Class D amplifier- what is important?

LG75

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• Jan 10,

• #1

Hi,
I considering ordering a three channel amplifier for my L/R/C from Audiophonics, the SPA-400ET that Amir really liked.
Now premium” (?) manufacturers has offering of triple mono design (three power suplies) and different “premium” OPA. The question does it matter? Would I be able to hear a difference if I’ll order three mono blocks? Does Sparkos OPA really sounds better than the LM used in Audiophonics design (they offer a stereo version with Soarkos).
Amir did his measurements on a stereo version with a single power supply and the LM OPA and the measurements looked great.

Cheers OP L

LG75

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• Jan 11,

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• #2

what sort of difference do you believe could be audible? What are the possibilities?

I believe that if you cannot empirically measure a difference or if the difference is negligible then you will not be able to hear it. However, you can find reviews of people saying that if an amplifier measures well it doesn’t necessarily sounds well and that they can hear a difference between OPA options,
Mono designs have the advantage to eliminate crosstalk due to not having shared components. I’m sure you’ll be able to see crosstalk effects in measurements, but in a well designed circuit it might be so small it won’t translate to anything audible.
I just wanted to see if I’m not missing something. Last edited: Jan 11,

DVDdoug

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• Jan 11,

• #3

With any amplifier the most important thigs are low noise and adequate power. Most solid state amplifiers have flat frequency response and low distortion unless over-driven into distortion.

lateralous

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• Jan 11,

• #4

To satisfy your curiosity, here is a direct measurement of op-amp options in a similar implementation:

Discrete OpAmp Review: Sonic Imagery vs Sparkos

Recently I reviewed the Nord Three SE 1ET400A Dual Mono Stereo Amplifier. That unit came with the Sonic Imagery's 990ENH discrete operational amplifier ("opamp") in the buffer stage (pre-amplifier). The owner then ordered a set of Sparkos Pro SS discrete opamps. This gave me a chance to...
To put it shortly, not something you should worry about and certainly not something to spend money chasing unless the stock op-amp implementation is bad, which is not the case here. Note that the function of the op-amp here is as an input buffer, it is reasonable to believe an op-amp could change the sound audibly if it really messed with the impedance your source is seeing but again not something I would pay for OP L

LG75

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• Jan 15,

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As an example to what I was referring, here’s Amir’s review of the VTV class D amplifier with tube. Based interface card: https://www.audiosciencereview.com/...ifi-amplifier-review-with-weiss-buffer./
Measurements was not as good as other Purify based designs and it didn’t get Amir’s thumbs up. Then there’s reviewers ,like Thomas, reviewing the amplifier, based on their subjective hearing test:
He really liked it in comparison to the Purity reference design. Thomas also has a different review of the Purify amplifier reference design He said it’s OK, but great when replacing the interface card with this one:

Purifi 1ET400A / Hypex NC500 Input Buffer

This Purifi/Hypex input buffer provides a fully differential output. It is an excellent upgrade on the Purifi EVAL1 and features extremely low distortion. The review:
I’m just bringing it as an example to times when measurements and subjective test don’t necessarily goes hand in hand, and for people who feels like the reference card makes a huge difference. As I said I’m a believer of data collection, based of measurements, but reviewer like Thomas always makes me wonder if I’m missing something.

BDWoody

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• Jan 15,

• #6

I’m just bringing it as an example to times when measurements and subjective test don’t necessarily goes hand in hand

When there are no controls used (levels matched, unsighted) these subjective comparisons have limited value.

