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Electronics-World-1959-05

www.americanradiohistory.com lows: the left channel will always be recorded on the inner wall while the right channel will always be recorded on the outer wall of the groove. The first band on Side A has channel identification tones to make certain that the left channel is connected to the left speaker and the right channel to the right -hand speaker. If the channels are reversed, merely interchange the speaker leads from the amplifiers. Interchanging the pickup leads will, of course, provide the same result. The second band has a metronome heat recorded on it. When played through a stereophonic system, the source of the sound should appear to come from half way between the two speakers. A slight differential gain (volume) adjustment may be required to accomplish this illusion. In order to make this test, the speakers should be placed several feet apart and the listener should stand equidistant from each speaker (in front, of course) about the same distance as the speaker spacing. The third band is a phasing test. A low- pitched tone (100 cycles) is recorded first in -phase on both channels and then out -of- phase. If your speakers are properly phased. the first tone should appear louder than the second. If the relative loudness of these two tones appears reversed, try reversing the leads to one of your two speakers and repeat paralleled as for playing a monophonic recording and connected to either of the two amplifiers. Failure to obtain a marked difference between the two 100 -cycle tones with the connection of Fig. 4 places the difficulty directly in the pickup cartridge. Before discarding the cartridge, try shorting the two hot terminals of the cartridge together right at the base of the cartridge. This eliminates defective connecting leads from the cartridge to the amplifier as a possible cause of the trouble. If there is still no appreciable difference in the two tones, the cartridge should be returned to your dealer for service and repair. The fourth band on Side A is provided for testing turntable rumble. Monophonic recordings are more tolerant of turntable rumble since only the lateral vibrations of the turntable are involved. Stereophonic 45/45 disc recordings are equally sensitive to vertical and lateral rumble and many older type record changers have far too much vertical rumble to be satisfactory for stereo applications. The rumble test band (the last band) has several reference tones at 100 cycles, which are recorded in successively lower steps, as an aid in judging how well your particular turntable is performing. The first tone is recorded at -20 db down from standard level (7 cm /sec. at 1000 cps). Almost any turntable should have a residual rumble of less than -20 db. PAIR OF 3 TO 5 FT. CLIP LEADS X 13 FLASHLIGHT BULBS (GREEN BEAD /FOR FLASHLIGHT) 3 CELL -308 0 +3DB USE 1F 13 BULBS i 3 7016 OHM SPEAKER VOICE 1.5 VOLT DRY BATTERY COIL TERMINALS Fig. 1. Simple lamp comparator as is discussed in the text. the test. Reversing one of the speaker leads will of course reverse the phasing between the two speakers by 180 degrees, In order to record this phasing test. it was necessary to cut the first 100 -cycle tone laterally (no vertical or difference component ) while the second 100 -cycle tone was recorded vertically (no lateral or sum component). In the event a definite phasing check cannot be made on the system, it would be well to test the pickup and amplifier as follows: connect the amplifiers as shown in Fig. 3 and leave the volume and balance controls as set previously on the metronome test. A definite difference in loudness between the two 100 -cycle tones will indicate proper operation of the pickup and two amplifiers. This leaves the possibility that the trouble may be in the dissimilarity of the two loudspeakers or their placement in a particular room. Relocation of the two speakers or at least a reduction in spacing may effect a marked improvement. A compromise with room acoustics should permit satisfactory phase results after relocation of the speakers. On the other hand, a failure to obtain appreciable difference in relative loudness between the two 100 -cycle tones with the connection of Fig. 3 is indicative of either a defective pickup or some differential phase error in the two amplifiers. The amplifiers can be cleared by connecting them according to Fig. 4. Here the two stereo pickup leads are Fig. 3. Method of using the phasing test as described above. DIM LLL IN BETWEEN BRIGHT 1.5 VOLT DRY CELL 3 TO 16 OHM SPEAKER VOICE COIL TERMINALS Fig. 2. A somewhat more accurate lamp output meter is shown The next rumble reference tone is at -30 db, then one at -40 db. and finally one at -50 db. A record changer in good working order should have rumble about -40 db. Only the better grade turntables will be better than -50 db. This special test disc was recorded on a HydrofeedV lathe to insure the lowest possible rumble level. Residual rumble on this lathe is better than -70 db. When making rumble tests do not confuse stray hum pickup with rumble. One method of differentiating between stray hum and rumble is to lift the pickup arm slightly off the record surface and note whether the apparent rumble remains. Do not move the pickup away from the playing area since stray hum pickup (from the turntable motor in most cases) will vary with the position of the arm. Turning off the motor will usually reduce the stray hum field. Excessive rumble may mean that your turntable should be repaired. Note: When checking for rumble disregard high - pitched surface scratch and clicks. After all of the above checks have been completed, one should have a fairly good idea of the performance of his hi -fi stereo or monophonic system. REFERENCE 1. "IRE Standards ou Rrrordi nq and Reproducing Methods of Calibration of 3f rehanieally Recorded Lateral Frequency Records, 195N." Proceedings of the IRE. December 19571, poge 19 ¡.1. _a_ Fig. 4. This phasing check eliminates the amplifier from test. R= 10 TO 20 OHMS (2 WATT) 58 ELECTRONICS WORLD

