SMPS for Audio

1©Copyright 2005 Switching Power Magazine

ne of the last bastions of analog technology

is in audio reproduction. Electronics hobby-

ists have delighted in building their own audio equip-

ment for decades, and it is still an area that is accessi-

ble to an engineer with basic power skills, and some

basic lab equipment. Most other areas of electronics

have been handed over to the microprocessor, and digi-

tal technology, which is far more difficult to engineer

for the hobbyist.

Most avid power supply designers have been tempted at

some time in their career to throw together an audio

amplifier, and in doing so, realize very quickly that most

of the amplifier consists of power supplies, thermal

management, and power management. The demands on

the power system are quite unique in audio, and solid

design experience is needed to make the system work

well. The domination of the power processing part of an

amplifier makes it an ideal target for more modern

power conversion, but for many reasons, it has been

slow to react.

O


Figure 1 shows the all-analog audio system. Aswe'll see in this article,the power amplifiers arenot very different frompower supply designs.Figure 2 shows wherethe large audio manu-facturers want to takethe future of audio sys-tems, at all levels ofpower and applications.In between these twoextremes are numerous opportunities for high-frequencypower conversion.

Despite the fact that digital audio has been around for25 years, we are only just beginning to see all-digitalaudio systems with embedded switched-mode powerand amplifiers and is not at all clear that this technologywill be embraced by the consumers as the hands-downwinner. Meanwhile, switching power supplies are final-ly making significant inroads into audio products. Inthis article, I will talk about the opportunities andpotential problems that arise in using switchers inaudio systems.

Power Dissipation IssuesCalculating the actual power supply requirements for apower amplifier is not easy. One reason is that ampli-fiers must drive a wide range of loads. A typical audioamplifier is specified to work into 2, 4, or 8 ohms.Older systems can be specified for 16 and 32 ohms,reflecting speaker technologies of the day.

Even calculating typical power dissipation can beextraordinarily difficult for an amplifier. Very few

speaker loads are purely resistive, and the nominal rat-ing typically refers to the minimum value of impedancein the region where the speaker is resistive. And nomi-nal really means just that– a 4 ohm speaker can be 2.4ohms when measured. Further complicating the situa-tion is the crossover network, which can also present asubstantial load to an amplifier.

Some amplifier manufacturers work closely with speak-er makers to optimize the rail voltage for the specificload. This has become more popular in many differentmarket segments recently, but it is still the excep-tion to the rule.

A simplified way to consider the output of a linearaudio amplifier is as a linear regulator, where the refer-ence is the audio signal. It is impossible to specify anoptimum value of the rail voltage for such a regulator.Low impedance loads require more current and lessvoltage, and much of the rail voltage results in dissipa-tion in the power devices. The amplifier can run much

more efficiently with a higherimpedance load that runs the out-put voltage closer to the rails.Widely-varying load requirementscan be much better dealt withfrom a dissipation and thermaldesign point of view with aswitching amplifier, one of themajor reasons that the industry ispushing in this direction.

Fig. 2: All digital system with fully integrated switchers and amplifiers. (The last step of thechain remains analog, of course.) Large initial investments will produce silicon-intensive solu-tions which close off the market to many smaller players, but allow high profits with mass pro-duction. Noise issues from the switching supplies and switching amplifiers will always provideissues with digital timing, and reconstruction of the analog waveform at the output. Tremendoussophistication in digital algorithms are needed to preserve acceptable sound quality.

Fig. 1: All-analog system - still the choice for some high-end consumers, but a very small piece of the totalaudio marketplace today. For most consumers, the front end has been replace with digital. Despite thisswitching, power supplies and switching amplifiers have been slow to penetrate this marketplace. This is nowchanging at an accelerating pace, with varying levels of success.

SMPS for Audio


Audio Market SegmentsIn the audio industry, many market segments areinvolved in audio power amplifiers. Here is a quickoverview of the different areas:

Consumer AudioConsumer audio includes home stereos, PC speakers,and surround-sound systems for home theater. Thename of the game in this area is severe cost reduction,with switched-mode amplifiers finding their way in theform of integrated modules in very low cost systems.For less than $100, you can buy a 5.1 channel system,with DVD and CD player, including biamped speakers,all operating at reasonable power levels. (We won't talkabout the quality of the sound– let's just say you getwhat you pay for.) In these low-cost systems, there islittle room for creative engineering. Snazzy electronicfeatures seem to drive sales, rather than sound quality.Switchmode power and amplification is essential here,since the cost of linear regulators cannot be absorbed,and the mass production levels demand high siliconintegration with minimum passive parts. IntegratedClass D amplifier modules are now commodity compo-nents for such systems.