The brain is a tricky beast, so isolating the ears and removing other clues is key. Our host did a video on the subject that might be good for some of these reviewers to watch.

ahofer

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• Jan 15,

• #7

As an example to what I was referring, here’s Amir’s review of the VTV class D amplifier with tube. Based interface card: https://www.audiosciencereview.com/...ifi-amplifier-review-with-weiss-buffer./
Measurements was not as good as other Purify based designs and it didn’t get Amir’s thumbs up. Then there’s reviewers ,like Thomas, reviewing the amplifier, based on their subjective hearing test:
He really liked it in comparison to the Purity reference design. Thomas also has a different review of the Purify amplifier reference design He said it’s OK, but great when replacing the interface card with this one:

Purifi 1ET400A / Hypex NC500 Input Buffer

This Purifi/Hypex input buffer provides a fully differential output. It is an excellent upgrade on the Purifi EVAL1 and features extremely low distortion. The review:
I’m just bringing it as an example to times when measurements and subjective test don’t necessarily goes hand in hand, and for people who feels like the reference card makes a huge difference. As I said I’m a believer of data collection, based of measurements, but reviewer like Thomas always makes me wonder if I’m missing something.

This happens a lot, so I have a copypasta response.

Welcome to ASR! You've made an unsupported assertion or a scientifically implausible claim that will cause most people in this science-oriented forum to react with skepticism (or scepticism if they are in the U.K.). Please don't take the reactions as overtly hostile - most of us are just frustrated with the many newcomers who have clearly come here just to "troll". Please do engage with the membership to find an objective, controlled method to support or discard your hypothesis. Our membership includes recovering subjectivists, manyengineers/scientists, and several famous figures in the world of audio engineering research. Generally, they can cite scientific, controlled research to support their views. Most believe in the fallibility of human sighted judgement, and think blind testing and measurements are critical ingredients for assessing equipment contributions to sound quality. We'd love to have you, but if all you want is a) to fight or b) to have others cheerlead for your subjective views or anecdotal evidence, I'd suggest you will be happier elsewhere.

fpitas

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• Jan 15,

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We get asked a lot why some reviewers like equipment that measures badly. The fact is, some people like the sound of certain forms of distortion, for example from (badly designed) tube amps. They have the right to like it, but you have to be a little careful. An amp like that is special purpose. Some tracks might be improved, but once you get a lot of different things going on in the music, the distorting amp creates intermodulation, which is the generation of a whole new set of signals not in the original music, and not even harmonically related. As long as the music is simple, say a singer and a guitar, that's not too bad. Get a lot going on though, and you have a sonic disaster. So, caveat emptor. Last edited: Jan 15,

ahofer

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• Jan 15,

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Some people like the sound of certain forms of distortion, for example from (badly designed) tube amps. They have the right to like it, but you have to be a little careful. An amp like that is special purpose. Some tracks might be improved, but once you get a lot of different things going on in the music, the distorting amp creates intermodulation, which is the generation of a whole new set of signals not in the original music, and not even harmonically related. As long as the music is simple, say a singer and a guitar, that's not too bad. Get a lot going on though, and you have a sonic disaster. So, caveat emptor.

Auditioning equipment on solo guitar or small ensemble music only is a mistake, I’ve learned. Some orchestral or dense electronica is a must.

fpitas

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• Jan 15,

• #10

Auditioning equipment on solo guitar or small ensemble music only is a mistake, I’ve learned. Some orchestral or dense electronica is a must.

I guess if you *only* listen to that kind of music, you're ok. OP L

LG75

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• Jan 15,

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This happens a lot, so I have a copypasta response.

I’m not asking questions to troll. I’m planning to buy an amplifier and some of Purify class D amplifiers seems to offer a good value - measures well and relatively reasonably priced for their performance.
I’ll probably go with my leading choice, Audiophonics as it did great in the measurements test and priced right. I just asked to see whether it’s worth considering OPA or interface card upgrade. I think I saw Amir did a comparison between Soarkos discrete OPA to the stock OPA without seeing a measurable improvement. I stick with the stock setup.

The fact is, some people like the sound of certain forms of distortion, for example from (badly designed) tube amps

That was what I’m thinking. In the examples I brought before, in one of the reviews the reviewer describes the amplifier as “dry”. I started thinking what does it mean? Why doesn’t it correlate to something seems in measurements. The only thing I could think of is that non perfect measurements of his amplifier of preference, like higher distortion, is something that might explain in

FlyingFreak

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• Jan 17,

• #12

Audiophonics looks like a great 'entry budget' company for those amp. I would follow that route too if I was in the EU.

