Facmogu F900 Amp

Class D Audio Amp

Overview

The Facmogu F900 is a tiny palm sized amp with multiple inputs (Bluetooth, 3.5 mm Aux, USB Audio) and a Class D Power Amp, all at the irresistible low price of $24 USD,
I was interested in it because it’s small and simple and cute (Nifty Industrial Design! Small! Blue! A Big Volume Knob on top! Real binding posts for the speakers! And did I say Cute?) and looks like the perfect thing for a guest room micro stereo. It’s based on the TI TPA3116 class D amp chip [ data sheet here: TI TPA3116 ]
It claims an optimistic 50 watts/channel… with a 4 ohm speaker load, 21 VDC supply, at a whopping 10% THD. (That means it’s a bit overstated in real world, listenable quality terms.)

I purchased mine on Amazon link here and it arrived promptly in a small light box with 3 cables. Observe it is shipped without a power supply, so…

First up: Get a good beefy power supply!

The output power of a power amp is strongly influenced by the voltage and amperage of the power supply. (ooh, ahh). Simply put, laws of physics (Ohms law) means a lower supply voltage will reduce your maximum speaker power.

Some quick testing of power output vs THD confirm this. Using a 12 volt supply into a 8 ohm resistive load, the amp puts out about clean 7 watts (at 0.05% THDZ), close to the theoretical max clean power for a good class D bridged design. However, at 10 watts the distortion skyrockets and THD is now an unlistenable 5%.

As expected, using a higher voltage supply helps considerably.
A 19 volt supply bumps the clean power rating to 20 watts/channel, a decent amount of power for a tiny pocket sized amp. (More on this later.)

Second Up: Hooking it up

Inputs

Sporting three inputs, Bluetooth, an analog 3.5 mm AUX, and a USB “soundcard” input, this little amp is an input winner. A pleasant voice announce the current input… Unless your volume is way up, in which case it can be startling.
The default input is Bluetooth, which is over-ridden by plugging something into the AUX in, which in turn is over-ridden if used as a USB “soundcard”.

I used the analog aux 3.5mm input for most of my testing.

The USB input … (EDNOTE: sample rate/bit-depth?)

The BT input … (EDNOTE: BT version, aptX?)

Outputs

Bridged amp topology means be cautious with speaker wiring wiring, there’s no common ground with this amp. Also, most sub-woofers won’t be compatible as they assume usually a common speaker ground as well.
A major annoyance for me was the speaker posts. Although they are nice looking speaker posts that accept single banana plugs, they were spaced too closely to accept my usual standard speaker dual banana plugs.

Third up: Testing and lab results

(specs and graphs here.)

Lastly, some listening notes

ABOUT SPEAKER SENSITIVITY

Sensitivity is a “standard” speaker measurement that can give you an idea of how “efficiently” a speaker translates input power into sound. It’s typically specified by measuring with a calibrated SPL at 1 meter while meter driving the speaker with 2.83 volts RMS (1 watt at 8 ohms).
Here’s where it gets tricky: To play about twice as loud perceptually, the speaker must be 10 dB louder…and it requires 10x the amplifier wattage to do it. Also interesting is a rather small 3dB increase in SPL requires double the amplifier power.

For example, a typical home stereo speaker’s sensitivity is around 85dB. Here’s a sampling of a few I have handy.
Alesis point sevens – 86 dB
Pioneer SP-BS41 – 85 dB
Radio Shack Minimus 7 – 81 dB
So… The Minimus 7 is 5 dB less efficient then the Alesis and therefore requires about 4 times as much power just to play at the same loudness.
That also means the same amp will run out of power much sooner on lower sensitivity speakers, which can be a concern with lower power amps like this one.

12 Volt supply
In my brief listening tests at about 1 meter (near field), the amp on a 12V supply outputs about 7 clean watts.
This is not enough for the power hungry Minimus 7’s (unless you’re only doing quiet background music.) The overall volume before distortion was too low for listening to acoustic jazz and celtic music, and the sound was lifeless and constrained.
The other two speaker samples were acceptable for near field, but sounded compressed and thin on high dynamic range content.
If you’re stuck with 12V use, I would advise sticking with speakers that are near field and 85 dB or higher sensitivity, otherwise this amp will simply run out power too soon.
19 Volt supply
I used an old laptop computer supply, rated at 19 V at 6.3 amps. The increased output power (about 20 clean watts) was immediately noticeable on all speakers; they sounded much more dynamic and the bass was more authoritative. I highly recommend a higher voltage supply like this one.
24 Volt supply
Still hunting for one with an adequate current rating, should be 4.5 amps or higher.(In process, more info soon)

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