This week, I wanted to at least touch on some of the other specifications you will typically see on a power amp. You don't see these exaggerated near as much...mainly because most folks never get past the wattage spec. But, they are there, and I thought you might like to know how them impact the way an amp sounds.
First off, let's take a look at the specs our trusty Crown XLS 802.
So, for a live sound application which specs are we really concerned with? While I can make a case to consider all of 'em, the first 4 are really the important ones. Let's talk about what they are.
Sensitivity Training
The spec for sensitivity of an amp is actually one of the easiest to understand. The spec for this amp is 1.25 volts. What that means is that if you turn the gain controls on the amp all the way up, it will take 1.25 volts at the inputs to drive it to full power. This is a reasonable spec, as most modern mixers and crossovers can output several times this.
The trick is that console outputs are almost always spec'd as dBu. To convert that to voltage requires....ah hell, I'll just give you the answer today. My Presonus StudioLive can do +24dBu. That works out to a little over 12 volts...plenty to drive the amp. (We'll do an article on decibels for non-math majors one of these days).
But decibels are funny critters, being logarithmic. That means they're kind of like the Richter Scale they use for earthquakes...where a 7 is a whole lot worse than a 6. If your console can output +18dBu, then that's about 6 volts. And if it can only do +4dBu...that's right at 1.25 volts...which means you'll have to have it floored to max your amps out. It also means you're completely out of headroom on your console and you're probably clipping the signal.
And once the signal clips at the console there's nothing you can do to clean it up later. ...you're S.O.L. at that point.
Freq Out
The frequency response of an amp is the range of frequencies that it can pass. Almost every amp will have that spec, but 80% of them are incomplete. A frequency response without a tolerance on it is really useless. The spec for this amp is 22Hz to 20kHz, but notice the numbers after that...+0 dB, -1 dB. That means the amp is flat to within 1dB which is incredibly good.
Most amps are spec'd +/- 3dB which is still perfectly acceptable for live work. But when you start seeing numbers like +/- 10dB...it's a BS spec. That means you're going to have to put a lot of EQ in front of that amp to get it to flatten out. And if there's no tolerance...well...that just means they're hiding something.
Now, how much frequency response do you need for live work? Unless you're using concert grade subwoofers and doing electronic dance music, you're not going to need to reproduce much of anything below 40Hz. And going to the other way, most PA systems start rolling off at 12 kHz.
So why do amp manufacturers spec the frequency response outside of this range. While you may not be able to hear (or a speaker pass) anything about 15kHz, an extended high frequency response can be an indication of how "fast" an amp is. There's a spec not mentioned about called slew rate, and you'll usually see it on consoles and preamps. Slew rate can be a scary looking spec, but what it amounts to is "how fast can this amp move on a waveform". It's usually expressed in volts per microsecond. An extended high frequency response can mean that the amp has a decent slew rate. What that that translates into sonically is amp that sounds clear and punchy. The slew rate along with the damping factor (we'll talk about in a bit) has a large impact on the "sound" of a power amp.
Bring Da' Noise
The signal to noise ratio (frequently abbreviated S/N) gives you an idea how much background noise the amp produces on it's own. Here again, a bigger number is better. For this amp, they're saying that the noise is 100dB below the signal. Putting that into more familiar terms, it means that the noise is 1/100,000th of the signal...pretty darned quiet. The difference between weighted and unweighted is that the weighted number is filtered and limited to what we can actually hear. The weighted number is more meaningful in the real world here. But even the unweighted is still pretty impressive.
Again, how much do you need? Keep in mind that your system S/N can not be any better than the worst component in the chain. So, if you amp is 100dB, but your Behringer console is only 55dB...then that's as good as your system can get. Personally, I look for 80dB or better unless the client just has no budget and is only going to be playing $#!+ hole clubs.
Distorting The Truth
When most musicians here the term "distortion" they think of the pedal a guitar player steps on before the intro to "Satisfaction". (OK, bad example...that's a fuzz box). But where I'm going with this is that while distortion is usually desirable to a guitar (and some bass) players, it's not in a PA system. So this is where a smaller number is better.
