April 1942 QST
 Table
of Contents
Wax nostalgic about and learn from the history of early electronics. See articles
from
QST, published December 1915 - present (visit ARRL
for info). All copyrights hereby acknowledged.
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 The National Company of Cambridge, Massachusetts,
was one of the first large distributors of electronics components and appliances.
It began life as the National Toy Company. Under the leadership of
Mr. James Millen, W1HRX,
National quickly became a market leader in designing and marketing high quality
radio products for both amateur radio operators and casual listeners of shortwave
radios. National Company very often ran full-page advertisements in the American
Radio Relay League's QST magazine, that were more in the form of an infomercial
than pure product promotion. This one, number 98 in a very long-running series,
extolls the virtues of their famous NSM Modulator. The majority of these ads were
written my Mr. Millen, but in this case it was done by Calvin Hadlock.
A bit of trivia: National Company was the first paid advertiser in The Radio
Amateur's Handbook.
National Company Advertisement
 (Number ninety-eight of a series)
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
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