The subjective review thing is really out there. It's super generous to give them the benefice of the doubt that maybe they prefer some sort of distortion.
Thomas, who I am sure is a great guy and do not doubt his integrity (as far as not being paid by company), hear differences between bit perfect streamers, perfect sounding dacs, etc.
I m no EE, I am a psychotherapist. From my professional point of view (not that I think anyone need a pro to see that), it is super easy to bullshit ourselves. I mean... without us bullshitting ourselves all of the time for everything, my profession would not need be. Another way to say it: if our job was to hear difference between cables and write about it, I am sure we would. I mean ... we would need to to make a living, so we would adapt and our mind would create differences. What we would actually describe would be a mix of preference in name of brand, status of said brand, look of cables, how much coffee we had that morning, our mood, the weather, etc. From a psycho biology point of view, it is much more efficient to lie to ourselves than to knowingly lie to others (it is less stressful).

People who say that they 'trust their ears' then go on hearing impossible things and proclaim themselves ready to die on that hill sounds like clients trying to convince me their abusive spouse is the greatest person in the world despite them describing the repetitive abuses and complaining about it in the first place. We are frail creatures and our mind is very far from our strongest features.

Vacceo

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• Jan 17,

• #13

If you hear an amplifier, something is wrong with the amplifier. Its task is getting a signal and, well, amplify it without changing it. That is impossible, but the change can be so small that you can only detect it with measuring tools.

Tailoring the sound to your liking is far easier with a combination of speakers, equalization and room interaction. That is where you should focus. For amps, as long as they provide the power you need, hopefully in the most efficient way possible and without altering the signal, that is just good enough. Last edited: Jan 17, OP L

LG75

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• Jan 17,

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Audiophonics looks like a great 'entry budget' company for those amp. I would follow that route too if I was in the EU.

It measured great so I wouldn’t classify it a “entry”, at least performance wise. I’m not in the EU either but exchange rate is good right now and the shipping cost are reasonable. The total price is competitive. If you know of US based alternatives in this price/performance point I’ll be happy to hear about it. VTV is probably is not much more expensive, but measured poorly compared to Audiiophonics and t similar

HarmonicTHD

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• Jan 17,

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It measured great so I wouldn’t classify it a “entry”, at least performance wise. I’m not in the EU either but exchange rate is good right now and the shipping cost are reasonable. The total price is competitive. If you know of US based alternatives in this price/performance point I’ll be happy to hear about it. VTV is probably is not much more expensive, but measured poorly compared to Audiiophonics and t similar

Buckeye.
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Vacceo

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• Jan 17,

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Apollon is another EU-based manufacturer with a great track record. Nord in the UK also does a good job.

If you're under the impression that Class-D amplifiers are just 'two letters worse' than Class-A models, think again: Class-D technology is making an increasing impact on the live sound world by offering more power with less weight than ever before. We find out how and why...

View Details

Every sound engineer has an opinion about microphones. And loudspeakers, they certainly all sound different. Mixing consoles? The differences there are down to facilities and operational convenience. But power amplifiers? Apart from 'bigger is better', who could get excited about an amp? The power amplifier, possibly several of them for a live show, sits in a rack and gets on with its job. The technology is mature and there isn't all that much difference between power amplifiers, so you simply choose a reliable manufacturer and the required power rating. Rack up, and you're done.

We should be thankful that power amplifiers are so uninteresting, because if they're uninteresting it means that they work well. Indeed, the power amplifier is one of the best-performing pieces of kit in the entire sound system, in terms of frequency response, distortion and noise. Modern power amplifiers are also generally a pretty reliable bunch. Amplifiers of old used to die on a regular basis as their transistors spontaneously combusted. But now, thanks to efficient cooling and protection circuitry, power-amp failure is a comparatively rare event. (If you don't find that so, look to your cooling — free air-flow is a must).