Professional Sound ReinforcementThis category includes standalone rack-mount ampli-fiers used for concerts, theaters, churches and night-clubs. It also includes powered speakers for the samevenues, in which the power supply and amplifier areintegral to the loudspeaker. The highest power productsare found here. The emphasis is on reliability, sincedowntime of the system is an unacceptable cost. Youmight initially think that sound quality is the numberone concern for sound reinforcement professionals, butsubjective sonic differences between different manufac-turers and different models are outweighed by the pri-mary concerns of reliability, size, weight, and cost.

Audiophile SoundThere is a small, but significant marketplace that cravesa sound reproduction quality that mimics a live musicevent. Emphasis is on audio performance, with little orno concern for cost. Systems can cost upwards of$100k. CD players alone can cost $40k, and speakersup to $75k. Standalone power amplifiers are used forhigh-end home audio, and home theater. A completerange of amplifier designs and technology can be foundcompeting with each other here, from horrendouslyinefficient tube designs which operate at less than 10%efficiency, to 90+% efficiency class D amplifiers fromrespectable audio companies such as Bang and Olufsen.Surprisingly, no one technology has won out over all

the others, and this market continues to mystify majoraudio companies that often find themselves comparedunfavorably with companies that are little more thangarage operations.

Musical Instrument/DJFor musicians, the amplifier is a subsystem built intothe overall product, such as in a guitar amplifier withspeaker, or powered keyboard. This category alsoincludes amplifiers for smaller speakers as used bymobile DJs and home recording studios. The emphasisis on cost. While better sound is very desirable, mostmusicians cannot afford the amplifiers that they wouldlike to buy, and the marketplace is very price competitive.

Automotive SoundAutomobile manufacturers drive the cost of automotivesound along with high packaging density. People listento audio without video more in their car than anywhereelse, and this is a very large marketplace. That doesn'tmean the sound has to be the best, since the car presentsspecial considerations. Ambient noise is high in a car,which means dynamic range is typically small. One ofthe major reasons that most modern music (and mostradio stations) compress their audio signal to a 10 dBdynamic range is for car listening. There is also a subsetof car audio that demands SPL and nothing much else,and there are even contests around the country for maxi-mum SPL at levels which would shatter normal carwindshields. Most of the time, (we hope) these areremote controlled without anyone in the car. Efficiencyis sorely needed, and switchers have penetrated deepinto this market.

Audio Power Amplifier TechnologiesOpportunities for applications of switched-mode powerdepend heavily on the type of amplifier that is beingdesigned. In the simplest case, the power supply can becompletely separate, just providing voltage rails for theamplifier to use. At the next level, the power supply canbe adjusted to follow the demands of the input signal.And at the most complex and involved level, the switched-mode power supply becomes the amplifier itself.

In this article, we are limiting our discussion to the mostwidely used topologies; there are many variations on thebasic topologies but a comprehensive treatment isbeyond the scope of this article. Class A, Class B, ClassG/H, and Class D topologies comprise most of theaudio power industry.

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Class A: (Fig. 3) In this topology, full peak output current is kept runningconstantly through the output power transistor(s). Thisoffers the advantage of keeping the output devices on all the time, and avoiding classic crossover distortion inan amplifier where devices overlap conduction. Class Aamplification is an extremely inefficient method, prima-rily used where the importance of fidelity outweighs size,weight and cost concerns, such as the audiophile market. As noted, full peak output current is kept running all thetime, so a 100W peak output amplifier will have a sup-ply that runs constantly at 200 W, even when the outputis at nominal 1/8 power running levels of 12.5 W.

For a Class A amplifier, the only role for a power sup-ply is to provide a clean regulated rail for the amplifierstage. Even for this class, a switcher offers significantefficiency advantages in handling wide input lineranges, and in regulating the rail voltage. However,since Class A is often dedicated to the audiophile mar-ketplace, acceptance of the SMPS has been slow.Recent high-power amplifiers which must meet ULrequirements and have power factor correction, do infact use switchers. This becomes a significant part oftheir marketing angle. Halcro Amplifiers is an example.Once the PFC circuit goes into an amplifier to meetagency regulations, the EMC issue rears its ugly head,and you may as well do the regulating stage of thepower supply with a switcher since the noise problem isalready there.

Class B: (Fig. 4) The class B amplifier runs lower quiescent current inthe output devices. It is much more efficient than ClassA, and very popular at power levels of several hundredwatts. For pure class B, there is no quiescent dissipa-tion, but this results in significant distortion in the out-put as the upper and lower devices turn on and off at thecrossover. This can be improved with feedback, if dissi-

pation is paramount, or more typically with both feed-back and increased quiescent current. This produces aClass A/B amplifier, which is Class A for small signals,and class B for higher output.

In the predominant half bridge version of a Class Bamplifier, two power supply outputs are alternatelyloaded as shown by the two current paths. At any giventime, only one of the supplies is loaded. This presentsdesign challenges to the power supply, and may causecross-regulation problems if the controls for each railare not independent. The linear transistors of the ampli-fier function as dissipative linear regulators and must becooled appropriately.

