TEST BENCH

LINN ISOBARIK DMSThis unusual and cos,tly loudspeaker

has becorne something of a bywordamong audiophiles in recent years,

partly because of its association with thefine Linn disc-playing system and partlybecause of ita rarity; only 400 or so pairshave been produced to date. Few reviewshave appeared in the press, as both thehigh cost and limited availability of theDMS has precluded their inclusion inlarge group tests. It must, however, besaid in Linn's defence that after a heavyspeaker such as this has undergone laband listening sessions, it has usuallysustained some veneer damage, and this ishardly something a supplier wouldrelish with speakers that cost over £I ,000per pair.

The DMSreviewed here is of recentproduction, and does differ from earliermodels in some respects. A number ofsmall improvements have been made,noteably to the driver output integration,colouration levels and frequency balance.

A 'PMS' version is also produced, the'P' intended to indicate professionalrather than domestic ('D') use, andindicating an active model where auxiliaryelectronic crossovers are employed, to-gether with multiple cable connectionsand three stereo power amplifiers (norm-ally Nairn units, but more recently alsoincluding models from Meridian).

Depending on the final choice of am-plifier, the working PMSsystem will costbetween £2,000 and £4,000 per pair and,by comparison with the standard DMSversion, it offers improvements in maxi-mum dynamic range, as well as superiordriver and crossover control, therebyoffering gains as regards clarity andreduced distortion.

No instructions are provided with theDMS, as the manufacturer intends thatthe speaker should be offered for hometrial by a specialist dealer before thecustomer makes his final decision. Wetook care to consult dealers and examinedthe leaflet provided by Linn. This statesthat the DMSis designed to [unction wellin a domestic environment and goes on tosuggest that the enclosure should bepositioned close to the rear wall some60-90cm from the nearest comer, with a'6-foot' distance recommended from thetop of the cabinet to the ceiling. Ideallythe front panel HF unit should be at earlevel (which requires the use of rigidstands some 30cm high, elevation fromthe floor being specifically recommended)while inwards angling of the enclosuresof up to 20°is also considered permissible.

Accordingly, on test these speakerswere tried in two rooms, one beingfairly small and a little on the hard sidein reverberation terms, the speakersset away from the corners and some25 em from the wall. In the main listeningroom, they were tried both against thewall and spaced well clear by O·8m.In allcases rigid 33cm-high stands were used.

In its usual guise the DMS measures38 x 38x 76cm, excluding 20mm of extragrille protrusion, but looks slightly smaller

an it really is. However, its massiveternal construction results in a weightf around 40kg-once positioned you

are unlikely to move it very often!A finely barred veneer is used for the

front upper section of the fascia, whilebelow the Declon foam grille continueswith a vertical bar pattern. More driveunits face upwards from the top surface ofthe speaker, and a second Declon grilleis used to mask them.

SPECIFICATIONLinn say very little about the technicalperformance of these speakers; forexample, the important impedance ratingis omitted, as is sensitivity, distortion andfrequency response. It is true that on thelatter subject Linn do state that in thedomestic environment the DMSis "broadlycapable of reproducing sound from 20Hzto 20K(sic) with a few dB variations", butanything more vague than this would bedifficultto imagine. The leaflet goes on tosay that the speaker will produce signalsdown to and below 10Hzwhile the recom-mended amplifier power rating rangesfrom SOWminimum to a 500W maximumper channel. High but unspecified soundlevels are claimed with "very low dis-tortion levels, particularly at lower fre-quencies than other loudspeakers canacceptably produce"~.

TECHNICAL DETAILSWhile six drivers are used for eachsystem, the DMSis essentially a three-waymodel with a quite straightforward cross-over operating at approximately 375 and3,oooHz. There are twin bass, mid and

treble drivers, which are paired elec-trically, with one pair of mid and trebledrivers disposed on the top surface, thetreble being nearest to the rear of theenclosure.

The front panel shows a conventionalvertical in-line format of the bass, mid andtreble units, with the sixth (the secondbass unit) hidden inside the enclosure.The latter configuration has been patentedby Linn. It's called an 'Isobarik' bassdriver, and is claimed to give low distor-tion levels, more typical of the mid band.The patent describes the configurationessentially as a pair of closely-coupled(via a small and hence high-stiffness airchamber) bass units, one on the front ofthe enclosure and the other inside. Theyoperate in tandem, electrically in parallelwith the infinite baffle (sealed box) en-closure volume loading the rear of theinside driver. In simple terms, the ap-proximate result at low frequencies is abass unit of twice the moving mass andsuspension stiffness, plus half' the im-pedance of one driver alone. The voltagesensitivity remains unchanged while thedoubled mass results in a reduction of thecut-off frequency of the sealed box en-closure of approximately 2. But in sodoing a penalty is incurred, namely thattwice the power is required to drive thesystem.

