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THE WAY I SEE IT By NEVILLE WILLIAMS Audio power ratings - right back to where we started from Having agreed some years ago that music power was a highly suspect rating, the world hifi industry settled on RMS watts as the true measure of amplifier power output. But since then, music power has gained totally new respectability and even been legitimised by the term "headroom"! I don't deny that there are supportable reasons for this apparent about-face but, in thinking them through, I was reminded of many other technical twists and turns that have confused the subject of audio power output ever since about 1930, when enthusiasts first began to take note of it as a parameter in its own right. Before then, audio output watts or, more fittingly, audio output milliwatts hardly rated a mention. Enthusiasts simply got to know, by experience or repute, that some valves were "louder" than others. The legendary 201-A detector/amplifier triode, for example, frequently used in the final stage of old-style battery sets, didn't even have an official power output rating. Figures were published for its derivatives, the 112-A (285mW max.) and the 71-A (790mW max.) but at a time when most receivers were powered by batteries or eliminators, the figures that really mattered were plate voltage and plate current. If either was unattainable or unaffordable, tough! For most enthusiasts, awareness of rated power output followed the release, in 1929, of the mains type 45 triode, the first substantial 16 SILICO N CHIP "receiver" type power valve. Under maximum supply conditions (330V) it offered a healthy 2 watts of output power in single-ended class-A, and up to 18 watts in pushpull class AB2 . Its dominance was shortlived however, because the 47 power pentode, released about eighteen months later, offered higher gain and 2.7W in single-ended class-A at the more manageable supply voltage of 270 - about twice the power available from the 45 at the same figure. Watts missing? But the 47 was not without its critics. Maybe it did have more gain but they didn't like its "tone" and it certainly didn't sound anything like twice as loud as the 45. The manufacturers were obviously having them on! But they weren't. Audio enthusiasts had yet to accept the sobering fact that a 2:1 (or any other) increment in audio power, or sound pressure level, is not subjectively apparent as such. Our ears spontaneously re-scale all such changes to an approximate logarithmic base - nature's way of enabling them to cope with a huge real-life range of sound pressure levels, from the merest whisper to the shattering roar of a not-somodern jet plane (modern ones are quieter). It was to quantify this natural logarithmic relationship that scientists devised the "bel" and its more practical derivative, the "decibel", abbreviated to dB. Taking the above 2:1 power ratio as an example, conversion to a decibel relationship involves taking the log (to the base 10) of 2, which turns out to be 0.3010; multiply this by 10 ·and call the result decibels say 3dB. And therein lies the answer to the fate of that supposedly missing watt: to the ear, with its logarithmic response, a 3dB change in level is perceptible but certainly not twice as loud. Watts twice as loud? If you're expecting a change in SPL (sound pressure level) sufficient to be described subjectively as " twice as loud", you need a decibel ratio of 10, or a 10-times boost in output power. To double the apparent loudness of that ancient 45 triode would call, not just for a 47 pentode but for a valve able to deliver 20W from a singleended stage or 180W from a pushpull pair - a rather tall order. If memory serves me correctly, no "receiver" type output valve has ever been marketed which could meet that requirement. Even today, to conservatively design a solidstate amplifier with an output of 20W single-ended or 180W in pushpull, you're still likely to end up with a parallel configuration! It's rather odd, when you think about it, that in terms of power output rating, most of the receivers and amplifiers that hifi enthusiasts have used during the past 50 years fall into a l0dB slot; from say, 10W for time-honoured push-pull 2A3s to 100W, covering virtually all other valve amplifiers and the vast majority of their solid-state counterparts. So be careful before you start boasting that your pet system can be twice as loud as the one next door. Unless you have 10 times the audio power and/or more sensitive loudspeakers, it can't! Watts and efficiency On the strength of that last reference, I should perhaps interpose the observation that in the context of loudness, many hifi enthusiasts are still unclear about the importance of loudspeaker sensitivity - the efficiency with which loudspeakers convert electrical drive power to sound. Loudspeaker sensitivity depends on design factors too numerous to catalog here but the intensity of the magnetic field surrounding the voice coil looms large as one of them. How sensitivity is measured is also beyond the scope of this article but it is expressed in decibels (referred to a sound pressure level of 12µbar at a distance of one metre) and curiously, the sensitivity range of domestic hifi