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10 Things about Audio Amplifiers You've Always Wanted to Know

by Alan Lofft September 18, 2008
Contributors:
Axiom A1400-8

Axiom A1400-8

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.

Yamaha RX-V2700For 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.

crossover distortionClass 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.

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.

McIntosh Tube Amplifer7. 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.

Frequency Response8. 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.

B&O I.C.E. Amplifier Module9. 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.

Alan LofftSpecial thanks to Axiom Audio for allowing us to reprint this article.

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Recent Forum Posts:

gene posts on September 20, 2010 15:48
Oh I guess it wasn't really made all that clear to the average joe who wrote the article….
Alan Lofft, Resident Expert of Axiom Audio


Just seemed like it was a bit biased towards Axioms product…

Probably but that's the issue often when I syndicate articles from manufacturers from time to time. I try to keep their product plugging to a minimal but its nice to get different content and perspective into the site whenever possible.
Warpdrv posts on September 20, 2010 15:12
gene, post: 752751
who is pushing the Axiom Amp? Two of my review samples blew up when I tested them which was noted in the review. Axiom won't even list my review on their website b/c they were so displeased with the results. I am still waiting for them to produce a bulletproof version of this amp b/c I feel it does have great potential, albeit a pricey solution.

I posted this article for them b/c I thought it was a good laymen article on amplifiers. Alan Lofft is no engineer but I thought it would be a good idea to take things down a notch for once to reach a broader audience. Now all of the sudden people are critiquing it 2 years after it was published


Oh I guess it wasn't really made all that clear to the average joe who wrote the article….
Alan Lofft, Resident Expert of Axiom Audio


Just seemed like it was a bit biased towards Axioms product…
Mike Ronesia posts on September 19, 2010 23:16
gene, post: 752751
who is pushing the Axiom Amp? Two of my review samples blew up when I tested them which was noted in the review. Axiom won't even list my review on their website b/c they were so displeased with the results. I am still waiting for them to produce a bulletproof version of this amp b/c I feel it does have great potential, albeit a pricey solution.

I posted this article for them b/c I thought it was a good laymen article on amplifiers. Alan Lofft is no engineer but I thought it would be a good idea to take things down a notch for once to reach a broader audience. Now all of the sudden people are critiquing it 2 years after it was published

We have plenty of other more tech based Class D amplifier articles for all of you propellerheads such as:

The Truth About Class D Amplifers

Switching Amplifiers Technology and the Issues
Good to hear. I read so few, less then stellar reviews in the AV industry that it's hard to really buy into them. That being said I realize most of these products work as advertised and it comes down to features, price and aesthetics most of the time.
gene posts on September 19, 2010 20:55
Interesting that Audioholics is still trying to push that Axiom amp here in this article against B&O class d “Ice” amps still…. Seems to me Axioms amps kinda fell off the radar after falling on their face so many times… RMK went through 3 of them and finally gave up, so many people had the same problems and discussion of them dropped off pretty quick with reliability problems…

who is pushing the Axiom Amp? Two of my review samples blew up when I tested them which was noted in the review. Axiom won't even list my review on their website b/c they were so displeased with the results. I am still waiting for them to produce a bulletproof version of this amp b/c I feel it does have great potential, albeit a pricey solution.

I posted this article for them b/c I thought it was a good laymen article on amplifiers. Alan Lofft is no engineer but I thought it would be a good idea to take things down a notch for once to reach a broader audience. Now all of the sudden people are critiquing it 2 years after it was published

We have plenty of other more tech based Class D amplifier articles for all of you propellerheads such as:

The Truth About Class D Amplifers

Switching Amplifiers Technology and the Issues
GranteedEV posts on September 19, 2010 11:14
Not a bad article, although i've already been sold on class D prior with this article:

http://www.psaudio.com/ps/knowledge/how-does-a-class-d-digital-amplifier-work/


But on that note, until I find a 180w RMS x 3ch class D amp for under 1000 dollars, i think I'll stick to looking at less efficient designs unfortunately. 4000 dollars for that axiom amp… it may have a demographic(although in those type of price ranges I usually think high-end Brick and Mortar), but that doesn't include a college student like myself. Which is strange, because Axioms tend to generally be on the affordable side of things.

No, I'm more interested in something like a trio of class A/B/G Outlaw 2200s, or a class A/B Emotiva XPA3s because of cost-to-performance ratio, as much as I'd like more efficiency.

The only class D amp i'll entertain for the time being is maybe an iQube for my headphones.
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