Still, manufacturers don't like to rest. They need to improve their products continuously, hopefully to gain an advantage over their competitors and encourage us to buy more of what they make. As it happens, there is still room for improvement in the field of power amplifiers. More Watts for the buck is one way they can be improved. Simply more Watts in a single amplifier is another. A third potential area for development? Make them lighter! If you've ever spent time rigging amps and amp racks, you'll know all about that.

Here's a good question: just why are power amplifiers so heavy? Electricity doesn't weigh anything, so how come amps have so much mass? There are two answers to that. One is the transformer that converts the mains voltage into a lower voltage suitable for the amp's circuitry. If an amp is rated at 500 Watts, for instance, the transformer has to be able to supply all that power, and more. So it needs substantial copper windings and a bulky core; it's bound to be heavy. The other potentially massive item is the heat sink. Not all of the electricity supplied to the amplifier circuitry is converted into useful power sent to the loudspeakers. Some is wasted as heat, and this heat has to be dissipated, otherwise the amp will cook. So the output transistors are bolted to a large, finned heat sink with a broad surface area that can lose heat easily, particularly when used in conjunction with a fan. It is true that heat sinks can be made from a lightweight metal such as aluminium, and clever design can make the heatsink part of the amp's structure. Even so, everything adds up and the typical power amp is a pretty weighty beast.

To reduce the weight of an amplifier, there is a simple solution: don't waste so much power. If no power were wasted, the transformer could be much smaller and a heat sink would not be required. Clearly, there never will be a 'no-waste' amplifier, but the more efficient an amplifier is, the lighter and smaller it can be. So in live sound, where power amps are used in quantity, efficiency in an amplifier is a very desirable quality. In other areas where power amplifiers are used, such as home hi-fi and studio monitoring, efficiency isn't such an issue.

This brings me to the topic of this article: Class-D amplifiers. The whole reason for existence of Class-D is efficiency. Plainly, there must also be Class-A, Class-B and Class-C, and one would expect these to be earlier developments, as they come before D in the alphabet. I'm going to explain how Class-D works, and why it is suitable for live sound. But first, I'm going to have to explain how all those other classes work.

In The Beginning...

If you don't know anything about electronics, don't worry. Well, don't worry much — I'm not going any deeper than an existing understanding of how a battery and bulb work will support. (Of course, a little knowledge of audio signals won't go amiss.)

In the beginning was the single-ended Class-A amplifier, as shown in Figure 1. I've simplified the schematic to show only the output device, which is where the differences between the classes are defined. In a simple amplifier like this, the audio input signal — a small alternating current (AC) — flowing into the base of the transistor ('b') controls a larger direct current (DC) flowing from the output of the amplifier's power supply through the collector ('c') and emitter ('e') to earth. The parts of the circuit I have left out 'bias' the transistor so that when there is no input signal, the output voltage (ie. the voltage at the collector) is half the total supply voltage. This allows the output voltage to vary both up and down to an equal extent, to recreate the AC waveform of the input. If the voltage with no input-signal present was to be anything other than halfway between zero and the full power-supply voltage, then inevitably one half of the waveform would run out of volts before the other, limiting the amount of amplification available before the waveform would be clipped.

Let's look at what happens when the input signal voltage to the transistor is low. The transistor will allow only a tiny current to flow between the collector and emitter, therefore the voltage at the collector will be almost the same as the full supply-rail voltage. So the load (the loudspeaker) is driven with a high voltage and a strong current — Ohm's Law, V=IR (Volts = Current x Resistance), dictates that current flow is always proportional to the voltage applied and the resistance within the circuit. Conversely, when the input voltage to the transistor is high, the collector-emitter part of the transistor will conduct. The voltage on the collector will become low, so the load (the speaker) is driven with a low voltage and only a small current (Ohm's Law again; the resistance of the speaker remains the same, but the applied voltage is now low, so the current flow is low). As the input signal waveform feeding into the transistor cycles up and down, so does the output voltage. The output voltage is a bigger version of the input voltage — which, of course, is the whole point of amplification.

