A 700Watt Amplifier DesignRobert Carver*

THIS ARTICLE DISCUSSES SOme ofthe requirements modern highfidelity systems impose on the

power amplifier. Renewed efforts tocope with previous shortcomings inpower amplifier performance has ledto a whole host of new technical problems and, necessarily, solutions. Theseproblems, their recognition, their practical solutions, and their significancewill be examined within the frameworkof existing power amplifier technology.

The author is an audiophile. a true-blue, dyed-in-the-wool high-fidelityenthusiast, lover of instruments andinstrumentalities, lavishly scored orchestral showpieces, choruses and cathedrals. The sonic details of a singledrumroll can put me into a state of rapture that Timothy Leary himself wouldhave envied. I have searched, like SirGallahad for the Holy Grail and T.R.Thompson7for the Lost Chord, for thelow frequency pedal note that wouldleave the trolls of Ireland impotent. Ihave sought high frequency performance that would interest and pleaseany passing bat. In short, I am a musiclover for whom the quest for realisticreproduction of music has taken on aconsuming devotion.

A look at the best of the availablebasic power amplifiers discloses thatbandwidth, distortion, and noise figuresextend far beyond the limits of audibility, and in some cases, even the limitsof laboratory measureability. It mightbe concluded that power amplifiershave reached such levels of perfectionthat further advances could not possiblyfActually, it was Adelaide Proctor. (Ed.)'President and Director of Advanced Projects,Phase Linear Corp.

24

provide any audible improvements andwould be simply "gilding the lily." Inany event, it would seem, for example,that decreasing distortion from 0.5%to 0.05% or increasing the signal-to-noise ratio from 90 dB^to 93 dB (twiceas quiet) would not have any audibleeffect.

This conclusion might be justifiedbecause the deviations from perfectionintroduced by the rest of the signal-processing chain (speakers, cartridges,and record surfaces) are several ordersof magnitude greater than those introduced by the virtually perfect (by comparison) power amplifier. Further, itwould seem reasonable to assume thattwo different amplifiers whose specifications in terms of power, distortion,frequency response, crosstalk, etc., arealmost identical should be sonicallyindistinguishable from one anotherwhen compared in listening tests.

However, high fidelity enthusiastshave long observed that different amplifiers do. in fact, sound different andthat some amplifiers seem to deliver amore robust low end along with sweeter,silkier highs. Yet their specifications,together with extensive laboratory testing and analysis can reveal no logicalreason. A mystery. An engineer recognizes that a mystery is really only alack of understanding born of insufficient data or the incomplete evaluationof existing data. In the case of the highfidelity power amplifier, it is simplythat the human ear is capable of hearingand resolving on-going musical detailthat has somehow eluded vast arsenalsof laboratory test equipment.

Our own experiences are illustratedin the following experiments and ex

amples. Extended listening comparisonshave resulted in several interesting observations. First certain high qualitytransistor amplifiers "sounded better" by a small margin than high qualitytransistor amplifiers of identical powerrating, in spite of the fact that the lat -ter amplifiers had far "better" electrical specifications in terms of distortion, damping, bandwidth, etc. Thisdifference in subjective sound qualitywas particularly dramatic when listening to high quality speaker systems usingelectrostatic components. Particularly,the high end was much airier and open,with much less apparent sibilance during high energy transients.

Let me digress a bit at this point andmention that particular care had beentaken to eliminate the last vestige ofcrossover distortion in the inferior amplifier. At that point, we were virtuallycertain that crossover distortion was notthe culprit. As shall be shown, this assumption proved to be valid.

The second observation was that twodifferent high power, rather expensivetransistor amplifiers introduced a mild"snapping" sound into the music whenused with some acoustic suspensionspeaker systems. The "snapping" occurred primarily during low frequencyhigh level passages and on some solodrum instrumcntals. Interestingly, thesnapping sound was not at all objec-tional during the drum solos—it tendedto give each individual drum beat anadded impulse and the illusion of tremendous transient response. However,on sustained low frequency notes, forexample, the pedal notes of a pipeorgan, the snapping was extremely

AUDIO • OUR 25H YEAR • FEBRUARY 1972

annoying and clearly an indication ofamplifier misbehavior.

