am..Brew

Number 12 on your F~ba ck ca rd
Good Mobile Audio­
For Pennies
Conveniently feed rig audio to your car speaker system.
by Paul M. Danzer Nlil
O
nee again. I put a new 2
meter rig in my car. and
once again, I was d isappointed .
There is always a lot of high
mobile ambient noise in a car.
Combine this problem with the
small size of the speakers provided
with most 2 meter rigs
and you end up with with a
sound system that just does nor
provide the dean distinct audio
we now routinely expect from
modern ham equipment.
Whether this is due to the
speaker or to the small size of
the aud io amplifier is debatable.
but the poor results are evident.
Years ago, there was plenty of
room to neatly add an external
speaker to solve the problem.
Today's automobiles are much
more cramped.
Solution
Fortunately. today's cars also
often h a ve fou r spea ker s
mou nted for the AM/FM radio
and cassette player.
The first thou ght you might
have is to open up one of these
speaker lines and share it with
the two meter rig. On second
thought. the problem of switching
back and fourth between the
ham gear and the entertainment
gea r gets quite messy.
The answer to this almost universal
problem is in the cassette
player. Figure I shows how to
do it.
Start out with a standard audio
cassette . If possible. obtaino ne which can
be opened by unscrewing 4 or 5 Phillips-head
screws. If necessary. you can use the glued
units. but this makes opening them up a little
harder.
Next, strip out everything in the cassette.
Phoro A. The assembled unit to pipe Jour n"g 's AF to your car speaker
system via the cassette tape pickups.
WIR E COIL \
GUID E P I~~ o )
(
-ec ex- WIRE\
/:
•
•
"
Now , when you look at the bottom half. you
will see something like Figure I, where on
each side ofthe opening for the tape there are
IWO guide pins.
Wind a small coil out of #28 , #30, or even
#36 enamel wire around the guide pins. The
"
• •
'-GAP '--CASE
OP EN CASE CBOT To ... HALF SHOWff l
Figure J. Inside ofthe cassette tape. Wind 4-5 turns of 28-36 gauge
wire around the guide pins.
kind of wire to use depends on
what you can get . and on what
you can handle. (For some pe0­
pte. scraping the enamel off #36
enamel wire is a bit of a task.)
For the winding. 5 to JO turns
will be e no ug h. The actual
numbe r isn't c ritical-you' ll
still end up with plenty of gain
for the audio.
The resistors in the figure
a re the re to prevent shortcircuiting
the audio amplifier.
I suspect that the resistors are
not re a ll y nece ssa ry under
ma ny c ircumsta nces. If the
small coil looks like a few ohms
(say 4 ohms), no resistors are
needed . I prefer 10 take no
chances so I raided my junkbox
and used a combinatio n which
provided a value of 2 ohmsjust
to play safe.
Finally, take a length of fl exible
fine wire. like the kind used
for miniature speaker or earphone
connections, and lead the
wire out of the cassette. Chose
the edge of the cassette which
will be clear when the cassette is
inserted into your cassette player.
For some units, you will
have to use one short edge. as
shown in Figure I. For other
units, the rear (long) edge is the
only o ne dear.
After dosing up the casscue.
connect a plug to the "f l ex"
wire to match you r rig's external
aud io output jack. Insert
the cassette , turn the player
and your rig on, and admire the audio. Two
front speakers, two rear speakers, front
and back fader, left and right adjustment,
bass control, treble control, a couple of
Watts per cha nnel - what else could you
want ? III
73 Amateur Radio • July,1989 43