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I'm using two Visatron FRS 5X-8 Speakers in a project. The amplifier being used is a Dayton KAB-215. Powering the whole thing from my power supply at 12V, I see a current draw of ~50 mA even at full volume. I'm a bit confused by this result.

My math suggests that for the two 5W RMS (8W Max) speakers I should see around 10W of power draw or ~.8A @ 12V. While I realize they may not operate continuously at this power during music, there is a big spread between 600 mW and 10W.

It seems like both the power supply (150W) and the amplifier board (2X 15W @ 8 Ohm) have tons of headroom here, so I don't really understand the low power draw. The bluetooth board doesn't have a volume knob on it currently, which the datasheet indicates should result in "full volume" on the amplifier side. My phone volume is maxed (when testing), and yet the power draw is super low. The speakers are fairly loud but not insanely so. I do have SPL measuring equipment that I can use to try to validate if the output is close to what it should be, but I'm still a bit perplexed.

I look forward to your thoughts on what is limiting the current draw, and whether this is a surprising result or not.

Update 1

I tested this with a couple other devices, and agree that it my phone bluetooth output is quite low.

Source: Galaxy S10, Via Bluetooth, "Audio Tools" App 100 Hz Sine Wave

Drivers: 2 Visatron Drivers, one in a 3D printed enclosure, one with no enclosure

Power Supply: In constant voltage mode at 12V, supplying power to both speakers Voltage: 12 V Current: .313 A

Single driver measurements + output: Voltage (Measued with true RMS meter in AC mode): 4.1V Driver impedance from datasheet: ~22 ohms Volume: 81 db @ (roughly) 1m, measured via the same phone app

I'm not quite sure how to translate the AC voltage at the speaker into wattage, given that the impedance is likely complex (right?). I read this post, but didn't gain a lot of insight from it.

The more I work on this the more I realize I don't understand, but I suppose that's part of the process. I also have a better measurement mic/setup, but I feel like I'm chasing bigger issues rather than small optimizations. I'm going to trace the graph, put it into WinISD and see what it expects from an SPL standpoint. I think that probably I just don't have that much input voltage from the bluetooth chip, and that everything else is behaving as expected, but I also don't understand the system well enough to conclude that.

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    \$\begingroup\$ Get a tone generator app for your phone, inject a low frequency that your meter will be capable of measuring - 100 or 200 Hz, say - and measure the current into and voltage across the loudspeaker. Update your post with the results. \$\endgroup\$ – Transistor Jan 18 '20 at 21:56
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    \$\begingroup\$ There are a lot of unknown variables here. You cannot assume that using 8 Ohm speakers, 12V power supply, and your phone on full volume, playing who knows what, will drive 5 watts into each speaker. You don't know what gain the D-class amp chip is set to, and you don't know the total gain from bluetooth receiver to D-class amp input. The board can use a wide range of supply voltages (12-24), and have some headroom for people who put 4 ohm speakers there (the amp chip can do it, but not necessarily this board). Chip is also not capable of more than 10W per channel at 13V supply. \$\endgroup\$ – Justme Jan 18 '20 at 22:20
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    \$\begingroup\$ "My math suggests that for the two 5W RMS (8W Max) speakers I should see around 10W of power draw" - was it loud enough to make your ears ring? If not then it wasn't at full power (10W into these speakers would be 96dBSPL at 1 meter). sengpielaudio.com/PermissibleExposureTime.htm \$\endgroup\$ – Bruce Abbott Jan 19 '20 at 1:27
  • \$\begingroup\$ @Transistor, Just did, thanks. I've tried to control more variables this time and added the measurements. \$\endgroup\$ – SpencerJoseph Mar 7 '20 at 18:52
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My phone volume is maxed (when testing), and yet the power draw is super low.

If you're using headphone out, that is usually less than line level (and in the EU a lot less than line level), so unless you have some make up gain somewhere in that system you won't get anywhere near the max output.

Test again with the line out from a CD player or similar device.

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Design Flaw

The Class D amp works best with a small battery or massive capacitor > 1 Farad with low ESR. A small Li-Ion 18650 cell can have 10kF then reduces with cells in series. Any time you put a boost DC-DC regulator on a Buck regulator source, you can expect problems unless there is a good large C low ESR buffer in between. A class D amp is a variable DC-AC inverter and has similar demands on a low source impedance.

Alternatively, a supply that can drive 10x the load you expect so you don't need to reply on large capacitors or a small battery. Try it on an HDD port from a PC with 12V.

There are many links to advance your understanding of concepts and test methods in my profile.

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