Now before you beat yourself up over this, just keep in mind that unless you've got an electronics background and understand how specifications are derived then terms like S/N ratio and Total Harmonic Distortion are just buzzwords.
Furthermore, consumer grade audio manufacturers are really bending the rules nowadays...all in the quest of that all mighty big number. The pro grade guys...not so much. Because the guys that are ready to spend a few thousand dollars on a power amp can spot a B.S. spec in a split second.
So for the next couple of weeks, I'd like to impart a little bit of this inside information to you. We'll talk about what certain specifications are, whether a bigger number is actually better (not always), and how a spec affects how a piece of gear sounds. We'll also talk about the ways that manufacturers "fudge" the numbers...especially wattage.
I had originally planned to cover all of the major specs of a power amp in one article, but after I started writing this I realized that it would wind up being a very long post. So we're going to split it up. This week, we'll talk about wattage and power ratings. Next week, the other specs.
Which Watt?
The output power of an amp is the most manipulated, massaged, and maladjusted spec on a power amp. After all, that's the first thing people look at. "Ooooo....2000 watts for just $100...I'll take it". Yeah, you'll take it all right...but I'm not going to say where you're going to take it.
First, let me clear the air here. A watt is a watt...period. There are no "tube" watts, "solid state" watts, "British" watts, or "American" watts. Anyone who uses these terms falls under the category of "repeats whatever they read on bathroom walls". The formulae for calculating power does not contain a variable for any of these terms (but maybe there should be an "moron" variable). You hear these terms used with guitar amps mostly, but be aware they're out there and they're bunk!
There are several variations of the power formulae, depending on the variables that you know, but at the end of the day it boils down to this:
Power (watts) = Volts * Amps
Like I said, there are variations of that if you know the load that the amp is operating into. The most frequently used is:
Power (watts) = Voltage Squared / Resistance (ohms)
You'll see that one written as P= E^2/R, and we'll be using that one later.
So with that said, lets look at a good set of wattage numbers, and then some not so good.
In With The Good
I consider Crown and QSC to be amoung the best amp manufacturers out there. You'll see their amps on a lot of pro touring rigs, and the care they put into those high end products are reflected in their consumer grade stuff. Since we've been talking about the Crown XLS 802 for the last couple of weeks, we'll give 'em a little more free advertising.
These are the average power numbers and are very conservative. Most amp manufacturers use RMS (root-mean-square) power (which would result in a slightly higher number). Either measurement is a good indicator of what the amp can pump out all day long without breaking too much of a sweat. Crown specs the frequency, the distortion level, and the loads they test into. If it looks simple, it's because it is. These are honest numbers without any trickery involved.
Taking A Peek At Peak To Peak
Prior to the mid 70's, amp manufacturers had no guidelines on how to spec power. So how did they do it? Easy, by using methods that would make for the biggest numbers! So you saw wattage ratings expressed in either Peak Power, or the notorious Peak To Peak Power.
So what are these numbers? This is definitely one of those cases where a picture is worth a thousand algebra equations, so let's look at a sine wave:
So lets say that I'm testing an amp into a 8 ohm load (typical for a loudspeaker), and I measure the peak value at 28.28 volts (there's a reason for that weird number). OK, so how much power is that? If you plug in the numbers you'll see:
(28.28 * 28.28) / 8
800 / 8
100 watts
So, the peak power of this amp is 100 watts. Any guesses as to the Peak to Peak power? Anyone? Yep, 200 watts. Sound pretty impressive, doesn't it. But there's a slight problem with this number.
Not So Peak Performance
The problem with the Peak power numbers is that for most amps they're not sustainable. What I mean by that is they can produce that power for a very short period of time (a few hundredths of a second). If you tried to produce that continuously, you would burn the amp out pretty quickly.
Looking at the sine wave above...you can see that the amp actually spends very little time (relative to the rest of the waveform) at the peak value.
At some point in the mid 70's, the Federal Trade Commission finally stepped in and put some rules in place about how manufacturers can specify the power of their amps. And with that came two new values...RMS and Average.
Sustainable Power
RMS and Average values are what an amp can produce over an extended period of time without destroying itself. So...what are they and how do they compare to the Peak numbers? Glad you asked:
Hey Man, What Happened
So lets use what we just learned on the example I gave earlier. Prior to about 1975, a manufacturer could have advertised the amp in my example as a 200 watt amp, and not told you how they came up with that number other than to say it was Peak to Peak. But what can this amp really do?
First, anytime you see Peak to Peak, divide it in half. Again...it's a BS number. Now, take the 100 watts you have left and multiply it by .7, which leaves you with 70 watts. That's a much more realistic number. But what's more impressive...70 watts or 200 watts (sounds like one of those phone commercials with the kids, doesn't it)? So now you know how the game is played.
Full Disclosure
A wattage rating alone is useless without knowing at a bare minimum the load you're operating into and the distortion. The example from Crown, that's a complete spec. But that's not so much the case with our next contestant. And I don't mind calling them out, because Pyle is one of the worst offenders as far as manipulating their numbers and playing off the average consumer's lack of electronics knowledge.
What A Pyle Of...
