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www.hi-liworld.co.ukHI.FIWOBLD MARCH 201610

REVIEW

At boftom sit output transformers either side of a mains trans'former, att with an industrtal black finish' Above them passoverhead input tines (white)" At centre le{t and fight you cansee the KT1ffi power output valve pairs, with big white cou-pting capacitors iust above. At top sits the discrete transistorinput circuitry, and centre banks of power supply capacitors.

bias' because it is part of what theywant to achieve in their Particularhouse-sound.

I found bias setting a bit fiddlysince you have to fish around insidea hole on the front panel to findthe screw head of an adilster, thenrwiddle it with a plastic 'screwd river'style tool, the like of which I haven'tseen for a long time, since using an

identical object to adjust the ferritecores of RF and lF coils in mY home_built communications receivers,

Apart from this littleperformance, the Reference 75SEs otherwise demands little userinvolvement.The power switch is asturdy front-panel rocker style deviceand power connects at rear througha !hree-pron8, U.S. style connector,not a European-style IEC connector.

The meter lights can be switchedoff, but the switch for lhis is on therear panel,

Like most power amps, there is

no volume control, the preamplifiermust have this. Unusually, though,there are no unbalanced Phono-socket inputs either; the Reference75 SE accepts only a balanced inPltthrouth an XLR connector, so it mustbe driven by a preamp with balancedoutputs. Needless to say,AudioResearch preamps have such outputs,as do many other preamps thesedays, especially in exPensive hiSh-performance systems.\ryhy? Becausethey eliminate hum looPs, providenoise cancellation and also providescreening for both siSnal conductors,resulting in better sound quality.

The use of balanced inputs and,presumably, all-balanced circuitD/,eliminates the need for an internalphase-splitter valve, cutting out a

whole stage. Hence the amplifieruses just two KTl50 power valves,

fronted by rlgged 6H30 double-

still show average music level wellenough to tive useful warning ofoverioad. Not that that is likely with75 Watts per channel of power ontap and most people will, I susPect,wonder why the needles stay at sucha low level over on the left side ofthe scale, because little more than 5

Watts is needed in most homes andsystems to play at normal volumeand this level is indicated just 25% upthe scale. Centre scale is 20 Wattsand that is veD/ loLrd.

Another use for these metersis to show bias level, because bias

on these amp needs occasionaladjustment, possibly every fewmonths under heavY use, as thevalves slowly ate.

I haven't used KTl50 powervalves (tubes) for any time in a 'fixedbias' amplifier like this to know fromexperience how they age, but as theyare a new audiophile desiSn and verysturdy, with huge power dissiPation,it is likely they'll age slowly, makinSthis a very occasional process,The heater and anode of any valve

degrade as their surface coatings(e.g. barium) wear away, in much thesame way as light bulbs wear out,the reason power valves have a lifeof around 2000hrs or so. Bias has tobe adiusted to compensate, in orderto keep the righ! amount of currentpassing through the valve. So anoccasional twiddle is required.

a spectacular looking andarnplifier - one not tosounding

be rnissedFixed bias is used because it Sivesmore power than the popular'auto-bias' circuit arrangement that makesbias adjustment unnecessarY, butthere's also an argument about soundquality in here.lt's commonly claimedauto-bias sounds soft and fi>(ed bias

hard, to put it simply.And ld go alongwith that as a broad outline of thedifference. I suspect Audio Researchchoose the inconvenience of 'fi)Gd

triodes workint as driver valves.Like mostAldio Research amPlifiers,however, there are solid-state gainstages in front of the 6H30s, makingthis a hybrid design.The use ofsolid-state up-front lowers noiseand measurement conflrmed theReference 75 SE is very quiet.

The new-design KTl50 Powervalves this amplifier uses update andeffectively replace the KTl2os that

MABCH 2l)I6 11

REVIEW

went before them.The l20s were arelatively recent updare of the post-war British GEC KT88 and a rathercrude one made by a factory in EastEurope.Whilst the KT88 soundedSreat - fast and clean, rather thanlush and warm - the KTl20 generallysounded clanky, to the point whereI chose not to review KTl20 amps.They despoiled the breed.