Class G/H: (Fig. 5, Fig. 6) One of the major problems in designing the power sys-tem for amplifiers is the widely varying load that can beencountered. Speaker impedances commonly rangefrom 1 ohm to 16 ohms nominal, and specifications areusually given over a range from 2 - 8 ohms resistive.Clearly, it is impossible to design an efficient Class Bamplifier to cover this entire range, and fixed voltagerails are not ideal for wide range loads.Recognizing this, class G and H amplifiers try toimprove the situation.

These are Class B amplifiers with increased efficiencyachieved by using multiple power supply rail voltages.The Class B amplifier normally runs from a low volt-age. This is increased on demand by a linear regulator(Class G), or a solid-state switch (Class H). To compli-cate the matter, G and H definitions are often swappedin some countries. As in the Class B case, some of the

power supply outputs are heavily loaded while others areentirely unloaded, again causing a crossregulation issue. While class G and H greatly improve efficiency, theyare not without problems. It is quite a design challengeto prevent the active power rail from distorting the sig-nal, and this class of amplifier is mostly used in highpower designs.

Fig. 4:Class Bamplifierwith DualSupply Rails

Fig. 3: Class Aamplifiers with Dualand Single SupplyRails. Although bipo-lars are shown as theoutput devices, youcan find MOSFETs,IGBTs, and even vac-uum tubes in modernproducts.

SMPS for Audio


Class D: (Fig. 7) This is usually a four quadrant nonisolated half bridgebuck converter, with the duty cycle of the power switch-es modulated at the audio frequency. In the half-bridgeversion, current draw alternates between the two sup-plies in the same manner as a Class B amplifier.

For all of the higher power technologies– Classes B, G,H and D– the load current alternates from one output toanother. This leads to unique challenges in the powersupply design. Ideally, we would use independent con-verters for each rail needed to ensure tight regulation.

Practical constraints typically confine this solution to asingle converter with multiple outputs. The unloadedoutputs tend to float to overvoltage while the loaded out-put tends to sag. If voltage feedback is taken across justone output, then the sag on the regulated output will below, while sag on the unregulated output will be unac-ceptably high. If the voltage feedback is taken acrossboth outputs in series then the sag will equal the float,which can be quite severe on a completely unloaded out-put. The problem with achieving cross regulation inher-ently is that one output remains almost entirely unloadedwhile the other is fully loaded.

The problem with post regulation schemes is that theyare expensive and consume space. In my experience,output loading can be used to partially mitigate thiseffect. An audio amplifier usually requires low voltagesupplies to power the opamps and display functions.While at first glance it seems easy enough to add a fewsecondary housekeeping windings, I have often usedsimple zener shunt regulators on the main outputs togenerate housekeeping outputs. Although they are ineffi-cient, they do provide some minimum loading to themain outputs, helping to minimize float and improvecross regulation. The same logic applies to ancillary dig-ital circuitry power supplies. They take more power thanwould be provided by shunt regulators, but by running abuck converter from the main outputs instead of addinganother output to the main transformer, we again providea load on the main outputs.

All of the above topologies may be configured as fullbridges, as shown in figure 8, thus requiring half of thepower supply outputs. This is significant when configur-ing a system, as the regulation scheme may be greatlysimplified by allowing a single output. The obvioustradeoffs are increased linear amplifier cost and com-plexity. At higher power levels, however, wewould naturally migrate to a full-bridge design.

Class D amplifiers are seeing the most rapid rise inapplications, in all areas of audio. This is the obviousstep in turning power amplifier design into a commodity,but it is not without its problems, as discussed in moredetail later.

Fig. 7:Class Damplifierwith DualSupplyRails

Fig. 5: Class G amplifier with Dual Supply Rails

Fig. 6: Class H amplifier with Dual Supply Rails

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Resisting the Switched-Mode Power SupplyThe audio industry has been one of the last holdouts ofconsumer electronics to resist the incursion of switched-mode power. There are several factors contributing tothe delay. Most of the unit sales in audio are in the con-sumer market, where power and performance are low(under 100 W) and the margins are thin. Until recently,a simple line frequency transformer has been adequate.This market has also experienced consumer acceptanceof heavy systems, where weight can be regarded asquality. More recently, the latest generation of portableelectronics consumers broke away from this requirement,expecting all of their electronics to be light and cheap.

In the musical instrument market, power requirementsare higher but margins remain thin. The average con-sumer (teenage guitar player or struggling musician)cannot afford more. For a guitar amplifier already pack-aged in a heavy wooden chassis, supply weight is not amajor issue until power levels climb. The size andweight benefits of a SMPS are most useful in the pro-fessional sound reinforcement market where both powerand unit cost are high. This market is relatively smalland is served by smaller companies, with commensu-rately smaller Research and Development staff andbudgets. At the same time it is much more focused onreliability concerns.

