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Electronics-World-1959-05

www.americanradiohistory.com By JERRY B. MINTER President, Components Corp. Here is how the "Electronics World" stereo -monophonic test record was made and how you can get the most from it. Making & Using a Stereo Test Record the past it has been customary to use frequency records for testing the frequency response of phonograph pickups. These records are lateral cuts of either stepped frequency tones or slowly varying (gliding pitch) tones. They may be made without pre -emphasis of the higher frequencies, in which case they are called "constant velocity"; or pre - emphasis may be included according to the RIAA curve for check of playback system equalization. Usually the pitch of the tones is identified by voice announcements or by bands on the disc between each tone. A standard reference level of 1000 cycles -per- second is usually recorded at the beginning for setting level (adjusting volume control). In order to make certain that the master record has been properly cut, measurements of the recorded velocity can be made optically, as shown in the photograph on the opposite page. The wider the reflected light pattern, the greater the velocity and the louder the recorded tone when played back.' Side B A lateral frequency record can be used for measuring stereo as well as monophonic pickups. To facilitate the use of ELECTRONICS WORLD'S test record No. 1, Side B for testing stereo pickup equalization, the tone bands have been divided into two groups: the first group of frequencies is recorded at the highest level which is tolerable for reliable tracing of 15,000 cycles. The tones below 1000 cps are recorded at a higher level to prevent masking, at the lower frequencies, by the presence of turntable rumble, hum, and surface noise. One -kilocycle reference tones are included prior to the start of each group for proper adjustment of level (volume). It is possible to make rough measurements by listening to the output from the speaker when the test record is played, but variations in room acoustics, placement of the loudspeaker, non -uniformity of human hearing, and many 56 Our author is shown cutting master frequency test lacquer. other variables reduce the accuracy of this approach. It is far better to use some sort of output indicator such as an oscilloscope, a.c. vacuum -tube voltmeter. or other a.c. voltmeter connected across the speaker leads (voice coil). Such a direct electrical connection eliminates room acoustics, etc. and permits the use of a lower output level without errors introduced by the variation in sensitivity of the ear at these lower levels (so- called Fletcher -Munson effect). The wide popularity of electronic kits has resulted in the home construction of many instruments- -many of which will serve nicely as output indicators. Almost any city block is sure to have one or more kit builders who would be willing to "oblige" if you have no equipment of your own. If such is not the case, however, the simple arrangement shown in Fig. 1 can be used for indication of output. The few parts can be purchased for less than a dollar at most ELECTRONICS WORLD

www.americanradiohistory.com radio parts supply houses. No special tools are required to connect these parts together. In operation this simple "lamp comparator" utilizes the fact that the human eye is an accurate instrument for comparing the relative brilliance of two bulbs. The reference lamp is operated from a battery at reduced voltage (moderate red color). The volume control of the amplifier is adjusted slowly upward from zero until the output lamp is the same brilliance as the reference lamp when the 1000 -cps reference tone at the beginning of Band 2 of the test record is played. While the various subsequent tones are played, the lamp should remain at approximately the same relative brilliance throughout the first group (down to 1000 cps) ; then the volume control must be reduced and reset on the next reference tone to prevent possible burn -out of the lamp as the level increases on the low- frequency group. Each stereo channel should be tested in sequence. (Note: Turntable rumble, hum, and surface noise may make it difficult to use a lamp or an output meter for the frequencies from 15,000 cps down to 1000 cps. In such cases, a listening test or the use of an oscilloscope is mandatory.) If the equalization for each stereo channel is set at "RIAA" and the lamp or other output meter indicates a non -uniformity of output, the treble control can be used to correct for the higher frequency group, while the bass control can be used to correct for the lower group. If the frequency response is not perfectly uniform, satisfactory stereophonic reproduction may be obtained, if the two channels are approximately balanced in response. For this purpose, each lamp can be connected to the output from each respective channel and the two channels thus compared for relative frequency response. The brilliance of the lamps should vary a minimum amount and in unison. If an oscilloscope is used as the output indicator, the observed waveshape should be sinusoidal. A departure from sinusoidal shape at the higher frequencies may indicate a defective stylus or insufficient tracking force (weight). At the lowest frequencies a departure from sinusoidal may indicate improper damping of tonearm resonance. If an oscilloscope is not available, the purity of the reproduced uniform than the acoustical output of most loudspeakers in common use. A serious deficiency in response at the higher frequencies accompanied by fuzzy tones trough, raspy tones) may he caused by a worn or dirty stylus assembly. See the next section on the use of the stylus test bands (Bands 1 and 3). On the same side of this test record there are two stylus wear test hands. These bands are recorded with a 1000 -cps tone at approximately 7 cm /sec. velocity. The outer band (Band 1) should sound pure and clean (without tone color) when played with an LP stylus at 33' rpm. If the inner band (Band 3) sounds fuzzy or rough by contrast, it is likely that your stylus tip is worn or flatted. If neither hand sounds pure (free from harmonics) have your stylus inspected at once. If inspection indicates that the stylus is not flatted and it has a tip radius of 1 mil or less, have your cartridge checked. Sometimes foreign particles will partially fill one of the gaps in a magnetic cartridge and create serious distortion. These particles can usually be removed with a soft brush. Cellulose tape can also be used to pick magnetic particles out of the gap. Simply fold a small piece of the tape, with the sticky surfaces outward, and insert in the gap. All dirt and other particles will cling to the adhesive surfaces. A poorly centered stylus moving system can also cause distortion. In certain pickups having damping blocks on either side of the stylus armature, inspect both blocks for rigidity. If one of the blocks is loose, asymmetrical damping results -and considerable distortion. Application of a minute quantity of Pliobond or other suitable cement will usually secure the damping block. In cartridges using a trailing cantilever arm for attachment of the stylus to the moving system, it is sometimes possible that the trailing arm is displaced to one side. If this arm does not line up (tangent) to the grooves at the center of the record, distortion will result on the inner band. Inspection of the pickup resting in the groove will disclose any such misalignment. In most pickups realignment of this cantilever arm must be done at the factory. Measuring width of reflected light pattern on test lacquer. tone or lack of tonal color will serve as an indication except at the frequencies above 5000 cps. Above 5000 cps the human ear does not detect small departures from sinusoidal response on single tones. Fig. 2 indicates a method of using three flashlight lamps to indicate relative output. The center lamp is connected to the speaker terminals and the volume control of that channel is adjusted on the 1000 -cps reference tone until the center lamp is midway in brightness between the other two reference bulbs. The ratio of power applied to the two reference bulbs is approximately 6 decibels -a commonly used measure of relative loudness as perceived by the human ear- -thus the system will be flat to within ±3 db if the center lamp brilliance remains in between the two reference lamps while the different tones are played on the test record. A system which is flat within 3 db electrically will be more May, 1959 Close -up view of the master recording lathe that was used. The recorded level of the stylus test bands is such that there should be no difficulty in tracking the outer band with a monophonic pickup while the inner band can be tracked if there are no large flats on the stylus. When testing stereophonic pickups an additional factor must be considered -the vertical response. To minimize distortion from this factor it is suggested that the two pickup leads be connected in parallel, as shown in Fig. 4. Connecting the stereo pickup leads in parallel should also be done when playing monophonic records in order to minimize the distortion which would otherwise result from the vertical "pinch- effect" component of groove modulation. Side A The record industry has standardized placement of the right and left channels with respect to the groove as fol- 57

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