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Arny Krueger
 
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Default simple Power Supply design help needed

"perry mason" wrote in message


Hi, I'm building a power supply to run a 12vdc car stereo in my room.
The power amp is a 300w ADS Power Plate 300. At 12.5v I need a 24A
supply?


Depends what you want to do with the power amp.

Actual current needs depend on the load impedance and the peak-to-average
ratio of the music you intend to play.

This seems wrong somehow. I thought I'd use the xformer from
a surplus 12v battery charger with an LM317 but I have the feeling
something needs review here.


Others have correctly pointed out that a single LM317 lacks the current
capacity required. You might want to look at a commercial 12 volt regulated
power supply to see what it really takes to do the job.

I'd appreciate any observations others may have. Thanks for your time, pm.


My first observation is that in this day and age, you can often buy for far
less than it costs to make. Paying $130 or so for a 12 volt 30 amp power
supply may seem like *lots*, but check out your bill-of-materials price for
a comparable build-your-own project.

Secondly, most car audio equipment will work well with 12.5-15 volts, and
more DC volts means more peak power which can provide a sonic advantage. 18
volts is the point where amplifier overvoltage protection if any, might
start causing issues.

Thirdly, a regulated power supply may not be required if you can get ripple
and regulation under control in a purely passive design. Car audio gear is
designed to work with relatively noisy power. There is generally another
switchmode power supply inside the amp, between the battery input and the
power to the audio stages. On a sonic-benefit per $$$ spent on capacitors,
it's the output of the switchmode inverter inside the of the amp where the
value often is.

Fourthly, the peak-to-average ratio of even highly compressed music is at
least 8 dB, which means that with a suitable capacitive reservoir (remember,
there is already one such reservoir inside the power amp) you might be able
to get away with 4-10 times less average current coming from your power
supply than what you would need if you were delivering sine waves into a
resistive load. In cars "stiffening capacitors" are often snake oil, but in
your application 100,000 uF or so can let you get away with a far more
modestly-sized power supply.