Happi|, someone must have feltthe same and decided better waspossible, hence the new KT-150.This is not a cheap clanker like theKTl20.With a distinctive gherkin-like glass envelope and strongnon-microphonic internal elecrrodestructure, plus large hith-powerdissipation anodes, the KTl50 js asophisticated build and was sellingat f90 apiece (a KT88 is {40), butin the amplifiers I have reviewed todate it offers a smooth and sophis-ticated sound that justifies this price.

Audio Research were wedded tousint the American 6550, a ruggedtetrode much like the GEC KT88,but they've obviously been convincedby the charms of the KTl50 so nowit has popped up in their expensiveReference amplifiers. I'll iusr makethe point here that unreliable valvesare a persistent background problemin valve amp manufacruring - andone best avoided. ln World Audio

A 6Hn driver double triode, wellknown lor ruggedness comhinedwith good sound. lt has anti-micro-phony damping ngs on here.

Desitn, we once had an entire batchof KT88s from East Europe thatfailed prematurely, causing a waveof complaints and relurns.To avoidsuch a thing, so preserving their

MARCH 2016

reputation and thecost of dealing withsuch a problem, js

almost surely whyAudio Research stuckwith the un-lovely butworkmanlike 6550in the past. lt alsosuttests that theyare happy with theKTl50 in rhis respect.

So that's just alittle aside to explainwhat you are gettinghere and why.Valvesarent transistoTs.Each one has its ownhistory and particularattributes, as wellas its own sound.So the valves AudioResearch use in theirnew Reference 75SE say much aboutlhe companyt designintentions in thisproduct, as well as

what bq/ers canexPect from it.

And finally,those unlovely blacktransformers youcan see in shots ofthe Reference 75 SE

internals.Audio Research manageSrea! results from relarively smalloutput transformers, the smallerblack blocks either side of the lartercentral mains transformen Quilehow they do this I do not knowbu! you can use feedback, high-quality core material and terriarywjndints to get "small stacks"whilst maintaining hith performance- and this they do.The result is fullpower and low distortion at bassfrequencies, an area where valveamplifiers traditionally struttle.Theuse of feedback also provides usefulloudspeaker damping to lessensoggy bass, although all this doesdepend on the loudspeaker used andits own level of acoustic dampingI find. Rear-mounted, gold-plaredloudspeaker terminals for I Ohmand 4 Ohm loudspeakers are fittedand they accept bare wires, spadepluts or 4mm banana pl!gs.

sout{D QuAurYThe Audio Research came run inbut we put 40 hours on it in anycase, and tave it a 30 minute warmup before listenint, as with allvalve amps.They were used withQuadral's superb Chromium Sryle Iloudspeakers featured in this issue,

KTlfi power output valves have very largeanodes (gtey) to dissipate heat, allowing them tohandle high powen

and Tannoy Westminster RoyalGR loudspeakers, through BlackRhodium cables.The source wasan Oppo BDP- l05D Blu-ray wirhits ESS Sabre32 DAC and inrernalvolume control, connected viabalanced cables. I have stopped usingCD altogether now, becomint tooaware of distortion, instead usinghigh-resolution files played by anAstell&Kern AK 120 hooked up tothe BDP-105D through an opticaldiSital cable. Even CD rips soundbefter than the original disc whenPlayed b), this system, likely dueto re-clocking in the transcriptjonProcess.

Classic Rock in the form ofFleetwood Macl 'Go Your OwnWay' (24195) teased out essentialelements of the Reference 75 and,in particular, its ability to make thetimbral qualities of instruments moreobvious than that of most amplifiers:Mick Fleetwood's kick-drumsuddenly had a skin being beatenby a pedal, instead of existing as anamorphous backtround thud, as I

so commonly hear it.The ReferenceSE iniected a sense of character toinstruments, removing the bleachedqualit/ common to so many oftodayt transistor amplifiers. lt! afast amplifier too, catchint the speed

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BEVIEW

The rear panel carries balanced XLRinputs at top, but not unbalancedphono socket inputs, so onlY Pre'ampliliers with balanced outqutscan be used - but most have themthese days- /Vote the big power inputsocket thrb is not an IEC design, buta unique U.S, socket. Gold Platedloudspeaker terminal posts acceptspades, bare cable or 4mm Plugs'Powet matching is to I Ohm and 4Ohm loudspeakers,

of Fleetwood and his inflections.Similariy, the slow kettle drum strikesin 'Jupiter', from Holstt 'Planets',had delicious dynamic Presence andhorns called out with a shiny rasP tothem.

