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About RF Cafe
Copyright: 1996 - 2024 BSEE - KB3UON RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while typing up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived... All trademarks, copyrights, patents, and other rights of ownership to images
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National Company Advertisement
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A bit of trivia: National Company was the first paid advertiser in The Radio Amateur's Handbook. National Company Advertisement
These days when all sorts of amateur gear are being adapted to serve new defense purposes; we wish to point out that the NSM Modulator has great possibilities as a public address amplifier. With a microphone and suitable speakers, it serves the same purposes as a megaphone for calling and giving instructions though its range and clarity put it in a class by itself. Its capabilities begin where the megaphone's end. The peculiar fitness of the NSM for this job lies in its automatic volume compression circuit. This circuit was originally designed to permit high modulation levels without danger of overmodulation. With minor changes this same circuit provides a high level signal for loud-speakers without danger of blasting. In our experience, this is a big help. When a microphone is used by a group of people with different speaking habits and with no previous microphone experience, some sort of monitoring is essential. An automatic device will do this job better than a manually operated control because it gives consistent results and is always on the job. The other features of the NSM also fit the specifications for a good public address amplifier. It has an undistorted output of 30 watts and is entirely self-contained. A four position tone control cuts highs or lows, or both, or leaves intact the normal range of 50 to 10,000 cycles. There are two input circuits, one of which provides ample gain for any of the commonly used microphones. In adapting the NSM for use with loud-speakers, the principal change is in the volume compression circuit. This circuit is, in effect, a delayed AVC. In a transmitter, the delay voltage is taken from the Class C plate supply, so that any change in plate voltage will automatically cause a corresponding adjustment in the modulation level. For use with speakers, where there is no Class C plate supply, the delay voltage can be obtained from the regular plate supply of the NSM through a suitable resistance network. The actual change is very simple, and we will send specific instructions on request. It is, of course, also necessary to provide an impedance match. The NSM is designed for a load of 1500 Ohms. In many cases, a series parallel arrangement of the speakers can be used to obtain this value. A matching transformer can also be used of course, but if the speakers are located some distance from the transformer do not use too low an impedance (such as voice coils in parallel), for the high current at low voltage will waste a lot of power because of the voltage drop in the leads. Calvin Hadlock
Posted May 11, 2017 Here are all the National Company advertisements I have:
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