By incorporating additional verticallydirected drivers, Linn's intention must beto improve the energy dispersion off-axisin the mid and treble where most speakersare weaker while, by continuing thevertical axis line, it is hoped that the stereoimaging will not be impaired. Inevitably,however, the polar response in thevertical axis will suffer, due to the addi-tional parallel radiating elements.

For the bass, the well-known KEFB139unit is employed, with a diecast frame andpolystyrene wedge diaphragm/cone. Themid unit is also from KEFin the form of thewell-established B110 - a lOOmmBex-trene cone driver with a neoprene rubberroll surround plus a light doping layerapplied by Linn. All the units are, in fact,of high power capability, the treble drivercoming from Scan Speak in Denmark anddesignated the D2008. It comprises ahighly sophisticated 19mm soft dome,loaded by a shallow horn phase corrector.

The internal volume of the enclosureapproximates to 55 litres, the extremelyrigid enclosure being constructed of19mm veneered high density chipboard,mildly damped by an internal bondedlayer of 12mmfibre board. The panels areextensively braced both at the seams andacross weaker areas to add stiffness nearthe driver apertures. In addition, fourtensioned 4mm steel bolts clamp lOp andbottom panels, with four more used frontto back. The front and side walls are alsobraced by internal panels, while the topsection almost forms a sub enclosure-afeature which may help to break-updominant standing wave modes.

The rear is theoretically removeable viacapped screws, but doing this damagesthe glue seals and would invalidate thecustomer's guarantee.

The drivers are neatly recessed and!>

Hi Fi fOr Pleasure 61

overall the standard of workmanship isgood. The mid-drivers are back-loadedvia a capped cylindrical tube some 137mmin diameter and 250mm deep, these filedwith acoustic absorptive material, whilethe mainenclosure is also filledwithabsor-bent - and generous - quantities of fineblack sheep's wool.

The design indicates a potential im-pedance rating of 4ohms, particularly inview of the paralleled driver lineup.Input connection is via 3-pole XLRcon-nectors; the large plug depth precludesreally close wall positioning. In addition, aconcealed bolt in the front midrange boxmust be removed.

LAB PERFORMANCEThe dual plane radiation axes of this modelrequired careful interpretation of theanechoic test results in order to extractuseful information.

At 2m, measured on the frontal highfrequency axis, the response showedsome interesting features (A). The 1mreferenced sensitivity was quite averageat 87dB/W - for example, higher than theRI05 - but the unusually low impedancewill in practice compromise the sensitivitydue 10 protection limiting with many

amplifiers. The low frequencies were wellextended, with the nominal -6dB pointat 30Hz. It was not possible to applylimiteto Ihe response owing to the lack ofnOrmally reflected output from the midand treble drivers although, if the toppanel output were to be reflected in full,the output at best could only improve by3dB and the curve suggests that therewould still be a shallow 2dBtrough in theupper mid range. The high frequencieswere quite good and well extended to20kHz, beyond which there was a sharpcut-off(the graph extends to 25kHz).

With 1/3-octaveaveraging (B), a betteridea of the subjective frequency balanceWiUI obtained, free of the interference andfine diffraction effects of the sinewaveresponse in (A). The main axial responsecontinued to show some prominence in the350Hz-1kHzrange, with some loss abovethis point, but the addition of an O·8m-square hard reflector position behind thespeaker to redirect the upper radiation into the forward axis produced surprisinglylittle effect (++++ line). Changes ofthe order of 2dBwere present from 200Hzto 3kHz,a region where such a reflectingplane could be expected to exert aninfluence, but the response trend did not

appear to be much improved.Response uniformity in the vertical

plane was poor, as evidenced by the 150above-axis trace (dotted), where grosschanges occurred up to IOdB, and thedriver integration between mid and treblewas also poor, this shown by the irregulari-ties between 3 and 6kHz.This result indi-cates that the frequency responce balancewill vary somewhat with listener ear-height. Above 6kHz,however, the off-axisresponse was very good, just l·5dB downat 15kHz.In the lateral or horizontal plane-the axis of symmetry for this system-the uniformity was much better at the300 measuring angle, and the output waswell maintained at just 2·5dB down at15kHz.

