EmoZine Article #1: Understanding Amplifier Classes
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Here at Emotiva, we believe that the more knowledgeable you are about how things work, the more likely you are to see what a great deal our products are; it’s really that simple. To that end, we’re going to start The EmoZine with a short series of articles about “how audio works”. The first topic we’re going to cover is one that seems to confuse a lot of folks: amplifier operating classes.
This article is going to focus on solid state audio amplifiers, but most of the basic concepts we talk about here also hold true for tube audio amps (the diagrams would be slightly different for a tube amplifier). We’re not going to say much about really obscure types of amplifiers, and amplifier types not normally used for audio at all.
[Figures 1 & 2 are attached.]
Figure 1. shows an audio signal: a sine wave (everybody likes a sine wave). You’ll notice that we didn’t label those up and down arrows anything except “+” and “-”. That’s because they could actually show lots of different related things. They could show the voltage at the input of the amplifier, or the voltage at the output, or the current going through the speaker (since the music at the output should be the same as the input, only louder, in a perfect world they should all look the same except for the values). Our sample starts at zero, goes more positive for a while, returns to zero, goes negative for a while, and eventually returns to zero. This is typical of music, although the waveform for music is usually a lot more complicated.
Figure 2. shows a very generic amplifier output stage. The output stage is the part of the amplifier that delivers the power to the speakers. It is also the part we’re usually talking about when we discuss amplifier operating classes. Our generic picture shows a single pair of transistors (regular “bipolar” transistors at that). Real world amplifiers may use other types instead (like MOSFETs), and many (including most of our bigger ones) use several transistors in place of each of the ones in the picture. The principle is, however, the same. Most amplifiers these days are direct coupled and use a split (plus and minus) power supply - like the picture.
All that stuff to the left (“drivers and the rest of the amplifier”) controls and drives the output transistors but, since we’re most concerned with the operating class of the output stage, we’re not going to talk about that stuff at all.
Since music goes both positive and negative, we have both positive and negative power supplies. If we didn’t, then the output wouldn’t be able to go positive and negative. (There are ways around this, using things like coupling capacitors and transformers, but those aren’t used much any more in serious amplifiers.) The two power supply connections are usually called “V+” and “V-” or “the positive and negative supply rails” or “the plus and minus power supplies”.
OK, now on to business... When our sample sine wave appears at the input of the amplifier, our mission is to make it larger and send something that looks pretty much exactly like it, only bigger, to the speaker. (The speaker will convert the changes in electric current into sound - one way or another.) A transistor is simply a block of stuff which, when properly motivated, can turn on and off to let current through it (or not). The important thing is that a transistor can turn full on, full off, or anywhere in between. We should, however, mention that when the transistor is partway on, with voltage across it and current flowing through it, it ends up having to “burn” some of the power passing through it as heat.
Class B Amplifiers
“In a Class B amplifier, only one output device is on at any given time.”
Let’s see what happens when we amplify our sample sine wave with a Class B amplifier:
A Class B amplifier is biased so that, when no signal is being amplified, neither transistor is on.
We start out at zero, with zero volts at the output, and zero current flowing through either transistor. As the signal starts to go positive, the transistor connected to the positive power supply (the top one) turns on and starts passing current to the speaker. The more positive voltage we want, the more we turn the transistor on.
With our example sine wave, as the voltage goes up, the top transistor will start from off, turn on more and more as the voltage goes up, turn on less as the voltage drops towards zero, and eventually turn entirely off when the voltage gets back to zero. As the voltage starts to go negative, that process will repeat with the transistor connected to the minus power supply (the bottom one). Finally, when the negative portion of the waveform gets back to zero, the bottom transistor will be off. At this point both transistors will be off. Since music is a continuous waveform, this will repeat...
Note that at no time are both transistors on at the same time. This is a pretty efficient way of doing things. Although each transistor does still have to burn some power when it’s partially on, at least they’re not both on at once, and neither one is on when there’s no music playing.
There are, however, a few drawbacks. For one thing, transistors operate most linearly (produce the least distortion) when they are biased halfway on, so each of these transistors is frequently running in an area where its distortion isn’t at its lowest. For another thing, each transistor actually has to shut off entirely part of the time. It so happens that transistors get especially sloppy right when they’re very near the point where they turn full off. The result is that, since both transistors aren’t quite perfect right where the signal gets handed off from one to the other, there may be a little rough spot right around there. This is called “crossover notch distortion” and is difficult to eliminate entirely in Class B amplifiers.
A Class B amplifier can be as much as 70% (or slightly more) efficient at most power levels.
Class A Amplifiers
“In a Class A amplifier, both output devices are always on and operating in their most linear ranges.”
