I purchased a small 2 channel amplifier for a DIY audio project (Amazon link). I'm hoping I can connect it to a power supply I already have to drive a pair of small speakers.
I would guess that the PSU would limit the 100w amp output to 33w, providing 16w to each 40w speaker. Would I need to attenuate the amp to 33% to support the supplied power?
It depends what you're going to use the amplifier for. If continuous full output sine wave, then the PSU would need to be able to supply the full output power. With less than the full supply voltage, that power would be rather less than the advertised maximum power.
If you are going to use it for music, then the full power peaks will only be required for a fraction of the time, and the average power supply requirement will be lower. As long as the amplifier input has a large reservoir capacitor, you'll probably be OK.
There's no need to 'attenuate the amp.' Just use the input volume control to adjust it to a level where it sounds OK, that is, not clipping.
The datasheet says "2 × 50 W Into a 4-Ω BTL Load at 21 V" on the first page.
If your power supply outputs 19V you'll get 2x40W, because power scales as voltage squared.
If your speakers are 8 ohms instead of 4, then you'll get half the power, or 2x20W.
If you listen to music and not continuous sine waves, then your laptop adapter will probably be okay. If not, get a more powerful one from the recycling center, they're free.
Would I need to attenuate the amp to 33% to support the supplied power?
Power scales as square of voltage, so that would be square root of 0.33, or 0.57, but... no need.
If you have a recording that is pretty low in volume it's nice to be able to turn it up.
And if the amp draws too much power, the switching supply should current limit, so it'll just sound really bad. You'll notice.
Note: If this is for a subwoofer, then yeah, get another supply.
Preservation of power says you're right, you're underutilizing the potential maximum output power of your speakers, and the potential maximum output power of your amp.
How much gain you can still do with your amp depends on the power that this results in, i.e. on the input signal. Also note that no amplifier is 100% efficient, so you get even less than your power supplys' power potential in power to the speaker.
Anyways, ~ 25 W of audio power is indeed not that little. Would work for a start, but you might overload your power supply.
Also not that 19V is a common voltage for good old classic barrel connector laptop power supplies, and these are basically available for free on the used / trash electronics market, with much higher output power than 33 W. So, get a powerful laptop supply and be done with it.
The datasheet for the TPA3116 amplifier says it is a stereo bridged class-D type. It shows the output power of 20W into 8 ohms per channel when the supply is 19V and the distortion is fairly low. It shows 90% efficiency. Then the power supply must provide 44W which is 44W/19V= 2.32A. If you play the amplifier loudly then your little 1.75A power supply is overloaded.
