AUDIOCLINIC (May 1981)

Phonograph Facts

Q. How many pressings is a master disc capable of producing before its original quality becomes degraded? Do record manufacturers make it a point to completely curtail use of masters which have reached the end of their quality-producing life? Is there a difference in the quality of a record when it is purchased from different sources (discount stores, record clubs, etc.) and at different prices? I have heard that records purchased from sources selling them at less than the full retail price are of lesser quality due to the fact that they are being supplied from masters near or past the end of their quality lifespan.

- Arthur Darrow, Troy, N.Y.

A. The master disc is not directly involved in the pressing of phonograph records. Rather, it is an intermediate step. The master disc is vacuum sprayed with a thin layer of silver only a few molecules in thickness. This coating makes the surface of the disc conductive and the conductivity, in turn, allows for the electro plating of nickel, is then stripped away from the lacquer master disc. Under some conditions the negative itself can be put into a record press, with the discs produced much like waffles are produced in a home waffle iron. This negative, however, is most of ten used to generate another plated met al part which becomes a positive. With appropriate equipment this metal component can be played. In fact; it some times is played to check quality so that the pressing plant is sure that parts produced from this positive can be used for record production. The positive, or mother as it is more often called, is once again plated, and the resulting negative is a metal stamper which is actually placed into the record press. When the stamper wears out, a new stamper is prepared from the mother. Thus, the number of discs produced from a master disc can number into the many thou sands.

The number of discs, however, which can be produced from any one stamper is limited. At one time it was not possible to produce more than 1,000 pressings of high quality from any one set of stampers. This is partially because of the nature of the processing of the raw vinyl material which is placed in the press. To day, with modern vinyl extrusion techniques and chrome-plated stampers it is often possible to produce 2,500 discs from a single set of stampers.

The use of chrome plating does have some drawbacks. Because it adds a small amount of noise, some producers opt for stampers which are not chrome-plated. If chrome plating is not used, it is likely that no more than 1,000 to 1,500 discs can be produced before noise begins to build up.

It is hard for consumers to tell whether or not a set of stampers has been run past its prime. When this is the case, I do not know how much of an attempt is made to separate the less-than-optimal discs from the rest. The cost of a stamper is relatively low, so there would be little benefit in servicing discount stores with discs which have been pressed with worn stampers.

Some record clubs have discs made especially for them--often collections of the albums and singles recorded by a given artist. These discs, however, are often recorded from original tape masters. The pressings are generally pro reliable houses.

There is one other source of phonograph records which is the bootlegger. Those engaged in this practice do not have permission from either the record companies or the artists. Generally speaking, the quality of such offerings is inferior to the authorized releases of these same titles. Though the prices of these illegally produced discs may be attractive, they should not be purchased. In part it is a matter of uncertain quality but there is another consideration: When one buys these pirated records rather than the authorized version, it deprives the artist, the song writer, and the publisher of the royalties which they receive when the authorized version of a disc is sold. In other words, these people are cheated out of a portion of their livelihood.

How Impedant of You!

Q. Very little seems to be written on the subject of extension speakers, except that equipment reviews will often say, "You cannot use more than two pairs of speakers at one time." Is there any way for an amplifier to be set up so that three or possibly four sets of speakers can be played simultaneously from one amplifier without running into amplifier overload problems or losing a significant amount of sound quality? Also, if two 8-ohm speakers are connected in parallel, is the resulting impedance four ohms or something else? If three 8-ohm speakers are in parallel, what is the resulting impedance? If there is a formula for working all of this out, I would appreciate your giving it to me.

-James Smith, West Palm Beach, Fla.

A. If two 8-ohm speakers are connected in parallel, their combined impedance is four ohms. If they are connected in series, this same pair of speakers will have an impedance of 16 ohms. If two pairs each of 16 ohms are in parallel, their combined impedance is 8 ohms. If three 8-ohm speakers are connected in parallel, this will produce a combined impedance of about 3.66 ohms.

The formula for obtaining impedances of speakers in parallel is the same as for finding resistances in parallel: The reciprocal of the resistance of speaker number one plus that of speaker number two, etc. The final answer is obtained by taking the reciprocal of the sum of all of the reciprocals. Thus, in the 8-ohm speakers, we take each of them and call them 1/8 (which is the reciprocal of 8); 1/8 plus 1/8 is 2/8. Taking the reciprocal of this, we now have 8/2, which is four, or four ohms.

If you wish to have four speakers connected to one channel of your amplifier, you must put two of them in series, put the remaining two in series, and finally, connect the groups in parallel. Use heavy-gauge wire to keep damping as high as possible. The damping will not be high in any case, but with many speakers this loss of damping will not matter. This series or parallel arrangement is the only way to have four speakers per channel operating from a single power amplifier, all at the same time.

Connecting three speakers to one channel of an amplifier is a real problem unless one wishes to waste power in the form of heat in a resistor. Where your amplifier has been designed to work into loads lower than four ohms, three speakers can be connected in parallel with no difficulties.

There is always the alternative of having two separate power amplifiers, each of which is fed to two speakers. This arrangement provides the best damping--in those instances where this is important.

Naturally, the greater the number of speakers per channel, the more that the available amplifier power must be distributed into each of them. If really loud listening is contemplated with all speakers operating, and if the speakers are relatively inefficient, it may be that a low-power amplifier may not have sufficient reserve power for this application.

Que Sera, Ceramic

Q. I recently decided to build the Leach moving-coil pre-preamplifier (Audio, February, 1978), and I would like to use the highest quality components. Mr. Leach specifies the use of ceramic capacitors in the circuit for r.f. interference suppression, and he specifically discourages substituting parts. Would the use of film-type capacitors--polystyrene or polypropylene--in stead of ceramic capacitors improve performance of this circuit?

-William E. Bobrick, Richmond, Va.

A. Ceramic capacitors were recommended because many other capacitors are inductive at radio frequencies, there by reducing their efficiency as r.f. bypass capacitors. Dielectric materials have been improved and ceramic capacitors have been made smaller for a given amount of capacitance This have cartridge having deteriorated. There is probably no phonograph stage in equipment which employs ceramic cartridges.

If your console employs a magnetic cartridge, check the stylus to see if it is properly centered or if it is bent, touching the cartridge body. This would re strict the motion of the stylus, leading to lower volume and considerable distortion.

If the magnetic cartridge proves to be all right, perhaps the phonograph preamplifier stage or stages are not operating as they should. Sometimes dirty switches are the cause of low volume from a phonograph system.

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(Audio magazine, May. 1981, JOSEPH GIOVANELLI)

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