PORTABLE SCINTILLATION COUNTER
FEBRUARY 1956
K
isvr".14
TELEVISION
SERVICING
HIGn FIDELITY
In this issue:
How
a
t)
Construct
Hartley "Baffle"
Easily Built
Echo Unit
Transistorized
Scope Calibrator
Remote Control
for the 630
35
and
CANADA
U. S.
A
"Three-Way" Bicycle Radio
(See page 4)
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AUDIO -HIGH FIDELITY
50L6 -GT
Side view shows all
components mounted.
A mechanical
sound-delay device
produces novel
effect in
echo
SPRING
DRIVER
PICKUP
music and voice
OUTPUT
TRANS
BUILD THIS SIMPLE
ECHO UNIT
By DANIEL M. COSTIGAN
THE listener can generally hear a
certain amount of echo effect in
most popular commercial recordings made within the past few
years. This is the result of a recording technique which, not too long ago,
was used only to create novelty effects.
The recording industry has discovered
that this technique, when skillfully
handled, can make a recording sound
richer and far more interesting than
older methods where reverberation was
purposely suppressed.
This echo effect is achieved by sev-
SOUND SOURCE
-i
eral methods, including the use of actual
hard -walled chambers, planned placement of microphones and special electrical devices which simulate an echo
by delaying the sound mechanically
before it reaches the microphone. The
sound -delaying mechanism is often a
metal spring or network of springs
with a driving unit at one end and
a microphone at the other. The unit
described here is based on this method.
The unit (see photos) consists entirely of such easily acquired items as
a screen-door spring, a disc-recording
ORIGINATING AMPL
."`,SPRING
MAGNETIC PICKUP
DRIVER
MAIN SPKR
ECHO AMPL
L-
Fig. 1 -Block diagram of apparatus for producing musical echo effect.
head as the driver, a discarded headphone for the pickup, a modified a.c.d.c. phono amplifier and a few rubber
shock mounts.
Fig. 1 shows the basic layout of the
unit. (Only the components shown
within the dotted lines are described.)
The driver is a low- impedance, magnetic type disc -recording head which
can be connected directly across the
speaker of the amplifier to which the
original sound is being fed. The spring
is of the normally compressed type
having fairly low tension. A simple
test for the required tension is holding
one end of the spring in the hand and
noticing how far the other end drops.
The farther the spring sags under its
own weight, the greater will be the
echo effect produced. The one used in
this unit is approximately % inch in
diameter and 1 foot long when unstretched.
The microphone (pickup) is a stand I,s'I.
2'
Ls'
I-
I
50L6-GT
12SQ7
.01
ECHO CONTROL
SEE
HIES
i
I
o
¶50
Fig.
T30
-U- DO
-------------
I
i
1.375'
FOR MOUNTING
"HOLES
FOR MOUNTING TO AMPL CHASSIS
3- Pattern for the subchassis.
NOT GND
RUBBER SHOCK MOUNT
35Z5-GT
NOTE: One side of the pickup lead and the
control jack are "hot" to ground. Terminals
must be inaccessible and shielded leads and
jack insulated, to avoid possibility of danger-
-The
o
o
CHASSIS TO ECHO UNIT
50n
I
2
fOR TÌIBE SOCK
i
Ì
I
JACK
TEXT
3.3K
Fig.
-moo
TERM STRIP
1.8K
3
.05
ous shock.
¡
I
FOR
TONE
270K
I
HE
r
5
220K
5
o b25'
I
I
Ir.D3v..
3
12AU6
o
.5;:i
I.5"
NUT
5
MACHINE SCREW
SPRING
B
12in
II?VAC-DC
2
7
SEE TEXT
50L6
125Q7
METAL CLIP
12AÚ6
GND
2
7
7
8
3
4
echo amplifier -circuitry within dashed lines has been added
STRAP
Fig. 4-Diagram shows how spring is
attached to its shock mount supports.
RADIO -ELECTRONICS
52
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AUDIO -HIGH FIDELITY
amplifier,
Phono
Underside view of
the phono amplifier and the at-
terminal strip and
output transformer
mounted on top.
tacked subchassis.
TONE CONTROL
ECHO CONTROL
JACK
e>
mit
12
Fig.
5- Cross-sectional
Ports for echo unit
Resistors: I-220,000, 1- 270,000
/2 watt;
watts.
