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If you have a problem or question on audio, write to Mr. Joseph Giovanelli at AUDIO, 134 North Thirteenth Street, Philadelphia, Pa. 19107. Cartridges & Phono Input Sensitivity Q. I have an amplifier with a sensitivity of 3.3 mV at the magnetic phono input. Attached to the input I have a cartridge which delivers an output of 1.1 mV per cm per second at 1 kHz. I wish to buy a cartridge of higher output. However, manufacturers use different methods to denote sensitivity. For example, some state output figures based on a stylus velocity of 1 cm per second; others, at 3.5 cm/sec., 5.5 cm/sec., or 7 cm/ sec. Therefore, it becomes difficult to compare output values. They all appear to use 1 kHz as a reference. I have the following questions: 1. Is there a formula that can be used to convert output ratings to 1 cm/sec. or to some other standard? 2. Based on a measurement of 1 cm/ sec. at 1 kHz, can you recommend what the optimum output of a cartridge should be to drive my amplifier to its maximum power output consistent with low noise and distortion so that the preamplifier is not overloaded? - Richard Garlatti, New Brunswick, N.J. A. If you have a cartridge which delivers 1.1 mV at 1 cm/sec. stylus velocity, you will have more than ample output to drive your amplifier when using that cartridge. If a disc is modulated at 7 cm/ sec., that disc is cut at zero dB recording level. On loud passages you can expect at least this much level to be supplied by the disc. Very often levels will reach 10 dB above this nominal zero-level value. In order to compare specifications of various cartridges you need to know the relationship between stylus velocity and the output voltage produced by the cartridge. This relationship is a direct proportion. If the stylus velocity doubles, the output voltage from the cartridge will also double. This assumes, however, that the entire electromechanical system record, stylus, and cartridge--are linear. You have to expect some non-linearity, but the amount will be rather small. For example, in the case of your cartridge rated as having 1.1 mV output at 1 cm/sec. at 1 kHz, this cartridge will provide 7.7 mV output at 1 kHz when playing as disc recorded at 7 cm/sec., or normal recording level. TV Sound Q. Is TV sound, as transmitted by television stations, similar in frequency range to FM sound? If not, what is the frequency range? -Joseph Weiglein, Milwaukee, Wis. A. Television audio is capable of producing a wide frequency range from 50 Hz to 15 kHz. TV sound is FM sound, but employing less carrier deviation. However, much of the material produced for television is recorded on optical sound tracks. This places high-frequency limitations on the resultant sound because optical sound systems are usually designed to cut highs sharply above 6 kHz. Video tape recorders, on the other hand, are capable of good high-end response out to 15 kHz. This will be fine if you listen to a live or recorded locally generated show. Network shows will be restricted to 5 kHz unless you happen to be listening to the station from which the broadcast is originating. The bandwidth of the telephone-line circuits is responsible for this high-frequency loss from network feeds. Microphone Switching Q. I am going to build a mike mixer-preamplifier. I want to switch mikes in and out without any clicking noises. Can you give me a circuit which will accomplish this? -Harold Grovesteen, Hyde Park, N.Y. A. Eliminating "clicks" when switching microphones in and out of "hot" circuits can definitely present problems, but it can be done. I would say that the job is practically impossible if the microphones to be switched are of the high-impedance type. However, no matter what kind of mikes are employed, the procedure is the same. You must obtain a low-capacitance key whose contacts are of the make-before-break variety. -------------- (Audio magazine, 11/1969) Also see: ABZ's of Stereo FM--Modern Switching-Circuit Decoder Electro-Voice (ad) = = = = |
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