loudspeakers, used over the past 50-odd years, also happens to be about 10dB wide - from around 85dB for comparatively insensitive systems to 95dB for models of reasonable efficiency. For a given level of audio drive (within overload limits), a 95dB loudspeaker system would be expected to sound twice as loud as one with a sensitivity of 85dB. On that basis, a venerable pushpull valve stereo amplifier, delivering 10 + 1OW RMS to a pair of those large and notably sensitive circa 95dB Wharfedale or Goodmans drivers of 25 years ago, would develop about the same SPL as a 100 + 100W solid-state amplifier feeding a pair of low efficiency (85dB) compact enclosures. These days, the "average" hifi FOUR POWER PENTODE valves from the late 20's and early 30's (from left to right]: type 33, 42, 45 and 2A3. Power outputs were in the region from 2-3 watts. enthusiast probably ends up with an in-between combination: around 40 to 60 watts of amplifier drive, a loudspeaker sensitivity of around 90dB and a marginal loudness advantage of about 2dB over the above extremes. In short, whether visualised in watts or decibels, audio drive power, on which we place so much emphasis, is only half the story. How efficiently we convert it is just as decisive! Watts available! But back to the 1930s: despite the levelling effect of ears and decibels, valve and receiver manufacturers of the period kept right on doggedly chasing small power increments, which would hopefully catch the buyer's eye, even if their ears were unmoved. For engineers at least, it was visually stimulating to see a larger pattern on the CRO screen and a higher reading on the output meter, especially in the knowledge that, to calculate the power resultant in watts, the formula required the voltage reading to be squared: P = E 2 /R That was fine but at an aural level, instead of squaring something, listeners were lumbered with logarithms, often ending up with a thoroughly unexciting 2 or 3dB an exercise that, if nothing else, served to separate technically in- dined pessimists from their opposite number. "It's not worth doing this or that", they would say, "because the difference won't be noticeable". To which the optimists would reply: "Rubbish! Put enough 2dBs end to end and, somewhere along the line, you'll end up with a difference worth having!" In a roundabout way, I guess, they were both right! Watts impedance? But valve or solid-state, with or without an output transformer, audio amplifiers normally end up feeding into a loudspeaker load one that is generally more reactive than resistive, having an impedance that can vary widely with frequency and which may approximate its rated value across only a limited portion of the spectrum. That's common knowledge, but I wonder how many realise the extent to which it compromises our oh-so-tidy thinking, based on those convenient but fictional resistive loads used for most power measurements . Watts or volts? By definition, any amplifier these days using adequate negative feedback has a substantially "constant voltage" output characteristic with loads of not less than a specified value. As a corollary, such an APRIL 1988 17 amplifier will also have a low output impedance and a high damping factor. . As such, it should have the potential to combine well with any ordinary hifi loudspeaker system, for which the normal design objective has been to achieve an adequate SPL in the listening room, when fed from a compatible "constant voltage" amplifier. Audio power, as such, does not appear in the above statement; nor can it in any precise way because, while the audio voltage at the amplifier/loudspeaker interface can be monitored and measured, the impedance and power factor of the load itself is subject to wide instantaneous variations with frequency. But, with rare exceptions (eg, SILICON CHIP, Feb. 1988, p.40) we simply pretend that loudspeakers are high-wattage resistors of, typically, 4 or 8 ohms. On that basis: (a) We quote their normal and maximum drive levels in terms of fictitious watts rather than the actual volts with which they are normally Did you fed and on which their frequency rating is based. (b) We likewise measure and rate amplifiers on their ability to deliver power into resistive loads which, in fact, they rarely encounter. Maybe I'm being over-pedantic. Maybe it's a bit like Santa Claus: easier to go along with the fiction than to complicate kids' lives by denying it! Admittedly, there are a few loudspeaker systems around in which the reactive components have been substantially balanced out but I wouldn't advise holding your breath until the technology becomes the rule rather than the exception. Watts - the genuine variety This brings us back to where this article began - the about-face in relation to amplifier power ratings. In its post-war resurgence, the hifi industry adopted the practice of quoting the audio power delivered to a load resistor across the output-to-voice-coil terminals at the onset of overload. The test was done under con- • llllSS tinuous tone conditions, using a lkHz sine wave and such other frequencies that the designer might choose. The figure obtained was commonly ref erred to as the RMS power output - a term that was challenged on principle by some who