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Power Amplifiers: Ten Things You Always Wanted to Know

Learn how power amplifiers work, the common types, and what to lookout for when shopping for amps.

Axiom Audio ADA1500 Power Amplifier Internal

1. Is Amplifier Weight an Indicator of Robust Amplifier Design?

You have to find out if the amplifier is a Class A/B analog amp or a Class D digital amplifier before you can generalize about weight and amplifier quality. The appearance of high-quality digital amplifiers such as the Axiom A1400-8 has changed the equation. Digital Class D amplifiers are much more efficient (90% or more) than analog Class A/B amplifiers (about 50% efficient). As such, digital amps tend to run much cooler and therefore do not require the heavy heat sinks associated with high-powered analog amplifiers, hence the overall weight of a digital amp may not be a reliable indicator of its intrinsic quality. For instance, Axiom’s eight-channel digital A1400-8 has received superb reviews. And while it’s not exactly light (58 lbs), it still weighs much less than its multi-channel analog competitors whose output power is often about 200 watts per channel, yet typically weigh from 90 to 125 pounds. Moreover, as an 8-channel power amp, the A1400-8 is much more compact than many analog rivals, one of the great advantages of high-quality digital designs.

For conventional analog Class A/B amplifiers, weight can be an indicator of good, robust design because it suggests that the amplifier’s important internal components — the power transformer, heat sinks, and storage capacitors — are large, and therefore have plenty of capacity to process and store large amounts of power to handle loud dynamic peaks without distortion.

By contrast, an analog amplifier that weighs less may use a smaller transformer with inadequate capacity and fewer or thinner heat sinks (heat sinks look like radiator fins and are used to dissipate output transistor heat generated by high power output and big dynamic swings).

2. Do Solid-State (Transistor) Amplifiers Sound Different?

They may sound different if they are used at high volume levels as they approach the limits of their output ratings, when the amplifiers’ distortion is rising and nearing the clipping point. However, if two different transistor amplifiers have the same smooth, linear frequency response, low distortion, and are operated within their output ratings, then they will tend to sound identical until they are called upon to produce great quantities of clean, unclipped power. With one amplifier, there may be a quality of effortlessness to the sound quality on big dynamic peaks in sound level, whereas another amplifier may start to sound strained or harsh on dynamic peaks because it cannot handle peaks free of distortion. Using music as a “test” signal, such differences may only appear as a need to “turn down the volume” rather than your hearing gross audible distortion artifacts. Note, too, that peaks can be as much as 12 dB louder, which will demand 16 times as much power from the amplifier, causing many lower-powered amplifiers or receivers to go into clipping.

3. How Does an Amplifier “Amplify”?

Perhaps one of the easiest ways to understand how an analog audio amplifier works is to think of it as a kind of servo-controlled “valve” (the latter is what the Brits call vacuum tubes) that regulates stored up energy from the wall outlet and then releases it in measured amounts to your loudspeakers. The amount being discharged is synchronized to the rapid variations of the incoming audio signal.

In effect, an analog amplifier is comprised of two separate circuits, one of which, the output circuit, generates an entirely new and powerful electrical output signal (for your speakers) based on the incoming audio signal. The latter is an AC signal of perhaps 1 volt that represents the rapidly varying waveforms of sounds (both their frequencies and amplitudes). This weak AC signal is used to modulate a circuit that releases power (voltage and amperage) stored up by the big capacitors and transformer in the amplifier’s power supply, power that is discharged in a way that exactly parallels the tiny modulations of the incoming audio signal. This signal in the amplifier’s input stage applies a varying conductivity to the output circuit’s transistors, which release power from the amplifier’s power supply to move your loudspeaker’s cones and domes. It’s almost as though you were rapidly turning on a faucet (you turning the faucet is the audio signal), which releases all the stored up water pressure — the water tower or reservoir are the storage capacitors — in a particular pattern, a kind of liquid code.

4. What Are the Different “Classes” of Amplifiers?

Class A designs have current constantly flowing through the output transistors even if there is no incoming audio signal, so the output transistors are always on. This type of amplifier has the lowest distortion of any but it’s extremely wasteful and inefficient, dissipating 80% of its power in heat with an efficiency of only 20%.

Class B amplifiers use output transistors that switch on and off, with one device amplifying the positive portion of the waveform, the other device the negative part. If there is no incoming audio signal, then no current flows through the output transistors. Consequently, Class B amplifiers are much more efficient (about 50% to 70%) than Class A designs, however there may be non-linear distortions that occur when one set of transistors switch off and the other set switches on.

Class A/B amplifiers combine the virtues of Class A and Class B designs by having one output device stay on a bit longer, while the other device takes over amplifying the other half of the audio waveform. In other words, there is a small current on at all times in the crossover portion of each output device, which eliminates the potential switching distortion of a pure Class B design. Efficiency of a Class A/B amp is still about 50%.

