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? This seems wrong somehow.

You may need _more_ than that. If the amp is actually capable of
creating 300 watts of clean audio output power, it's likely to
actually be drawing 500-600 watts of total power (amps are not 100%
efficient), and you may need 50 amps or more.

If the "300w" is marketing puffery, or if you're willing to run the
amp at quite a lot less than its full output capacity, the actual
needs might be a lot less than that.

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.

Darned right. An LM317 by itself is limited to 1.5 amperes, I think.
It's not going to cut it.

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

You're likely to need a hefty DC supply to run an amp of this calibre
indoors.

Using a battery-charger transformer and rectifier bridge is probably a
reasonable starting place. You'll need a good-sized filter capacitor
following the rectifier bridge. You'll need a hefty set of pass
transistors - the commonest linear-supply design of this sort uses
multiple 2N3055 transistors (TO-3 cases) in parallel, with ballast
resistors, and a set of big heavy heatsinks. Take a look at the LM317
data sheet - I believe you'll find details on how to use external
transistors such as 2N3055s as external pass-elements, to boost the
LM317's current capacity. For supplies in the 25-50 ampere range I
suspect you'll need an external driver transistor or two inbetween the
LM317 and the pass transistors, to provide the necessary amount of
base drive current. You'll need to design in current-limiting
circuitry (straight limiter or foldback) to keep your supply from
frying its transistors, or starting a fire if the outputs are
accidentally shorted.

If all of this still seems a big overwhelming, you could just buy an
Astron switching power supply (see www.hamradio.com for one source),
or just decide that using an auto amp in a home setting is more
trouble than it's worth, and buy an amp designed for 110 volt operation.

--
Dave Platt AE6EO
Hosting the Jade Warrior home page: http://www.radagast.org/jade-warrior
I do _not_ wish to receive unsolicited commercial email, and I will
boycott any company which has the gall to send me such ads!

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

get your supply to put out around 14V and you can get by with less amps.
Check the documentation on the amp. It should specify what the working
voltage range is.
----------------
Alex

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

get your supply to put out around 14V and you can get by with less amps.
Check the documentation on the amp. It should specify what the working
voltage range is.
----------------
Alex

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

perry mason:
You will very likely need a hell of a lot more amps than a LM317 @ 1.5A can
deliver. Also it is very unlikely that your surplus 12 volt battery
charger can deliver enough current to be useful in building the kind of
heavy duty power supply that is certainly going to be required for your set
up....... 24 amps at an absolute minimum with an actual requirement of
almost double that if indeed your 300 watt amp can really deliver 300 watts
RMS ... and that you intend to run it at high volume levels and are
expecting clean performance.
To build a safe and proper 12 volt power supply that can deliver enough
current for your apparent needs is not going to be an easy, novice task, nor
will it be cheap.
Do a search on the internet for power supply and electronic equipment
suppliers and most certainly Ebay for power supplies that are already built,
you can most probably buy something cheaper than if you built it.... and if
you purchase a quality built unit it will likely include important safety
shutdown features to protect your stereo equipment, your home and the power
supply against over-voltage and fire starting short circuits.... very
important when dealing with this kind of metal melting current.
A possible low-tech solution for you would be for you to use that 12 battery
charger that you have to keep an automotive battery continually topped
off.... then you can run your high power stereo system through a suitable
fuse directly from the battery. If the charger is an "automatic" type that
will reduce to a trickle charge when it senses that the battery reaches a
fully charged condition you can leave it hooked up all the time whether you
are using the stereo or not. The main drawback to all of this would be
that if you intend to run your stereo at very high volume levels on a
continuous basis the battery may run down after approximately an hour or so
unless you give it a chance to charge up with the stereo shut off or turned
down to easy listening levels.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


..
..

"perry mason" wrote ...
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? 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. I'd appreciate any observations others
may have. Thanks for your time, pm.

From what we can perceive as your understanding of
electronics, even if you had an adequate transformer (which
seems unlikely from your description), a proper schematic,
and a kit of all the components, you would still likely find it
very difficult to successfully build such a power supply.

Where did you get the 24A figure? What is the fuse rating
on the amplifier? 24A seems too low assuming the unit is
really 300W because it is very unlikely to be 100% efficient.

"perry mason" wrote ...
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? 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. I'd appreciate any observations others
may have. Thanks for your time, pm.

From what we can perceive as your understanding of
electronics, even if you had an adequate transformer (which
seems unlikely from your description), a proper schematic,
and a kit of all the components, you would still likely find it
very difficult to successfully build such a power supply.

Where did you get the 24A figure? What is the fuse rating
on the amplifier? 24A seems too low assuming the unit is
really 300W because it is very unlikely to be 100% efficient.

"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.

perry mason wrote:
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? 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. I'd appreciate any observations others
may have. Thanks for your time, pm.
Run it from a car battery with charger.

A

perry mason wrote:
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? 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. I'd appreciate any observations others
may have. Thanks for your time, pm.
Run it from a car battery with charger.

A

"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.

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

perry mason:
You will very likely need a hell of a lot more amps than a LM317 @ 1.5A can
deliver. Also it is very unlikely that your surplus 12 volt battery
charger can deliver enough current to be useful in building the kind of
heavy duty power supply that is certainly going to be required for your set
up....... 24 amps at an absolute minimum with an actual requirement of
almost double that if indeed your 300 watt amp can really deliver 300 watts
RMS ... and that you intend to run it at high volume levels and are
expecting clean performance.
To build a safe and proper 12 volt power supply that can deliver enough
current for your apparent needs is not going to be an easy, novice task, nor
will it be cheap.
Do a search on the internet for power supply and electronic equipment
suppliers and most certainly Ebay for power supplies that are already built,
you can most probably buy something cheaper than if you built it.... and if
you purchase a quality built unit it will likely include important safety
shutdown features to protect your stereo equipment, your home and the power
supply against over-voltage and fire starting short circuits.... very
important when dealing with this kind of metal melting current.
A possible low-tech solution for you would be for you to use that 12 battery
charger that you have to keep an automotive battery continually topped
off.... then you can run your high power stereo system through a suitable
fuse directly from the battery. If the charger is an "automatic" type that
will reduce to a trickle charge when it senses that the battery reaches a
fully charged condition you can leave it hooked up all the time whether you
are using the stereo or not. The main drawback to all of this would be
that if you intend to run your stereo at very high volume levels on a
continuous basis the battery may run down after approximately an hour or so
unless you give it a chance to charge up with the stereo shut off or turned
down to easy listening levels.
--
Best Regards,
Daniel Sofie
Electronics Supply & Repair
- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -


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