attempt to operate a digital device over its zero level the result will be obvious and quite nasty sounding. An example of such a device would be a digital electronic crossover such as the dbx Drive Rack. Even though it is analog in and out, the internal processing is 24 bit digital, thus the metering reflects digital jargon, not analog. In digital processing there is virtually no headroom above its zero level. However, the good news here is this: If you send an analog zero level, +4 dbv signal into a dbx Drive Rack or other digital processing device, you will not see 0 db on its metering. Instead you will see about –15 db. This means that the digital device is allowing the zero analog signal to pass through the dsp digital processing at 15db below its hard clip (distortion) point. Therefore, zero analog level in will equal zero analog level out on this device even though it’s metering is reading –15 db. The professional method of referring to this digital level would be stated as –15dbfs, which means 15db below full-scale output. Another important point to make here, digital devices have their lowest noise and best distortion performance the higher the internal level is, provided it never exceeds zero. For example, the lowest noise and the best distortion figures for a dbx Drive Rack will be right at its zero metering, or 0dbfs. Obviously we cannot operate our sound system without some safe measure of headroom, so we strive to keep the signal passing through these devices at –15dbfs, or there about. Many people make the mistake of lowering the levels much to low in digital devices, and this can make your audio sound grainy and harsh. Thus, you should maintain your line level outputs all the way to the power amplifiers, as this is the preferred place to make the necessary headroom changes.
POWER AMPLIFIERS
Now we come to power amplifiers, which can also be confusing. Power amps are rated in terms such as a specified input level produces a specified amount of watts into a specified load, the load being the speakers themselves. The 8ohm rating you see so very often on speaker cabinets is the rating of impedance or stated another way, AC resistance to ground. In other words, the term “ohm” is a measure of resistance. If you were to measure the resistance, or impedance if you will, of a piece of wire, you would quickly learn that its resistance would be zero, no resistance whatsoever, a dead short. And too, if you connected that piece of wire across the terminals of a power amplifier you would quickly cause the power amp to overheat, and likely burn itself out. So, when we connect an 8ohm speaker to the terminals of a power amplifier, we have installed a load of 8ohms of resistance to ground on the amp. If we had two such speaker cabinets, each being 8 ohms, we would then have a load of 4 ohms (not 16 as some tend to think). Looking at it another way, we are now 4 ohms closer to a dead short. Add two more 8ohm cabinets for a total of four 8 ohm speaker cabinets across the terminals, and you have a 2 ohm load, which is very, very close to a dead short. Not good!
Ok back to our power amplifier level considerations: Now we have a zero level signal coming from the mixer, down the chain of processing including graphic equalizers and crossovers, and we wish to take that signal and plug that direct into our power amplifiers and hear beautiful music coming out the other side of those speaker cabinets. Not so fast. There might be a problem. Power amplifiers are rated in some respects, like digital processing. For instance, our group uses a certain brand of power amplifier that is rated as +4 dbv input produces a maximum output of 425 watts into 8 ohms. If we were to operate our sound system, with the gain/level totally tweaked correctly, except the power amplifier’s volume control (gain control) is set to wide open, we would be driving +4 dbv, or zero level, direct into this power amp. Any peak above that level, even though the mixer and its associated chain of processing can handle an increase of up to 15 db above +4, the power amp cannot abide that much input level, nor can your speakers. The amp has a maximum output of 425 watts into 8 ohms, and a +4 input is where that occurs. To keep the headroom of 15 db, we must lower the input gain on the amplifier to –15db. Setting the power amplifier gains down to compensate for headroom factors forces you to do two things. First you must run your output master level control a bit hotter than you would have to do otherwise, and secondly, it forces you to operate the mixer and the processing chain at a level more conducive for proper performance. Now, when we have a peak coming down the mixer chain of say +10 over zero, which is fairly common, the amplifier is still working well within a margin of safety. Also, we have kept the levels going through our processing chain to a safe, comfortable level that provides plenty of headroom and the best possible compromise of noise performance versus distortion. An alternative to turning down the input controls on the power amplifier is to select the –10 setting (assuming you have such a setting) on the last device in the chain, just before the power amps. In this configuration, you could turn the amplifier gains almost all the way open, or no more than –1 db down. The –10 db setting is effectively accomplishing the same thing for you as it is in effect a 14 db pad. Take note, this should be only done on the last device in the audio chain before it reaches the power amplifiers. Nowhere else in the system is this a good idea.
If we you are using self powered speaker cabinets such as Mackie’s, EAW. JBL or Meyer’s, the input to the power amplifier in the cabinet is not actually direct into the power amp section, but more likely, the signal first must pass through the inherent electronic crossover and dsp processing. In most case such as this, the input to self-powered cabinets is considered true professional line level, and the headroom factor is built into the processing, in other words, not to worry. But, to be on the safe side, check the specifications on the self-powered cabinets you have to determine what input level is considered typical for your particular model.
CONCLUSION
Alright, now we have all of our system levels set properly. And for the most part, you would think you would be good to go. However, there are other considerations such as how to drive subs, what crossover point to use, how to use a graphic equalizer on a speaker system properly (not smiley faces), and many other considerations. In months to come we will look at these various issues and try to take the perceived magic out of the equation and make it far more easy for everyone to comprehend.
Until next time,
Ben Harris
http://www.southernsoundquartet.com
Author: Ben Harris
| Author's Website: www.southernsoundquartet.com
Written: 03/01/2009
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Category: Sound Advice
Comments: 1
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You know your stuff! I had to reread this 3 times and may have to print it out and carry it around with me for reference.lol
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