Now, how many of you actually know what "distortion" is? The technical definition for the intellectually curious is any non-linearties introduced into a signal. For the mere mortals, it's probably easier to explain how it's measured.
To measure distortion in an amp, you input a single tone (1kHZ is the standard, but some folks use 400Hz) and measure the output signal level. Then, you filter out the original tone at the output and measure the signal level again. The crap that's left over...that's your distortion.
Again, how much (or little in this case)? It depends on the application. If this amp were going into a studio or other critical listening environment, I would want 0.1% or less. But for live work, I'll generally accept anything at 1% or below. This amp has 0.5%, so it's fine.
But remember, for the distortion numbers to be meaningful it has to be a rated power. And this is where you can get tripped up. The Crown spec above is written to say that distortion will never be worse than half a percent. But other manufacturers will bound the distortion measurement by doing things like spec'ing it at a lower power or not over the entire frequency bandwidth of the amp.
If you don't see this number on a spec sheet...it's likely because the manufacturer doesn't want you to know that it sucks.
Et All
The other numbers (IMD, Damping, Cross Talk, and Input Impedance) are still important, but unless you're putting a very large system together (I'm talking 10 power amps or more), don't concern yourself too much. Most folks getting into a system that big are going to hire a geek like me to do it for 'em anyway. But just for giggles, here's what they are and how the affect what comes out of the speakers.
- IMD - Intermodulation Distortion. Dang...that hurt. IMD is one of one of those distortions that is really uncool. To explain it, you have to have a quick primer on the concept of Intermodulation. Whenever you mix 2 different frequencies together, you wind up with 4! Yep...it's magic. You get the original 2, plus the sum and difference of them. So..if you mixed 800Hz and 1200Hz, you would also get 400 Hz (1200-800) and 2000Hz (1200+800). To bad we can't do that with currency. Anyway, it's an annoying artifact, so the more negative this number is the better.
- Damping Factor - I would tell you how this number is derived, but you would just want to throw something at me. Let's just go with this...the damping factor is an indicator of how well an amp can physically control a speaker. Think about what's happening with a speaker...it's travelling one direction and then all of a sudden has to go the opposite way. If the amp doesn't have enough control to do this, then the first thing you notice is that the low end is muddy and undefined. The Damping Factor (sometimes written as DF) is one of those "bigger is better" numbers. There are a lot of other things that affect the system Damping Factor (when you add in the speaker and the wire that hooks it up). If you see an amp with 200 or better, you're in good shape. Don't look for this spec on a budget amp...it would reek if they printed it.
- Crosstalk - This one is actually pretty easy to understand. On a stereo (or multi-channel amp), the crosstalk is a measure of how much of the signal on one channel gets into another one. Again, the more negative the number the better. I only pay attention to this spec if the amp is going into a critical listening environment (like a studio). But live...all the noise coming off the stage is going to mask the minor amount of crosstalk in the PA system. As long as it's at least -40dB I'm not gonna sweat it for a PA.
- Input Impedance - This is almost useless to spec now unless you plan on hooking up 10 amps in parallel. Back in the 60's when stuff had real transformers in it for the audio, this mattered. Now...no so much. It's just another big number to confuse the average user.
Next week...I promise no (OK...very, very little) math. We'll talk about working with sound and lighting companies. I recently played a show where we hired in the production (and man oh man was it nice to not have to pack a PA at the end of the night). A lot of what we'll discuss also applies to playing on house systems at a club.
Until then, keep the meters out of the red.
Ken
Ken Carver has been a musician and performer since the early 70's, and involved with live music production since the mid 70's. He worked for 15 years as a broadcast engineer, building numerous studios and transmitter sites around Texas. He's also worked in Critical Care Communications for the medical industry, R&D for an automated lighting manufacturer, and owned Project Lighting & Sound in the 80's. He currently heads up an R&D Hardware Technician Team at National Instruments in Austin, and still performs on the weekends in the Central Texas area. You can reach Ken at itsjustlogistics@gmail.com
Good info Ken!
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