Emulating car battery output through home power?

[Jak Nikols]

I'm setting up an amplifier which is intended for use in a car, and I'm not using it in one. I'm trying to power it through a typical DC 250 volt wall outlet. What kind of transformer should I use? (I'm also trying to power a car stereo with it as well)

 

[NascentOxygen]

You mean 250v AC, I assume?

How many pins does your wall outlet accommodate?

 

[Jak Nikols]

You mean 250v AC, I assume?

How many pins does your wall outlet have?

3 pins. 2 regular and the optional ground.

 

[berkeman]

I'm setting up an amplifier which is intended for use in a car, and I'm not using it in one. I'm trying to power it through a typical DC 250 volt wall outlet. What kind of transformer should I use? (I'm also trying to power a car stereo with it as well)

3 pins. 2 regular and the optional ground.

You buy a power supply module that is already safety approved for the 240 (not 250) Vrms wall AC Mains output, and provides you with the output voltage required by the stereo or other load. What is the input power for the stereo specified for?

 

[NascentOxygen]

You need a lot more than a transformer. You need a heavy duty 12v DC regulated power supply; this is a box containing not only a transformer but also transistors, ICs, large capacitors, and more.

If you indicate how many watts your amplfier can put out, I'm sure someone here will be able to say what amp rating the regulated supply will need. If you plan to simultaneously power a second power-hungry device, its power needs must also be included in your calculations before deciding exactly what you need to buy.

 

[Jak Nikols]

You buy a power supply module that is already safety approved for the 240 (not 250) Vrms wall AC Mains output, and provides you with the output voltage required by the stereo or other load. What is the input power for the stereo specified for?

On the page in which I'm buying the stereo from, it doesn't specify what the input power is. It just takes whatever the car battery gives it, hence why I'm wondering to just make a wall output act like a battery.

 

[berkeman]

On the page in which I'm buying the stereo from, it doesn't specify what the input power is. It just takes whatever the car battery gives it, hence why I'm wondering to just make a wall output act like a battery.

So the input voltage is 12V. What is the power output specification of the amp?

 

[Jak Nikols]

So the input voltage is 12V. What is the power output specification of the amp?

The amp is 1,500 watts.

 

[berkeman]

The amp is 1,500 watts.

Holy crap that's a lot of power. I = P/V, so that gives you the specification for the output current from an equivalent DC power supply.

 

[Averagesupernova]

Be careful with amp manufacturers specs. There are plenty of ways they can inflate the spec to more than it actually is. If it truly is able to drive 1500 watts into a load then it is unlikely you will be able to find a 12 volt power supply capable of running this amp. If the amp were 100% efficient (it's not) you would need a power source capable of supplying 125 amps.

 

[jim hardy]

On the page in which I'm buying the stereo from, it doesn't specify what the input power is.

Find out what is the current rating of the fuse in its input wire.
Get a supply with current rating at least equal to that of the fuse.

something like this should do
https://www.amazon.com/dp/B001YI95EY/?tag=pfamazon01-20

 

[anorlunda]

A 12v auto battery charger may also provide what you need, and they are relatively inexpensive.

I agree that 1500 watts sounds excessive. Your car would have a hard time supplying so much current.

 

[jim hardy]

A 12v auto battery charger may also provide what you need, and they are relatively inexpensive.

The transformer type charger is not "filtered" so he'd want to add a "supercapacitor" at charger's output. That's available in any car radio shop, even Walmart.

 

[anorlunda]

The transformer type charger is not "filtered" so he'd want to add a "supercapacitor" at charger's output. That's available in any car radio shop, even Walmart.

I would try it first without the filter Jim. The auto 12v environment is very noisy so the amp may have lots of it's own filtering. If it doesn't work well, the capacitor could be added.

 

[jim hardy]

Indeed.
Transformer type chargers hum, too.
Inverter type is quieter both electrically and acoustically

 

[nwaelder]

P = E^2/R, to deliver 1500W RMS using a 12V supply voltage, the resistance (impedance) of the connected speakers would have to be about 0.1 Ohm. With a more practical 2 Ohm speaker impedance, you would need more than 55V to deliver 1500W.

 

[Averagesupernova]

P = E^2/R, to deliver 1500W RMS using a 12V supply voltage, the resistance (impedance) of the connected speakers would have to be about 0.1 Ohm. With a more practical 2 Ohm speaker impedance, you would need more than 55V to deliver 1500W.

