BUILDING OUR NEW AM/FM PORTABLE RADIO PROJECT

NEWSLINKEAD 4.95BUILDING OUR NEWAM/FM PORTABLERADIO PROJECT .i I9 312966 123633., 144411,ettredEllop kgt.PIKIA,

Video & Audio: The Challis ReportARTEMICS AS6OM STEREOVALVE POWER AMPLIFIERHow good were the best valve or 'tube' stereo amplifiers of the 1960's, before they were replaced by solidstate designs? And how do today's 'new generation' valve amplifiers compare with the latest solid stateamplifiers? This month Louis Challis tries to answer these and related philosophical questions, whilesimultaneously reviewing the impressive Artemics AS6OM amplifier.One of the deep questions which eachof us has asked ourselves during ourlifetime, and on which many religions areso firmly based, is 'whether there is a lifeafter death'. If we set aside the immortalityof our own lives, which most of us do atsome stage, then one of the most outstanding, if not downright peculiarrebirths — after being pronounced deadby nearly all the relevant commentators- is that of valve amplifiers, whichseemed to have been dealt a death blowby the development of outstanding transistor amplifiers.Thirty years ago, transistor amplifierswere all the rage and valve amplifiers hadbecome passe. Anybody who wasanybody was discarding their large, hotand fragile valve amplifiers for small,cool, and — generally far more expensive— transistor amplifiers. Mullard andPhilips had released their OC16's, whichwere then 'state of the art' power transistors, and I and many others used them todesign and construct relatively simplepush-pull amplifier output stages, whichcould easily produce 20 watts of relativelycool output.Whilst the first and second generationpower transistors proved to be well suitedfor loudhailers and other mundane undemanding PA applications, they had insidious characteristics which renderedthem far less appropriate when attemptswere made to apply them to high-qualityapplications. Those particular applications were only then being qualified bythe four-letter word 'hifi' — whereas incontrast, the transistor amplifiers whichwere offered for similar applications wereall too often labelled by far more pejorative four-letter words!It took another 10 years, to a periodsomewhere between 1973 and 1975,before the majority of serious reviewersand equally serious consumers wereprepared to accept transistor poweramplifiers on an equal footing with the8ELECTRONICS Australia, November 1993best valve amplifiers, in the 'highfidelity stakes'. During that 10-yearperiod from 1963 to 1973 a prodigiousamount of research and development wasapplied to the development of low distortion, low noise transistor poweramplifiers. The objective was to obviatethe very clear problems which plaguedthe first, second and even third generationof these amplifiers.Valve amp featuresNow the best valve power amplifiershad a number of significant attributes, themost important of which was mostprobably their adoption of 'Class A' circuitry. This ensured smooth, seamlesscrossover characteristics in push-pulloutput circuits, well controlled andreasonably low third-order harmonic distortion, and a 'mellow' sound whicheverybody loved, and which evokedwidespread acclaim.That doesn't mean to say that allvalve amplifiers were good. Rather, thatthe best amplifiers were exceptionallygood, whilst the others were generallyreasonably good.In the decade between 1955 and 1965,high fidelity valve amplifier circuit designhad made great strides, and the best ofthose amplifiers (like the ultra-linearamplifier designs developed byElectronics Australia, the high-poweredKT66 amplifiers developed by AWA, andthe Murray amplifiers developed by theElectrical Engineering Department at theUniversity of Sydney), were all outstanding even when assessed by the higherstandards that prevail today. The amplifierdesign techniques developed during thatdecade put the best valve amplifiers at theThe rear of the amplifier provides terminals for speakers of different loadimpedances, as you can see, plus the usual RCA input connectors and an IECmains input connector with switch (far right).

very top of the class, so that the early transistor amplifiers faced what then seemedto be an almost insurmountable hurdle inbridging the gap.But given sufficient time, when therelevant research is lubricated by appropriate funding, it is wonderful whatcan be achieved. In the following 20 yearsthat gap was bridged, so that today thefinest transistor amplifiers are every bit asgood as (some say even better than) thebest valve amplifiers of 1965.By 1990, there were relatively fewvalve manufacturers left in the world,as relatively few people were eitherdesigning, let alone manufacturing valveoperated equipment. If you had askedme in 1990, what the prospects werefor somebody (anybody for that matter)marketing valve amplifiers in opencompetition to transistorised amplifiers,my response would have been courteous but brief.If you had asked me the same questionin 1993, the answer would still have beenequally courteous, but it would no longerbe as brief. I am now aware that for somealmost inexplicable reason, valveamplifiers are suddenly the flavour of themonth. But explaining why valveamplifiers have risen 'like the proverbialPhoenix from the grave', and have re-asserted a position in the marketplace, becomes a trifle more difficult for me toexplain in objective and sensible terms.As a child I was entranced by the softred glow of the valves installed in myfamily's radio in the living room. The feelings of wonderment and novelty that2kHz Tone Burst0.5rnsfdivisionthose valves imbued in most children(who became many of today's adults)were real, and perhaps still are the 'stufffrom which legends are made'.Transistors are relatively cold andmetallic, and without any grace-savingvisual attributes. Valves are warm, illuminated even when not working, andthus have the ability to excite both ourauditory and visual senses. With that as astarting point, and when appropriatelymarketed on the basis of their 'newness',their 'difference' and their claimed 'superior characteristics' when compared withtransistors, we have the makings here of asuperior marketing program — one thatno transistor amplifier can ever hope tomatch in quite the same way.There are of course some relativelyprestigious people who have jumped ontothe valve bandwagon. In the developmental league, people like Bob Carver arenow producing CD players with valves inthem (what next!).And as Selwyn Sayers of EA itself hasjoyfully pointed out to me, Edge of'U2', Slash of 'Guns & Roses', EddieVan Halen, Jimmy Barnes, John Entwhistleof 'Who' and Brian May of 'Queen' allapparently use valve amplifiers inpreference to transistor amplifiers —either for their own pleasure, or in somecases for live performances.With that sort of recommendation,who am I to argue with the conceptor the underlying philosophy of valvesELECTRONICS Australia, November 19939