OK, we saw a good wattage spec. How's about something from the other end of the scale:
Two things to point out. First, they are not doing anything illegal. Second, keep in mind that they are advertising this amp as a 1,400 watt product.
Under the section for "Continuous Output Power", they list the amp as being able to produce 70 watts per channel into an 8 ohm load, and 110 watts into a 4 ohm. That alone tells me that the power supply for this amp is under designed. On a decent amp, it would produce about twice the wattage into 4 ohms as 8. On this amp, the power supply runs out of gas before then. That's a red flag.
There's another flag on this as well. In the 8 ohm mode, they spec that the amp can deliver 70 watts over the full audio bandwidth (20Hz to 20kHz). But, on the 4 ohm ratings, the 110 watt figure is only spec'd at a single frequency. So that does that mean? It means that the distortion figure likely goes to hell at frequencies above and/or below this. That's a bad sign.
Anyway, we have power and load. The Total Harmonic Distortion (THD) is listed as 0.1% at "rated output power". So, that infers that it's at the 70 watt / 110 watt figures. But then they throw out "Maximum Power" for the two loads. And guess what, there's no definition for "Maximum Power" other than what they define. Most likely, that wattage is at a higher distortion value...probably 1% (which still isn't terrible).
And then there's that Peak Power....which I'm at a loss on how they came up with. If you take the 100 watt figure as an RMS value and work the math backwards, the Peak value I come up with is 142 watts. So how the hell did they come up with 700 watts?
Hey Rocky, Watch Me Pull A Spec Out Of My A$$
Here's how they did it. If you measure the power supply rail of the amp at idle, it's likely coming in at around 75 volts (a typical value for a 200 watt amp). Now, if you plug that number into the power formula we used earlier, then you get:
(75 * 75) / 8
5625 / 8
703 watts
So in effect what they are saying is that the amp has the "potential" to produce a peak output of 700 watts. However, there's a whole lot wrong with this:
- As soon a you put a signal in the amp, the supply rail is going to drop a bit
- Even if the supply rail didn't drop initially, it will in a few thousaths of a second
- Assuming they've got a really good power supply that maintains it's voltage under load(which they don't), you're going to have switching losses through the output transistors.
So the 700 watt spec is valid...until you actually try to amplify something. That's real useful...not!
Levelling the Playing Field, And Then Not
As I said earlier, the Federal Trade Commission standardized how wattage could be advertised around 1975. Amp makers either played by the rules or got fined for false advertising.
However, in the last couple of years the FTC appears to be bowing to pressure from electronics manufacturers. They are now considering no longer requiring the distortion figure in print or radio advertising. Their "rationale" behind this is a crappy as Pyle's power numbers:
- Most equipment produced today has a low distortion value.
- Most consumers don't understand that number
- Consumers that want to know the figures can find them on the Internet.
A Dead Give Away...
I've gotta give Behringer the "Most Exaggerated Wattage" award for their new iNuke series amps. And here's why.
There's a basic law of energy conservation that says you can't get more power out of something than you put into it. Keeping that in mind, and the fact that a watt is a watt regardless of how it's being used take a look at this spec sheet for what they claim is a 3000 watt amp:
If there's one number you can usually trust on a spec sheet, it's the power consumption number. You see, if the manufacturer wants to get a UL (Underwriters Laboratories) sticker or equivalent, they have to state the max power the amp can draw under worst case conditions.
So, what Behringer is saying is that they can put 350 watts into the amp, and get 3000 watts out. Really? Where did the other 2,650 watts come from? OK...here's how they do it.
They're using the same trick as Pyle, and basing that 1500 watt per channel number on the power supply rail voltage (probably about 110 volts) The power supplies used in light weight amps (switching power supplies) can deliver a lot of power instantaneously, but can only do it for maybe a 1/10,000th of a second. And then that's it...it's out of juice until it recharges on the next power cycle. So it's a completely useless number other than for marketing. If someone could make a device that could continuously output more power than it took in, they wouldn't be jacking around with amps. They would be making trillions of dollars selling cheap energy to the world.
The bottom line here is that you can't have more continuous power out than you take in. If you read anything to the contrary, it's more marketing BS. In fact, the true power output of an amp should always be less than the power it draws from the AC outlet. No amp is 100% efficient. Some really good (and expensive) ones approach 90%...but most are in the range of 50% to 70%. So, expect a true 500 watt amp to draw between 700 to 1000 watts. The difference between the two figures is what's lost as heat.
Oh, and what do our friends from Pyle list as the power consumption for their "1400 watt" amp:
Wow...it's blank! Go figure. Losers....
I realize we've covered a lot of stuff today, but to me this is important stuff. I'm really tired of seeing decent working musicians spend their hard earned money with these companies thinking they're getting 3000 watts of power only to find out it's really less than 1/10th of that.
In fact, it bugs me so much I'll make this offer. If you're considering buying a power amp and you're not sure of what you're getting, then email me at the address below. Either tell me the make and model or the amp or send me a PDF of the spec sheet. I'll read it and tell you what you're really getting, not what the magazine ad wants you to believe.
Next week, we'll wrap up our discussion on specifications and how they impact the way an amp sounds.
Until then, keep the meters out of the red. Especially if you don't have as much power as you think!
Ken
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