This really isn't relevant. Car audio amplifiers in this range have built in switch-mode power supplies to step the voltage up to appropriately drive 2 and 4 ohm speakers. I suspect the amp in question is a single channel class D designed to drive a subwoofer.

 

[nwaelder]

This really isn't relevant. Car audio amplifiers in this range have built in switch-mode power supplies to step the voltage up to appropriately drive 2 and 4 ohm speakers. I suspect the amp in question is a single channel class D designed to drive a subwoofer.

Ah - The OP is "setting up" an amplifier. Thanks for pointing this out.

 

[mr166]

Well as a low cost solution why not connect a 60 amp battery charger to a car battery and use that . The battery should provide enough filtering.

 

[sophiecentaur]

Well as a low cost solution why not connect a 60 amp battery charger to a car battery and use that . The battery should provide enough filtering.

That would be the cheapest solution by far and, as long as you use fat enough supply leads, it would be truly representative of how that amp is going to work in the car.
Does the amp manufacturer have any recommendations about how to supply the amp when mounted in the car? If they don't then their quoted Peak Power value is pretty suspect.

 

[anorlunda]

60 amps times 14 volts is only 840 watts.

1500 watts is still hard to believe. I looked at the owners manual for a 2500 watt car amplifier for a sanity check. It says 2500 watts peak power. It also says "current draw 1.4 amps". It also says that under seat mounting is fine. I believe that the continuous power load is only 17 watts, not 1500 watts. It should be possible to power that amp with only a trickle charger, instead of a welders supply.

http://audiobahn.com/phocadownload/Manuals/Amps/ClassD.pdf

Notwithstanding the above, I have seen cars driving past with the sound so loud that the metal of the roof visibly bounces with each drumbeat. How on Earth can someone sit inside without ruining their hearing in seconds? My own hearing was ruined by a factory job where the noise level was much lower than the music in those cars.

 

[jim hardy]

Marketeers have no qualms about outright lying.

To engineers, power rating of an amplifier is its capability at reasonable distortion without overheating.
That would be continuous RMS watts.
Peak power, calculated using the peak instead of RMS voltage and current would be twice that. That's a modest marketing lie.

To marketeers it is permissible to unabashedly lie by advertising the "Peak Instantaneous Power", or "Music Power" .
That is the absolute maximum volts X amps the amplifier can deliver during a transient lasting only an instant , disregarding distortion and duration of the transient.
It'll be probably the sum of all the channels of a multichannel amplifier because that makes the number bigger yet
Some crum-bums will even give a theoretical value that's calculated assuming power supply startup transient voltage !This paper by Texas Instruments explains the "dirty tricks" .
http://www.ti.com/lit/an/slea047a/slea047a.pdf
and gives actual test results.
.
Just looking at one of the devices in that paper
• TAS5142 – 4 half-bridges (2 BTL channels or 4 single-ended [SE] channels)
let's use the BTL (bridged) rnumbers

That IC can be rated
87 watts RMS continuous , no clipping (not noticeably distorted )
108.5 watts RMS continuous at 10% distortion
174 watts peak power (twice continuous as mentioned)
318 watts peak instantaneous power using the maximum rated voltage for the device. There being two BTL channels in the device, it could be marketed as 636 watts, maybe even more in some countries.

So - TI's VERY respectable 87 watt per channel bridged digital amplifier should be powered by about a two hundred watt supply, ~15 amps at 14 volts
even if it was bought as a "636 watt" amplifier
That's why i suggested OP look at the fuse.

and it's why i posted that preposterous supply . (well, also partly because i found one last week at my metal recycle yard , cost me 30 cents a pound not quite twelve bucks)

I would be surprised to find OP's "1500 watt" amplifier actually capable of 150 honest watts at 1% distortion.

Check out that TI paper , it's an eye opener.

old jim

 

[jim hardy]

Interesting
i just looked at the link anorlunda posted
http://audiobahn.com/phocadownload/Manuals/Amps/ClassD.pdf

that manufacturer publishes RMS watts.

1500 watts into 1 ohm is 38.7 amps
My preposterous supply would be about right for their AT12001DJ

Audiobahn looks like a name to respect.

 

[anorlunda]

Yes Jim, but I think it can supply that much power only in short bursts, like a drum beat. Averaged over time, the current draw is 1.4 amps at 12 volts.

