Nice find with these speakers.
I'd say the previous owner's "handiwork" is also a nice surprise, at least if you are a tech/solder monkey like me.
That said, I can attest to it about you and Matth79 being right in that these weren't repaired/modded properly.
Yes, you can use a 12V AC power adapter to get roughly +16V and -16V, but not the way it's wired currently.
The "proper" way to do it when you don't have a 12-0-12V AC power adapter (i.e. 24V with center tap... or dual 12V AC output) is to put one of the transformer's AC output wires on DC ground, and the other onto the one (or both) of the AC pins of the bridge rectifier. Then have the bridge rectifier (+) pin go to +15V on the board and (-) pin go to -15V on the board. Essentially, this creates a voltage doubler circuit from the 12V AC source. 12V AC produces roughly 16.5V DC. So you get +16.5 V DC and -16.5V DC.
But before you go into trying out various power supplies / adapters, let's first analyze a few things here, based on the pictures your posted.
For starters, I see Jackcon caps as what appears to be the main filter caps for the +/-15V rails. I'm pretty sure these aren't the originals. It's quite possible the originals went back and the owner replace them. Or maybe he blew up the originals with the 12V AC adapter mod he/she did (quite possible, since the 0 / ground will indeed float the way it's wired from the bridge rectifier, as Matth79 noted, thus making it possible for one cap to receive all of the voltage and the other to get nothing.) Anyways, my point being is that these are very low quality Chinese caps... so you might want to replace them with something better (please don't use Amazon or AliExpress, as these are notorious for having either fake/counterfeit or garbage quality no-name caps.) But!!! Before you do that, we need to analyze even further, unless there's a schematic diagram for one of these somewhere on the internet (a quick search on my part shows nothing... though I will admit searching for schematic is one of my weaknesses.)
So, what's next?
- Take a look at components U3 and U4 - what are they? (i.e. part number written on them.) I suspect they are linear regulators of some sort (likely 78xx and 79xx series), especially judging by the darkened PCB around them - looks like they ran hot.
Based on what U3 and U4 are, this will clue us in on what kind of voltage input these speakers accept. As a general rule for 78xx and 79xx series of linear voltage regulators, the supply voltage needs to be at least 2V higher than the intended output. In other words, if these are 7812 and 7912 (for generating +12V and -12V), then the minimum voltage they would get on their V_in pin should be +14 and -14V in order for them to regulate properly.
On a related note, do R11 and R12 have the same resistance? And how are they wired. At this point, it might help to post more pictures - possibly ones from both sides of the PCBs, if you can. To me, these look like dropper resistors before U3 and U4... or maybe not, with R12 being rated for 510 Ohms (green, brown, brown, gold for the color code.)
Next up, the amplifier chip, U1 - what is it / what part number is written on it? This will give us further clues about the mix/max operating voltage of the circuit.
Lastly, the speaker impedance ratings and power rating (Watts), if any are listed. This will give further clue about what power supply would be suitable.
For the typical case of a 4-Ohm 10 Watt speakers, a +/-12V DC power source should be enough for up to around 12 Watts RMS.
Paar wrote on 2025-04-22, 16:15:
Unfortunately I cannot test it because the 12VAC adapter doesn't work. I have disassembled it and there is only transformer inside, but it doesn't generate any current.
First, make sure you had your multimeter set to measure AC voltage and not DC (common mistake I've seen people do and sometimes that I do myself.)
If that's not it, then it's possible the transformer was over-loaded and gone open-circuit on the primary side. If it's a half-decent transformer, it should have either a thermal fuse or just a plain time-delay fuse on the primary side/winding, typically right where the AC wires enter the transformer, under a few layers of tape/insulation. If the transformer does have a thermal fuse or time-delay fuse and if it's open-circuit, then changing this should bring it back to life. If you want to try it as a test, you can just put a wire short across it and that should bring it back to life. Of course if the wires on the transformer looked burned on the primary winding, then probably not worth trying this.
Paar wrote on 2025-04-22, 16:15:
* Build a new power supply myself. Honestly, I'm not confident enough I would do it justice, even though you can find nice schematics on the internet. I'm not that skilled with electronics, especially with parts dealing with high voltage.
For speakers like these that use amplifier ICs, 99.9% of the time you won't have to go with that option.
Just looking at the datasheet of the amp ICs will tell you what voltages may be suitable... and from there, based on the impedance and power rating of the drivers, you can also approximately estimate the current rating for each channel... and after this, easily pick a power adapter or two with the appropriate voltage and current ratings. I actually did this just very recently for a home theater amp I was repairing. In my case, I ended up using two 19V 3.46 Amp laptop power adapters hooked up in series on the secondary/output in order to give me +20V and -20V. Not neat-looking... but after exploring several other options, this one made the most sense in the end.
