Originally published at: https://boingboing.net/2020/08/27/this-bt-5-0-amplifier-drives-m.html
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This looks awesome, Jason (@jlw) . So your phone connects bluetoothfully to the amp which is wired to and powers the speakers ?
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Yes, I connect to the Fosi via BT and play music via wired speakers.
I like the look. A real Radio Shack vibe there.
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but is it water proof? you say this is on your deck. in a safe box?
And the BT amp is mains powered, I assume? I’ve been looking for something similar (and it does not need to be an especially powerful amp) but which is battery-powered.
Haven’t found one yet though this is on the possibles list but is out of stock and has been for long time.
Although there is a single speaker version…
Glad you found a solution for putting your speakers to good use. I wanted to add another perspective from a similar situation.
Last Winter I thoroughly researched Fosi BT amps, even emailing with their technical support to get full specs on their Class D amplifiers. Fosi amps were very enticing for their amazingly tiny size and high efficiency.
In the end though, I decided on the Sony STR-DH190. It’s a standard amp/radio/phono with a BT interface tacked on.
Advantages:
• Sony knows after so many decades how to make a reliable, performant amplifier section.
• When run at 85% power, the distortion is far below the Fosi’s.
• When it fails, I can return it to a local store rather than ship to China.
• The remote control has proven very useful. Receiver and phono functions if ever needed.
• Speaker protection relay for power on/off.
• UL listed.
So far, flawless performance.
Disadvantages:
• Costs more. ($99 on sale)
• Is a clunky stereo component that needs a shelf.
• Puts more carbon into the air to manufacture and run.
• Loss of the nerdy cool factor of going Class D.
YMMV of course.
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Most D-amps require a DC power range where the range is specified by the design and higher input voltage=higher power output. For example, here is an amp I threw together in an emergency while our main stereo amp was in the fritz; permissible power is 12-26 volts, the power supply I have on it (box on the right) is a 24v LED power supply, but that was probably overkill with our efficient speakers.
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On behalf of every neighbor within earshot of your awesome outdoor speakers that you love, we hate you and your speakers. I’m sure there are exceptions but 99.9999% of outdoor speakers are irritating the hell out of anyone not at your gathering. This is not kind, but outdoor speakers are also almost never kind.
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In English, please? 
What is a D-amp?
I think what you’re saying is that batteries may not deliver enough power to enable a sufficient amplifier sound ‘power’ output? Ergo, any decent amp must be mains-powered?
The rest was gobbledegook, I’m afraid … ‘permissible’ power? LED power supply? (LEDs are lights, aren’t they?)
Off to bed now. I’ll probably have weird dreams about this stuff. 
The technical term is “Class D.” This creates sound by digitally interpreting the input signal and then digitally generating an output signal, without any actual connection between the two. Think of it like Star Trek transporter technology for audio signals, where of course there’s no reason why, when Kirk gets beamed back up, the transporter can’t create a Kirk that’s twice normal size.
D amps used to be sneered at by audiophiles – even the genuine ones who “phile” the “audio”, and not just the equipment – but they have improved enormously in recent years. I can hardly tell the difference between the amp in my photo (the main board of which cost me $15) and our vintage Denon-NAD combination it was temporarily replacing.
I think what you’re saying is that batteries may not deliver enough power to enable a sufficient amplifier sound ‘power’ output?
No, my amp will run easily off a car battery (for example), and an inexpensive “buck converter” can easily change output voltage up or down from a given input voltage if you need something higher. Lots of people run D amps off batteries. However, the more power you want from your amp, the bigger your batteries need to be, whatever the nominal voltage. Class D amps are very very efficient, but you’re not going to get big sound out of a few AA batteries. I’ve seen D-amp projects using battery packs from cordless power tools, but I don’t know how long the projects ran on those batteries.
The rest was gobbledegook, I’m afraid … ‘permissible’ power? LED power supply? (LEDs are lights, aren’t they?)
This amp (and many such) don’t require one fixed voltage; you can feed it any voltage in a specified range, though at the lowest end there are stability problems and at the highest end you risk overheating. The power supply I have on it is the same technology as a laptop power brick, though it was designed for powering strings of LED lights.