maintained that there was no such quantity as RMS power. It has persisted, however, on the grounds that it refers to power derived from a measurement of RMS voltage. Continuous tone testing was recognised as a conserve tive method but justified because it indicated the ability of an amplifier to handle sustained passages of loud music as, for example, the heavy bass pedal notes of a pipe organ. It was a comfortable enough rating for generously designed amplifiers but an embarrassing one for " budget" models in which sustained loud signals of any kind caused a reduction in the internal supply voltage and, with it, a reduction in the measured power output. Watts - music and peak In consequence, some manufacturers ignored the RMS convention these issues? Issue Highlights January 1 988: 4-Bay Bowtie UHF Antenna; Dual Tracking Power Supply; Custom Phone Ringer; Subcarrier Adaptor for FM Tuners . Please send me a back issue for □ November 1987 □ December 1987 □ February 1 988 □ March □ January 1988 t 988 Enclosed is my cheque or money order for $ ... ..... or please debit my □ Bankcard □ Visa Name .. .. ..... ... ... ... .. ....... .... .... ..... .... .......... .... .. ... ... ...... .... ... ...... ..... .. Address .. ... ... .. .. ..... ...... ........ ... ........ ... ..... ... .. .. ............... .. ...... ....... . Suburb/town ...... .... ... ... .. .. ..... ..... .. ... ... ... ....... .... Postcode .. .... ... .... .. . Card No .. ..... .. ... .. .. ... .. ..... .. .. .. .. ... ........................... .. ..... .. .. .... ... ... .. . Signature .... ... .... ........ ....... ... ....... ..... Card expiry date .. ... .. / .... ... / ... ... . February 1 988: 200 Watt Stereo Power Amplifier ; Deluxe Car Burglar Alarm; End of File Indicator for Modems; Simple Door Minder; Low Ohms Adapter for Multimeters. March 1 988: Remote Switch for Car Alarms ; Telephone Line Grabber; Low Cost Function Generator; Endless-Loop Tape Player. Price: $5.00 each (incl. p&p). F ill out the coupon at left (or a photostat copy) and send it to: SILICON CHIP , PO Bo x 139, Collaroy Beach 2097. ~---------------------------------------18 SIUC() N CIIII' and began rating their amplifiers in "music power", signifying the output available at the onset of distortion during short musical phrases. It could typically be from 10-30% above the RMS rating, so that a humble 10 + 10W amplifier might well be rated at 13 + 13W music power. To create a still better impression, the two figures could be added together to yield "26W total music power" . But then someone else realised that the instantaneous power at the peak of a sine wave was twice that . of the sine-wave itself, permitting the aforesaid humble 10 + 10W amplifier to be re-rated yet again to 26 + 26W peak music power or "a massive 52 watts of total peak music power". Such tactics gave the hifi industry a thoroughly bad name and consumer pressure in the US eventually led the Federal Trade Commission to rule in 1974 that amplifiers must be rated in terms of continuous power outout. Further, in testing the power output of an amplifier. it would be subject to an hour's preconditioning at 33% of its rated power. This caused great consternation among amplifier manufacturers because it meant that amplifiers had to be designed more conservatively, particularly as far as their power supplies and heatsinks were concerned. Subsequently, music power was re-defined and recognised in the 1978 IHF (Institute of High Fidelity) standards as a supplementary rating, together with a new term, headroom, which referred to the decibel ratio of music power to continuous power. But music power was still regarded by many as a Clayton's output: watts you had when you really didn't! Watts - transient peaks? But, as I indicated right at the outset, the recording scene has changed drastically during the past decade, as also have our expectations of amplifier performance. Around the mid-70s, audio engineers became increasingly aware that, while their analog tape decks were producing ostensibly clean master recordings, they were subtly crushing the high amplitude transients - a problem of dynamic range for which there appeared to be no ready answer. Then quite suddenly, through the efforts of resourceful professional recordists, hifi enthusiasts were confronted with a sequence of new "ear popping" audiophile discs, some direct cut, others dubbed from digital master tapes. It so happened, at that opportune time, that I acquired a then-new Technics SU-V 4 70 + 70W integrated amplifier, fitted with fluorescent peak-hold level indicators calibrated to 100 + 100W. For the first time, in a home listening situation, I was exposed to recorded transients that not only gave pianos, acoustic guitars and other percussion instruments a startling sonic presence but which, from no more than ample room volume, flicked the level meters to 50 watts or more - well above the reading for a sustained, subjectively loud, organ or orchestral fortissimo. Clearly, a new era had dawned, which was soon to be perpetuated by compact discs. Taken at face value, the above observation suggests that a hifi enthusiast who likes to listen at a generous domestic level, using typical loudspeakers (1 W, lm, 90dB), now needs several hundred watts per channel to be reasonably sure of coping, not so much nowadays with fortissimo passages, but with the high amplitude transients that characterise some modern recordings. Such amplifiers are very expensive and can easily run into many thousands of dollars, particularly if they fully conform to the US FTC standards. There has to be another way, and there is. In the last few years a number of manufacturers have looked at the concept of headroom. Why not design an amplifier with far more headroom than previous designs offered? Instead of having a headroom figure of say, ZdB, which is fairly typical of current designs (such as the Studio 200 described in the February 1988 issue of SILCON CHIP), why not go for a headroom figure of 6dB? 