Early SMPS in the pro audio industry were designedand manufactured by engineers with little or no experi-ence in SMPS. This led to reliability problems, tarnish-ing the image of SMPS in the industry and reinforcing thenotion in the marketplace that bigger and heavier are better.

Advantages of SMPSFor audio power products, some obvious advantages ofa SMPS over a simple line frequency transformer arelisted below:

Size and weightIn the professional audio industry, a typical 2kW ampli-fier can reduce its weight from 80lbs to 20lbs. This isparticularly significant in touring sound where a systemis constantly shipped at great expense, and for DJshand-carrying gear. Size can shrink from 3 rack units(RU) to 2RU. A standard RU has a 1.75" chassis heightin a 19" wide rack. Rack depth ranges from 12" to 20".

Home audio is a little different. Switch-mode powersupplies have been used for many years in the Europeanmarketplace where size of equipment must be reducedto fit in the typical home environment. In higher-endsystems, the size and weight of the amplifier is some-times considered a mark of "quality", and some homeaudio amplifiers can weigh in excess of 300 lbs. As anexample, see the Mark Levinson No. 33 amplifier atwww.marklevinson.com, which is a mono unit weighingin at 350 lbs.

Line and load regulationThis is more easily implemented in SMPS with highefficiency. It is commonplace to design equipment forworldwide application, and this is a reasonable require-ment for switching power supplies. Wide range univer-sal voltage inputs are simply not possible without tapchanges in linear power systems. Widely varying loadscan also be handled with switchers, although this willalso provide unnecessary stress in many cases, witheither too high a voltage rail for low impedances, oroversized current capability for high impedance speakers.

Power factor correctionIt is now mandated for CE compliance at higher powerlevels that the system must draw power so that it emu-lates a resistive load. Peak rectification current pulsesare not acceptable. SMPS can be small and efficient,while linear power supplies cannot achieve reasonablepower factor correction without very large reactive com-ponents. The system will be tested with a standard resis-tive load driven at 1/8 of maximum power output. If theinput power draw is greater than 75 W under these con-ditions then the absolute harmonic limits must be satis-fied. Since the limits are now simple maximum limits,products may not require active PFC until their fullpower rating rises above 2 kW. As always, the higher

Fig. 8: Full Bridge Arrangements of Class B, G, H and DAmplifiers

SMPS for Audio


order harmonics can be just as much of a problem as thelower order harmonics.

Line frequency interferenceWhile normally regarded as noise generators, a SMPShas the ability to actually reduce line-frequency inter-ference both internal and external to the product.Everyone is familiar with the classic 60 Hz hum ema-nating from line frequency transformers, and this is exhibit-ed as both audible noise, and radiated magnetic fields.

The large magnetic field inside the metal chassis of apower amplifier with a large line frequency transformercauses hum pickup by the low level circuits, and circu-lating currents in the chassis itself. I've measured up to2 Amps. This can lead to system grounding problems.The result is a hum in the amplifier output. The effectsare reduced with the use of a toroid rather than an E-core. The tradeoffs are increased cost and usuallyincreased sag under load due to the larger winding DCRassociated with toroids.

The large external line frequency magnetic field associ-ated with a guitar amplifier comes from the combina-tion of a large line-frequency E-core and a woodenchassis that offers no magnetic shielding. The problemhere is that the electromagnetic pickups on the electricguitar itself will pick up this field when in close prox-imity to the amplifier.

CostLarge line-frequency magnetics can be expensive, espe-cially low-noise toroids. Associated large passive ele-ments such as capacitors and smoothing chokes can addto this cost.

SMPS Disadvantages:Development timeUnderestimating development time of a power supply isa real problem, especially when in a corporate environ-ment that has little experience with SMPS. The tenden-cy to minimize the effort that goes into a SMPS iswidespread in the audio field. The line-frequency trans-former solution is often a small part of the job for anaudio product designer. A linear power system can oftenbe designed in a single day and expected to work prop-erly. The SMPS usually requires a second design engi-neer working in parallel with the amplifier designer.SMPS development time can be reduced in many ways,but will never be close to the line frequency transformerdesign time.

Product complexityFully half of the complexity of a SMPS audio poweramplifier is contained in the power supply, and that'sjust for the dc-dc part. For a quiet and efficient powerfactor correction plus actively adjustable power rails, asdescribed later, a miminum of 50% of your develop-ment time should be devoted to the power system. Andthat is assuming that you have extensive design experi-ence. Design is complicated severely by the peak pow-ers that must be provided during transients. This mustbe done reliably and usually with low cost.

ReliabilityAt lower power levels, reliability of a switcher willalways be less than that of a linear regulator. At higherpower levels, linear regulators run hot, and the switcherscan offer advantages of less thermal stress on the semi-conductors. However, even with the thermal advantage,you can expect the reliability of the electronics to belower simply due to parts count and system complexity.This is especially true in the cases where power supplydesign difficulty is underestimated, the developmentprocess is inadequate, and engineers without relevantexperience undertake complex designs.

