On Jan 21, 8:27*am, "William Sommerwerck"
wrote:
No one mentioned putting a string of silicon diodes in series to lower
the voltage. *Unlike a resistor, the voltage drop would be reasonable
fixed. *But, no fancy parts, capacitors, etc are required. *Sort of
crude, but why wouldn't it work? *7 diodes would drop approx 4.2 volts
bringing 13ish down to 9ish. Or, maybe 8 diodes to be safe. *13.8 down
to 9. *Just wondering why no one mentioned the possibility. Heat
would have to be handled, but no worse than a three terminal regulator
and the heat would be spread across all 8 diodes.

Excellent idea. 1N400x rectifiers should work.

I tried running a small 9V transistor radio years ago at work from a
12V supply. I used a zener and a series resistor. Even with such a
small radio the pair got much too hot. The OP plans to power a boom
box. At the least he'll need to use a three terminal regulator or a
series pass transistor with the zener in the base circuit. Whatever it
turns out to be may need a heat sink, must be properly polarized,
fused, adequately wired, (so as to not set the vehicle on fire), and
I agree with William that this project, although simple to a seasoned
technician might be beyond his capabilities. Lenny