In the third observation, in which avacuum tube amplifier rated at 60 wattsRMS per channel was compared witha transistor amplifier also rated at 60watts RMS per channel, it was observedthat the tube amp sounded somewhatmore powerful. We discovered that wewere able to increase the sound levelsignificantly before objectionable distortion occured when using the tubeamp. In fact, the 60 watt/channel tubeamp sounded the same, exactly thesame, as a fine transistor amplifierrated at slightly over 100 watts/channel.Two tube amplifiers were used, bothvintage models, a Citation II and aMarantz Model 9. The transistor ampwas of modern design and is very highlyregarded.

When all of the transistor amplifiersincluded in our listening test were compared, an interesting pattern emerged,which was two-fold. First—relatively lowpower transistor amplifiers, those under60 watts, and those built into receiversand integrated amplifiers, never exhibited any form of overt of obviousmisbehavior. The only "fault" it waspossible to render judgment against wastheir low power and consequent inability to produce satisfying music levelswithout severe overloading. On the otherhand, two of the high power units exhibited the "snapping" phenomena andthis we considered to be overt misbehavior and a grevious fault. (As we shallsee later, the trouble was due to the

Fig. 1—Amplifier #1, a 60 W/chan.uni t w i th regulated power supply,opera t i ng i n to an 8 ohm load a tclipping point, which 60 watts. Bothchannels operating.

protection circuits.)The second part of the emerging pat

tern was that, given two transistor amplifiers of similar power ratings, it wasfound that units with a regulated powersupply sounded the least powerful; thatunits with separate power supplies (twopower transformers) sounded subjectively more powerful; and, interestingly,units with a single, common powersupply sounded the most powerful.Without exception, the units that appeared "most powerful" sounded significantly "cleaner" and more "open"when compared with the other units.All amplifiers were operated at identical listening levels, and each was operating just below the point of audibleoverload using the "most powerful"sounding unit as the reference. All ofthese units had similar RMS powerratings. (As a matter of course, the a.c.power line voltage was adjusted slightlyto give each amplifier identical continuous sine wave power output.)

At this point, the task at hand is toidentify the reasons for the subjectivedifferences on a rigorous, scientificlevel, and approach the problems froman engineer's viewpoint.

The first investigation centered arounddetermining why some amplifierswith identical power ratings did notsubjectively sound equally powerful.For our tests we used a commerciallyavailable 60 watt/channel amplifierwith a regulated power supply andcompared it with a unit specially builtand designed for the experiment. Itwas designed to deliver 60 watts/channel with both channels in operation andit used a single unregulated powersupply. An oscilloscope was installedacross the speaker terminals and thetest was arranged in the familiar A-Bfashion. It was possible to switch fromone amplifier to another instantly whilesimultaneously listening to music andobserving the output of each amplifieron the 'scope.

It became immediately obvious whythe second unit sounded more powerful. We observed that the second unit'soutput voltage would rise considerablyhigher prior to clipping than the unitwith the regulated supply. It soundedmore powerful because, in fact, it wasmore powerful when operated withmusic into a high fidelity speakersystem.

To understand this, it is necessary tomake a detailed examination of how

the power supply of an amplifier effects the available output voltage swing.

The absolute value of the powersupply voltage is what determines themaximum output voltage swing. If thepower supply voltage is, for example,63 volts, then the amplifier can deliver at its output terminals up to 63volts peak to peak. Once current begins to flow, as the amplifier is delivering power to the load, internal lossescause this voltage to plummet downward. In the case of a 60 watt/channelamplifier whose power supply is unregulated, the supply must be able tomaintain 63 volts under full load withboth channels operating into 8 ohmload resistors. Since the supply is unregulated, and yet it must somehowsupply 63 volts, it must necessarily bedesigned to deliver a substantiallyhigher voltage during no-load or higherimpedance load conditions in order to"make up difference" due to internallosses. In the case of a typical transistoramplifier, voltage losses in the powersupply are approximately 30%. Hencethe power supply voltage must be anunloaded 95 volts.

A regulated power supply can bethought of as "loss free" because itsoutput voltage remains constant anddoes not vary from a no-load conditionto a full load condition. In the case ofthe 60 watt/channel amplifier, it is regulated to 63 volts—never higher, neverlower.

Never higher, never lower. Thereinlies the reason that the amplifier withthe unregulated supply sounds morepowerful. Speaker systems are not ofconstant impedance; they vary over awide range, from below 8 ohms andclimbing to a high of 30 ohms or more.At resonance, the speaker impedance isat a maximum and if substantial poweris to be delivered, the amplifier musthave substantial output voltage capabilities. The expression for power isP = V7R. From this it is readily seenthat if the impedance R increases, thevoltage must increase or the powerdelivered will decrease. If the powersupply is regulated, it cannot increase,and the available power under dynamicconditions is severely curtailed.