EMC IssuesA linear power amplifier with a line frequency trans-former gives very little cause for concern with respect toemissions, outside of the line frequency itself. Once theSMPS is added nearly every other aspect of the amplifi-er must change. The chassis must be designed with radi-ated emissions in mind. The line level inputs must bedesigned while considering cable clamps and radiatedemissions. A line filter is required, for example,where SMPS designs radiate frequencies from theswitching frequency on up. These are also carrierfrequencies for line-frequency and audio frequen-cy harmonics.

Plan to add an entire EMC design phase, plus extensivetesting time in an EMI lab to meet agency requirements.This step cannot be overemphasized. Emissions are cru-cial in audio applications where high gain amplifiers,microphones, and other circuits are highly susceptible toemissions. Also, be very wary of emissions control tech-niques designed to 'cheat' the measurement equipment.Frequency-shifting techniques may get you past emis-sions standards, but they don't stop the bursts of RF energyfrom interfering sporadically with high-gain systems.

SMPS for Audio


Cost of Development It is tempting to look at the cost of commodities such asPC power supplies, and conclude that a SMPS can bevery inexpensive. However, you are not likely to find anoff-the-shelf power supply that will meet your specifi-cations, and the unit cost will include significant devel-opment investment. This investment is often amortizedover relatively small production quantities. Ultra low-cost power can be achieved only when produced inmass quantities overseas, and when performance iscompromised severely for cost goals.

Amplifier Design ConsiderationsThe biggest difference between a standard SMPS andan audio power amplifier SMPS is the peak-to-averageloading ratio. For a typical SMPS, peak capabilities arevery close to average capabilities, and the supply canrun indefinitely at full power without degradation.When the typical SMPS designer enters the world ofaudio power design, the power supplies are often over-designed, and hence, overpriced.

In an audio power amplifier, peak power and averagepower vary greatly, presenting some unique designopportunities and constraints. Let's look at a typical"100 W" non-audio power supply. It can deliver 100 Wsteady-state at a given ambient temperature, and has ashort-term overload capacity just somewhat higher thanthis, needed for transient regulation, design headroom,and startup. We usually design a SMPS power supply tocurrent limit at around 110-120% of its rated load.

A "100 W" amplifier, however, is capable of producinga 100 W sinewave before clipping. This corresponds toa 200 W instantaneous power peak that must be sus-tained, from one power rail only (typically there aretwo), near the peak of a 20 Hz (50 mS) sinewave. Ifthere is minimal energy storage in the design to savecost on capacitors, two power supplies, which average50 W load each, must be able to produce 4 times thisamount on an instantaneous basis.

But it also means that typical audio program materialwill produce an output power between 1/8 and 1/3 ofthe 100W rating, or about 12W to 33W. To test an audioamplifier for UL or CE purposes, it is loaded to 1/8power using pink noise.

So, if it were 100% efficient, our "100W" amplifier willhave a peak draw of 200 W from one output and anaverage draw of 12 W taken over both outputs. Thisobviously allows us to scale down some of the magnet-ics design, and the heatsinking. The magnetics designchanges are considerable. While we still need to design

for peak currents and peak flux density, copper may bescaled back considerably versus a design that mustcarry full power continuously (ie a normal switched-mode design). This has its limits, though, as the com-ponent will still be subjected to high copper losses dur-ing full power operation. The time duration of fullpower operation that the design must withstand is dif-ferent for every application, but the acceptance criteria must beidentified early and become part of the design and test cycle.

Consider a power transformer designed to run at 1/8power continuously with the maximum permissibletemperature rise, roughly half due to copper losses forthe purpose of illustration. When the transformer sees afull power test, its current will be increased by a factorof 8. This assumes an ideal amplifier or a constant effi-ciency class D amplifier. The copper losses soar to 64xthe steady state value, which can quickly develop a hotspot and cause insulation failure.

This multiplying effect can actually be more pro-nounced in a class D design where the 1/8 power dissi-pation is very low. A comparable class B amplifieralways runs less efficiently at lower power, and thepower supply is more conservatively designed withdecent thermal capability. That makes the power sup-ply draw vary less than the 8:1 spread illustrated. A similar constraint exists for heatsinking, in whichpower semiconductors may have reduced heatsinkingversus full power requirements. The heatsink must beadequate for full power operation for a specified timeinterval. The transient thermal resistance of theheatsink due to specific heat or "thermal mass" isimportant in these cases to handle peaks in music thatare of relatively short duration.

It is tempting to scale down the power semiconductorsas well, but the low frequency signals involved have along enough time constant that the silicon will be cho-sen for peak outputs. Similarly, capacitance will bechosen as a function of peak power, although ripplecurrents may be scaled for average power.

