reducing ac voltage with transformer and resistors

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  #1  
Old 12-06-13, 02:49 PM
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reducing ac voltage with transformer and resistors

recently on my stereo system in my garage my ac/ac adapter went out. It was a 120v ac to 13.5v ac transformer. Upon trying to locate a replacement, I discovered that it was an obsolete product, but figured a person should be able to find an ac transformer with the same specs. I was only able to find a 12.6 volt and a 25.2 volt transformer though. I have the 25.2 volt and some wire wound resistors, 2 10 ohm and 2 50 ohm. As I recall, at least with DC voltage, that you can use resistors in line to lower voltage. I need to get down to 13.5 volts and am wondering if there is a formula to calulate the resistance of the resistors needed if it is possible to do it that way. If not I am wondering what components would be needed to do it.
 
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Old 12-06-13, 04:31 PM
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A 12 volt AC wall wart of adequate amperage should work fine and is as common as dirt.
 
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Old 12-06-13, 04:36 PM
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It would make more sense to just buy the proper transformer for your stereo. A quick search on eBay comes up with quite a few available depending on your amps required.
 
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Old 12-06-13, 05:34 PM
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The current draw of audio equipment varies with the loudness. Meanwhile the "output" or delivered voltage of your makeshift power supply (whether AC or DC) will vary with the current draw when you use resistors in series. The end result is erratic operation and distortion of the sound.

Transformers all by themselves also suffer output voltage variation as load varies but not as profound as when adding resistors to the mix.

(Voltage variations can be reduced with AC input DC output power supplies employing series resistors by using capacitors between "hot" and "neutral." For these types of power supplies the combination of series resistors and capacitors is intended to "filter" or smooth out pulsations in DC derived from AC via rectifiers or diodes.)
 
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Old 12-06-13, 10:39 PM
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Buy the largest wattage/amperage 12VAC adapter you can find.

Like: 12VAC, 1200mA AC-to-AC Wall Adapter Power Supply
 
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Old 12-08-13, 12:45 PM
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I actually tried a 12.5 volt ac power supply from the same manufacturer, which I know works because it works on the other speaker set. I know the transformer was burned up, as it was obvious because of the smell, the fuse inside the amplifier was fine. I was hoping to get it working though because I have no other method of running my extra speakers other than running 3 speakers on each output in series parallel, though the impedance will not be exact, and may be a little lower than running just one.

It seems to be that I have to have an exact voltage for some reason, my only other thought would be to use a potentiometer to mess with the voltage.
 
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Old 12-08-13, 12:58 PM
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Usually with AC transformers the exact voltage is not critical as the unit internally converts it to DC and then regulates it. A potentiometer would not work in your case. You'd be back to several resistors at several ohms and 10watt+
 
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