I
-3,300 ohms,
-I
1
1
I-
Idisc- recording
-low- impedance
head; 1-1,000ohm magnetic headphone; -output transformer to
match 5016; I-2- terminal barrier strip; 2- rubber
shock mounts; 1-subchassis (see text); 5- rubber
% -inch grommets;
/= s 3 x 20 -inch plywood mounting board; 3- ' / ?-inch rubber grommets;
cabinet
for mounting unit.
I
I
I
-'
.
view shows method of shock -mounting echo unit.
Capacitors: -.001, -.005, -.05 pf, 400 volts;
1-8 pf, 50 volts, electrolytic.
1- a.c.-d.c. phono amplifier;
Miscell
:
12AU6 and socket;
screen -door spring (see text);
1
(.
°+( ? '#
Sn
megohm,
ohms, I
watts; I -125 ohms, 10
2
{:
I-
and 1,000 -ohm headphone unit with its
cover and diaphragm removed, the
spring replacing the diaphragm as the
vibrating element. It will probably be
necessary to do some experimenting to
determine the spacing between the
spring and the pickup. I found that
the most gain could be had by letting
the eyehole at the end of the spring
rest firmly on the phone casing. This
particular type of pickup was chosen
because it has no direct mechanical
contact with the spring and therefore
allows the spring to vibrate more freely
and prevents any damage to the pickup
in case the spring should vibrate excessively.
The recording head, or driver, is
energized by the output of a standard
amplifier and the resultant vibrations
are transmitted through the spring to
the pickup. A stiff piece of wire, extending from the needle chuck of the recording head through one end of the spring,
provides the necessary mechanical
coupling. The voltage generated in the
pickup by the vibrating spring is then
amplified and fed to an auxiliary
speaker which operates simultaneously
with the one to which the recording
head is connected. The echo amplifier
is a standard a.c.d.c. phono amplifier
modified by the addition of an extra
stage of voltage amplification.
Fig. 2 is a schematic of the modified
amplifier. The only changes made, other
than the added stage (shown within
the dashed lines), were replacing a
200 -ohm ballast resistor in the heater
circuit with one of 125 ohms, and
altering the heater wiring to accommodate the 12AU6. Also, since I intend
to use the unit with an electronic organ,
the gain control on the original amplifier
was replaced by a phone jack and the
circuit changed to accommodate a shunt
type control which will be mounted on
the organ console and connected to the
echo unit by a shielded cable. This
change, of course, is optional.
The added stage was built on a subchassis measuring 2 x 2 x 114 inches
(Fig. 3) and attached to the phono
amplifier chassis at the end nearest the
first amplifier stage. I had to cut a
rectangular opening in the side of the
echo unit to clear the large electrolytic
capacitor protruding from the bottom
of the chassis. The shielded lead connecting the pickup to the amplifier is
also run through this opening.
Shock mounting is a necessity because even the most feeble external
vibration reaching the spring will be
picked up and amplified. In this unit,
the spring was stretched between two
rubber shock mounts of the type used
in military communications equipment
and available on the surplus market.
Each end of the spring was attached
to its mount by a machine screw and a
U- shaped metal clip (Fig. 4). The
spring, when mounted, was stretched
to about 1% times its normal length.
The rest of the shock mounting was
done with ordinary rubber grommets.
The pickup, for example, was disassembled and the two screws which
hold the entire unit together were replaced by longer ones to allow a pair
of grommets mounted on a metal plate
to be added when the unit was reassembled. Two holes, large enough to
clear the grommets, were then drilled
through the mounting board and the
pickup assembly fastened over them.
The mounting board measures approximately 3 x 20 inches. The driver was
mounted on a metal plate which, in
turn, was isolated from the mounting
board by grommets. This whole unit,
consisting of driver, spring, pickup and
mounting board, was then isolated by
grommets (Fig. 5) from the base of
the rectangular case which houses it.
The driver was mounted in the middle,
instead of at one end of the unit, to
give added support to the otherwise
flimsy spring. I found that changing
the position of the driver had very
little effect on the amount of echo
produced. Of course, if it is mounted
too close to the microphone, there will
be an inductive coupling which will
decrease the echo effect considerably.
All metal objects within the immediate vicinity of the microphone, including the spring and the pickup casing,
should be grounded to provide hum
END
protection.
53
EBRUARY, 1956
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