Oscilloscope photographs in Figs. 1through 6 graphically illustrate theseeffects. Referring to Fig. 1, the 60 watt/channel amplifier with the regulatedsupply is being driven to the clippingpoint (point of overload) with a con-

26 AUDIO • OUR 25TH YEAR • FEBRUARY 1972

Fig. 2—Amplifier #2, a 60 W/chan.un i t w i th dua l power supp ly. Theclipping point is the same as Fig. 1,60 watts, both channels operating.

Fig. 3—Amplifiers #3, a 60 W/chan.unit with a single power supply. Theclipping point is also 60 watts, bothchannels operating.

F ig . 4—Ampl i f ie r #1 w i th low f requency tone burst simulat ing drumbeat. Clipping occurs at 60 watts. Theload is an 8 ohm acoustic suspensionloudspeaker.

Fig. 5—Amplifier #2, with the sametone burst and speaker load, deliversabout 20% more voltage than amp #1unde r dynam ic mus i c cond i t i ons .Clipping is just under 90 watts.

tinuous sine wave signal. Both channelsare operating. Only one channel isshown in the photo. In Fig. 2, the unitwith two power transformers in itspower supply is being similarly drivento the clip point. Fig. 3 shows the amplifier with the unregulated power supply similarly driven. Notice that theclip point is the same for all three units,63 volts peak to peak. In Figs. 4, 5 and6, a low frequency tone burst is used tosimulate a drum beat. The load is aneight ohm acoustic suspension loudspeaker. The unit with the regulatedsupply (Fig. 4) clips at its previousvoltage level, 63 volts. However, theunits with the unregulated suppliesFigs. 5 and 6 are able to deliver a highervoltage prior to clipping. The unit with

Fig. 6—Amplifier #3, with the sametone burst and speaker load, deliversabout 30% more voltage than amp #1under dynamic mus i c cond i t i ons .Clipping is over 100 watts.

the single unregulated power supply isclipping at a voltage level approximately30% higher than its sine wave continuous clipping level. Thirty percentis almost a xh increase, and since poweris proportional to the square of thevoltage, the power increase is approximately 1.3 x 1.3 = 1.69. Almost seven-tenths more effective power is available from this amplifier.

From another point of view, for anygiven average power level, the secondamplifier will be clipping significantlyless during musical peaks, and is thereby generating significantly less distortion. This is why the second amplifier sounded "sweeter and airier."

We repeated these experiments withour vacuum tube amplifier and com

pared results. It turned out that thetube unit behaved in a manner similarto the amplifier with the unregulatedsupply, with an interesting exception.When the tube amplifier was verylightly loaded, with a load of around30 ohms or higher, its voltage swingcould go extremely high, producingalmost 50% more than the fully loadedcondition. This high impedance loadis the load condition that an electrostatic midrange or tweeter unit imposeson an amplifier. The power demandsare rather moderate because the loadimpedance under dynamic conditionsis relatively high, and therefore thevoltage requirements of the electrostatic screens are high. Present day electrostatic mid- and high-range screensrequire their power at high voltagelevels, precisely where a vacuum tubeamplifier excels.

These findings are summarized ingraph form in Fig. 7. Notice that the"best sounding" amplifier (7C) hasavailable additional operating areathat is "forbidden" by the amplifierwith the regulated power supply (7A).Figure 7B shows the operating area ofan amplifier with two power transformers, and Fig. 7D depicts a 60 wattamplifier idealized to represent a "Perfect" 60 watt/channel amplifier. Noticethat the available operating area isalmost double that of the unit with theregulated supply. The "perfect" 60 watt/channel amplifer would have, as adesign goal, a very high voltage powersupply. It would sound very clean andvery powerful. The high voltage designapproach produces the very bestpossible "sounding" amplifier, but, asis often the case, there is a tradeoffagainst other desirable characteristics.

28 AUDIO • OUR 25TH YEAR • FEBRUARY 1972

1- 1 1

K u j

o f

1 1

a. => y , O P E R A T I N G / ) . 1/ / A R E A • > * A ,

AMP OUTPUT VOLTAGE

AMP OUTPUT VOLTAGE

UNREGULATED,HIGH POWERVOLTAGE LINES

°- ^ /V OPERATING <^— nV / A R E A < T ~//• t € t < t ( f ft

AMP OUTPUT VOLTAGE

Fig. 7—Why four different amplifiers,each rated at 60 watts/channel soundsubjectively different. A, this area isforbidden because of the tight powersupply regulat ion. B, This area isavai lable to an amplifier with dualpower supplies. C, This area is availab le an amp wi th an unregulatedpower supply. D, This area is availableto an amplifier with an "idealized"design.