Let’s see what happens when we amplify our sample sine wave with a Class A amplifier:
A Class A amplifier is biased so that, when no signal is being amplified, both transistors are biased halfway on. This means that, when there is no signal at all, about half of the maximum current the amplifier can deliver flows from the V+ supply, through the top transistor, through the bottom transistor, and to the V- supply. Both transistors are biased in the middle of their operating range, which is the best spot for them to be in terms of lowest distortion. Since equal current is flowing through both transistors, the output voltage is zero and no current flows through the speaker.
As the signal starts to go positive, the transistor connected to the positive power supply turns on more, while the transistor connected to the negative supply turns on less. The result is a “net gain” for the positive side and positive current flows. As the signal goes more positive, the current in the positive transistor goes up, and the current in the negative transistor goes down by the same amount. When the signal goes less positive (and back to zero), the current in the two transistors goes back to being equal. When the signal goes negative, the negative transistor turns on more, the positive transistor turns on less, and the output goes negative. All of this time the current is varying symmetrically between the two transistors; one goes up, the other goes down, both about the same amount.
When we get going loud enough, each transistor is getting near the point where it isn’t carrying much current, but they both spend more time near the point where they are at half capacity, which is the point where they have the lowest distortion and the best sound. Even better, in a lot of that middle range, the small amount of distortion created by the two transistors tends to cancel out, giving us even less distortion overall. Sounds like the perfect answer, right?
Well, not quite. Our Class A amplifier has lots of current flowing through both transistors all the time, and the total power it consumes averages out to a lot more than with its Class B cousin. Even worse, while the Class B amp had no current flowing when there was no signal, the Class A amplifier has about the same amount of current flowing whether it’s at maximum output or zero output. This means it runs hotter, needs bigger heat sinks to get rid of that heat, needs a bigger power supply to supply all that extra power, and works a bit harder all the time.
A Class A amplifier is about 50% efficient or less at full power, but, since it uses about the same power at any output level, gets just as hot even when it’s idling.
Also, those big heat sinks, and output devices, and power supply mean that Class A amplifiers tend to be quite expensive.
Class A/B
“In a Class A/B amplifier, both output devices remain on at all times, but one or the other is usually doing most of the work
at any given time.”
Maybe we should consider a compromise. Let’s see what happens when we amplify our sample sine wave with a Class A/B amplifier:
In a Class A/B amplifier, we basically start off with a Class B amplifier, with both transistors off. Then we adjust things so that each transistor is always on just a little bit. When there is no signal at all, a little bit of current flows through both transistors (just like in Class A). For small amounts of power, as the current in the top transistor goes up, the current in the bottom transistor goes down, and vice versa - also just like we described for the Class A amplifier. However, as we go above a certain point in the positive direction, the current in the top transistor keeps going up, but the bottom transistor is already almost shut off. Because of the way we have things set up, that bottom transistor will always stay on a little bit, but it won’t be able to reduce its current at the same rate that the top transistor raises its current anymore. Likewise, as we go below a certain point in the negative direction, the current in the bottom transistor keeps increasing, but now the top transistor is already almost shut off. This time, because of the way things are configured, that top transistor will always stay on a little bit, but it won’t keep lowering its current as the bottom transistor increases its current.
Most people refer to anything past the point where both transistors are operating symmetrically as “the Class A/B range” for that amplifier. By keeping a little current flowing in both transistors all the time, we’ve avoided that area where both transistors are off at the moment one switches to the other (and the very non-linear area right near it), so we’ve dodged that crossover notch distortion - which really was the biggest problem with Class B amplifiers. However, we’ve lost some of the other benefits of Class A. Even though, for the little bit of power down where both transistors are operating symmetrically, we are technically operating in Class A, it’s still at a point where the transistors are not in their most linear range, and, of course, we’re only in that Class A “sweet spot” for a watt or two (maybe five or ten with luck). Let me stress, however, that Class A/B amplifiers can sound really really good. That crossover notch was our one major problem, and we have entirely eliminated it.
Switchable Class A and Class A/B
Now we come to amplifiers like our new XPA-1L:
Most people, with typical speakers and average listening habits, spend most of their time listening to music at an average power level of a few watts to a few tens of watts. However, at a given average level (again with typical music), you occasionally have peaks that need as much as ten to twenty times the average power to avoid peak clipping and compression. So you need five or ten watts most of the time, with the occasional need for fifty or a hundred watts to deliver a loud peak.
How about if we could take a real 35 watt Class A amplifier (which would deliver 35 watts of real superb-quality Class A
sound), and combine it with a 250 watt Class A/B amplifier (which could deliver 250 watts of excellent sounding Class A/B power when we needed it)? That would give us the best of both, right?