This raises some questions. Firstly, why is the load (the loudspeaker) not simply connected between the supply rail and the collector? The answer is that, if it were, a current would always flow through the loudspeaker, even when no input signal was present. That would a) be wasteful, and b) displace the cone of the loudspeaker from its rest position, even when there was no signal. The second question is why is there a capacitor (C1) between the collector and the loudspeaker? The answer is to prevent the actual (DC) current of the power supply reaching the speaker, as it's only the changes in voltage that we are interested in, and a constant applied voltage would, as above, offset the cone from its rest position. (For the sake of simplicity, I'll leave out the explanation for the resistor.)

This simple amplifier is known as 'single-ended, Class A'. It has only one output device, and when there is no signal the current through the output transistor is at least as great or greater than the maximum current that ever flows through the loudspeaker. So the Class-A amplifier is working flat out even when there is no signal! A Class-A amplifier can only ever be 25 percent efficient, according to the mathematics. So even working at its best, three quarters of the input power is wasted.

What's Class-G Amplification?

It's the dream of every amplifier designer to come up with a design that one day will be described as a 'class'. The classes have actually advanced far beyond Class A, B, AB and D. Classes E and F are used in radio transmission, so we can ignore them. But Class G and Class H are relevant to audio.

We know that Class AB is inefficient, or at least isn't as efficient as we would like it to be, considering that at power-amp levels there's a lot of power being wasted. The reason it's inefficient is because the instantaneous signal level is controlled by transistors that resist the flow of current, essentially by dissipating it as heat. So, in simple terms, when a transistor is only passing half the current it could, the other half has to go somewhere — and that 'somewhere' is a small but significant contribution to the second law of thermodynamics and the eventual death of the universe. Who would have thought an amplifier could do that?

But here's an idea: what if small signals could be delivered by a small amplifier and large signals by a large amplifier? The small amplifier wouldn't have to dissipate too much power, and neither would the large amplifier, since when called upon it would deliver its power to the loudspeaker. In practice this can be achieved by using two or more pairs of power-supply rails. One pair of rails supplies a low voltage for small signals. When the clipping point of these rails is approached, the amplifier switches over to a higher-voltage pair of power supply rails. And there is no need to stop at two pairs of rails. Clearly, switching could be an issue and a potential source of audible defects, but the gains in efficiency can outweigh the problems in certain applications.

AISP contains other products and information you need, so please check it out.

Class-H is a development of Class-G (to be honest, they are both just developments of Class-AB, but let's not get too picky about it). In Class-H, the signal is used to vary the power-supply rail voltage. So when the signal is at a high level, the power-supply rails are also at a high voltage, in readiness. This avoids the switching involved in Class-G. Interestingly, the generation of the rail voltage incorporates circuitry very much like Class-D in nature.

You Push, I'll Pull

Figure 2 shows an alternative strategy, in the form of a push-pull amplifier output stage. One transistor 'pulls' the voltage up on the positive half-cycle of the waveform. The other transistor 'pushes' the voltage down on the negative half-cycle. Well, that's the kindergarten explanation. Let's look in a little more detail...

In this version, I have shown both a positive supply rail and a negative supply rail, as well as an earth exactly in between in voltage; zero volts in fact. A single-ended (positive- or negative-only) power supply can be used, but a dual-rail supply is better, as no DC-blocking output capacitor is necessary. This is because, when there's no signal, both terminals of the loudspeaker are at zero volts, so no current flows and there is no DC to be blocked. You will notice that the transistors are slightly different to each other. The upper transistor (Q1) is what we call 'npn', meaning that it will conduct between collector and emitter for a positive voltage at the base. The lower transistor (Q2) is 'pnp', meaning that it will conduct between collector and emitter for a negative voltage at the base. If you're into electronics already, you will have noticed that there is another difference between this and Figure 1. In Figure 2, the loudspeaker is connected to the emitters of the transistor, rather than the collector of the transistor in Figure 1. This means that all the voltage amplification has to precede this stage. This part of the circuit is responsible for delivering a high current to the loudspeaker. But don't worry too much about that; it doesn't affect my explanation of the amplifier classes.