Fig. 8—Output of a high power amplifier operating at 40 Hz into an 8 ohmresistive load. Power level is 1 50 watts.

Fig. 9—The same amplifier as Fig. 8operating at 40 Hz into an 8 ohmloudspeaker load. The tearing at thecenter of the waveform is due to falsetriggering of the protective circuits.The power level is again 1 50 watts.

The liability assumed with the highvoltage amplifier is that the normaloperating temperature of the amplifiermust be higher than with the conventional design. If the designer is willingto accept this drawback and is willingto design into his unit an extra marginof thermal stability, an amplifier usingthis approach would be without peer.

A detailed examination of high powered amplifiers in the 100 to 150 watt/channel range reveals shortcomingsand problems unique to these units.

A severe design problem that must beundertaken when building a high poweramplifier is to design an adequateprotection device for the unit. All highpower amplifiers must incorporatesome form of protection circuit toprevent their destruction in the eventof an accidental overload. The protection circuit must limit the outputof an amplifier if it is operated intoan improper load, but it must not inany way limit the output of the amplifier when operated into a proper,normal, or loudspeaker load. Thesetwo conditions represent conflicting requirements imposed on a protectioncircuit, and it shall be shown that inmany instances these conflicting requirements have resulted in protectioncircuits that do not completely protectthe amplifier, or worse, often limitthe output in some manner that resultsin an audible degradation of the musical signal. In the most severe cases,outright amplifier misbehavior results.

Fig. 8 is a 40 Hz output signal delivered into an 8 ohm resistive load.The power level is 150 watts. Note

Fig. 10—The same ampli f ier as inFigs. 8 and 9 with the same loudspeaker as Fig. 9, but with the amplifier's protection circuits removed. Notethe perfect waveform.

that the signal is perfect. Fig. 9 is thesame amplifier operated into a complexload whose impedance is also 8 ohms.The load is an 8 ohm acoustic suspension loudspeaker. Notice the largespikes that are occuring on the downward slope of the sine wave. Thesespikes are caused by false trigger actionof the protective circuitry built intothe amplifier and cause the "snapping"sound mentioned previously. The spikesare called "flyback" pulses and aregenerated as follows. As the sine wavereaches its peak value and begins todecrease, the energy that has been storedin the magnetic field associated withthe inductive component of the loudspeaker impedance is forced to flowback into the amplifier. The protectivecircuit senses this reverse energy flowas an overload and commands the amplifier output stage to shut down. Thishappens instantly, and when the outputstage turns off, the energy is preventedfrom flowing back into the amplifier.The result is a large voltage spike dueto the collapsing magnetic field in theloudspeaker. An easily understoodanology is an automobile spark coiland a set of points. When the pointsinterrupt the flow of current, the collapsing magnetic field inside the coilgenerates a high energy spark. In ananalogous manner, the voltage at theloudspeaker terminals rises until clamping diodes (built into all large amplifier output stages) conduct and preventany further increase. The audible effectis the "snapping" misbehavior and occurs during heavy low frequency demands. Fig. 10 is the same amplifier

30 AUDIO • OUR 25TH YEAR • FEBRUARY 1972

Fig. 11—A 15 KHz tone burst outputof a high power amplifier into an 8 ohminductive (loudspeaker) load. Note theperfect response.

but with the protection circuits disconnected and operated into the loudspeaker load. Observe that the sinewave is again perfect.

Figures 11, 12 and 13 show the output of an amplifier rated at over 100watts operated into a loudspeaker system. In Fig. 11, the signal consists ofa 15 kHz tone burst whose on and offtimes are equal. Observe that the amplifier output response is perfect.

