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Did you ever attend a "silent" dancing party?

The dancers wear

earphones and only they hear the music. The effect is eerie...

Y ES, you can hear loud and clear with

no physical connection between your
earphones and radio or hi-fi. What's more
through the transmitting coil, they gener-
ate a magnetic field around the coil which
varies in proportion with the currents. The
you can hear when others cannot. The loop field produced by the transmitting coil in-
system is great for getting the sound from duces currents in the receiving coil which
your television without interrupting grand- are a facsimile of the signals applied to the
ma's nap. With loop listening a housewife transmitting coil. These currents in the re-
can keep up with her chores while hearing ceiving coil are applied directly to phones
her favorite programs without trailing or an amplifier for further amplification.
wires and without having the radio or hi-fi The action is exactly the same as a trans-
blasting through the house. Here's how former.
your loop system works and how to build
it. For Maximum Range, the transmitting
loop should be as large as possible and con-
In Operation, as the signals flow sist of many turns. To wind the coil, trace a
JUNE, 1963
1. WIRE loop and speaker to select. 2. SCHEMATIC for transistor amplifier which boosts sound.

line conforming to the desired overall di- The Receiving Loop should be as large
mensions on your workbench. Drive 1-in. in diameter as possible. Since the receiving
nails equal distances around the marking loop will normally work into medium to
to form a coil form. When the winding is high impedance inputs, it should have as
completed, remove the coil from the form many turns as are practical as this will in-
and secure its turns in place with tape. Re- crease both its sensitivity and impedance
move the insulation from the leads and match. As mentioned earlier, the receiving
attach them to a convenient length of loop can be connected directly to a pair of
ordinary "zip cord." phones for short range operation. The
Due to its low impedance, the transmit- phones should have an impedance of be-
ting loop is connected to the transformer tween 500 and 2,000 ohms.
terminals of the particular amplifier being A self-contained amplifier can be used to
used. Due to the low impedance of the out- considerably boost the operating range.
put transformer secondary, #20 or heavier With the transistor amplifier between the
wire should be used to wind the transmit- receiving loop and phones, the operating
ting loop. The coil should not consist of range was extended to about 20 feet. A five
more than 50 turns. If you like, a S.P.D.T. inch coil wound with 100 turns of #30 wire
switch can be included in the setup so that yielded an operating range of about 15 feet.
either the loop or speaker is connected to The transistorized amplifier is straight-
the output transformer. forward with the exception that a common
base input stage is used rather than the
more conventional common emitter con-
figuration. This provides a better imped-
ance match between the receiving coil and
the amplifier's input. The output trans-
former shown in the schematic matches the
last transistor to the four ohm stereo
Placement of the receiving coil need not
be a problem if a reasonably sm,all loop is
As Much Power As Possible should be
used to drive the transmitting loop in order
that the amount of amplification between
the receiving loop and phones can be kept
to a minimum. Excessive amplification at
the receiving end can cause an objectional
amount of hum and spurious noise. The
ratio of the energy emitted to the sur-
rounding radiation should be as high as
possible. •