Class D amplifiers, although there are a number of different design variations, are essentially switching amplifiers or Pulse Width Modulator (PWM) designs. The incoming analog audio signal is used to modulate a very high frequency PWM carrier that works the output stage either fully on or off. This ultra-high frequency carrier must be removed from the audio output with a reconstruction filter so that no ultra-high frequency switching components remain to corrupt the audio signals. As previously mentioned, Class D designs are extremely efficient, typically in the range of 85% to 90% or more.

5. Do Amplifier Class Names Represent Performance Ratings?

No. Nor do the Class letters signify anything. They are just a convenient way of differentiating types of amplifier circuits. For example, “D” does not stand for “Digital” in a Class D amplifier, although there seems to be some conflicting evidence on this. In any case, in discussions, the “D” seems to have taken root as signifying a so-called “digital” design.

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6. What Are “Digital” Amplifiers and How Are They Different From Analog Amplifiers?

An analog amplifier works in analogous fashion, regulating the output stage devices (transistors) to release power from the amplifier’s power supply to the loudspeakers in a manner that exactly mimics the tiny incoming audio waveform. Digital amplifiers use high-frequency switching circuitry to modulate the output devices.

7. Why Do Some Audiophiles Insist on Tube Amplifiers?

Tube amplifiers distort in a different manner from transistor amplifiers, generating musically agreeable even-order harmonic distortion that may lend a sense of so-called “warmth” to sound quality (the “warmth” is still a distortion or coloration; it’s not present in the source signal) and it’s this characteristic that most tube aficionados prefer. While tube amplifiers are often not as smooth or linear in frequency response as transistor designs and have other liabilities as well, when pushed near or past their output limits, tubes tend to gracefully distort, without the harshness associated with transistor clipping. However, tube amplifiers are limited in output power due to the tubes and output transformers.

Solid-state amplifiers, when pushed past their output limits, “clip” the audio waveform producing potentially harsh-sounding odd-order distortion that can be quite grating or unpleasant to the ear. On the other hand, kept below their maximum rated output, transistor amplifiers are very neutral and smooth and have none of the complex impedance interactions that may affect tube devices.

8. What Are the Most Important Attributes of Any Amplifier?

One primary attribute is a ruler-flat smooth frequency response from the deepest audible bass signals at 20 Hz (or lower) to the highest frequencies we can hear, at 20,000 Hz. A smooth, linear frequency response means that the amplifier will treat every incoming audio signal, whether it’s a bass-drum signal at 30 Hz or a cymbal’s high-frequency harmonics at 10,000 Hz exactly the same way, increasing the electrical strength of each tiny signal by exactly the same amount. Low total harmonic distortion (THD), below 0.5%, is essential so that any distortion artifacts remain inaudible with music. Finally, generous power output from a robust power supply so that the amplifier can handle the huge range of soft-to-loud dynamics present in virtually every type of music and soundtrack. “Generous” could be defined as a minimum of 50 to 100 watts per channel or more. For realistic music reproduction, more power is always desirable.

9. What Are “ICE” Amplifiers?

The Ice Power division of Denmark’s Bang & Olufsen (B&O) holds patents on its “ICE” amplifier, which is basically a Class D switching design (Pulse Width Modulator) with variants that B&O claims reduce distortion to levels associated with Class A amps, while retaining the high efficiency of Class D switching designs. ICE amps use a very high switching frequency of 384 kHz, which B&O says is 20 times as high as the highest frequency the ear can detect. The ICE amps also use feedback control to minimize the effects of the PWM design. Axiom’s engineering division took a different approach in the A1400-8 amplifier design. Axiom worked with International Rectifier to develop new silicon output devices and drive the MOSFETs in the output stage in such a way as to produce a perfect Pulse Width Modulated square wave at the output before the reconstruction filter. This approach also simplified the feedback network which made the amplifier more robust in its operation without being subject to oscillations or instability. The A1400-8 also uses a very high clock frequency (450 kHz) to allow for excellent transient response and non-aliasing in the audio band. The massive power supply is able to accurately output very high current and voltage to the loudspeaker over extended time periods.

10. How Do Small, Low-Powered Amplifiers Put Speakers at Risk?

Initially, it seems contradictory — how could a low-powered amplifier burn out speakers, when amplifiers of 200 or 400 watts per channel would seem to put speakers at much greater risk? The reason is that a small amplifier of 10 or 20 watts per channel can easily be driven into distortion and “clipping” with even moderately loud playback and dynamic peaks in loudness. The clipping cuts off the waveform and turns the output signal into an almost pure constant DC signal, which can quickly cause the fine wires in the speaker’s voice coils to overheat and melt. A large amplifier outputs clean power to the speakers — distortion-free AC audio signals — that the speaker voice coils will accept on a momentary basis without damage.

By Alan Lofft (bio), Axiom Audio

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