In this configuration, a high input voltage will cause Q1 to conduct, bringing the output voltage close to the positive supply-rail voltage. A zero input-voltage will cause neither transistor to conduct. The output is at zero volts. With neither transistor in conduction, clearly no current is available for the loudspeaker. But since there is no voltage across the terminals of the loudspeaker, it doesn't need any! (A handy coincidence.) When the input voltage is low, Q2 conducts, allowing the voltage at the output to descend almost to that of the negative supply-rail. From this, you can see that Q1 handles the positive half-cycles of the waveform and Q2 handles the negative half-cycles. This is Class B.

The beauty of this arrangement is that it is much more efficient. When the input signal is zero, there is no current flowing either through the loudspeaker or through the transistors. The maximum theoretical efficiency for a sine-wave input is 78.5 percent — a vast improvement over Class A.

But there is a fly in the ointment. A transistor will conduct hardly at all if the voltage on the base is less than 0.6 volts (minus 0.6 volts for a pnp transistor). So input voltages between ­0.6 and +0.6 volts will not stir either transistor into conduction. Figure 3 shows the consequence. That flat spot in the middle of the waveform is called 'crossover distortion' and is an intrinsic feature of Class B. Fortunately, there is an answer, and that is to 'bias' the input to the two transistors, as in Figure 4. Those two new components between the bases of the transistors are diodes. Their effect is to separate the standing voltages on the bases by 1.2 volts, thus overcoming the intrinsic 'inertia' of the transistors. The input signal now only has to twitch and the transistors will respond. This is a simplification of a real-world circuit, but only slightly so. In a real circuit, the voltages on the bases of the transistors would have to be slightly further apart, and adjustable to set the 'quiescent current' (the constant current when no input signal is present).

The benefit here is that crossover distortion is almost eliminated, at the expense of a slight standing current when the signal is at zero level. It is interesting to note that the biasing could be arranged so that the transistors carried a very high current for a zero input signal. When the input moved, this current would be diverted through the load. Guess what? This is Class A again. It's a push-pull Class-A output stage, no more efficient than a single-ended Class A, but more practical to implement, and entirely lacking in crossover distortion, which is why it is admired by high-end hi-fi enthusiasts. The compromise situation of biasing the output transistors so that they are just in conduction is called Class AB. Class AB is far and away the most common type of amplifier. It's reasonably efficient, and its sound quality is excellent, surpassed only by Class-A amplifiers that run as warm as an Aga and cost the earth — both to buy and to run.

To round off this section, Figure 5 shows a simple Class-C amplifier. It drives a resonant load and is thus very efficient at the resonant frequency of the load — over 90 percent. However, since it only works over a narrow range of frequencies, it is entirely unsuitable for audio. Class-C amplification is actually used in radio transmission.

He Ain't Heavy, He's My Class-D Amplifier

Now that we know how classes A, B, AB and C work, we can look at Class-D. Clearly, classes A to C are all in the same family, but Class-D is completely different. In Classes A, B and AB, the problem is lack of efficiency. Some power is wasted, and we would prefer that it could be sensibly employed in driving the loudspeakers to ever-higher sound pressure levels — or, at least, not converted to heat. Where power is wasted is where a transistor is in partial conduction. When a transistor is fully conducting, it's like a piece of wire, and a piece of wire loses hardly any power. When a transistor is fully off, it doesn't conduct at all, and if it doesn't conduct at all, there's no power to waste. It's the in-between stages that cause the problem, where the transistor wastes power and gets hot. So what if we could find a way for transistors to be used only in their fully-on or fully-off states. If that were possible, no power would be lost. But is it possible...?