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Just send $2 + 50c p&h and your return address to: ARIST~ ELECTRONICS PTY LTD PO BO_X 191, LIDCOMBE, NSW 2141 A l'lllL HJBtl l9 There is also a large illuminated rocker switch on the variac which is a convenient feature. The price is just $175 which is something of a bargain in today's market. For further information contact Geoff Wood Electronics, 229 Burns Bay Road, Lane Cove NSW 2066. Phone (02) 427 1676. Sony's limited edition Video 8 Camcorder To mark the Australian Bicentenary, Sony has released a limited edition of its Video 8 Camcorder, the model CCD-AU200. This is dressed up in the green and gold Bicentenary colours. The CCD-AU200 uses the standard 8mm Video 8 cassette giving up to three hours of recording time (depending on the battery pack) and has a charge coupled picture sen- The Way I See It - Light sensitive switch for outdoor lighting If you wish to turn a light on at dusk and off at dawn this unit from Arista is the solution. It combines a bimetal switch and large photocell to provide automatic switching. It is energised from 240V AC by connecting it in series with the lamp to be switched. It is rated for incandescent lamp loads up to 2kW; up to lkW for mercury discharge lamps with high power factor; and up to 1.8kW on mercury discharge lamps with low power factor. Power dissipation in the switch itself is two watts. It is claimed to switch on for light levels below 20 lux and off for light levels above 100 lux. The NLS2 light sensitive sor which will operate down to a minimum light level of 19 lux. The camera has automtic white balance, automatic iris and automatic focus so that picture recording is just a matter of aiming and squeezing the trigger. In addition to these features, the camera has a manual zoom lens (2.5x) with macro capability for close-ups. It also has compensation for backlighting which normally ctd from page 19 power output four times as high as the continuous power rating. Now we are starting to talk about a really worthwhile increase in power, or are we? Designing an amplifier with such a headroom figure means that the power supply and heatsink requirements are quite different from those for an amplifier expressly designed to meet FTC standards. Instead of having a power supply which is comparatively well regulated (ie, varying in voltage between no signal and full power by only 10% or less), we have a power supply which is designed to deliver maximum output for a very short period of time which under the IHF standards is only 20 milliseconds. That means you can use a small power transformer and still come up with spectacular power outputs. For example, with a power transformer, output transistor and heatsink configuration which might normally be expected to give 50 watts per channel, you could now get 200 watts per channel but only on very short term basis. This is just what you want for handling those short term transients which characterise pianos, guitars and other percussive instruments. That is just what a number of manufacturers have done. By various means, companies like Carver, NAD and Proton have come up with very large figures for music power and headroom. And therein lies the basis for what I described at the outset as a supportable reason for an apparent about-face, and a totally new switch is available at the recommended retail price of $17.95 from Arista outlets. tends to darken the subject; just push the back-light control to correct the picture. The versatile black and white electronic viewfinder has indicators for function and lighting conditions and is adjustable to suit left-handed or right-handed operation. Again, to mark the Bicentenary, the CCD-AU200 is priced at $1988. (Get it?) respectability for once-maligned music power. But, having thus completed one ride on the technical merry-go-round, we find ourselves heading for another, this time to do with the watts/decibels dilemma of the 1930s. What's the best way to describe and rate this extra output? The options are "music power" or "dynamic power" in watts, hopefully supported by the endorsement [IHF-A-202). Or, again, "headroom" in decibels, arrived at by similar methodology. As it happens, the manufacturers in question are specifying both music power and headroom but the tendency is to emphasise the watts and forget the headroom. The way I see it, we still prefer the watts we can boast about to the decibels we can actually hear! lb Al'lllL 1988 93