Have a question about multimedia or audio? Ask our Expert, Ken Boyce

Ken,

All the class-D ICs I have seen the datasheets of seem to give outlandish figures for power, only to find out at the back of the datasheet in on of their charts that distortion is in the double figures at anything approaching moderate. Do you see Class D as the future of audio amplification?, or do you see some other class/hybrid class becoming the standard in future audio amplification?.

Regards,

John, Farnell Technical Team.

John,

You have asked an excellent question, but the answer is not so simple.

One of the principle reasons to use Class D audio is to generate large output power without having the power dissipation and heat generated by Class A, B, AB, G, and H audio amplifiers.  So Class D is a very attractive option.

The often forgotten spec in audio is the infamous 10% distortion figure for output power.  This is an industry standard and is a common (almost required) spec point in all audio amplifiers.  You will find it in data sheets for practically all audio amps.  In case you didn't know, the 10% distortion figure is THD + N, or Third Harmonic Distortion + Noise,  and is the level at which most people will clearly notice that the sound is distorted somewhat.  This is because the 10% level is also the level where the output signal waveform tends to start clipping.

Unless something else is radically wrong with the amplifier, the THD+N drops off as you lower the output power.   So the industry has a 1% distortion spec as well on amplifiers.

I have heard some very good Class D amps but all were operating at 1% distortion level output power, or even less.

For audiophile quality audio, most people will not choose Class D since they are sure (1) they can "hear" the distortion even at lower output powers, and (2) somehow Class D amplification will modify the sound vs. pure linear amplification, and some other reasons that are hard to prove technically. Kinda like gold plated audio cables sound better :>)

As equipment becomes smaller and lighter, choosing linear amplification for audio is a hard sell due to lower efficiency and heat dissipation.  So.. increasingly, Class D is being chosen.  A good example would be the "bookshelf" type amplifier systems in the home versus the older larger equipment.

A good example would be audio amps in thin TV's.   It is well known that if heat generating linear audio amps are close to the LCD screen, the color near that location can be slightly modified and noticeable.  Also, power supply requirements are bigger if those are linear, and they also generate heat.   Class D allows TV makers to put acceptable sound in a thin package.

Cell phones and items of that type also benefit greatly by using Class D amps, even at 1W (1% THD+N) to 2W(10% THD+N).  In today's environments with large amounts of ambient noise, having higher output levels of sound is a good thing.  Some of the Class D  IC amps today have 10X the output power (10% THD+N) in the same die size as older linear amps at 0.5W.

To avoid using Class D amps, designs have appeared using variations of linear amps - Class G, and H.  These types of amplifiers attempt (and succeed) to reduce power losses and heat generation by varying in some way the supply voltage to the amplifier based on the input signal level.  The higher the input, the higher the supply voltage is allowed to go, and vice versa.  But they do not match Class D in efficiency or heat reduction.  But they are linear and can have better power output at low THD+N.  Class G switches the supply voltage based on some input level or levels in multi-level Class G, while Class H tracks the input signal and changes the supply voltage accordingly.

The other thing to remember is that Class D is not that beneficial at very low output powers.  An example would be headphone only amplifiers.  Most of these are linear, either capacitor output coupled, or direct coupled (+ and - supply voltages internally generated by a charge pump circuit).  Since the headphone or earbud is very close to the ear canal, any distortion and Class D switching noise would be easier to hear and be downright annoying.

By the way, if you think datasheet high output power ratings are somehow misleading, try figuring out what Peak Music Power Output (PMPO),  Peak Envelope Power, RMS power, Sine power (the most reliable measure of the output power capabilities of an amplifier), DIN power (European standard) means when you see them on some consumer audio products.   An example would be a small bookshelf type speaker amp that offers 100W PMP when the amps clearly can only put out less than 10W Sine Power.

I hope this answer helps you more than add confusion to the mix.

Ken

John,

You have asked an excellent question, but the answer is not so simple.

One of the principle reasons to use Class D audio is to generate large output power without having the power dissipation and heat generated by Class A, B, AB, G, and H audio amplifiers.  So Class D is a very attractive option.

The often forgotten spec in audio is the infamous 10% distortion figure for output power.  This is an industry standard and is a common (almost required) spec point in all audio amplifiers.  You will find it in data sheets for practically all audio amps.  In case you didn't know, the 10% distortion figure is THD + N, or Third Harmonic Distortion + Noise,  and is the level at which most people will clearly notice that the sound is distorted somewhat.  This is because the 10% level is also the level where the output signal waveform tends to start clipping.

Unless something else is radically wrong with the amplifier, the THD+N drops off as you lower the output power.   So the industry has a 1% distortion spec as well on amplifiers.

I have heard some very good Class D amps but all were operating at 1% distortion level output power, or even less.

For audiophile quality audio, most people will not choose Class D since they are sure (1) they can "hear" the distortion even at lower output powers, and (2) somehow Class D amplification will modify the sound vs. pure linear amplification, and some other reasons that are hard to prove technically. Kinda like gold plated audio cables sound better :>)

As equipment becomes smaller and lighter, choosing linear amplification for audio is a hard sell due to lower efficiency and heat dissipation.  So.. increasingly, Class D is being chosen.  A good example would be the "bookshelf" type amplifier systems in the home versus the older larger equipment.

A good example would be audio amps in thin TV's.   It is well known that if heat generating linear audio amps are close to the LCD screen, the color near that location can be slightly modified and noticeable.  Also, power supply requirements are bigger if those are linear, and they also generate heat.   Class D allows TV makers to put acceptable sound in a thin package.

Cell phones and items of that type also benefit greatly by using Class D amps, even at 1W (1% THD+N) to 2W(10% THD+N).  In today's environments with large amounts of ambient noise, having higher output levels of sound is a good thing.  Some of the Class D  IC amps today have 10X the output power (10% THD+N) in the same die size as older linear amps at 0.5W.

To avoid using Class D amps, designs have appeared using variations of linear amps - Class G, and H.  These types of amplifiers attempt (and succeed) to reduce power losses and heat generation by varying in some way the supply voltage to the amplifier based on the input signal level.  The higher the input, the higher the supply voltage is allowed to go, and vice versa.  But they do not match Class D in efficiency or heat reduction.  But they are linear and can have better power output at low THD+N.  Class G switches the supply voltage based on some input level or levels in multi-level Class G, while Class H tracks the input signal and changes the supply voltage accordingly.

The other thing to remember is that Class D is not that beneficial at very low output powers.  An example would be headphone only amplifiers.  Most of these are linear, either capacitor output coupled, or direct coupled (+ and - supply voltages internally generated by a charge pump circuit).  Since the headphone or earbud is very close to the ear canal, any distortion and Class D switching noise would be easier to hear and be downright annoying.

By the way, if you think datasheet high output power ratings are somehow misleading, try figuring out what Peak Music Power Output (PMPO),  Peak Envelope Power, RMS power, Sine power (the most reliable measure of the output power capabilities of an amplifier), DIN power (European standard) means when you see them on some consumer audio products.   An example would be a small bookshelf type speaker amp that offers 100W PMP when the amps clearly can only put out less than 10W Sine Power.

I hope this answer helps you more than add confusion to the mix.

Ken