 

[davenn]

Agree with your seconds last post, Jim

NONE ... I repeat NONE of these hi power car amps can supply the stated wattage
its all sales hype PMPO Peak Music Power Out blah blah blah
its been a sales con job on customers who know no better for years, like the OFC (oxygen free copper) speaker cables garbage

in all seriousness you are probably lucky if it produces 700W peak

 

[jim hardy]

Yes Jim, but I think it can supply that much power only in short bursts, like a drum beat. Averaged over time, the current draw is 1.4 amps at 12 volts.

Hmmmm i suppose RMS could be for a millisecond interval at 1khz frequency...

seems over the top, though.

 

[jim hardy]

that manufacturer publishes RMS watts.

1500 watts into 1 ohm is 38.7 amps
My preposterous supply would be about right for their AT12001DJ

hmm

450 watts into 4 ohms is 42 volts RMS, 60peak

which means that thing must have a power supply capable of 60volts and almost 40 amps, 2.4KW.
If so , even my preposterous supply would be way undersized for Audiobahn's AT12001DJ .

What size input fuse does it have ?

 

[meBigGuy]

From the manual:

"Due to the power requirements of the amplifier, this connection should be made directly to the positive (+)
terminal of the battery. For safety purposes, install an in-line fuse holder (not included) as close to the
battery positive (+) terminal as possible with an ampere rating not to exceed the total value of fuses in the
amplifier."

Nowhere do they give a specific input fuse value.

There are 4 30A fuses in the 1500W unit. There are 5 30A fuses in the 2500W unit. The units can be bridged.

I have no insights to this monster.

 

[jim hardy]

From the manual:

Thanks.
The manual never finished loading for me, i was lucky to get to that page i snipped from.It's amazing what one can buy nowadays.
My '49 Buick radio had push-pull 6V6's , capable of maybe twelve watts, with a single 8" speaker . I put in a nice Jensen with 16 oz magnet and it sounded great.
In high school i could name the major US symphony orchestras just by their distinctive sound, over that radio.
Alas , such acute hearing disappears with age.

 

[NascentOxygen]

Don't forget that OP indicated, I believe, that he wishes to simultaneously power two things from this 'battery substitute':

(I'm also trying to power a car stereo with it as well)

I take it that an amplifer and a stereo must be two different beasts?

 

[sophiecentaur]

hmm

450 watts into 4 ohms is 42 volts RMS, 60peak

which means that thing must have a power supply capable of 60volts and almost 40 amps, 2.4KW.
If so , even my preposterous supply would be way undersized for Audiobahn's AT12001DJ .

What size input fuse does it have ?

If you were to use a push me pull you arrangement, you might expect 24V peak to peak swing (less the inevitable drops across the amplifying devices), which would give 576W (absolute Peak Power) into a 1Ω load. I have to wonder about the efficiency of 1Ω LS units, too. What flummoxes me about this is that diminishing returns will set in if you try to obtain high voltages for the supply, from a 12V source. Do the manufacturers use inverters to get more oomph?
It seems to me that their advertising is more high power than their amplifiers. boomboom.

 

[jim hardy]

Do the manufacturers use inverters to get more oomph?

yes. I helped son fix one, it made 40 volts DC so was capable of 80V peak to peak.
Somebody had reversed battery which wrecked the PWM IC and inverter transistors. Audio power IC's on other side of SMPS transformer were okay.

It seems to me that their advertising is more high power than their amplifiers. boomboom.

Agreed.

boomboom?
Given that a speaker has DC resistance ~ 80% of its nominal 8 or 4 ohms, it's hard to envision more than 1/5 of the power it receives actually being turned into sound.
That means hot air from both ends ?

 

[Averagesupernova]

Couple of things:
-
There ARE amps out there that will produce spec'd output power.
-
The 1.4 amp spec is probably in standby with no output.
-
There is no such thing as RMS power although it is spec'd in the car audio world routinely. Power is always average. Even peak envelope power in an AM transmitter it is average taking the RMS volts and the RMS current at the peak of the envelope. The reason car audio equipment got into spec'ing things as RMS is because people started to realize the games that were being played.
-
My guess is that the Audiobahn amp will do what it says. It is a class D amp designed only for sub-woofers. Not a very large portion of the audio spectrum.

 

[jim hardy]

There is no such thing as RMS power although it is spec'd in the car audio world routinely. Power is always average.

