The primary job of any power amplifier is to boost the signals it receives at its input from a preamp to the level needed to drive the speaker or speakers connected to its output. Put that way, this brief introduction may make the amplifier's task, or that of the power amp, sound simple. Nothing could be further from the truth.
In the case of an electrodynamic loudspeaker, which is the operating principle of practically every speaker used in professional sound reinforcement, the speaker is driven by an electric current flowing through its voice coil. That current is supplied by the amplifier, which in reality follows the voltage variations at its input (the ones it receives from the preamp) as faithfully as possible and reproduces them at its output at a level several times higher. Only then does current actually begin to flow through the speaker's winding. In other words, a power amp amplifies voltage, and when it finds the speaker winding at its output, current flows. This is where Ohm's law comes in, establishing the relationship between voltage, current and resistance, or impedance in the case of alternating current, which is what concerns us here. So, if we divide the voltage at an amplifier's output terminals by the impedance of the speaker or cabinet, we get the current flowing.
Buying Criteria for a Power Amp
When you are about to spend your hard-earned money on a power amp, or on any piece of audio gear for that matter, the one thing that really matters is that the last word belongs to your ears.
That may sound odd to some, but the reason I say it is the well-documented subjectivity involved in judging the sound of any piece of audio equipment, cables and connectors included. If it sounds good to you, great, at least for you, since you are the one putting up the cash. But let's not kid ourselves: the opinion of someone well versed in the subject is not the same as that of a layperson, and if an audio unit has poor technical specs on paper, it will almost certainly deliver poor results sonically. That is why I'm going to break down some of the main specifications you will find on power amps, so you have the best possible idea of what to look for when choosing one, or at least can compare a number of candidates on paper, since it is not always possible to try them out on site.
Interpreting Power Amp Specifications
Whatever the amplification topology (Class A, B, AB, C, D, etc.) used in a power amp, its spec sheet will largely define how it behaves in operation. As for topology, the most sophisticated power amp designs are pure Class A. You don't usually find amps designed this way in professional sound reinforcement, but it is not unusual to see them in serious recording studios.
Frequency Response and Output Power
This figure, which may be given as a number or as a graph, shows the range of frequencies (power bandwidth) the amplifier can reproduce. It should be at least the range of human hearing (20 to 20,000 Hz), along with the amount of deviation in +/- dB it holds across that range (frequency response linearity) relative to a perfectly flat response (the theoretical ideal) of 0 dB. The wider the bandwidth, the better, since you get better harmonic reproduction. The theoretical ideal would be a power amplifier able to reproduce a bandwidth from 0 Hz (direct current) to 8 Hz (infinity).
This figure matters especially for power amps that will drive the reference monitors in a recording or mastering studio. As for response curve linearity, the closer to the 0 dB mentioned above, the better.
As an example of the above, here are the actual figures from the spec sheet of a power amp from a well-known manufacturer.
Power and T.H.D.
As you can see, output power varies with the load impedance (of the speaker or cabinet) connected, and it is based on defined total harmonic distortion (THD) levels, in this case 0.1%, and note, at a specific test frequency of 1 kHz. Notice how output power increases as load impedance decreases, but watch out, because this is no cure-all.
For example, if that held true in this case, delivering 280 W per channel into 8 ohms, changing the load to 4 ohms would yield 2 x 280 W = 560 W. It doesn't, although it doesn't fall far short. This detail gives you a clear idea of how good or bad the design of the amp's power supply is. That said, a scrupulously designed power amp mathematically doubles its output every time the load impedance is halved (Ohm's law followed to the letter). It's worth saying that this rarely happens in professional amps, so there's no need to split hairs.
Why do I bring this up? Simply because it would be the norm in recording studios, where nothing takes priority over quality. I'm not saying a professional power amp of the kind commonly used in, say, a PA system isn't of very good quality, or that you can't use one of these no-compromise designs; it simply doesn't reach the extreme precision of a high-end amp, and trying to use those across the board would be downright absurd, since given their high price the budget would be plainly unsustainable, not even for the Rolling Stones. Although I can assure you that on rare occasions some musician has included one in his backline. Keep in mind, too, that while the design of a pro amp is geared toward ruggedness (vibration in transport, knocks, etc.), an extreme-fidelity amp is conceived with other goals in mind.
On the other hand, looking at output power over a 20 Hz to 20 kHz bandwidth, you can see that it turns out lower than the figures above, while the measured THD is clearly lower, and that it rises somewhat with a 4-ohm load. Consequently, this power amp will give its best performance loaded at 8 ohms.
