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Electronics-World-1959-05

From any Point of View,

From any Point of View, more Experts choose ACROSOUND ULTRA -LINEAR 11 60 watt amplifier DESIGN The combination of patented ULTRA.LINEAR circuitry -plus new HYBRID FEEDBACK principle- VARIABLE DAMPING control, and ULTRA STABILITY, represents a new high in the art of amplifier design ... an example of ACROSOUND'S latest achieve. ment in AMERICAN Know.How. This superi. ority of design now enables anyone with or without any previous knowledge of electronics to assemble for himself or herself .... (yes its that easy!) ... the finest of amplifiers and at a most reasonable cost, to only two hours! PERFORMANCE By listening test, or by in. struments ... second to none in clarity and frequency response. Normal level distortion is virtually unmeasurable -IM 1% or less at 60 watts, 120 watts peak. Completely stable ... unaffected by loads, perfect square waves. _ - QUALITY Every part going into the assembly of critical and even non-critical circuitry is tested and checked to allow no more than 2 if% variation from ACROSOUND'S standards. Specialized test equipment unavailable commercially was designed in ACROSOUND'S laboratories to achieve this result. Every printed circuit board is placed in trial opera tion on a laboratory amplifierOutput tubes are matched by trial and do ible checked. COMPONENTS ACRO'S newest 10.600 out. put transformer with special hybrid winding - separates functions of output circuit and feedback circuit. Heavy duty. completely assembled. and thoroughly tested. printed circuit board assures uniformity of perform. ance. Low distortion EL34 output tubes are operated well within their ratings ensuring long tube life and optimum performance. PRICE In preassembled kit form so that you may save money, learn while doing, and have the proud satisfaction you built the best for only 175.50 net ... or if you feel you would prefer it laboratory assembled it still repre. sents a bargain at S10S.S0 net. HEAR IT AT YOUR DEALER NOWT 128 BE READY FOR ACROSOUND DISTORTIONLESS PRE AMP DESIGNED FOR THE STEREO.PHILE Experts know ithe ACRO i.s best! Others ... Learn uwhr.' Write to ACRO PRODUCTS 369 SHURS LANE PHILA. 28. PA. Experiments in Stereo for P.A. Systems By JACK THORNTON DECENT comparison tests with stereophonic music systems suggested a re- investigation of a two -channel technique in public- address system work. The results were quite interesting and the idea seems worthy of further experiment by those working with auditorium amplifier setups. Such a system uses two separate sound channels set up in a "left" and "right" relationship which is essentially the same as that used in stereo music recording and playback. Auditorium public- address systems usually use a loudspeaker enclosure at either side of the stage. The two sound sources obviously give better sound distribution and coverage. But the system remains monophonic because both speakers are fed by a common microphone and amplifier. Consider a person speaking from a stage without a sound system. The audience experiences the normal acoustical effect, that is, when the speaker turns slightly to his left, the audience in that direction hears the voice projected a little more toward them. The audience the other way hears the voice diminish slightly. Even though this effect may not be too pronounced, natural sound does depend upon it. When the same person uses the ordinary single -channel sound system, the loudspeakers override his actual voice so that listeners hear only what comes over the p.a. Now when he turns slightly, his voice fades in both directions as he goes "off mike" and the sound diminishes in both speakers. The sense of movement and liveness is impaired. Of course there is the visual sense of direction because the audience can see the person and his movements. But they still hear the sound through a monophonic system. A dual- channel sound system seems to offer one method of improvement. This holds true even though the source of sound moves very little. First tests of p.a. stereo were run in adjoining radio studios with a window between them. A curtain could be drawn for comparison between impressions when the observers could or could not see the speaker. Matched microphones fed companion amplifiers in studio A. Matched speakers were placed eight feet apart in studio B. A switching arrangement allowed single - or dual -channel operation, with both speakers operating either way. Observers had no difficulty identifying the "stereo" setup, even with the curtains drawn. The person talking was twelve inches from the mikes. Mike separation varied from two inches to two feet. The larger separations gave exaggerated effects. About ten inches seemed the most natural in that particular studio. More checks were run in an auditorium. Microphones were Shure 556 cardioids to allow somewhat greater mike - to- subject distance without audio feedback. The systems were balanced by having an observer at the center of the auditorium indicate when the sound seemed to come from the middle of the stage. With speech as the program material, trial and error finally placed the microphones on the speaker's rostrum twelve inches from the voice, separated from each other ten inches, and angled in slightly toward the sound source. The closer the mikes are to the voice. the closer they must be to each other if a "ping-pong" situation is to he avoided. (It's quite an effect to have the speaker turn his head slightly and seem to jump thirty feet to the other side of the stage!) If the microphones are too close to the voice, most of the stereo effect is lost since any movement will put the speaker off both mikes and you'll be back to a monophonic system. Unless the speaker has a weak voice or the room is a bad one for feedback, the greater mike distance will give better sound anyway. When music is the program material, both the volume of sound and the size of the group producing it must be considered. A vocal or instrumental solo usually allows greater mike distance: two or three feet. Mike separation is accordingly greater. As the musical group becomes larger so must microphone distances become greater. Large orchestras or choirs seldom need amplification and if a sound system is used with these, it's usually for a soloist and the solo settings can be used. Operators who wish to duplicate and expand these experiments could use this guide: microphones, amplifiers, and loudspeakers should be as identical as possible. Identical models may not be matched; one unit may be new, the other a veteran of rough service. Tone controls are set the same for both units, although some compensation for unmatched units can be obtained by tone setting variations. With mikes about twelve inches from a voice and separated about ten inches, balance the two channels by having an observer indicate when the sound seems to come from the speaker. Then vary the mike distance and separation for best effect with the program material to be used. Such a setup is an interesting sidelight on the stereo boom and, although it won't hide poor p.a. systems or practice, it offers a worthwhile improvement in the naturalness of sound reproduction. -30r ELECTRONICS WORLt,

