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[SOLVED] Basic question about AC Adapters

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Newbie level 6
May 1, 2011
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I apologize in advance if this question doesn't belong in edaboard at all. If it doesn't please tell me a better place to ask it.

Can anyone tell me how to determine how many volts and milliamps an AC Adapter needs to provide if you have the device that needs the AC Adapter but no specs for the adapter itself?

I am NOT an electronics hobbyist, just someone trying to help a friend. My friend has a pair of computer speakers but has lost the AC adapter for it. He would like to buy an AC adapter for the speakers but doesn't know exactly how many volts/milliamps it should provide. The information is not marked on the speakers and he long ago lost any documentation that came with it which might say what the specs of the AC adapter should be. I've googled the speakers in the hopes of finding a manual online or even tech support from the manufacturer but came up empty. These speakers are pretty old and date back to his first computer; I'm guessing the company is no longer operating. I believe the company (or at least the make of the speakers) is Quick Shot or something similar; I've forgotten the name since I was at his place a few days ago.

I'm hoping there is a way to determine the proper AC Adapter, perhaps by using my very basic analog multimeter and touching the contacts to the jack where the AC Adapter gets plugged in. Or maybe a way to get "close enough" via trial and error using other AC Adapters. I have a variety of old AC adapters from dead devices and am hoping that one of them will work with my friend's speakers.

I have nearly no knowledge of electronics at all so please keep the technical jargon to a minimum in your reply ;-)

How many speakers are there, 2,3??
Nothing is written on speakers module are you sure?
If not atleast plz mention the size of speakers.

First of all does the computer speaker need DC or AC? If you switch your meter to highest resistance range and try the jack with the red lead to the centre and black to the ring, note the reading. Switch the leads and repeat. If the readings are the same then it requires AC, if they are different, then DC.
A pair of computer speakers would be rated at 5W each, so combined, 10W, allowing for inefficiencies the amp will be using, say 15 Watts.
Now for the voltage, try low(5V), there won't be any damage but the output could be low and distorted. It is most likely that the voltage will be in the region of 12-19V, if the loudspeakers are of high power , say 20W/ channel then the voltage may be higher (20 - 50V). One way of having an educated guess is to open up the loudspeaker where the power is fed to. In it on the PCB, there will be some little can components (normally with a blue or black sleeve but with an aluminium centre which the plastic sleeve does not cover. These will have their value and voltage rating on them, such as 100MF 16VW, 100 MF is its value, 16V is its voltage working rating, so if the highest voltage rating you can find is 16V, then its certain that the amplifiers operating voltage is lower then 16V, most likely 12V. So in this case, 15 W at 12V = 15/12 Amps or 1.25 A 12V , AC or DC as per para. 1

First of all does the computer speaker need DC or AC?

It depends, for example my Creative Inspire T10 have an AC input and use 11,5v/1.6A power adapter, the rectifier bridge is inside the speaker.


EDIT: sorry, I thought you meant that all PC speakers use DC, my mistake
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Speakers from that far back usually ran on 6 or 9 Volts. I agree with Chuckey's method for determining the requirement for AC or DC and using the capacitor ratings to define an absolute maximum voltage. If you are able to open the case on the speaker that has the electronics in it (you would have had to in order to see the capacitors), the circuit board will probably have just one integrated circuit on it (rectangular black thing). Use a light and magnifier to read the markings on that IC and report back.

On the other hand, if the case is securely glued so it's not feasible to open it, just try 6V. Using the line level output from the computer as an input to the speakers, turn the speaker volume up all the way. If the speakers aren't very loud or distort badly, try 9V. Most of these cheap, no name types will get plenty loud but will start to distort a little or even rattle when operated at their intended voltage so if you can't get that at 9V, go to 12V.

Unless the speakers are unusually large, they will not likely need more than 200mA each, but a higher rating on the adapter does no harm.

By the way, I often see pairs of good quality computer speakers for sale at thrift stores for less than $5 and power adapters for less than $3.
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How many speakers are there, 2,3??
Nothing is written on speakers module are you sure?
If not atleast plz mention the size of speakers.

There are two speakers. I didn't see anything written on the speakers that mentioned volts or mlliamps. I did see "DC" in raised letters near the jack where you connect the AC Adapter.

I didn't see any other writing on the speakers but I will have another look - and take some photographs - when I go over to my friend's place the next time, hopefully later today if he is around.

---------- Post added at 10:22 ---------- Previous post was at 10:19 ----------

I'm obviously going to have to go over to my friend's house again to try your suggestions. I'll print off the advice that I'm getting and bring it with me. I'll report back here when I've been to Doug's house.

---------- Post added at 10:27 ---------- Previous post was at 10:22 ----------

I have a few old AC Adapters that I'm not using so I'll just give him one of those if we can find one that works reasonably well. But we will consider buying another adapter or speakers if that doesn't pan out.

If the plug input says DC you should try with a DC power supply.
After all if the input should be AC and you connect DC then you will just have a slightly lower voltage in the circuit becauae of the forward voltage drop of the diode bridge.
On the other hand if your speaker input is for DC and you connect AC you will damage the internal electrolytic capacitors and maybe the circuit too.


Just a quick note to tell you all that the problem is resolved. I went over to my friend's place this afternoon, confirmed that the speakers had "DC" on them, and then tried powering them up with an old 300 mA 9V AD adapter I had lying around. It worked very well!

Thank you all for your help!

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