RegulationWe actually have decisions to make regarding line andload regulation when powering audio power amplifiers.As mentioned above, most applications require multi-ple outputs alternately loaded from 0-100%. Cost limi-tations usually prohibit active post-regulation schemes,so requiring regulation definitely imposes a burden onthe design. Why should the switching power supplyregulate? A proper treatment of the subject is not possi-ble here; reference [2] provides a more thorough treat-ment but I will cover some basics below.

SMPS for Audio


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Audio power amplifiers are characterized by theirpower outputs at several load impedances, usually from8 ohms to 2 ohms. 100W into 8 ohms requires a 40Vpeak sinewave output, whereas 100W into 2 ohmsrequires just 20V peak output. If we had perfect loadregulation and delivered 100W into 8 ohms, we wouldget 400W into 2 ohms, illustrating a benefit of load reg-ulation: for any given 8 ohm power rating, load regula-tion gives far more 2 ohm power. Of course the reversesituation illustrates a disadvantage of load regulation:with 2 ohm power held constant, regulation gives farless 8 ohm power. Optimizing the supply schemerequires more knowledge of the amplifier's voltage andcurrent limits, but an unregulated supply is often a rea-sonable compromise in which the amplifier ratings showless than the 4:1 spread predicted with load regulation.

One way to fully utilize the amplifier's voltage and cur-rent capability involves selecting the supply voltage thatis optimum for the load impedance actually being used.I've done this on several products. The drawback is thatthe flexibility adds complexity and requires user input.Nevertheless, the products had the same rating at 2ohms and 4 ohms, which gave them an advantage overunregulated products.

Line regulation has the advantage of allowing theamplifier to provide the same performance at low linevoltages while unregulated amplifiers see a 20% powerloss at 10% low line. This has a disadvantage, however,when a design is scaled to maximize the capability of a15A outlet. In this case the maximum line draw allowedis just 12 A and will be determined by driving theamplifier at 1/8 maximum power and measuring worstcase draw over the +/- 10% line voltage range. In theunregulated case, low line causes low supply rails andslightly less current draw, whereas the line regulatedsupply will draw 10% more current at 10% low line.This is a disadvantage that can cause reduction in thepower rating at nominal line voltage.

A disadvantage of an unregulated supply is the presenceof line frequency related ripple voltage, especially whenunder load. This means that under clipping conditionsthe amplifier output will not have just the harmonicallyrelated distortion predicted by flat-topping a sinewave;since the clipped peaks will track the supply and its linefrequency related ripple there will be distortion presentthat is not harmonically related to the fundamental,which could be quite unpleasant.

Alternative regulation schemes I have used include adiode summing network that let the voltage loop

respond to the lowest rail. This was used for a Class Bamplifier with good results. A variation was neededwhen the unloaded rail float became too much andapproached the breakdown voltage limit of the outputtransistors. The voltage loop responded to the lowest railuntil the highest rail hit an overvoltage limit at whichpoint the highest rail dominated the feedback loop. [4]

Another regulation trick I have used involved foldingback the supply voltage in response to amplifierheatsink temperature. This allowed the amplifier to avoidthermally induced shutdown by scaling linearly. [4]

Noise, Noise, NoiseThere is no doubt that the biggest impact of introducingswitched-mode power into an audio power amplifier isthe noise. Like it or not, switching power supplies arenoisy. For most applications, the noise is clearly visibleon the output ripple, and we bypass load semiconductorswith capacitors to reduce the noise locally.

The trouble with an audio system is that we can hear thenoise, and it directly degrades the intended performanceof the amplifier. An audio amplifier generally needs aclean supply output. Not just with respect to switchingripple, but also particularly with respect to high frequen-cy spikes corresponding to switching transitions presentas differential and common mode noise. This usuallynecessitates a common mode filter on the power supplyoutput. In addition, a common mode filter on the audioamplifier output is required as well.

Even though the switching frequency is usually muchhigher than the audio bandwidth, it can interfere withperformance in several ways. While audio performanceis generally considered from 20 Hz to 20 kHz, the dis-tortion measurement bandwidth is usually up to 80 kHzto ensure capturing several harmonics of 20 kHz. Thismeans that a 65 kHz SMPS will need to control switch-ing frequency contamination more rigorously than a 130kHz SMPS.

Even if the fundamental problem is painstakingly elimi-nated– there is the problem of high frequency switchingtransition spikes. Although these occur at the switchingfrequency, they are comprised of small bursts in theMHz range, and are difficult to suppress. If they wouldsimply average they would result in a small amount ofswitching frequency, and would have negligible effecton performance. However, the switching frequencyspikes are modulated by the controller of the power sup-ply as it actively regulates line voltage variations. Thesesignals can be demodulated when they encounter an

SMPS for Audio


active P-N junction in much the same way that an old"crystal set" radio works. This results in line frequencyharmonic content, which is often worse than the linearpower supply produced.