Fig. 12 consists of the same 15 kHztone burst but with the following characteristics. The off time is very longcompared to the on time. The repetition rate of the tone burst is verylow. In this instance, 500 Hz. This particular tone burst simulates the simultaneous output of a low frequencymusical note (the repetition rate) anda high frequency musical note (theinternal tone burst frequency). Thiswould, in a musical sense, correspondto the simultaneous reproduction ofa low frequency woodwind note and,say, a harp. Notice that the first fewcycles of the tone burst are limitedand distorted. This is because the protection circuits in the amplifier confusethe simultaneous low and high frequencies with an overload, falsely trigger,and limit the amplifier output. It isonly under certain combinations ofsimultaneous low and high frequencydemands that the protection circuits arefalsely triggered into operation. Fig. 13shows the same output as Fig. 12, butwith the protection circuits removed.Again, note that the response is perfect.

The audible effect of this particularamplifier misbehavior is much moresubtle than the previous example. Theeffects range from a slight "edginess"and "stridency" to outright breakupassociated with the highs. As Mr. C.G.

Fig. 12—The same 15 KHz tone burstwith the repetition rate set for 500 Hz.Note the limiting and distortion on theleading edge of each burst. This iscaused by the protection circuits confusion of the simultaneous low repetition rate and high burst frequency withan overload.

Fig. 14—Instantaneous single sweepphoto of an opening piano allegro.Note the flattening that is occurringnear the peaks as the amplifier overloads. The average power is about 38watts. The amplifier is a 120 W/chan.unit.

McProud put it, "It's when an operasinger hits her high C at the end of anaria."

A general review of all of these photographs graphically indicates the needfor improvement in current power amplifier performance. These shortcomingsserved to solidify the goals and objectives an amplifier design should reach.

A perfect amplifier should be powerful enough never to overload, even during low frequency passages and onmusical peaks. The protection circuitsshould never falsely trip and generatehigh distortion or amplifier misbehavior,yet they should safeguard the amplifierin the event of an accidental short circuit or from any other form of abuse.An amplifier must have a mechanism toprotect the loudspeaker from accidently

Fig. 13—Same conditions as in Fig.1 2, except that the protection circuitshave been disconnected.

Fig. 15—Same type of photo as inFig. 14, but the amplifier is a 350W/chan. unit. Note that the peaks arenot clipped.

dropped tonearms or amplifier failures,and all of these qualities must be builtin.

How powerful? This question hasbeen asked and answered more oftenand in more ways than is easily imagined. Simply stated, an amplifiershould be powerful enough to preventoverload and clipping when operatedat a satisfying listening level. Instantoverload recovery is not enough.

The best speaker systems today obtain their smooth, wide range low distortion performance by significantlysacrificing efficiency. That's simplephysics. The best are incredibly good,but they require large peak power andoutput voltage capability of an amplifier. Figure 14 shows a 120 watt amplifier reproducing the opening allegro

32 AUDIO • OUR 25TH YEAR • FEBRUARY 1972

piano note from Part III, of Beethoven'sEmperor concerto performed by Rudolf Serkin. The volume level hasbeen adjusted so the piano volume levelapproximates a live piano. The speakersystem is a modern unit which employs active equalization. Observe thatpiano note peaks are being clipped.This leads to harshness and may causelistening fatigue. Figure 15 is the samepassage but with a 350 watt amplifier.Note that clipping does not occur. Thesound is smooth, sweet, and open. Thesubjective volume level is identical inboth cases. The average power levelin both cases is approximately 38 watts.

If the goal is to eliminate the severeamplifier distortion that occurs onmusical peaks and during low frequencypassages, and if the best wide rangespeakers available today are utilized,a minimum of 200 watts/channel isrequired. A maximum of over 500watts/channel is required when usingsome of the very latest, highly inefficient speakers. The important pointto remember when dealing with thesehigh power levels is that the peak toaverage power ratio of musical material is approximately 10 : 1. Thismeans that when a 200 watt/channelamplifier is operating full tilt into aset of loudspeakers, the long time average power delivered to the loudspeakeris only 20 watts. In addition, theseloudspeakers are designed to safelysustain extremely high impulsive powerlevels. For example, the new AcousticResearch LST loudspeaker system cansafely sustain 1000 watts for brief timeperiods. It is this capability that willallow a high level drum beat, a lowfrequency pedal note, or an opera singerhitting her high C at the end of an ariato be safely accommodated by theloudspeaker system.