www.americanradiohistory.com Hi -Fi Crossovers (Continued front page 55) Records and plays Professional Stereo. Two or 4 -track heads. All -aluminum construction. 14) 0. 4/ C(3 c cá' FREQUENCY IPS FLUTTER NOISE RESPONSE SPEED d WOW RATIO ± 2 db 30 -30000 CPS 15 .06% 57 db x 2 db 20 -20000 CPS 71/2 .08';;. 54 db 3 db 20 -11000 CPS 31/4 .18';;.. 44 db (Recording qualify for 4 -back slightly less) Write tar Catalog RN -SB $695 Less Case .Staccdaui awe( Seetee Nradtld INTERNATIONAL RADIO 8 ELECTRONICS CORP. ELKHART, INDIANA PURCHASING A HI -FI SYSTEM? Send Us Your List Of Components For A Package Quotation WE WON'T BE UNDERSOLD! A I I Inc] chain' ise is brand new, factory fresh & guaranteed. AIREX RADIO CORPORATICN PARTIAL LIST OF BRANDS 64-R Cortlandf St., N. Y. 7, CO 7-2137 May, 1959 IN STOCK Altec Lansing Electrosoice Jensen Hartley University Acoustic Research Janssen Wharfedale Korlson Viking Concert one Bell G.E. Weathers Harman- Kardon Eico Pilot Sherwood Acrosound Fisher Bogen Dynak it H. H. Scott Pentron Ampio Leak VM Challenger Wall.nsa Garrard Miracord Glaser -Steers Rek -O -Kat Components No re leo Fairchild Pickering Gray Audio Tape Full Line of Cabinets while at 20,000 cps this figure was 50 watts. This comes very close to expectations in that, at the crossover point, the power should be 3 db down from maximum. As indicated in Fig. 9A, the actual measured crossover point for these two electrolytics in series turned out to be 700 cps low (from 5000 cps) which would indicate that the electrolytic combination was about 139; off from rated value. What is of major importance is the fact that a full 50 watts could be delivered through such a back -to-back configuration of polarized electrolytics. Just to be contrary, the back -to -back direction was reversed so that where in the first case, the positive terminals were tied together, in the second case the negative terminals were connected. There was no change in performance. There is apparently no reason why non - polarized electrolytics cannot be used in crossover networks. But, now. how about the polarized type? With the same test setup as described previously, a test run was made on only one of the 8 Pfd. units of the previous test. Again during the run the amplifier was adjusted to give the maximum undistorted waveform at the load resistor. The results were exactly the same as in the case of the non - polarized combination. There was no waveform distortion at a full 50 watts input to the load resistor and at the crossover point (in this case 2500 cps for 8 Pfd. for 8 ohms) there was a clean 25 watts which was the expected 3 db down. To tie the matter down even more firmly, a test was made on three different 50 pfd. polarized electrolytics rated at 150, 50, and 25 volts respectively. No differences in performance could be observed as far as waveform at maximum output was concerned (50 watts). They were all clean, as observed on a scope. The last question that was to be resolved was the matter of the rising impedance of a large value of electrolytic at the higher frequencies, which would have the effect of reducing the voltage at the tweeter terminals. Fig. 9B is the curve of maximum undistorted waveform voltage at the load resistor from which it will be seen that for this 50 pfd. capacity, there is a drooping voltage characteristic at the load. When, however, this capacity was shunted by a small paper capacitor, the voltage characteristic was respectably evened out. Looking prejudice squarely in the eye, there would seem to be no reason as yet for not using electrolytics, polarized or non -polarized, for network construction. This, along with Chart 1 should make the matter of collecting the necessary components for a home - built network a fairly simple and straightforward operation. -30- FREE HOME TRIAL proves Columbia CD stereo cartridge sounds better Now you can prove it to yourself. The Columbia CD is the better stereo car- tridge. We believe this so strongly that we have arranged with your CBS -Hytron distributor for you to test the Columbia CD... free in your own home. We are sure you will agree this cartridge does sound better than any other ceramic or magnetic stereo cartridge. It is superior in linearity ... separation ... compliance ... low mass ... freedom from hum ... output level ... ruggedness. Your ears will tell you quickly what elaborate curves, facts and figures prove. The Columbia CD simply sounds better. Make sure by your own tests that you invest in the better stereo cartridge. This offer is for a limited time only. Ask your CBS -Hytron distributor to let you try the Columbia CO, Model SC -1D, today! COLUMBIA19 Model SC ID Cost $24.25 CBS -HYTRON, Danvers, Massachusetts A Division of Columbia Broadcasting System. Inc. 129

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