Your transformer looks like it has 3 wires coming out on the primary side, and 4 wires coming out on the secondary side.
The secondary side is easy : you have two separate windings, and each could have a different voltage and different amount of current (voltage x current = number of watts that winding can provide).
The primary side is a bit tricky, since you have only three wires.
I think this transformer is either designed for 110v or for 230v. but the extra wire is there to give you the choice to use this transformer in countries that have slightly different voltage.
For example, the first wire would be 0v, the second wire could be 100v and the third wire could be 115v - so if you're in a country like Japan where they have 100v AC , you would connect the mains wires between 0v and 100v , and if you're in US or a country with 110-120v you would connect the second mains wire to that wire, leaving the middle wire unconnected.
It could also be a transformer designed to work with 110v or 220v, in which case the middle wire would be in the middle of the primary winding. In this case, on 110v you would connect the first and last wire to use 110v with all the wire of the primary winding, and if you're on 220v you'd connect the first wire and the middle wire, using just half of the wire in the primary winding.
This is not how it's normally done, normally such transformers have two separate primary windings like you have on the primary side , and if you have 220v you connect the windings in series, and if you have 110v you connect the windings in parallel (so the same amount of wire is used in both cases)
Based on the text on the transformer, it's probably a 25VA transformer. If the secondary windings are equal (same voltage , same current) and what I assume is true, that means each of those windings can provide up to 12VA to something connected to that winding.
This VA is an AC unit of measure .. basically, if the secondary winding is 12v AC and you have an incandescent lightbulb rated for 12w, it means the lightbulb will work perfectly and be as bright as it was designed.
Electronics don't work with AC voltages however, so these voltages have to be converted to DC. That's what those groups of 4 diodes do on the circuit board you shown us ( here :
http://prnt.sc/em1z38 ) and the large capacitor after those four diodes convert the 12v AC voltage to a voltage that's probably around 10-12v DC (depends on the size of that capacitor).
In the picture, I see a LM7808V chip, which is a linear regulator that outputs 8v as long as the input voltage is at least 9-9.5v, so that tells me at least one of those windings is big enough that after converting the AC voltage to DC, you still have about 9-10v DC.
But, this conversion from AC to DC isn't without losses, so if you have 12v AC and about 12VA on the secondary winding initially, after this conversion you have maybe 10-12v DC but only about 7.5-8VA (8 watts at 10-12v DC)
So if your audio amplifier needs at least 12v or something like that, this transformer may not be adequate for you, in some conditions the DC voltage may be too low for the transformer to work.
No, you can't replace an audio amplifier chip with another, they're different inside and require different components.
If it was up to me, considering you're a beginner, I'd suggest going and buying a couple of TDA2050 chips and the components in the schematics in the datasheet .. they are pretty easy to use and require few components and they can be made to work with either positive and negative voltage up to +/- 25v or with a single power supply ( up to 50v) and the datasheet has a schematic inside for both versions of power supply, and the chips are also super easy to find on eBay (you can buy 2 for 1.2$ , 10 for 3.3$ or 50 for 12$).
Here's the datasheet:
http://www.st.com/content/ccc/resource/technical/document/datasheet/1d/71/14/1e/5d/cf/47/01/CD00000131.pdf/files/CD00000131.pdf/jcr:content/translations/en.CD00000131.pdfHave a look at the example circuit board for split power supply on page 4 , or the single power supply on page 6 ... in your case, i'd connect the secondary windings in series and create a single power supply and feed both amplifiers from it.
THis is a MONO amplifier, so you need two identical circuits for each channel, so you have to buy 2 of each you decide on. and later when you find a more powerful transformer you can simply replace it (the parts accept up to 50v DC like I said).
Here's a couple more circuits that may be even simpler than the ones inside the datasheet :
http://www.eleccircuit.com/tda2050-amplifier-stereo-35w-75w/And here's another design with split power supply (connect the secondary windings together and the middle becomes ground, and the ends become V- and V+) :
http://diyaudioprojects.com/Chip/DIY-TDA2050-Hi-Fi-Chip-Amplifier/It's a very good chip to start designing something.
ps. and most components (like resistors) are standard values which you'd probably find in other circuit boards you have around (tvs, old radios etc , broken computers). You could grab a multimeter, desolder components and measure the values and you'll build yourself a nice collection of basic parts and then order only what you need.
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