I always took RMS to infer average over one complete interval of the test signal as opposed to the instantaneous measurement at a transient's peak.
I tell friends who ask: "RMS is an honest measurement, don't believe any power number that's not marked RMS."
If power is energy/time , and a watt is a joule per second,
a marketeer who wants to stretch the truth can claim X microjoules per microsecond at a peak. X is RMS X crest factor^2 .
Sinewave of 8 volts RMS across 8 ohms is 8 watts
at the peak of the sinwave though it's 16 watts instantaneous.

If he can find a waveform with higher crest factor than a sine, what can he claim ?

it's limited only by his internal power supply voltage !
Red and blue traces have same RMS value but red one's peak is 3x blue one. He could claim 9X more "music power" than his honest competition's RMS for the exact same amplifier...
The worst of the abuses described in that TI paper is to calculate power using the internal power supply's highest transient voltage... liar liar pants on fire...

I agree with you on that Audiobahn amp,

 

[Averagesupernova]

The car audio world is a tricky place. Some amp manufacturers have spec'd their amps at 1 watt. In car audio competition there are various classes and amplifier size is often criteria to determine what class you get put in. db drag racing as it is known is a competition to see who makes the loudest bump. If you have an amp rated at 1 watt which is capable of delivering 1000 times that power how do you think you will fair against those who have amps that are honestly rated? I believe there is a standard 'burp' that is played over the system. It is not just a 50 mS transient. It is probably a couple seconds long.
-
For those of you who think a 1500 watt amp is extreme, I did a quick google:
https://www.google.com/search?q=out...ved=0CAgQ_AUoAmoVChMIpLPQtZ30xgIVB48NCh0JRASR

 

[sophiecentaur]

Power is always average.

Average over whatever time interval you choose to specify. Rate of doing work may be very relevant, even though it only applies for a brief period.
But I am not 'forgiving' HI manufacturers. They are mostly selling snake oil. Their Peak Power may well be relevant to someone, somewhere. Loud impulsive sounds may get through where low organ notes will exhaust the power supply and make it sag.

 

[Averagesupernova]

The main reason I want to specify that power is average and not RMS is because we cannot take the instantaneous power at the crest of a sine wave and multiply it by .707 and expect a correct result which is what we typically do with the voltage and current. It should go without saying that average will be defined over a specific period of time and this time varies due to many reasons.

 

[jim hardy]

we cannot take the instantaneous power at the crest of a sine wave and multiply it by .707 and expect...

Sinewave is well behaved, its crest factor is √2.
So, for sine with resistive load peak power is twice true power.

Square wave would have crest factor of 1because peak = average.

In self defense speaker manufacturers defined a test signal for rating their speakers
http://www.jblpro.com/pub/technote/spkpwfaq.pdf

...at end of that document

clip a sinewave and it approaches a square wave with same peak, containing twice the power

 

[sophiecentaur]

The main reason I want to specify that power is average and not RMS is because we cannot take the instantaneous power at the crest of a sine wave and multiply it by .707 and expect a correct result which is what we typically do with the voltage and current. It should go without saying that average will be defined over a specific period of time and this time varies due to many reasons.

I think the rationale is that a typical audio signal mostly consists of bursts of mid range frequencies and impulsive sounds and it covers many octaves of spectrum. Mostly, it doesn't consist of long periods of single sinewave. For brief periods, in most audio programme, the Power level will be much higher than the Power Level that you would measure with a long term 'thermally' based Energy / Power meter. So it isn't altogether 'bad' to talk about peak power. Many circuits will cope with short peaks of high power without folding. More demanding programme material will sort them out, though.
Interestingly, to give a consistently 'loud' sounding audio signal, the phases of the various components can be tinkered with so that the high peaks are ironed out so the mean level can be turned up to avoid clipping. This, introduces some distortion and the process will also have to introduce some delay but it is useful when, as in AM, there is a fixed maximum that can be handled by the modulation process. this is referred to as Audio Compression.

 

[sophiecentaur]

I want to specify that power is average and not RMS i

To be precise, the RMS value, over a given interval is the Power. RMS is the Sum of the Squares of the Volts (say), divided by R, averaged over that time, whatever the waveform happens to be. If you do that sum on a set of samples of a waveform, you will get the same answer (i.e. correct value) as if you measured the power with a calorimeter. It is common to assume that 0.707 of peak applies to every waveform and many cheap and cheerful meters will do that, assuming a sinusoid, every time.