THE CHALLIS REPORTversus transistors (I'll have more to sayabout that later).By now you must be starting to questionmy sanity (that is unless you are alreadysold on the concept of valve amplifiersand have traded in that ubiquitous 100 or200 watt transistor amplifier for one ofthese new 'you beaut' valve amplifierswhich are now turning up in increasingnumbers at some of the better h if iretailers! Well, let's try to put the wholeissue into some sort of perspective.The Artemics ampAs it happens, EA's editor was recentlyoffered for review an Artemics AS6OMStereo Tube Power Amplifier, which usesfour KT88 pentodes in the output stages(two in each), and which is undoubtedlymore attractively constructed than anyvalve amplifier that I have seen in recentyears (let alone 30 years ago). Jim and Iboth thought this high-quality amp wouldmake a good representative of the 'reborn'valve amps, both to evaluate their performance and to compare them with moderntransistor technology.The Artemics AS6OM uses specialoxygen-free copper (OFC) custom woundoutput transformers, 1 /0 precision MILstandard resistors and computer qualitycapacitors, for both the power supplyfiltering as well as for the activeelectronic circuitry.I carefully read the brochure provided,did a double-take at the way some of thewords were spelt, and then realised thatthe Artemics AS6OM amplifier is manufactured in China. I suspect that a largeproportion of the intending purchasers(quite apart from the manufacturer's personnel), may have not previously seen avalve amplifier, and would consequentlyregard this amplifier as being 'state of theart' esoteric equipment.In their day, the KT88's were a mightypotent valve, and when appropriatelyconfigured, could provide power outputsof 60 - 70 watts. In those days this wasregarded as BIG POWER.Today many people are blase aboutpower amplifiers which produce less than200 watts continuous output and I notethat the very same Bob Carver who hasjoined the valve band wagon, has justdeveloped a new line of transistorisedamplifiers, which are capable of delivering 800 watts into a four-ohm load. Poweris obviously important; but potentially asmidgin less important than most peopleare prepared to accept, and especiallywhere quality sound 'rules supreme'.The Artemics AS6OM is relatively neat,using a strong and beautifully constructedchassis, with three large and neatly boxedtransformers along the back of the chassis.The internals of these transformers arepermanently hidden from view, but theyadd so much weight that the amplifier'scentre of gravity is disturbingly shiftedtowards the back.In front of the two output transformersand central power transformer are the fourbulbous KT88s, directly in front of whichare pairs of 6DJ8 and 12AU7 driver valves— which I must admit I hadn't seenaround for more than 20 years. On theneat front panel of the amplifier is a littlegreen LED, whilst at the back are thepower 'on/off' switch and multiple springloaded terminals providing the ability tomatch load impedances of 16, eight andfour ohms respectively.The valves are provided with a supplementary perforated cover whichprovides very essential physical protection, particularly when moving theamplifier. As I confirmed, its presence ensures that you don't destroy your brandnew toy by dropping discs, records, orchildren's toys on top of those fragile glassenvelopes. (Nowadays, valves like theKT88 are surprisingly expensive!)Objective testingThe objective testing of the AS6OMamplifier proved to be far morestraightforward than I would have expected. As I soon discovered, the frequency response was far better than I wouldhave expected in terms of my prior experience (which stems back more than 30years, to the days when valves were 'Kingof the Castle').The AS6OM has an extremely smoothresponse over the critical frequency rangeof 25Hz-20kHz, with an overall deviationof generally far less than 0.1 dB, whilst itsoverall frequency response is /-1 dB from5.6Hz to 80kHz. I have to admit that I wasimpressed by that performance, as themajority of valve amplifiers I have testedin the past were hard pressed to achieve a30kHz bandwidth.I next progressed to assessing theamplifier's power output, which the glossybrochure claimed as being 70 watts perchannel into an eight-ohm load. In contrast, my measurements revealed that theamplifier's output at the onset of saturation (as determined by comparing the output directly against the input using the X-Ydisplay of the cathode ray oscilloscope, todetect the onset of clipping) was 41.4watts per channel.That power output figure left me a trifledisappointed, as I had visions of the earlyJapanese radios (circa 1960) for which themanufacturers measured and gliblyquoted the square-wave output into a10ELECTRONICS Australia, November 1993