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Running a 12V car amp from the mains


SamJUK

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Time was when PC power supplies produced shedloads of 5V power, and a bit of 12V for running a few disks.

 

Modern motherboards now power the CPU from the 12V supply via a switching reg, so some power supplies can produce prodigious amounts to 12V power.

 

Warning: many / most? / all?? PC power supplies will regulate terribly if there isn't at least the specified minimum load on the 5V output, s power resistor or two is the way to load this output.

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Warning: many / most? / all?? PC power supplies will regulate terribly if there isn't at least the specified minimum load on the 5V output, s power resistor or two is the way to load this output.

Or even a couple of fans, so that 5v is doing something useful. most fans will spin up at 5v and be useful and virtually silent.

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The battery chargers that we "reverse engineered" for teaching purposes had no smoothing or voltage regulation of any form whatsoever. Putting a capacitor across the load will help to some extent (but is not the complete solution), but it is no match for the smoothed and regulated output of a switch mode power supply.

I agree about chargers, but this is a car amp designed to take the variations in supply that come with power being delivered from an alternator/generator with, in electronic terms, fairly crude voltage regulation. It shouldn't need a switched mode PS.

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I've tried using amateur radio (designed for in car use) as a base station with a car battery and a mains charger several times for different radio events (JOTA etc). Each time there has been mains hum which increases when you transmit (ie increase the load) as the battery voltage falls. as battery voltage falls then the ripple from the rectified AC becomes evident and results in hum

On receive (light load) there is no problem as the battery act as a large smoothing capacitor. What did work was using a regulated DC 10A powerpack set to 14v to charge the battery.

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I agree about chargers, but this is a car amp designed to take the variations in supply that come with power being delivered from an alternator/generator with, in electronic terms, fairly crude voltage regulation. It shouldn't need a switched mode PS.

 

OK - but the characteristics of an alternator with even basic voltage regulation and connected to a lead acid battery are rather different from that of a battery charger output connected solely to an audio amplifier.

 

The thing about the computer SMPS is a) it's cheap by virtue of its ubiquity b) readily available in a variety of power capacities c) is relatively quiet d) the amp gets a fairly clean voltage to run off

 

By comparison, car battery chargers hark back to a more primitive age of electronics ;-)

 

Simon

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As others post, you will need not 12 volts, but about 13.5/14 volts for full performance.

Regulated power supplies of this voltage are widely sold by Maplin and others, and are probably best if you want an off the shelf solution.

Cheap power supplies may be rated in chinese amps, therefore it would be sensible to get one with a CONTINOUS output of 25 or 30 amps for a 19 amp load.

 

A common application of such power supplies is for ham radio, and they tend to assume full power only briefly when transmitting.

 

It might be worth measuring the current draw, it may be a lot less than expected if the amplifier is not worked at full power.

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I was sure the suggestion of a PC PSU would spur a raft of pro's and con's and I was initially determined to steer away from it!

 

I'd say suck it and see first, and if you get weird results (like the psu exploding, singing or motorboating from the audio) then you'll know where the problem lies. Car amps have dc-dc converters in there that oscillate at a high frequency which may beat with any ripple off an external SMPSU, especially one where the main 5V is unloaded so the regulation is wacky. It's impossible to say given the mish-mash of PC power supplies available to us at the moment.

 

Did you say why you specifically needed to use a car amp and not a proper 4-channel mains amp? This would be a better solution.

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Hi Sam

 

After reading all the posts so far seems a little strange to use a car amplifier! The whole reason why it is 12 volts (13.8 volts) is because it is for a car. You have mains electricity available! The worst thing about all this is that you will be converting mains electricity down to 12 DC and then when it goes into the amplifier it then gets converted back up again to something like +-30 volts rails to drive the amplifier. So high voltage down to 12V and then back up again - Up - down - Up again.

 

I think the options are:

 

Use a 4 channel amplifier that is mains powered something like - http://www.apart-audio.com/Products/produc...ails.aspx?id=74

This may be a little expensive! but I sure there are other options out there.

 

You could go for a HIFI amplifier http://www.richersounds.com/product/amplif...3/camb-a1v3-blk You could buy 2 of these for less than £100

 

The car amplifier you have you could modify it, removing the step up switch mode power supply part and just supply the amplifier part direct with the correct voltage rails using a transformer bridge rectifier and capacitors. - This may well be slightly out of your depth, it would also void the warranty on the car amplifier of course.

 

If I was going to do something like you are doing, I would have gone for a mains powered amplifier.

 

I wish you all the best with the project.

 

Stuart

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