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Fun!!
A recent rumble box 
I swore off my audio impulses twenty years ago. Then recently I changed my mindset.
Music therapy FTW
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I bought one of these to power some small outdoor speakers, stuck it in a cigar box and powered it with an old laptop power brick.
$20AUD, and it’s defintely good enough for bluetooth.
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It’s case by case right? For example I have freebie car stereo that has good input options and two channels rated 50 watts, lol, I alligator clamp it to a car battery and it will play music for two three 12 hour work days and the voltage of my battery is still 12.3 volts. Now that is a good long run, and you can do it in junk 6x9’s in a box, ok, or if you love your music you want quality speakers.
Speakers are the soul. Like volume? Get speakers with high SPL or sensitivity as in 96 decibels or more 100+Db plus! These sort of speakers make FAR more sound! I have heard systems running 30 or less watts per channel and it was audio Nirvana. Trouble is, those drivers are costly and there is the rub. Watts are cheap they say, speakers are not, however for battery powered rigs watts must be kept to the minimum. Loud speaks are a good way to compensate. A volume knob turned down but still plenty sound means less power consumed. These extra high SPL levels and Bluetooth enabled class D circuit boards allow AirPods and wireless headphones to be capable while tiny and lightweight.
Class D, T, H amps can sip power and drive great sound. They even make freak amps that are hybrids but don’t mind that.
Now, say I decide to hook up the old 18” sub @ 1500 mono watts and a couple towers for stereo say some nice 3 ways drinking 1k stereo watts together. This means multiple amps, yeah? One mono block for the sub, and either 1kw stereo amp or two 500w mono amps for left and right… How long do THOSE batteries last?
Who cares? I mean, disclaimer: don’t do this in town without a permit! I only do this to chase off the ground squirrel 
that are undermining the barn foundation, at parties. Ok I was rambling?
Well if I use home stereo or competition car audio amps then I am looking at class A or class B or AB hybrid circuit topography, great sound but my same car battery is shot in an hour or few. If I use class D amps instead I retain great sound however it goes for enough hours for a fine hootenanny.
I advise high sensitivity speakers because you need less power to get the sounds. I agree with everyone that class D is silly energy efficient. Bluetooth AUX RCA input amp let’s you skip having a receiver, very cool. I like old passive analog mixers which are almost free get good sound from almost any speakers and they draw basically zero power. Not portable though.
If you want portable check this sorta thing out…
For my fellow audio lovers who like playing with wires and building speakers… A homebrew passive crossover that sends just the right signal to each driver is pure magic, is it not?
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I hope that y’all are familiar with the relationship between DC amplifier voltage and speaker power. The power ratings of most low-cost audio amplifiers are exaggerated.
A 24VDC power supply feeding a bridged output amplifier can produce 36W RMS into an 8 ohm load. The 100W that they claim is a lie. Blame physics.
The car trunk amplifiers that produce hundreds of watts from a 12V car system have internal power supplies that feed 50VDC or more into the amplifier circuit. That’s the only way to make more power into a speaker.
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Maybe the Fosi amp has an internal PSU like the car amps?
These are.
http://www.dualav.com/marineaudio/amplifiers.php
A marine grade UL listing from the factory will cost extra. May lack Bluetooth or other features. For this reason many makers DO put it in a box, why not? It works and can be very cheap.
The Fosi amp says right on the front panel that it uses the TPA3116 chip, whose data sheet is easy to find.
https://www.ti.com/lit/ds/symlink/tpa3116d2.pdf
This chip is rated at 26V maximum input voltage, so it will not run at a higher voltage than the 24V that is supplied with the included power adapter.
It’s lies, all lies.
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They do specify 4 ohm speakers, which are somewhat uncommon.
The TPA3116 datasheet specifies an output power of 50W per channel, or 100W if the channels are bridged. Perhaps they use two TPA3116 chips, running each bridged? (That 100W rating is into a 2Ω load, though…)