Hi -Fi Product Test Report NEW RIBBON MICROPHONE THE Ercona Corporation has just announced importation of the "Mark III" line of British -made Reslo studio microphones. Included in this line are various models with the differences being mainly in output terminating impedances. Our particular interest, since they are all of the ribbon type, was in determining first -hand the durability of these microphones. There have been comments within the industry that ribbon microphones today are not as delicate as they were in years gone by. We tested one of the models, the CRH, which is designed for high -impedance output, by dropping it several times from a height of four feet directly on to a cement floor without it exhibiting any deleterious effects performance -wise. This obviously backs up the many comments we have heard. In the event that the ribbon element is damaged, the ribbon cartridge is detachable and replaceable. Although these microphones are designed for p.a. applications as well as studio or home recording, care should be taken when used outdoors. The adverse effects of a strong blast of air against the ribbon still remain a problem. In the case of this microphone, however, the ribbon is shielded by nylon and wire -cloth screens and the perforated shell. Proper rotation of the microphone will, of course, alleviate part of this difficulty. According to the manufacturer, the frequency response of this microphone is basically linear from 20 to 15,000 cps. Its characteristics are bi- directional but this may be altered by use of internal pads. Its bi- directional effects vary with frequency: at 1 kc. it has 2 db more sensitivity in front and at 10 kc., 20 db more. This feature can be used to advantage since the rear can be employed for low - frequency pickup and the front for the higher frequencies. The output level is -58 db. Although our tests were limited (as we did not have an anechoic chamber available) , we feel quite confident that these microphones are entirely suitable for top -quality applications. Prices, depending on the model, range from $64.50 to $84.95, audiophile net. ELECTRONICS-- LAB TESTED HIGH- COMPLIANCE SPEAKER SYSTEM [OR those who are interested in a compact, high- compli- I ance speaker system, we would like to call attention to Allied Radio's "Knight" KN -2000. The system incorporates a specially designed 12 -inch woofer, an 8 -inch mid -range unit, and an exponential- horn -loaded compression high - frequency tweeter. The enclosure is fully sealed and lined with acoustic material. All three speakers are fed by a three -way crossover network with an externally operated control to adjust the high- frequency level. The system has an input impedance of 16 ohms and will handle 20 watts average power. It is a compact unit measuring 1375" x 2615" x 12,'4" and is available at a cost of $84.50. The speakers used in the "Knight" KN -2000 system have obviously been made especially for this unit. The woofer seems to be extremely well designed with a long -throw voice coil and, as is usual with units of this type, it has an extremely low resonance frequency -approximately 38 cps unmounted. The mid -range speaker seems to be a fairly conventional unit except that the basket is a complete solid enclosure around the cone, producing a hermetically sealed chamber. The tweeter appears to be a conventional type. The over -all response is quite smooth, particularly at the low- frequency end. The system does not have the exaggerated bass that can be found in some bass -reflex type speaker enclosures which, incidentally, some people may prefer. Audibly, the system seems to take hold at about 55 cps and extends at the high end beyond this reviewer's limit of hearing. To be extremely critical we did note a slight attenuation of response between 100 and 200 cps and at approximately 2000 cps. Again, only a very critical ear could determine this and then only by A -B testing against another speaker unit known to be flat in this range. Another point of importance concerns the efficiency of the speaker system. Normally, high -compliance units go hand -in-hand with low efficiency. Apparently, and justifiably, Allied has compromised in this case since the efficiency of the system was not as low as was expected. STANTON STEREO "FLUXVALVE" PICKUP cartridges are still coming in at a high rate. STERk.:0 We have four on the bench now awaiting our tests. The most recent one checked is the Pickering Model 371 "Flux- valve." It is a moving iron magnetic type and is an improved version of a previous model. The pickup has a .7 mil diamond stylus and is recommended for use with a tracking force of three to six grams. Our tests showed that it has uniform response up to 3000 cps, dropping slightly by 1.2 db at 7000 cps, followed by a resonance that increases the output to +4.2 db at 12,000 cps. Response then drops off rather rapidly to 15,000 cps. The sharp rise is inherent in quite a few of the cartridges we have tested in the past and, according to Pickering, it is not detrimental since it is at the very high end. Actually, this (Continued on page 153) May, 1959 59

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