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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.

  • PSU: 19V 1.75A DC (33w?)
  • Amp: 5-24V 2x 50w (100w)
  • Speakers: 2x 40w 8Ω (80w)

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?

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  • \$\begingroup\$ If you tried to take too much power the power supply may fail. \$\endgroup\$ – Andy aka Apr 17 at 17:40
  • \$\begingroup\$ It's a little worse than you imagine. Assuming you are using a bridged arrangement (the 2x 50 W), those amplifiers would require a DC rail much higher than 19 V. Even if those are class D amplifiers. So I frankly don't believe their specifications. It's much more likely that you may get about 15 W from what you have. \$\endgroup\$ – jonk Apr 17 at 17:43
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    \$\begingroup\$ Obviously you cannot violate the conservation of energy by having overunity efficiency. So the question is, does your amp need 100 W of input power to function? Probably not. You can probably play it at modest volume with your 33 W supply. \$\endgroup\$ – mkeith Apr 17 at 17:46
  • \$\begingroup\$ Bear in mind that you’ll notice clipping at peak power levels; unless your musical taste doesn’t extend beyond a steady sine wave, the average level will be significantly lower than this, perhaps an order of magnitude. So even if you are operating close to clipping level (dependent on the supply voltage) you may be well within the power rating (dependent on the supply power rating). Some power supplies respond better than others to momentary overloads, yours may be happy to provide extra power, or the output voltage may drop, or it may shut down altogether. \$\endgroup\$ – Frog Apr 18 at 4:04
  • \$\begingroup\$ @Frog I ended up going with a 65w power supply in the end but even with that if I drive the signal too much the amp cuts out. The product description mentions output short-circuit protection but that doesn't sound like what I'm describing. Any chance I'm damaging the amp by overloading it? \$\endgroup\$ – Jonah M Apr 21 at 2:23
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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.

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  • \$\begingroup\$ Yes it's for music. I'm not sure how large of capacitors you're talking about, but these ones are 35v 1000μF. If clipping is a safe indicator to go by, I'm happy with that. \$\endgroup\$ – Jonah M Apr 17 at 18:08
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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.

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  • \$\begingroup\$ My speakers are 8Ω and no, I'm not trying to power a sub. Thanks for the tips! \$\endgroup\$ – Jonah M Apr 17 at 18:11
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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.

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  • \$\begingroup\$ I have several laptop PSUs, and I'm trying to use a wall-wart style one instead of one with a big brick in the middle. I realize that means underutilizing the amp/speaker potential, but I suspect it will be enough for my purposes. \$\endgroup\$ – Jonah M Apr 17 at 18:05
  • \$\begingroup\$ well if you already know you're ok with underutilizing your audio reproduction system, and you're kind of OK with risking a supply by overloading it, what exactly were you wondering about? (33W in a wall-wart form factor does seem a lot, so thread carefully with these specs) \$\endgroup\$ – Marcus Müller Apr 18 at 10:41
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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.

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