In view of the manufacturer's claim oflow distortion and wide dynamic range,the second and third harmonic distortionresults(C) were disappOintingat 96dBSPL,1m (equivalent to a 9 walts input levelreferred to 8 ohms). Careful checking ofthe drive amplifier output assured us thaithe graphs were representative of thespeaker and not the test equipment.The important third harmonic wastypically 8% over much.of the mid rangeand upper bass, this characteristic often

62 Hi Fi for Pleasure

related to saturation of the crossoverinductor cores. At 35Hzthe third harmonicmeasured 15%, the results for the secondharmonic being broadly similar, priorexperience with the KEF units involvedhaving shown that lower distortion is poss-ible at this lower sound level. The lowerpair of distortion curves were taken withsound level reduced by four times to the9OdB,and revealed that the third harmonicwas better than 0·5%, 35Hz,and typically0,2% or less elsewhere, with secondharmonic at a maximum of 1·3%, 75Hz.These results demonstrated that theIsobarik distortion was quite normal atlower levels and thus toa good standard.

On the impedance graph (D), a normalsealed box system resonance at 31Hzwas present with a cruelly low character-istic at higher frequencies, with dips to3ohms, 100Hz, and 2'3ohms, 4kHz. Theaverage value was barely higher than 4,with the 'nominal' value very low at 3ohm.These results show that the speakerrepresents a very difficult load to driveand will also be critical of the impedanceof both the accompanying connectingcable and amplifier output: it was fortunatethat the amp used for the listening testswas, in fact, capable of driving this load

quite happily (a Sansui AU919 II), andthat low impedance speaker cable wasused. Hardly suprising, in view of allthis, that the Nairn amplifiers with theircapability to drive near impossible loadshave emerged as one or the few devicescompatible with the OMS!

SOUND QUALITYIt quickly became apparent that the DMSpossessed an individual balance andcharacter that differed from our acceptedreferences, attracting greater diversityof panel opinion than did any of the othersystems in the report, while also demon-strating a greater variability and inter-action with different programme sections.However, while the scores were belowaverage for the group, this speakercertainly possesses some virtues. Forexample, it did not sound as dull as thesimple axial response measurementsmight have suggested, the extra contrib-ution of the upper drivers in a particularlistening environment undoubtedly beinga factor here.

Significantly, the higher level resultswere the least favoured-a fact whichaccorded with the lab-measured dis-tortion. With regards to its programme

interaction, the effects on spoken anasolo singing voice, as well as piano andsmall string orchestra, were preferredto those on larger orchestral, churchorgan and massed choir.

The bass and treble registers were wellpresented, the former with good depthand clarity, and the latter with good detailand smoothness, although the lowertreble did appear forward, this lendinga sibilant, almost metallic, quality. Themidrange sounded uneven and, onoccasion, rather sharp, with the pianoreproduction 'chilled' with a ratherclinical effect overall. Colouration didnot appear serious, being mainly of the'tube' and 'sharp' type. ,Stereo imagingshowed notable differences by com-parison with more conventional systems.Classical crossed-mic recordings weredistorted in perspective, lending a vagueover-wide effect with diffuse centralpositioning, although with a pleasingincrease in height. Muiti-mikedand poprecordings were given an enhancedambience with interesting spatial effectsand an impression of detail; in particular,it seemed as if the DMScompensated forthe excessive 'weight' and 'closeness'ofmany rock recordings. !>

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CONCLUSIONSIconclusions toiethiso reportariif~ one

system had sounded Significantlypoorer than the others, as this wouldenable me to draw some comparisons andcontrasts; in the event, however, this didnot prove to be the case.

The least favourably received were theIsobarik and the SEAS DD Tower. Thelatter did not provide the anticipatedstandard as regards bass quality anddynamic range, this partly due to thepresence of some colouration and re-sponse imbalance which prevented theenjoyment of high sound levels. Stereodepth and precision were also a trifleweak considering the price level, but inits favour were the easy-to-drive imped-ance as well as the unusual styling. Itsparticular qualitymay suit some systems.

The Isobarik DMS was a differentmatter and required careful qualification.At the outset, I can only repeat that thenominal 30hms loading impedance iscertainly punishing, and accordingly thematching amplifier must be chosen withgreat care. The 'different' character ofthe DMS, together with its unusual dis-

persion and spatial properties plus'forward' balance, necessitate skilledmatching to the rest of the purchaser'shi-fisystem, and Linn's own recommend-ation of a prior demonstration at home ismostpertinent in thiscontext.

Despite the brochure's claim, the DMSdynamic range and distortion levels werenot particularly good, and I can under-stand why those who do achieve satisfyingresults from this system are so enthusi-astic about its brother, the more costlyPMSor active version. Eliminationof thepassive crossover must solve the imped-ance and dynamic range problems, andoffer a much reduced distortion level,particularly in the mid band; however,strictly speaking the PMSis not directlycomparable with the DMS.The latter isworthy of auditionbut will require carefulconsideration of its pros and cons, not theleast ofwhichis its high price.

Overall, the RI0S II comes out ascertainly the best system as yet tried in myessentially neutral listening room. Itsparticular strengths are accurate stereopresentation, with at times an uncannydepth and ambience, plus the absence of

any obvious subjective faults or exag-gerations. Clearly its character is basedon balance and compromise, while theengineering achievement is also com-mensurately high and justifiesthe price.

