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  #1  
Old 09-07-2011, 04:29 AM
crankcase crankcase is offline
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Default Plus and minus 18 volt DC power supply

Hi All

I’m looking to rig up a power supply for a set of computer speakers that is missing its original. The speakers use + 18 volt DC, -18 volt DC, and ground (I think) power supply. I plan on getting four 9 volt batteries to test them with but am thinking of how to achieve a more permanent power supply. I guess the amp in these speakers need well matched voltages?

Any suggestions?

I’ve been thinking of a 36 volt power supply hooked up to a 2 resistor voltage divider.
Maybe two 18 Volt supplies (my favorite cheap approach).
Possibly a computer supply to see if the +-12 Volts and +-5 Volts could be hooked together without destroying the supply.
As a last resort, I suppose I could build something if I could get it done for less than $20.00, especially If someone could suggest a complete project design (I would really appreciated it).
Also, the supply is no longer available new, none is on ebay, and I have not found a work around on the web.
Thanks
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Old 09-07-2011, 09:00 AM
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Welcome to the forum
There are two things to keep in mind when finding a suitable transformer for an amp.

1. Find it's rated voltage
2. Figure out it's needed amperage.

1. Finding voltage.
If it is a transistor based amp (output stage) it is a little difficult and I need to know the circuitry. But there is another approach which to find the working voltage of the filter caps. The voltage needed for the Amp to work will be around 10% less than the rated voltage on the filter caps. If the filter caps are not there, then u seriously needs to know SAO of the output devices.

If the amp is an IC driving the speaker, then all y need is the data sheet of that chip.

2. Amperage.

If like above, if the amp is an IC, then the amps needed can be found using the data specs. By using ohms' law will give u a rough idea on how much amp it will draw at peaks.

Same goes for the transistor output stage types. The Ic of the out put device can give a rough idea on how much it can handle depending on the load used.
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Old 09-07-2011, 10:11 AM
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MPJA carries these regulated 18vdc, 1A plug supplies for $6.95/ea:
http://www.mpja.com/prodinfo.asp?number=18253+PS
That would be a fairly cheap way of getting your 18v supply. Lots easier and more efficient than trying to build your own, too.
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Old 09-09-2011, 07:31 AM
crankcase crankcase is offline
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Default Blew up the amp


Thanks for the ideas, but alas, I blew up the amp using two 18 volt cordless drill batteries. I'm ok with how I had it wired, I think I just guessed wrong at the polarities. I had the + and - tied together as the "ground" which left me with a - 18 and + 18 remaining. It measured 36 volts from plus to minus. If I had fused the lines perhaps no smoke would have escaped.

I blew an electrolytic cap. and a larger NPN transistor. The NPN seemed to be matched with a PNP that did not physically look blown. I'm not sure what kind of amp it was, but there was one chip with about 40 pins sitting under a large heat sink. I assume it must be one of the "new" class D jobs. I'm not real familiar with these, but I guess they work similar to a switching power supply. I wish I could have known an easy way to determine polarity.

I wish I knew more of what was going on, but the amp was highly proprietary, and judging from the info. on the net, the manufacturer was perfectly happy with people blowing up their amps since they they no longer sold replacement power supplies anyway.

I'm going to throw a couple of more dollars at it, see if I can learn something, and call it a day. I don't understand the exact reasons for the + and - power supply nor how to determine polarity when no technical documentation is available.

Thanks
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Old 09-09-2011, 11:48 AM
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The markings on the electrolytic capacitors is a big hint as to what the polarity should be. Electrolytic caps won't tolerate reverse voltages; they will have a very high leakage rate (sort of like a forward-biased diode), and that will cause heating of the electrolyte, which will boil and cause the enclosure to rupture, spewing conductive confetti over your circuit board.

Class "D" amplifiers use rather large inductors in their output section, usually toroids.

You have not provided a manufacturer, model #, photo, or any other such information for the amplifier, so there is not much more we can do to help you.
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Old 09-09-2011, 12:20 PM
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Quote:
The voltage needed for the Amp to work will be around 10% less than the rated voltage on the filter caps......
In case the rating of capacitor is higher or lower then, what to do..??
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Old 09-10-2011, 04:33 PM
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Only a moron would use caps with lower rating Pritesh.
It is advisable to use higher voltage rating capacitors for long service life.
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