"perry mason" wrote in message

On Fri, 7 Nov 2003 11:02:17 -0500, "Arny Krueger"
wrote:

"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.

Thanks, that's really what I was looking for. Roughly, what kind of
voltage rating do the caps need?

25 volts working voltage generally suffices, IME.

I assume I would have to parallel
values to come up with a 100,000 uF total.

Actually you can find commercial caps that run up past 1,000,000 uF - that's
a Farad or more!

The really big caps are called "stiffening caps" and can be found in car
audio shops and on eBay for around $50-100. I found a surplus cap that is
rated at something like half a Farad for about $30 on eBay. It does a nice
job of stiffening a surplus (eBay, natch) 15 volt 20 amp switching power
supply to power a (true and genuine) 60 wpc carsound amp for home use. I
think said amp is called a "200 wpc" amp by some but after you dust off the
spec sheet, its about 60 wpc.

One problem with all this shucking and jiiving with car sound amps is that
by the time you provide them with power, your investment is beginning to
approach or exceed the cost of a cheap 100 wpc stereo receiver.

Also, the amp has a switch
labeled "low power". I'm not sure what that does technically but if
you depress it while it's playing the volume drops drastically so
maybe the design could be a lot less demanding. It doesn't have to be
very loud. 3/4 volume with the low power switch in is still plenty
loud, it just doesn't have the visceral punch car stereo owners look
for. I see now there is an automotive style 20A fuse plugged into the
bottom of the power amp. The exact model is a Power Plate 120, so
maybe that's 60W per side. A whole different requirement.

Well a fuse will carry more than rated current for short periods of time,
but periods of time that can give the perception of increased loudness.
Suffice it to say that with suitable backing capacitance, a power supply
that can provide enough continuous power to blow the fuse would be
sufficient. I think you'd find that the classic 30 amp 0-15 volt power
supply commonly used to service car audio components would do a nice job for
you.