The first step in designing a 700 wattamplifier was to evaluate existing design approaches to high power. Primarily, the problem is one of obtainingthe required high output voltage. Threehundred fifty watts @ eight ohms, twochannels, requires an unregulated powersupply capability of over 200 volts. Untilvery recently, the very best existingtransistors had sustaining voltages ofonly 120 volts, and, at that, a designerwas pushing the state of the art to buildan amplifier with a 120 volt supply (150watts). The standard solution to highervoltages is to use low voltage transistorsand use a step-up transformer or auto-transformer at the output of the amplifier. The disadvantages of this approach are many. Excessive phaseshifts through the transformer generatestability problems, increase distortion,reduce the bandwidth, and transformers

are excessively heavy and expensive.We computed that an amplifier usingstep-up transformers or auto-transformers would weigh over 130 pounds!

A second design approach consistsof connecting two or several low poweramplifiers together in series to obtainthe required high output voltage. Amplifiers connected in this fashion aresaid to be "in bridge" and their separateoutput voltages add together or double.Since power increases as the square ofthe output voltage, doubling the voltagewould quadruple the power. For example, two 150 watt units in bridgewould yield four times 150 or 600 watts.

This design approach is a fairly workable one, but it too suffers severe andfundamental drawbacks. The input andthe output grounds are not common.Rather, they are "floating" above chassisground and any attempt to use suchan amplifier in a multiple unit installation would raise havoc with thegrounding system. Another drawbackis that a stereo amplifier would requirefour separate amplifiers connected internally to obtain two channels; thisplus the required double power supplywould even further add to the complexity, weight, and cost.

Solving the primary problem of transistor voltage breakdown required closework with a major transistor manufacturer. The basic power transistorused in the 700 watt amplifier design isa 600 volt television horizontal sweeptransistor. This basic power transistorwas modified extensively in order toobtain the best suitability for highpower amplifier application. Energybreakdown levels, current gain, pulsesafe operating area, and other transistor parameters were carefully adjusted inorder to optimize their use.

Another of the many problems associated with transistor amplifier designis the problem of crossover or "notch"distortion. Historically, this was a severe drawback in early transistor amplifiers. It was successfully solved byallowing the output transistors to operate in a mode which was somewhat lessefficient than ideal and represented oneof many engineering compromises. Inorder to eliminate crossover distortion,it was necessary to allow a small amountof idling current to flow at all times.This idling current would generate asmall amount of heat but was perfectlyacceptable for small, low voltage amplifiers that had at most two pair ofoutput transistors. For a large 700 wattamplifier, the amount of heat thatwould be generated by idling currentflowing in 24 output transistors wouldbe excessive. It was necessary to incorporate a novel biasing circuit that

would allow the output stages to operatewithout idling current (pure class B)and to simultaneously completely eliminate crossover distortion. This biasing circuit is used in integrated circuit "op-amps" but had previouslynever applied to power amplifiers. Thesuccess of this approach depends on thecareful attention to specific powertransistor parameters in the low currentregion. Crossover distortion appears ashigh intermodulation distortion at lowlevels. The best transistor amplifiershave attained IM figures of well below0.05%. Production 700's attain IM figures at 750 milliwatts of between 0.01%and 0.02%.

Speaker protection is accomplishedby a "crow bar" circuit in which heavyfault-current (for example, caused byaccidently dropping a tone arm, or anoutput transistor failure) is forced toflow through a pair of fuses rather thanthrough the loudspeaker. Since the "crowbar" forces heavy fault-current to flowthrough the fuses and not throughthe loudspeaker, they open immediatelyand prevent any possibility of damage.

The problem of amplifier misbehaviorcaused by false triggering of the protective circuits was solved by incorporating a totally new protection circuitdesign which monitors, from microsecond to microsecond, the energy* thatis being absorbed by the output transistors during normal operation. Allprevious protection circuits have monitored the current, or in the case oflarge amplifiers, the power. The energylimiting approach results in an amplifier that can provide approximatelythree times as much power as an amplifier equipped with current limitingor power limiting circuits.

Whenever a loudspeaker engineermakes an attempt to extend or smooththe frequency response of his design, orto lower the distortion, the laws ofphysics demand that the loudspeakerbecome ever less efficient.

These two facts of life, the conflictingrequirements of sonic perfection versus loudspeaker efficiency has alwaysset an upper practical limit on loudspeaker performance. The recent availability of truly high power amplifiershas allowed speaker designers a newfreedom, and without question, thebest speaker systems of tomorrow willbe capable of truly awesome perform a n c e . R .

'Energy is the time integral of power. Expressedmathematically. Energy, E= VI dt, whereV = Voltage, I = current, and with the limitsof integration chosen to be over one half cycleof the waveform.

34 AUDIO • OUR 25TH YEAR • FEBRUARY 1972