Need help downgrading amp to lower power

[TortuousAugur]

I have a pair of speakers from an old trashed stereo (wish I had kept the radio part of it now...) that I would like to build an amp for. The problem is, I have zero electric theory knowledge. I am a quick learner, and would probably be able to figure out what goes where, but I would need help with parts conversion to change the rated power output from what I am looking at (32w), to what the speakers say they can handle (12.5w & 25w, @ 6Ω).
I'm reading a little into an electric starter book, but trying to remember the equations and other math of it is loosing me.
I realize I am asking someone to do all my homework for me, and won't be mad if no one wants to help.

I was looking at a couple of instructable diagrams and parts lists, this one being what I would like to achieve, if possible. Would I have to make two separate amps to satisfy the different power ratings for the speakers?

 

[duke37]

If the speakers are a pair, they will have the same specification.

You will not damage the speakers if you drive with care and not use excessive volume.

To halve the peak power of the amplifier, reduce the supply voltage to 70% of that specified. Power is proportional to the square of the voltage. 0.7 * 0.7 = 0.5 roughly..

 

[john monks]

You may try simply placing a resistor in series with the speakers. I placed 49 ohms in series with my speakers and still have plenty of volume. This is not perfect, may add some distortion, but it is quick and easy, and it works for me.

 

[alfa88]

Duke's solution is exactly what I was thinking. A LM317 regulator will work nicely.

 

[TortuousAugur]

Each speaker is identical, yes, but each speaker has a normal speaker, and a lower frequency sub woofer speaker. The sub woofer speakers are the higher rated power pair.
Taking an educated guess, you mean for me to replace the power supply's LM78XX with an LM317? If I replace just that part, everything else falls into place for the power I need, or do I still need to watch how much I turn it up? I'm able to use my multimeter where the speakers hook up at to check how much power is coming off it, correct?
I'm interested in making it exactly what the speakers need, because I intend to make this a dedicated amp for ONLY this pair of speakers, as I don't really see myself using them anywhere else.

 

[alfa88]

The advantage a LM317 has is that you can adjust the voltage. So if I'm figuring out Duke's calcs right you could adjust the Voltage to 8.4V and you're done.

 

[duke37]

If you insist on using a high volume setting and limiting the voltage with the regulators, you will need to dissipate extra power so getting more heat - not a good thing.

To dissipate this heat, use Tom's solution of an external resistor in series with the speaker, 10 ohm 5W should be enough. You can test if this will give detectable degradation of quality by by-passing it temporarily.