That must have been playing fairly loud? What speaker impedance was used? And how did it sound before the failure? Is there any visible sign of failure?
So others can learn from this experience without repeating it.

The load was 3 ohms. It sounded great, until it didn't sound at all. It was almost inevitable. I can see what they've done and why they've done it. I have another version of the board (also driving a 3 ohm sub) the specifications show 12-24v (50w+50w+ 100w) - but it does have a small heat-sink. On the blue-tooth version they decided it could cope with max 20v without the heatsink if they reduced the claimed output to 25w+25w+50w. Why wouldn't they - it saves money.

Despite the general anti-Chinese sentiment going around, I have great respect for them. They don't mess about. I told them it blew-up, they said, "Your money will be refunded within 3 days."

 

I am not opposed to Chinese products, but I seriously take issue with misrepresentation and outright lies.
In addition I find the whole concept of selling products with no adequate information at all to be quite obnoxious.

 

Is it rated to drive 3Ω? Often the minimum is 4. If that is the case here, the failure is your fault.

 

With the stereo amplifier that you are using in mono, why not have one channel drive the woofer and have the second channel drive the tweeter. Mix the stereo into mono only at the inputs of the amplifiers.

 

Is it rated to drive 3Ω? Often the minimum is 4. If that is the case here, the failure is your fault.

The quoted resistance of a speaker driver is 'nominal', and measure in DC. However speaker drivers are AC devices. Inductance varies with frequency. From memory: to be categorised as 4ohms a driver's inductance must not fall below 3.2 ohms at any frequency. To make the whole calculation moot, a driver's characteristics change when you put it in a box. Subwoofers offer a different challenge. Inductance often peaks at the resonant frequency. Due to the narrow bandwidth being processed nominal inductance can change wildly. e.g. a 4ohm driver behind a 50hz low-pass filter may have a nominal inductance of 12ohms.

 

A speaker has low inductance that affects only high audio frequencies.
A 4 ohms speaker has an impedance of 4 ohms at about 400Hz. It resonates with a much higher impedance at a low frequency and its inductance causes the impedance to rise at frequencies higher than about 400Hz.

Why try to connect the stereo amplifier channels in parallel? That is used when a single speaker has an impedance of 2 ohms.
I think the amplifier failed because it is a cheap Chinese copy or because the complicated paralleling was wrong.

 

A speaker has low inductance that affects only high audio frequencies.
A 4 ohms speaker has an impedance of 4 ohms at about 400Hz. It resonates with a much higher impedance at a low frequency and its inductance causes the impedance to rise at frequencies higher than about 400Hz.

Why try to connect the stereo amplifier channels in parallel? That is used when a single speaker has an impedance of 2 ohms.
I think the amplifier failed because it is a cheap Chinese copy or because the complicated paralleling was wrong.

I think you're missing the point. Typically, , (refer to your own graph), a subwoofer behind a low-pass filter receives virtually no signal @ 400Hz. Please understand it is a 2.1 system (3 channels) stereo output is a prerequisite.

Your assertion that the component failed because it's Chinese . . . I don't know where to go with that. Wake up! Every electronic device in your life as Chinese components: you car, your phone, your stereo. The fact that these things are far more reliable than they were 20 years ago doesn't track with your racism. When it comes to manufacture of semi-conductors the Chinese lead the world.

 

Where was the heat sink??? Even at 90% efficient, the heat at 50 watts out is 5 watts, on the tiny chunk of silicon that will get hot in a hurry. And it is clear on the data sheet that provision is made for a heat sink.
Running in the parallel mode is a special case and clearly double the dissipation, thus it would need more heat sink capability.
I read about the function of the mode control pin and it still is not clear, except that The pin should not be left floating.

 

I doubt that you purchased the cheap Chinese Bluetooth amplifier in a reputable audio store. Instead you probably bought it on ebay or AliExpress and it has no manufacturer's name. They sell many fake items.
Where did you buy it?
What are its detailed audio specs?
What is the part number of its audio power amplifier IC?