Fig. 9 shows the audio output waveform at idle for aClass D power amplifier with no audio output commonmode filter using a SMPS with no dc output commonmode filter. As with most power supply output voltages,the switching frequency ripple is negligible compared tothe edge transition spikes. These spikes will cause prob-lems with other gear, as well as violating emissionsstandards. As mentioned before, although these spikesare caused by ringing frequencies in the MHz range,they have sidebands in the audio frequency range thatcan be demodulated by the amplifier circuitry.

Fig. 10 shows the noise on a Class B amplifier poweredby a switcher. Since this is a linear amplifier stage,there is no switching ripple frequency as on a Class D,but the voltage spikes are just as pronounced and willcause problems. There is no common-mode filtering oneither the power supply output, or the amplifier output.

Fig. 11 shows the noise on a Class B amplifier after acommon mode filter has been added to the power sup-ply. There is more than an order of magnitude of noisereduction in this case. Note the different scales– 1 V/div

without filter, and 50 mV/div with the filter added.

Fig. 12 shows the noise on a Class B amplifier after acommon mode filter is also added to the output of theamplifier itself. These two stages of common-mode fil-tering are essential for high quality audio design withswitchers in the system. Despite the cleaner waveforms,there is still no guarantee that all audible noise will beeliminated. As with bypass caps on conventional powersupply loads, the noise is locally reduced, but it can becausing problems elsewhere in the system.

If the SMPS is sharing a chassis with the analog circuit-ry, then precautions must be taken to avoid contamina-tion through all vectors. The output filtering is the mostobvious source, but since the supply will likely not getan enclosure, care must be taken with other noise trans-mission mechanisms. An E-core power transformer (orinductor) is a good noise source, and should be physi-cally remote from any sensitive circuits. It should alsobe fitted with a flux band to reduce leakage. Anyswitching device heatsinks should be tied to a quietpoint, such as -bus or ground. Do not allow a devicewith a high frequency squarewave present on its metalmounting plate to have a conductive connection to aheatsink, even if it is a dedicated isolated heatsink. Thiswould become an E-field source.

Fig. 9: Class D amplifier output voltage and inductor current,showing ripple voltage and edge transition spikes and ringing.

Fig. 10: Class B amplifier output noise with SMPS, no commonmode filter on SMPS output or amplifier output.

Fig. 11: Class B amplifier output after adding common mode filterto SMPS outputs.

SMPS for Audio


Fig. 12: Class B amplifier output, common mode filter on SMPSoutput and amplifier output.

Figure 13 shows the system layout of a 3 kW audiopower amplifier with SMPS. In addition to placing theline filter in an isolated metal compartment, the outputfilter is also placed in a separate metal compartment, asis the low level audio input circuitry. This assures thatthe electrical signal path through the filter is not cir-cumvented by radiated paths, foiling the filter.

Fig. 14 shows the SMPS module used in Fig. 13 withthe top of the fan air tunnel removed to show the place-ment of the power transformer and power inductorbetween two heatsinks. This arrangement helps to con-tain stray E fields, while the core bands help containstray B fields. This supply has the four outputs neededfor a half-bridge class G, which are filtered with a fivewinding toroidal common-mode choke in the upper

right corner. The AC line wiring also gets a commonmode choke before leaving the board, shown in thelower right corner. This cleans up the AC wiring, mini-mizing its contribution to system noise.

For this reason SMPS outputs need additional outputfiltering, particularly for the common mode content.Two winding common mode chokes are not effectiveon multiple output converters, since currents may origi-nate on one output and return on another output; athree winding choke is used for a two output class Bsupply and a five winding choke is used for a four out-put class G/H supply. Since the noise is generally highimpedance in nature, a C-L-C configuration worksbest. A common mode filter is often needed on theamplifier output for the lowest noisefloor. Long cableruns internal to the product frequently need ferriteclamps; I often place 0805 chip style ferrite beads onthe PCB in the design process, with the intention ofshorting them out if not needed (but they usually are).

Further complicating the matter is the fact that mostferrites are microphonic, that is, they translate mechan-ical vibration into an electrical signal. This is usuallyonly a problem in a high impedance input line with amore massive (with respect to an 0805) multi-turncommon mode filter choke in place; I've never seen an0805 cause a problem. Imagine my surprise when Ifound that an SMPS guitar amplifier wouldrespond to a punch in the cabinet with an audible"thwack" from the speaker.

Measurement of the audio performance of the systemcan be complicated by SMPS. Audio Precision makesan industry-standard line of measurement devices.Recognizing the high noise levels from the SMPS, theyhave recently introduced an accessory to limit the highfrequency out-of-band signals produced by a noisySMPS or a class D amplifier. Without this filter thefront-end amplifier could be tricked into providingerroneous results, either a reading higher than the actu-al noise, or just as often, a reading much lower than theactual noise. Similar results were seen for THD+N,total harmonic distortion + noise, although it must benoted that a carefully designed amplifier/SMPS doesnot cause measurement errors, especially if it is quietenough to pass CE testing.

