Tape Recording Magazine - AmericanRadioHistory.Com

... here are the solutions to some of the most annoying
hum problems that plague tape recordists.
by
JAMES H. MILLS, SR.
e
OF all the questions received by our "Questions and Answers"
editor, fully fifty per cent of them have to do with
the elimination of hum. Most of the hum troubles appear
when the recorder is connected to a radio or TV set or
another recorder.
The most prevalent and annoying source of trouble, noise
and hum is caused in part by the "AC loops" which may
be in the equipment itself. The inequality of line resistances
and grounding systems in the home or business power wiring
and ineffective line drainage methods used by manufacturers
of radios, TV sets and other audio devices also add
to the causes.
One of the most essential requirements for click, noise
and hum -free recording is a common source of commercial
power. This may be obtained by connecting all devices into
the same double outlet or through the use of a three -way
adapter which is available at your hardware store or electrical
dealer's.
Almost everyone's electrical supply is heavily affected
by commercial users, long exposed lines pick up noise, local
appliances contribute clicks and current ups and downs,
and even radio and TV stations can cause trouble as their
current demands are heavy and fluctuate rapidly.
In almost every case a material improvement in the
quality of recordings will result if you use a "brute force"
line filter in combination with your multiple outlet plug as
shown on page 45. These are the "Tobe" or "Cornell -Dubilier
-IF series" units connected as shown in the sketch below.
Be sure a good ground is provided for the filter as directed
in the instructions that accompany the unit.
After the three -way plug is inserted, connect the recorder,
radio, etc. by plugging them all in the same plug. Transpose
the plugs until minimum hum is achieved in all units.
Once the hum is cleared from the incoming power you
can turn your attention to the audio connections.
The following arrangements and circuits are not specific
in every case but rather general. In some particular types
of equipment they must be varied or experimentally applied;
however, the principles involved may be fitted to almost
any case by one fairly adept at hooking things up or by
a radio or TV mechanic.
The popular type tape recorders have a conventional
input arrangement which is fairly broad in tolerance. It
usually runs anywhere from 15,000 to 500,000 ohm impedance
with only minor losses in quality over this range.
These input jacks are usually designated "Microphone,"
"Radio," or "Phono" and require a driving level, or voltage
input, of from .01 (in the case of a microphone) to 1 volt
(in the case of a radio or TV pickup).
Because of this wide variation, and because there is no
standard ratio between these inputs from brand to brand of
recorder, you may have to experiment to find the jack that
gives you the least hum. It may be necessary to plug your
phono connector into the microphone jack on the recorder,
even though you have a jack on the recorder marked
"phono." This would be true where the phono input was low.
By plugging it into the mike jack you do not have to turn
the volume up so high.
ANT
For the elimination of hum, clicks
and noise from the power lines the
use of an AC line filter is advised.
A three -way adapter should be
plugged into the unit and all the
audio devices hooked into it as
shown. The plugs should be transposed
until minimum hum is secured
from all units. The line filter
should also be connected to a good
ground.
0 AMR4 RADIO iVO
PHONO
- THU E -WAY
ADAPTER
WALL OUTLET
(S)
RECORDER
1- GROUND OpR
F ACE PLATE
C Fw
43