Because of space and the complexity of the term, I'll only say that T.H.D. refers to the average harmonic distortion across all orders (2nd harmonic, 3rd harmonic, ... nth harmonic), and that the lower it is, the better. It is usually expressed as a percentage, as you can see in the table, although it can also be given in dB, which is less common.
RMS Power
The RMS (root mean square) rating is also commonly called effective or continuous power. It refers to the power the amplifier can deliver continuously over long periods, normally tens of hours, without any problem. It is called RMS because it is the result of dividing the peak power by the square root of 2.
Effective or RMS power = Peak power / 20.5
Peak power = RMS x 20.5
20.5 is the same as the square root of 2
Never Confuse Effective Power With Peak Power
Peak power is what the amplifier can deliver for a brief interval without being damaged. Also, when the amp reaches this operating regime, the integrity of the speaker connected to it is in serious danger.
Some manufacturers publish the peak power figure instead of the effective or RMS figure to give the impression that their amps are super powerful. I insist, don't get tangled up in this, because it is very important: keep in mind that you need a speaker to connect to the amp you plan to buy, and it has to be matched to it, otherwise your budget might not be properly worked out.
You may also see other ratings used to specify output power, such as music power, program power and a thousand and one tall tales. Pay attention mostly to RMS power.
Bridge Mode
Bridge mode refers to the fact that practically all professional stereo amplifiers offer the option of "summing" the two channels into one, delivering higher power so the amp, configured this way, can be used as a mono block. See the table.
Slew Rate and Rise Time
This parameter shows how fast a power amp's output can swing (how quickly it amplifies) and is specified in volts per microsecond (V/µsec). The higher this value, the better. It is also related to another spec called rise time, which tells you how long it takes the amp in question to go from its resting state to full power. Rise time is specified in microseconds (µsec).
Notice that in bridge mode the slew rate also doubles, which is useful whenever you want a faster response from the amplifier.
Damping Factor
A coil, like the one in a speaker, resists the establishment of current flowing through it and likewise resists the disappearance of that current once established. Put simply, a speaker's winding stores current (magnetic energy) for a brief interval, and you want that interval to be as short as possible. It is determined by the resistance through which the coil discharges. The higher that discharge resistance, the longer the discharge takes, and vice versa.
Well, the impedance (remember we're talking about alternating current) through which the speaker coil discharges is precisely the amplifier's output impedance (not to be confused with load impedance). To avoid overwhelming you with electrical terminology, I'll just say this is what's known as back electromotive force. And one more important point: the cable we use to connect the speaker to the amp has some resistance of its own, in series with the amplifier, so it adds to the discharge resistance, and for the reason I explained earlier the damping factor goes down.
Dividing the load impedance by the power amp's output impedance gives that amp's damping value. Damping = Zload / Zout. The point isn't to calculate it but to understand the concept. In any case, what internal impedance does this power amp have? How would damping be affected if I changed the load impedance to 4 ohms?
Signal-to-Noise Ratio
The name says it all: it is the ratio, in dB, between the amplified incoming signal (usually at maximum power = 0 dB) and the amplifier's own noise when there is no input signal, with the gain also kept at maximum, that is, 0 dB. The voltage is measured in both states, the output voltage is divided by the input voltage, and the result is converted to dB. The signal-to-noise ratio, or S/N, determines how quiet a power amp will be. The higher the S/N, the better.
Before wrapping up this article, I want to tell you something quite important that, although it has nothing to do with any technical specification, measurement or parameter on our subject, is genuinely relevant when buying an amp, not to mention one or several amplifier racks. Have you ever asked yourself: so how much amplifier power do I need? How powerful an amp should I buy?
Electrical Power Required
I have always seen a lot of confusion around this idea of EPR, and I've heard more than one outrageous claim about it, such as "1 watt per person," said by an "expert" on the subject, so I'll explain it, if only very briefly (space forces my hand).
To determine the amplifier power you need, you must know:
1) The distance to the farthest listener you want the sound to reach.
2) The sound pressure level (SPL) you need at that distance.
3) The sensitivity, output or efficiency of the speaker or speaker system, in dB.
These three figures are absolutely essential for calculating EPR. You would also need to factor in the acoustic absorption of the space being covered, and a few other things I won't mention for now because they fall outside the scope of this topic. I consider this significant, which is why I've pointed it out.
I hope you find this article useful. If you already knew the subject, a refresher won't hurt, and if you didn't, you have at least picked up a few new terms that may give you a foundation to keep digging. Take care, and until next time.
