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Old 01-14-2012, 03:37 PM   #1
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Default Help me find a 12v source for my amp

I'm using this little guy from ebay:

to power my old speakers. It needs DC-12V .5A, see?

Currently using this wall wart to power it:


As you can see, that's supposedly 3 volts and 400mA higher than what the amp needs, but it works perfectly. I can get the system way louder than I need it and quality is exactly what I want.

Problem is any 12 volt source I find in the car is of a much higher amperage, and distorts the sound at any volume higher than what my iphone can do on it's own. Found one that was barely better, and it caused the check engine light to pulse with the music, which can't be good.

Any idea where I can get a decent 12v .5-1a source? I'm sure the stock stereo wiring would work, but that's long been cut out. There's still lots of stuff under the dash, just don't know where to tap it from.
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Old 01-14-2012, 03:50 PM   #2
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Amperage is current draw, it isn't forced. In other words, providing a source of higher current capability doesn't mean the device drawing from it will be overpowered, it will only draw what it needs to operate, so that isn't your problem.

My guess is voltage fluctuation caused by your alternator when plugging it directly into your car's electrical system is what is causing the distortion. i.e. -> your amps internal reg and power supply filtering are ----.

I can power my detcans from a wall adapter or from batteries and get crystal clear sound all day long, but plugging them into the cig lighter port on a car makes them pretty much unusable.

What you probably need is a good sized cap on the power feed line to suppress the noise. You can pick them up on ebay for pretty cheap, like <$10 bucks.
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Old 01-14-2012, 03:53 PM   #3
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Yeah I was thinking the same thing has Jeff. Get a capacitor
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Old 01-14-2012, 03:55 PM   #4
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That's not what I wanted to hear!

Can you link me to something like what you were talking about? Maybe I have something around here that would work.

I know when I wired in a cigarette lighter to some random 12v15a power source off the fuse box it would work for my DC-AC power inverter, but blow up my phone chargers.
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Old 01-14-2012, 03:56 PM   #5
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Quote:
Originally Posted by curly View Post
Problem is any 12 volt source I find in the car is of a much higher amperage, and distorts the sound at any volume higher than what my iphone can do on it's own.
I sense a fundamental misunderstanding of how current (in amps) is expressed and measured.

The amperage rating on the device (0.5a) tells you that, when operating at 12v, the device will consume a maximum of 0.5 amps (continuous) when operated at its highest level. Because of this, you must feed it from a circuit which is capable of supplying no less than 0.5 amps. You can connect it to a circuit rated to deliver 1 amp or 100 amps, and the device will perform the same.

Think of an air compressor. Voltage is air pressure (PSI), and current is air volume (CFM).

Now, say that you have a small grinder that wants 40 PSI at 10 CFM. We all know that you need to stick a regulator in there to reduce the pressure of the air from whatever is in the tank down to 40 PSI or you'll probably damage the tool. But does it matter if you have it plugged into a compressor which is capable of delivering 50 CFM vs one rated only for 20 CFM? Of course not. So long as you don't over-pressurize the tool, it's only going to consume 10 CFM, regardless of what the compressor is capable of supplying it.


Same concept in electronics. At a given voltage, any device is only going to consume as much current as it wants to, and no more. You could connect that amp directly to the battery (which is capable of supplying at least 1,000 amps) and it would work just fine.



Sidebar: Ohms Law tells us that Power (in watts) is equal to Voltage (in volts) x Current (in amps). So an amplifier which draws 0.5 amps at 12 volts will be capable of delivering an average power of 6 watts. (Figure normal system voltage at 13.8v instead of 12v and you can get all the way up to 6.9 watts.)



edit: I see that in the time it took me to write this, several others have chimed in with similar opinions. Curse my obsession with verbosity.
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Old 01-14-2012, 04:18 PM   #6
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Don't worry, I understood from Jeff that I was doing it wrong, you clearly spelled out why.

So Joe, or anyone, does the fluctuating voltage from my alternator sound like the issue?

My revised question: does anyone know of a steady 12 volt, anyf'ing amp circuit I can draw from?
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Old 01-14-2012, 04:22 PM   #7
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You have switched power like 5" away, at the MegaSquirt. Just put a fuse on the wire you run.
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Old 01-14-2012, 04:31 PM   #8
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You can get a light tester and test a bunch of those wires you said you cut out for the radio. Find a good ignition on source and run it to that. If you still have an issue with sound quality then I agree it needs a cap.
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Old 01-14-2012, 04:39 PM   #9
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Quote:
Originally Posted by Ben View Post
You have switched power like 5" away, at the MegaSquirt. Just put a fuse on the wire you run.
Lol. Deal.

Checking...