It is, and the solution is what we call Class-D. Figure 6 shows a simplified Class-D amplifier. First, let's look at the similarities between this and what we've already discussed. You can see two transistors, in push-pull configuration, as before. The transistors look slightly different because they are MOSFETs (Metal Oxide Semiconductor Field Effect Transistors) rather than 'ordinary' transistors. The output transistors have to be fast, so that they can switch very quickly between fully on and fully off. It also helps for the on and off states to be 'really' on and off. The closer the transistors can get to full conduction or full non-conduction, the greater the efficiency of the amplifier will be. Clearly, though, as well as the similarities there are some differences.

Let's start at the output. What's not going to happen is that the transistors create a high-voltage version of the input signal. What is going to happen is that they switch alternately to lift the output all the way up to the positive supply rail, then all the way down to the negative supply rail, as quickly as possible, with no in-between voltages. This is clearly going to be a pulse waveform. Now here's the clever bit: if the width of the pulses can be made proportional to the input signal's instantaneous level, the power delivered to the loudspeaker, averaged over time, will be the same as if the input signal had been amplified in the conventional way. Think about that for a moment, because it's key to how a Class-D amplifier works.

Going further, if the output is filtered to remove the high frequencies and sharp corners of the pulse waveform, the original input signal will be reconstructed, exactly the same shape as it was, but bigger. The net result is an amplified signal that you couldn't distinguish from that produced by a conventional Class-AB power amplifier.

But how is the pulse waveform produced? OK, it isn't simple, but it isn't rocket science either. First we need a circuit building-block known as a comparator. A comparator has two inputs: let's call them Input A and Input B. When Input A is higher in voltage than Input B, the output of the comparator will go to its maximum positive voltage. When Input A is lower in voltage than Input B, the output of the comparator will go to its maximum negative voltage. Figure 7 shows how the comparator operates in a Class-D amplifier. One input (Input A in my example) is supplied with the signal to be amplified. The other input (Input B) is supplied with a precisely generated triangle wave. When the signal is instantaneously higher in level than the triangle wave, the output goes positive. When the signal is instantaneously lower in level than the triangle wave, the output goes negative. The result is a chain of pulses where the pulse width is proportional to the instantaneous signal level. Magically simple! We call it 'pulse width modulation', or PWM. And that's all there is to it. You now understand how a Class-D amplifier works, and if anyone tries to pull the wool over your eyes and convince you that the 'D' stands for 'digital', you can tell them how wrong they are, with confidence. Class-D is not digital.

That's Not All, Folks

Clearly, there is more to know. For instance, it's important to know that the switching frequency must be very high to achieve the necessary resolution. A switching frequency of around 300kHz, which is around 15 times the highest audio frequency of general interest, is typical. The dynamic range and signal-to-noise ratio of the Class-D amplifier are controlled by the switching frequency — the higher the better. Clearly, the greater the rate of pulse generation, the more closely the pulse width will be in proportion to the instantaneous signal level. However, the drawback of increasing the switching frequency is that the amplifier will be less efficient. Optimum efficiency would be achieved if the transistors could switch instantaneously, so that they were in either their fully on or fully off states, where almost no power is consumed. But in the real world it takes a little time for the voltage to swing, and during that time some power is dissipated. So the more often the swings take place, the more opportunity for waste. Even so, the efficiency of a practical Class-D amplifier can be better than 90 percent, which is significantly better than a Class-AB design (78.5 percent at best and typically closer to 50 percent).

Coming full circle, because a Class-D amplifier is more efficient than the conventional Class-AB one, it can be lighter. And that, in a nutshell, is the reason for Class-D's existence. Lighter also leads to smaller, and to achieve the high switching speeds necessary, the circuitry has to be physically small. Look inside a Class-D amplifier and you'll find a transformer. Look hard enough and somewhere in there you'll find the circuit too! 

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