The Reference 75 SE was airYand spacious in its portrayal of thesoundstage, a property supportedwell by the ribbon tweeter ofQuadral's loudspeakers, makint clearthe posilion of instrlments withinthe London Symphony Orchestra,playinS lupiter'.

Audio Research don't do 'softand warm', as I mentioned earlier,

and this power amplifier has a lightopen quality about iq even the'dark'recordings of Diana Krall seemedto have a window opened ontothem,letting light into the shadowsBass was relatively stront throughour giant Vy'estminsters but milderthrouth the Quadrals, althouSh stillfast and punchr - Queen's 'RadioGaGa' (241aa.2) opening synthdrum sequence lightinS up ourlistening room with a sense of tithtlycontrolled power, sharPly timed as

you'd hope from a machine. FreddieMercury hung above the QuadralChromium Style 8s caught in a crisP,

clear lighc the Reference 75 SE isn'tbright but it has a sparkling clarity.

c0Ncrusr0llThe new Audio Research 75 SE is

a spectacular looking and soundingamplifier - one not to be missed.With big illuminated power metersand gorgeously finished casework itis a statement in the home-

DeliverinS fabulous sound qualityfrom a new-design KT 150 valves - ortubes should I say in this case - it'sa power amplifier that demonstratesjust how good it Sets when a tubeamplifier is so well honed.

MEASURED PERI(IRMANCTlhe fleference 75 delivered 78 Wattsinto 0 ohms and 72 Watts into 4 ohmstrom its 4 ohm tap {l% thd), so itslatod output of 75 Watts is accurate, ifnot conservative. h uses Kn50 valves

in push-pull runninq with fixed bias toachieve this.

oistortion was low across the audio

band. as valve amps go that is. At I kHz

distortion hovered around 0.1% below 5

Watts outpul rising progressively to 0.4%

{-ldB below tull rated output), comprisinglow order harmonics - mainly second and

third.ln spite of compact output

transformers the Reference 75 produced

little bass distortion, much like other

Audio Research anps, but unlike many

other valv€ amps where core maqnetic

hysteresis produces third harmonic and

eventually maqnetic satuntion thatlimits bass power ln contrast to this the

Reference 75 delivered full bass power

with ease and, at low powers, very littledistortion.

As power rose, so did distortion,reaching 1% atfull power - still a

reasonable figure as valve amps go. Since

at 5 Watts and below bass distortionmeasured less than 0.2% the amp willsound clean in practice. High frequency

distortion was under good control also,

again measuring 0.2% or less below5 Watts, and 1% at full Power. mostlysecond hamonic - see our analysis at IWatt output I 0kH2.

Frequency response rolled downslowly above 20kHz to a -1dB point at35kHz, from both 8 ohm and 4 ohmoutputs, into I and 4 ohm loads respec-

tively of course.An unusual discrete, balanced

tmnsistor input staqe ensures low noise

of -105d8; there is no unbalanced phono

input and therefore no phase splitterstage in this amplifier - it is all-balanced.

The Audio Research Reference 75power amplifier measured well in all

areas, in keepino with the company's

standards. NK

Distortion 0.!%Sensitivity 1.4VDampiq lactor 4

FFEOUE'VCY RESPOA'SE

DISTORTION

AUDIO RESEARCHREFERENCE 75 SEf8698eooec0UTSTANDING - amongstthe bcst.

VERDICTA big power amplifier withmassive sound, yet cleanand fast.

F08- appearcnce- standard ot Jinish- sound

AGAINST- very big- no unbalanced inputs

Absolote Sounds+44 (0)20 8S71 3909l@w absolutesounds,com

t

PowelFrequoncy responseSeparaiionNoise

75 Watts41lz- 30kHz

88dB"105d8

MARCH 2OI6 13