Editor's Review (Feb. 1973)

Focus is on amplifiers for this issue and there is no doubt that they have come a long way since the days when 1 watt at 20% distortion was The State-of-the-Art! In the late thirties, high fidelity amplifiers of the day usually put out five to ten watts and then we had the famous Williamson design in the forties with about 15 watts. High voltage tubes like the 6L6, KT-66 and EL-34 raised the wattage higher and then the advent of solid-state brought a gradual increase in output powers, 100 watts (real watts, not Music Power) will soon be rated as medium power! It is only fair to say that present-day loudspeakers do need much more power, as every improvement in linearity and frequency range has had to be paid for in reduced sensitivity (To avoid an acrimonious letter from you-know-who, I must exclude horn loaded systems which have a high inherent efficiency). I am often asked, why do amplifiers sound different but yet measure the same? The answer is: they don't. If there is an audible difference, then the measurements are incorrect or incomplete.

Assuming we had two amplifiers with identical major parameters--including bandwidth, THD and IM distortion at all levels, same proportions of harmonics, damping factor, input impedance and signal handling capacity, stability margin and so on, then there are several possible explanations. One concerns overload characteristics. Amplifier A may clip cleanly without causing the dc supply voltage to fall unduly, but amplifier B might have an inferior power supply so the voltage will not only drop on sustained peaks but the smoothing will become inefficient and so a kind of sawtooth hum component will be superimposed on the signal. Another point sometimes overlooked concerns deviations from the standard RIAA equalizing curve: even with selected components the divergence might be 1/2 or 1 dB. If these tolerances went in opposite directions, the total variation is large enough to be heard on a A-B comparison.

Incidentally, these A-B tests would have to be made with the same program source and same loudspeakers and due attention must be paid to phono input lead lengths as an extra three foot can introduce enough capacity to make quite a difference in high frequency response.

MCA DiscoVision

MCA have just demonstrated their long-awaited Disco-Vision video disc which appears to be very similar to the Philips laser system mentioned recently in this column. The laser is a low powered helium-neon type but, unlike the Philips system, the beam is electronically "steered" to follow the information spiral. The disc itself is 12 inches in diameter and can be made of PVC or other plastic by thermo-stamping or embossing just like conventional records. Density is 12,500 tracks per radial inch and playing speed is 30 revolutions per second. Playing time is 20 to 40 minutes-depending whether color is used. In addition to the full TV bandwidth, the discs can accommodate two audio channels. Two playing units will be available-a single disc player and an automatic model which will take 10 discs and prices are expected to be around $400 and $500 respectively. In operation, the output signals are fed to a TV set via the antenna input and then switching to an unused channel.

MCA have a film library of over 11,000 titles and presumably some of them will become available on discs--which incidentally ought to be relatively inexpensive. The question is--will a laser' system eventually replace our present recording system? How long would a MCA disc play with just two or four audio channels? And would such a system be immune from the pops, crackles and scratches that defy our record cleaners? It's an interesting possibility anyway.

Response Curves

Seen outside a Philadelphia so-called Adult Bookshop, a large sign advertising "8-tract tapes, $2.99." Somehow, I doubt whether they would have any of Billy Graham's though ...

-G. W. T.

(Source: Audio magazine.)

Also see:

Dear Editor (Apr. 1974)

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