New-York tribune., 1922-05-28
Pick With Care
The Parts for
Loud Speaker
Radio Fans Warned to Pick
Apparatus With Care and
Avoid Causes of Distor-
ton When Planning Set
By B. Bradbury

| HG0lO EnDlncCf tcHcTHt EteCtTtO
After some weeks or months of lis
toning to radio broadcast programs by
the use Of head receivers, it generally
becomes desirable to install q loud
speak ing device which viii make the
signals audible throughout the room
and enable several people to hear them
at the same time. There is also the
| further desirability Of securing signal
intensities loud enough for dancing or
concerts, where the music must be
heard throughout d fairly goodsized
hall
| To secure this amplifcation and at
i the same time maintain the original
quality Of the signal, it is necessary
to observe number Of precautions in
the selection and operation Of the ap-
paratus used.
Assuming that the signal directly
from the detector is of good quality
and loud enough to be heard distinctly
in the head receivers, our problem is
chiefly concerned in the maintenance of
this quality since it is comparatively
| simple matter to obtain the necessary
intensity.
| In order to eliminate or minimize
loud speaker distortion it is necessary
to know the causes involved. The source
of trouble may be IN the amplif1er, in
the loud speaker tnelf, or in both
| For good amplifcation of telephone
signals the interstage transformer
should be capable of giving a uniform
transfer of voltage from about two
hundred to three thousand cycles a sec
ond. A good many transformers in use
do not do this, and their operation is
ono Of the principal causes for un-
satisfactory signals. For any particu-
lar plate voltage applied to an ampi-
! fer tube there is a corresponding value
ltf negative grid voltage which should
Be applied in order to operate upon
straight portion of the characteristic
curve, and thus secure uniform ampli-
ficstion. with forty volts on the plate
and the negative grid bias should be
about one volt, and with 100 volts on
the plate the grid bais should be
about four volts negative. Standard
amplifier eNbes will generally operate
with two tnoes in cascade without
! overlooking them. Where more vol
ume is desired for r..ry strong signals
it is preferable to use for the third
stage smallpower tube>r,,ith three or
four hundred volts on the plAte Inc
grid voltage in this case sh oN., . be
ABOUT twenty to twenty-five volts 1ie,-a-
tive This will hold the plate curreh,
down to normal value and operate upon
strai ght portion Of the characteris-
TIC. 1t is also we'll to use reactance
coupling between the second and third
tubes, as this will cause very little
distortion and will give good amplif-
cation.
For connecting to the loud speaker.
especially when using q power tube for
the last stage, it is preferable to use
an intermediate transformer with the
proper constants, instead of placing
the loud speaker directly in the 1rbe
plate circuit. This keeps the plate cur
rent out OF the loud speaker winding
and prevents the polarizing effect that
It would have upon the diaphragm
tWo-stage amplifer will generally I
give enough intensity to operate a loud !
speaker in an ordinary living room. !
while the loud speaker mechanism it. (
setf may be of the head telephone type /
with suitable horn. Some receivers
operate very we'll at moderate intensi- +
ties; but when very strong signals are
applied to them they vibrate so vio-
lently that the diaphragms hit he
pole pieces and produce harsh sounds (
instead of good reproduction of the (
original signals. To handle the proper /.
amount of energy, therefore. the con- |
struction must be such that reasonably |
strong vibrations will not force the |
diaphragm to its limit Of travel. The !
material and shape of the diaphragm |
are also important in determining the !
resultant tone qualities. since whatever
natural periods of vibration it NAV
have Will bring out certain tones in
greater proportion than they should be !
to the remaining tones of a musical
selection.
The size. shape and material Of the |
horn arc likewise important factors in /
the final result. The high natural pe I
rlod of q small horn will tend to ac. !
centuate the higher tones to the ex- !
cIuslon of the lower ones. The ma
terra from which the horn is made |
should be some substance which which |
VIII not vibrate readily and produce |
rafting sounds