Tech Forum

One method that I have used successfully is to look for a large electrolytic capacitor inside the unit being powered. Many times there is a filter capacitor connected directly across the power supply input. By checking the voltage rating, you can determine the maximum voltage that can be applied. Since the original design would allow for some margin (let's say 30%), the normal applied voltage would be somewhat lower. A 16 VDC rated capacitor would likely be used with a 12 VDC power supply. Usually the next lower standard supply rating will work.

A voltage regulator at the input can also give you a clue to the power supply rating. If you can read the number on the chip, you can determine its output rating. The input voltage usually needs to be at least 2 VDC higher than the output for proper regulation. This sets the minimum voltage.

You can also use the capacitor polarity markings to get the polarity right if the jack is not marked. Sometimes you will find a bridge rectifier at the input which makes the input polarity insensitive. Hope this helps.

The easy way is to use a variable voltage supply and start with the lowest standard: 4.5V, 6V, 9V, and 12V. So, here's how I've done it: Start with ~ 3.5 volts with the speakers "on" and supplying some input, computer, mp3 player, or a an audio frequency generator. Increase the voltage until it starts playing, then increase the voltage to the next standard that is a higher voltage than the voltage you are currently using. Example: The speakers starts playing at 7.7V so the next standard voltage is 9V so increase it to that and notice how it sounds. If it sounds good at both low and high volume, you're done, now notice the current being used, at lowest and then at highest volume, don't forget to increase the audio output of your supplying device, computer, mp3 player, audio generator (be careful with this one it probably can go way too high for the input to the speakers).

Now build or buy a power supply with the correct voltage and current and get the needed power plug as well. (I have a collection of plugs. I bought ~ 2-3 of each from a supplier in a box with the part numbers, so I can test, use the one, and then order replacements when I use the last one.

NOTE: If the type of voltage, AC or DC, were unknown, then the same steps would work. If it were AC in then the polarity of the DC supply would not matter because behind the socket has to be a rectifier, which could even be one diode and if you then had the wrong polarity it simply would not turn on at all, no matter what reasonable voltage you used. In all probability, they would be using a full-wave rectifier so polarity won't matter and again you just need to find the working voltage and the needed current.

Unfortunately, there are no rules about what sort of wall-wart connector supplies what voltage, so unless you’re reeling lucky, you’ll have to get into the speaker which takes the power plug. (Marlin, RS or Farnell in the UK should have a matching one). It’s very rare to find AC output adaptions these days, except for printers. A speaker will normally expect DC to minimize hum pickup, as also inferred from the socket label.

Looking inside, check for the voltage ratings on the first few power smoothing capacitors. Check for a local voltage regulator and its spec. If there is one, then you know the maximum you can apply, but as long as you supply, say 5V more than its fixed output, it should be enough. More will dissipate more heat, which may cook it! (Engineered down to a price!) If there’s no regulator, check the amplifier chip spec and choose something lower.

The current rating is usually whatever you can find to replace it with. Facetious, I know, but unless you have a power supply and meter, it can only be guessed from the regulator and amplifier spec sheets. More current capability is better, as regulation will be better.

The info you need might also be on Google?

Sure! You already know it’s DC and the polarity. You can make an educated guess that the voltage is between 5 and 12. So connect a variable power supply and go up a volt at a time until the speaker works adequately. Then get the appropriate wall wart and put the variable PSU back on your workbench.

There are three possible solutions.

First and easiest is to contact the speaker brand or supplier with the model number. They will give you the exact supply information and may even offer to sell you a replacement.

Second, open the speaker that has the DC input and look at the electrolytic capacitors. Search their labels for their WVDC rating, and multiply by 0.8 as a safety factor. For example, if the rating was 15 WVDC, then the maximum power supply voltage would be 12 VDC which is very common. The current rating depends on the load when the volume is maximum, but a good guess to begin with is 0.5 A (500 mA). Get the new power supply, test the speakers with a signal and increase the volume. If the sound gets muddy or distorted, then use a higher current rating, probably adding another 0.1 A or more. The speaker will not use any excess current, but the new power supply must not drop the voltage at the highest volume.

Third and most diffcult is to use a variable DC power supply. Start with 6 VDC and power up the speakers. If the volume is too low, and a higher setting distorts the sound, the circuit is limited (clipping) and needs more voltage. Apply increments of 0.5 VDC and repeat the test until the sound is good at all levels, then note the applied voltage. If it is a strange voltage (for example 8.5 V) then add another 0.5 V and test again.
If satisfactory, then buy or build a new power supply with that voltage and add 0.1 A (100mA) over the current measured on the variable power supply. Good luck and have fun!

Two ways I can think of:

1. Can you open the speaker case to access the electronics? If so take a look at the electrolytic and note the voltage ratings - use a supply that is lower voltage than that (observe polarity).

2. Plug the speakers into an audio source, plug in a variable supply (observe polarity), start at 3V and raise it until you get enough output level from the speakers.

A typical supply for these things is 9-15 VDC @ 300-500 mA. I suggest taking a 9V battery, attaching a power plug to it (probably a coaxial), plugging it in and turning the amp on. Chances are it’ll work fine, but the 9V cell won’t last long if you use the speakers for extended periods.

If you’re leery of this shotgun approach, open the main unit and examine the WVDC of any electrolytics on the board. From that info, choose a DC supply 3-5 volts LOWER than the WVDC rating on the caps. IOW, if the caps say “16 VDC”, use a 9 or 12 VDC REGULATED adapter and choose a beefy current rating (i.e. 800 mA or more!) - trust me, peaks in the incoming audio WILL cause sags in the power supply (and distortion), so having plenty of current from the supply is a must!

Without any details of your computer speakers I’m going to guess they are about 3.5” x 3.5” x 5” and use a LM2822M audio amplifier IC. Assuming that’s the case, the IC can run of a voltage between 3 and 15V. I’d try a 6 to 9VDC supply if you open up the enclosure with the amplifier and confirm its a LM2822M and note the of impedance the loudspeakers. I’d use 6 to 9V when 4 ohm loud speakers are used, or 9 to 12V for 8 ohm loud speakers.

You should find the negative supply terminal is connected to the stereo jack plug sleeve (common signal ground/screen). To use a low voltage AC supply, the amplifier board would need the addition of a bridge rectifier and capacitor large enough to give the required DC voltage with minimal ripple adding a cost that’s easily avoided by using a DC adaptor.