The L1S0surprised us all. The obviousJBLsound ofthe past has been supersededand the result was a creditably balancedsystem with good stereo and excellentrendition of detail. In this respect at leastit is not unlike the broadly comparableYamaha NSI000M. Like the Yamaha, theLIS0 is also capable of ar, impressivedynamic range, remaining in full contra]at high volume levels and excelling itselfin the bass, where the depth, power andprecision of its performance were onlytoo apparent on test. It proved to be alittle critical of vertical listener position,and showed a mild colouration as welI asa 'sharpness' whichdid notparticularlysuitstring tone, but the design is nonethelesswellworthauditioningespecially for use inlarger rooms. On rock programme it cansound pretty devastating, while the classi-cal performance is also quite satisfactory.

theenear_uItrasonictrseblepeakthough!Hi Fi for Pleasure 67

ISOBARIK'S DISTORTION REPRIEVEDIn April HiFi for Pleasure we publishedexhaustive tests on four emment loud-speaker systems - the KEF RI0S MkII,JBLL150,LinnIsobarik DMSand SEASDDTower. Regrettably, the Isobarik distor-tionresults have since been proven wrong.All four speakers underwent lests at theacoustics laboratory of GEe, Hirst Re-search, in Wembley, London, using theircalibrated laboratory amplifier. Duringthese anechoic tests, the Isobarik OMSyielded disappointing distortion results at96dB at 1m, although figures were fineat 9OdB.

Preliminary investigation indicated thatHirst's amplifier was not to blame for thisdiscrepancy, but subsequent informationtends to refute this. Recent discussionswith Linn revealed that, while the overallresults were not in general in dispute,those for distortion were consideredunrepresentative. Accordingly Linn sup-plied us with a curve of a DMStaken innon-anechoic conditions which showedthe maximum third order harmonic to bearound 2.8% 30Hz, with typically betterthan 1% above 50Hz.The second harmonicpeaked at around 5%, 70Hz,being typically3% below 70Hz, then falling to a 1-1.5%level over the remaining frequency range.

These results are, in fact, similar incharacter to those printed in our reviewof the DMSas regards the 90dB reading,though naturally with some scale correc-tion for the different power levels.

Raising our original 90dB SPL result by10dB, for example, thereby increasing1% to the 3% level, in fact provides closeagreement with the Linn data.

Linn suggested that the drive amplifierwas responsible for the problem, andstated that, in their experience, the Nairn250 was one of the few models found notto exhibit interface problems with the DMS.

In order to rapidly check the source ofthe problem, if one existed, a preliminarycheck of the distortion performance ofour review samples was made, using adifferent amplifier at the full 96dB/metrelevel in a non-anechoic but well dampedenvironment. Using an Ivie 30A real-timespectrum analyser it was possible for usto establish that the Linn data were in factmore typical of the DMS than our ownoriginal readings, indicating that ourpublished results at 96dB were incorrect.

For example, the worst-case result for2nd harmonic at 80Hzgave a figure of 4%with the 3rd order about 0.4%, and in themid band (700-800Hz),where we originallyprinted figures of 3% 2nd and 8% thirdharmonic, the new readings were 1.3%and 0.3% respectively; again quitesimilar to the curve supplied by Linn.Finally at 5kHz where 10% or so wasoriginally recorded the data now gave0.8% second and 0.25% third harmonic -much better results than before.

The load matching problem of the DMSwould appear to have resulted in a

faulty performance when driven by thelab amplifier at the higher 96dB soundlevel but, in all other respects, the graphstaken under less demanding impedancesconformed to Linn's specification.

To conclude, the comments concerningthe DMS'distortion printed in the originalreview last month clearly need modifica-tion. From the new data the Isobarik canbe seen to offer respectably low levels ofdistortion throughout the range, particu-larly as regards the more subjectivelyimportant third harmonic. This is inkeeping with the results for other highquality models in the DMSsize and pricerange. Assuming that the Isobarik was notat its distortion limits at 96dB,it could thusbe driven to produce a wide dynamicrange provided that the amplifier chosencould sustain the loading. (As a matter ofinterest, to produce a 105dBAsound levelat 1 metre with the DMS,peak currents ofup to 16amps are required, and thus verylow resistance cables are also animportant consideration.) Me

* * *Some news has also been received fromJBLconcerning the L150speaker coveredin the same review. JBLhave stated thatthe minor peak in the treble at 19kHzwillsoon be under control, and furthermore,the crossover has been improved by theaddition of low loss high-frequency

capacitortsctooallotheencriiticalunit.

Hi Fi for Pleasure 49