Braineack built harness, which one Ben?
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Old 01-14-2012, 05:10 PM   #10
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It's probably best to get the power right off the battery....
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Old 01-14-2012, 06:54 PM   #11
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Link to amp you bought please?
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Old 01-14-2012, 09:33 PM   #12
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http://www.ebay.com/itm/120426423251...84.m1497.l2649

If you're interested, it really does work great with a wall wart, can't wait to have some tunes for my next drive to ORP, or if I decide to drive it to work.
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Old 01-14-2012, 09:50 PM   #13
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So what would I need to do to use this with 2 door speakers in Miata? Just power it and hook directly to speakers? Oh and what's a wall wart?
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Old 01-14-2012, 09:55 PM   #14
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Quote:
Originally Posted by curly View Post
So Joe, or anyone, does the fluctuating voltage from my alternator sound like the issue?
A cheap amp sounds like the issue, and I'm not being sarcastic here. An automotive electrical system is a harsh, noisy place, and that needs to be borne in mind when designing a high-gain analog device that's going to be connected to it.

Quote:
My revised question: does anyone know of a steady 12 volt, anyf'ing amp circuit I can draw from?
Well, the whole electrical system of the car is, in theory, at the same potential. A pair of wires run directly from the + and - terminals will have the cleanest power, as they connect to the one point in the system which is "furthest away" from everything noisy, but I wouldn't imagine you will see much variation no matter where you connect.

If noise is indeed the issue, it's possible to filter the power with various combinations of L and C, however it's easy to start spending money this way.


Quote:
Originally Posted by curly View Post
Braineack built harness, which one Ben?
Hmm. Which one wire in a standard ECU harness contains switched +12?

(Hint: it's the white/red wire on every Miata until '01.)
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Old 01-14-2012, 09:57 PM   #15
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Quote:
Originally Posted by jbrown7815 View Post
So what would I need to do to use this with 2 door speakers in Miata? Just power it and hook directly to speakers?
Yup.


Quote:
Oh and what's a wall wart?
Generic term for any power supply that plugs directly into the wall, rather than having a separate AC cord. It sort of looks like a wart on the wall, and usually serves the additional function of blocking access to the receptacle directly adjacent to the one into which it is plugged.
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Old 01-14-2012, 09:58 PM   #16
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Isn't that an "in" power? I was thinking about finding a 12v coming form the MS to hopefully be a little cleaner. I'll try the white/red when I get a chance.

I was hoping to avoid plugging the wall wart into the inverter into the cig lighter into the car, but it might come to that.

Jbrown, a "wall wart" is what I call a 120v AC to somerandomvolt DC adapter. Comes with most electronics these days.

I thought it was a fairly common term?
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Old 01-14-2012, 10:00 PM   #17
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Quote:
Originally Posted by jbrown7815 View Post
So what would I need to do to use this with 2 door speakers in Miata? Just power it and hook directly to speakers? Oh and what's a wall wart?
And like Joe said, yup. It really is **** with the wall wart. If I can find some clean power it'll be a gold mine. Might wanna wait for a bit though, if I can't get it to work, I might blow $30 on one with a slightly better internal power regulation rather than more money on capacitors.
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Old 01-14-2012, 10:04 PM   #18
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I'll stay tuned then, wouldn't mind upgrading the garbage speakers in my car...Thanks!
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Old 01-16-2012, 04:50 PM   #19
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Quote:
Originally Posted by curly View Post
Isn't that an "in" power?
Isn't what an "in" power? I don't understand what's being asked here.


Quote:
I was thinking about finding a 12v coming form the MS to hopefully be a little cleaner.
There's really no power filtration anywhere in the entire car (unless you count the one capacitor near the ignition system), so the "cleanliness" of the power will be roughly the same anywhere.

Here's a quick thought experiment: Consider the factory radio. It's not plugged into anything special, power-wise, and yet it sounds fine. It also draws more than 0.5A, which should give you some clue as to the quality / power-delivery of that amplifier you've got there. From the pictures in the first post, it seems to say something like "500W" on the side, and that's just an absolute fantasy by nearly two orders of magnitude.
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Old 01-16-2012, 10:20 PM   #20
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I realize the crapiness of the amp, and care not what the actual watt output is, only that it cost just under $13 shipped. I drive this thing twice a month max outside of track days, I wanted a sub $20 system to power the speakers I already had and to have it run off my iphone.

You pretty much got what I meant about the "in" power. It's going into the MS, which means it hasn't been capacitorized (is that a word?) yet. But you also said the MS doesn't really do that, so scratch that idea.

I will try white/red wire, see if it's any better than anything else I've tried. If not, which I'm expecting it won't, I'll try some sort of capacitor. I wonder if I have an old camera laying around I could yank the flash out of. Dumb idea?

I really feel bad for myself, I'm not as electrically stupid as I make myself sound in this thread. Oh well, I'm certainly not doing anything to prove otherwise.
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