Meet the XPA-1L.
In Class A mode, the XPA-1L delivers 35 watts of real Class A power, then transitions seamlessly and automatically to 250 watts of still great sounding Class A/B power (250 watts into 8 ohms,
500 watts into 4 ohms) when you need it.
Other Classes
As you may recall, we mentioned that in a Class A, Class B, or Class A/B amplifier, when either output transistor is partially on it has to “drop” the voltage between what the supply is delivering and what it is sending to the speaker. It’s also sort of common sense that the maximum voltage the output transistors can deliver is limited by the voltage of the power supply they are connected to.
For example, one of our big amplifiers, which can deliver hundreds of watts with ease, has to have almost +/- 100 volts available from the power supplies. However, if that same amplifier happens to be running quietly at 8 watts, it is delivering only about +/- 12 volts (at most) to the speaker. At that instant, the output transistor is dropping most of the voltage between the supply voltage and the voltage required at the output. This isn’t a major problem, but does generate excess heat (so the amp runs warmer and needs bigger heat sinks), and contributes a bit more to global warming and your electric bill. With very high powered amplifiers, this can be something worth improving.
Note that the following are the industry standard definitions for these amplifier classes. Some manufacturers have created their own (slightly different) definitions.
Class G
With a “Class G amplifier”, the output stage of the amplifier itself is still either Class A, Class B, or Class A/B; the Class G designation refers to the fact that the power supply that is connected to the output stage can be switched between different sets of supply rails that deliver different voltages. For example, the output stage may be connected to a +/- 20 volt supply when it is running quietly at under ten watts, but, if it’s called upon to deliver more power, it automatically switches to a separate +/- 100 volt supply. Some designs use three or more sets of power supply rails, and they handle the process differently, but the principle is the same. Switching between rails happens automatically, and very quickly. Unfortunately, while there have been successful Class G amplifiers, Class G tends to cause more problems than it solves (and there are better alternatives), so this type of topology has more or less been abandoned.
Class H
With a “Class H amplifier”, the output stage of the amplifier itself is still either Class A, Class B, or Class A/B. However, in a Class H design the power supply rails are continuously varied as the output stage needs more or less voltage to produce the required output voltage. The term “Class H” describes this sophisticated power supply topology. [So, while some manufacturers and uninformed audiophiles may refer to “a Class H output stage” the reality is that it is a Class H power supply topology powering a “normal” output stage.]
Emotiva’s Optimized Class H™ topology
Many of Emotiva’s high-powered amplifiers (the XPR line) use our highly advanced Optimized
Class H™ power supply topology. The output stages on our Optimized Class H amplifiers are all purely linear Class A/B output stages. The Class H power supply continuously varies the supply rails to provide superior efficiency and cool operation. While, in a normal Class H topology, both power supply rails are varied together, Emotiva’s Optimized Class H™ topology actually varies the supply rails separately, which delivers even greater efficiency. You get real Class A/B sound with better efficiency and less waste heat.
Class D
Class D is actually a general term that covers a wide variety of “digital switching amplifiers”.
In a Class D amplifier, the power supply is switched rapidly on and off by a high-speed transistor or FET operating as a switch. Because the switch is either on or off, but never part way in between, it never has to “burn” power. The length and timing of the pulses is carefully calculated such that, after they are passed through a special filter, the result is an exact copy of the audio input. (There are several different variations on how the pulses are calculated and controlled.)
Class D amplifiers share several benefits, including very high efficiency (sometimes up to 95%) and light weight (they can avoid heavy transformers and often also use a switch-mode power supply). A lot of Class D amplifiers sound sufficiently good for professional sound reinforcement, but few deliver true audiophile sound quality (there are some Class D amplifiers that audiophiles consider good, but not many).
Sometimes the term “digital amplifier” is used to refer to Class D amplifiers. It can, however, be confusing since that term can also be used to refer to any sort of amplifier with a digital input.
A note about preamps:
The drawbacks of Class A power amplifiers are almost entirely due to the fact that there is significant power involved. In contrast, it is simple to design line-level and low powered circuitry to operate in Class A , and many preamps, headphone amplifiers, and line stages operate in Class A.
One final note:
Just to avoid confusion: Stereophile Magazine uses a product rating system that rates products by “class”: (“Class A”, “Class B” , etc). This is strictly a rating of how well they liked each product, and has nothing whatsoever to do with amplifier operating class (although amplifiers that operate in Class A usually receive good ratings).
EmoZine Article #1:
Understanding Amplifier Classes
March 15, 2013
Keith Levkoff
Emotiva Audio
www.emotiva.com Attachments:
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