5196106219

ELECTRONICS Australia. September 1987

FORUM

dises, she said. They certainly keep coming back for morę!

The manager of a large specialist musie storę was somewhat morę analyti-cal. Complaints were few and far bc-tween but, yes, he could remember the odd occasion.

Did the complaints have to do with sound quality or tracking problems?

“Mainly tracking problems,” he said, “and apparently justified because the dises didn’t track too well on our shop player either. We simply replaced them with a new pressing and that was the end of the matter.”

Perhaps morę to the point:

“We get occasional complaints about all recordings . . . black dises and tapcs as well. My impression is the CD’s are better than average, not worse!”

Another man with a technical back-ground told me of a friend whose CP player was equipped with a LED indica-tor which flashed whenever it had pro-cessed a major fault — presumably w hen the error proccssing logie had flagged “uncorrectable — interpolate”-.

His friend had said that he could usu-ally pick a record that had morę than its share of data errors. When I asked about the 1000 errors-a-minute ratę, that sort of figurę had apparently not even been considered.

“Good heavens. The LED’d be on all the time!”

Maybe others with access to a similar player could enlarge upon these obser-vations.

In the meantime, I come back to that BER = lfr³ figurę, representing inter-polations equal to a tiny fraction of 1%. If we discount the fact that interpola-tions are “educated” Computer guesses and regard them as totally incongruous distortion components, they would still only constitute a tiny fraction of 1% — and that’s less identifiablc distortion than one would expect from an FM transmission, a black disc player or a loudspeaker system.

In saying that, Fve just completed the fuli circle, with tongue firmly back in cheek and a conviction that interpolar tion distortion is exactly what it started out to be: a “phurphy”!

Self-adhesive quality!

While in the generał subject of compact dises, I have to hand a letter from D.H of Waterloo, NSW. After saying how much he enjoyed the dissertation on the “golden ear” brigade in April,

28 he feels that they are about to be up-staged by the “flashed platinum eared” group. As evidence of this, he encloses a clipping from the “Western Mail” newspaper (March 21-22, page 3) in which the writer says that a self-adhe-sive plastic foil, pasted on the label side, can improve the sound from compact dises.

According to the Western Mail writer, compact dises pose a problem in that they are both valuable and smali enough to be slipped into a pocket by a light-fingered visitor. Dises can be “per-sonalised” by lightly engraving or mark-ing the label side only with a felt pen, but ink marks may prove to be erasa-ble. Sticking bits of paper to the disc is not a good idea, either, “because they can come off and get caught up in the works”.

The preferred approach, according to the Western Mail, is to mark the label side with a felt pen and then to pastę over the side a self-adhesive plastic foil “damper”, available • from most hifi Stores' for $5 for a pack of five. It will protect the ink from erasure and “will improve your sound at the same time”.

To be frank, I have ncver scen these plastic dampers, nor have I felt the need for them. To the best of my knowledge, nonę of the “friends” who visit my house are of the light-fingered kind, anyway.

But frankly, I’m not enamoured with the idea of pasting a foreign coating.on the surface of my compact dises. If it’s very thin, I fail to see how it could do much for acoustic damping. But, if its suitably thick and heavy, I would be concerned about the possibility of it being positioned sufficiently off-centre to prejudice balance and promote vibra-tion at 5(X)rpm.

And what of its long-term Chemical and physical properties? Are they com-pletely compatible with those of the disc or might they ultimatcly set up stresses with possiblc warping. Pm afraid that, like D.H., I am not anxious to join the “many people” who are said to be “working through their collection of dises, gradually treating them with the coating”.

Damping problems

A.J. of City Beach, W.A., to whom I referred earlier, also expresses little sympathy for the “golden-ear brigade”, but in the context of supcr-heavy loudspeaker cables. He makes the point that extreme electrical damping is neither necessary with well designed loudspeak-ers, nor is it attainable by simply using heavier cables, because voice resistance

itself becomcs the limiting factor. Says

A.J.:

"// the super-cable addicts (or vic-tims?) are dinkum, they’U have to buy loudspeakers with wice coils madę of similar wire”.

A.J. might be interested to learn that a pretentious American console receiver (Gulbransen, if I remember corrcctly) imported for the luxury market during the late ‘20’s, used a formidable dynamie loudspeaker, with the lowest resistance voice coil I have ever heard of. The receivers were being traded in and scrapped during the brief period that I worked in the E.F. Wilks radio factory in the mid ’3()s. ‘

Instead of a voice coil, as such, the speaker used a single strip of copper (or alloy) so punched and formed that it provided a 7/8th-tum in the magnetic field and two flexible legs which fed and supported the moving “coil”. The sensi-tivity was very Iow but this was compen-sated for by an amplifier boasting a pair of type 50 output triodes, similar to those then being used for theatre sound systems.

But, contrary to what A.J. suggests, this set-up would not necessarily have madę for improved electrical damping:

(a)    Because of the Iow efficiency of the magnetic Circuit, and

(b)    Because, the impedance of the 7/8th-turn copper strip would have been so Iow that, in terms of damping, it would have posed a near impossible challenge for the output stage, trans-former and connecting leads.

In short, loudspeaker damping de-pends on the relationship between the source and the output load, rather than on absolute values, A.J. then goes on:

The irony is that the same salesmen who will advocate heavy cables will also push expensive dynamie headphones for the “ultimate in fidei i ty”, and the same “golden ears” will happily listen to them while they are plugged into an amplifier with a source resistance of some hun-dreds of ohms (viz. the usual series headphone jack attenuator, as in the EA 60160 Playmaster) where the damping factor is virtually zero.

My own approach to the provision of adequate damping has been simply to shunt a 4VL resistor across each headphone. /U a bonus, the extra loss re-duces to near-inaudibility the level of output stage hum, otherwise frequently evident at the headphone jack. Such a shunt should really be built into all am-plifiers.

To someone who is ostensibly being supportive, I may appear to be some