Industry TrendsSeveral industry trends are impacting the designers ofaudio power amplifiers and SMPS. The most obviousis the outsourcing of the design and manufacturingprocesses, typically to China. No high-power amplifiermanufacturer has successfully outsourced the design

Fig. 13: 3 kW Class G amplifier showing system layout.

Fig. 14: Switching power supply for 3 kW Class G amplifier Notethe extreme small size of the power supply in Fig. 14. This is due tothe fact that the power supply is not designed to thermally withstandfull power for long periods of time. .

SMPS for Audio


cycle of a power amplifier with SMPS yet, but most aretrying. All major companies have already transitionedthe manufacturing process to China for class B ampli-fiers with line frequency transformers, and several havesourced these types of designs with varying degrees ofsuccess. The complexity of the product has kept thehigher technology amplifiers with SMPS from movingto China yet, but this will come to pass eventually. AsChinese engineers gain more experience with the opti-mization of SMPS for audio power amplifier use, thedesign cycle of these will probably move there as well.

Another trend is the increasingly pervasive use of ClassD amplifiers as part of an all-digital system, as shownin Figure 2. Since these amplifiers produce so muchEMI by themselves, and have poorer audio performancethan linear methods, the burden of a quiet SMPS infront of the amplifier itself is substantially relaxed, ifthat is perceived as an advantage. There is no doubt thatcompanies want to push audio to fully digital systems.This brings the technology firmly in the control of thelarge companies with high development budgets, andallows profits to be captured with mass marketing. Justas it is with the soon-to-be-extinct VCR, a phenomenalamount of technology can be had for a small amount ofmoney when production is done in the hundreds of mil-lions. Digital processing and efficient switching willallow this step, and remove many of the costly partsfrom the system.

But this will not dominate the entire audio marketplace.There are severe issues remaining with digital audio,mostly related to clock jitter and synchronization ofmultiple digital components. The problems show upwhen converting back to analog, which, unfortunatelyfor the digital engineers, must be done with extremeprecision. This has plagued the industry since the inven-tion of the CD, and it will become much worse with theswitching noise introduced by the Class D amplifica-tion. Some companies are even talking about futuredevelopment of Class D amplifiers where the filter itselfis removed from the amplifier, and the characteristics ofthe speaker are used as a low-pass filter. EMIproblems will no doubt cause some severe issueswith such a scheme.

Research has shown that the human ear can discernclock jitter down as low as 10 ps, and it gives rise toaudio frequency harmonics that are particularlyunpleasant. Putting a switcher in close proximity toprecision digital clocks can lead to seriouslydetrimental results.

There is also something inherently flawed in the func-tion of the switching amplifier in its normal range ofoperation. Most of the music produced is at very lowpower levels relative to the amplifier's capability. Thatmeans that the switching transistors of a full bridgeoperate at close to 50% duty cycle, and a high-voltagerail is chopped alternately in the positive and negativedirection to produce a very low output voltage. Theadvantages in dissipation of such a system are apparentat peak power outputs, but at low powers it can be veryhard to achieve low distortion figures.

ConclusionsThe incursion of switching power supplies into audiodesign is obviously going to continue strongly in thefuture. No single technology is going to completelydominate the marketplace due to the advantages anddisadvantages of each approach, and the diverse needsof each marketplace. I have one piece of advice forcompanies in the audio industry: don't underestimatethe magnitude of the task in designing a switch-ing power supply into an audio system. The inte-gration into the system and EMI control requiresubstantial investment.

If you are designing just a bias supply for a Class Bamplifier, the design task is very different from design-ing a full-blown switching amplifier. There are manysubtle issue relating to sound quality that arise in aswitching amplifier, and companies such as Crown,QSC, and Bang and Olufsen that have products on themarket have invested signifigant R&D dollars to reachthe level of performance that they have achieved.

ReferencesReferences [1] - [5] are available atwww.audiopowerelectronics.com

[1] Computer Aided Design and Analysis of Class B and Class HPower Amplifier Output Stages, Audio Engineering SocietyPreprint 4329, Eric Mendenhall 1996[2] Power Supply Regulation in Audio Power Amplifiers, AudioEngineering Society Preprint 5694, Eric Mendenhall 2002[3] Audio Power Amplifier Output Stage Protection, AudioEngineering Society Preprint 5695, Eric Mendenhall 2002[4] US Patent 6,611,169: Power Supply Regulation and ProtectionCircuit for Audio Power Amplifier, Eric Mendenhall 2003[5] US Patent 6,621,253: Amplifier Having a Variable PowerFactor, Eric Mendenhall 2003[6] High Performance Audio Power Amplifiers, ISBN 0 7506 26291, Ben Duncan 1996[7] Class D amplifier and pulse modulation technologyhttp://www.icepower.bang-olufsen.com/sw1022.asp

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