Direct from Shenzhen – Bluetooth amplifier

By Ben Everard. Posted

This author once converted an old 1940s radio into a Bluetooth speaker. It looks marvellous and sounds good, but it needed separate Bluetooth and amplifier modules wired together. Not the most onerous task, but when we came across all-in-one receiver and amplifier modules, we decided to find out if we could use them as a ready-made device for upcycling old speakers into wireless audio receivers. In principle, these modules should let you add wireless audio by just connecting up the wires. Let’s see if they really work.

We bought a ‘TDA7492P 2 x 5W Wireless Bluetooth 4.0 Audio Power Amplifier Board Digital Amplifier Module with AUX Diy Electronic PCB Board’, from diymore Official Store on AliExpress, for £5.40, plus £2.15 delivery to the UK. This board integrates a Bluetooth receiver and 2×25 watt stereo amplifier. The only additional hardware needed is an 8–24 V power supply and speakers. There’s a 3.5 mm audio output if you want to use it with an external amplifier.

From a functional point of view, this module worked really well. The Bluetooth came online as soon as the unit was powered on, and paired easily with our phone. As soon as we piped music through it, it started playing. There are five control buttons (play, back, next, volume up, and volume down), and again, these worked well. These buttons are small, and lower down than some of the higher components, so they’re not particularly easy to operate once the module is in a case. We’ve seen laser-cut cases with extenders for these buttons, or alternatively, it would be fairly easy to solder onto this board to have additional controls that could be mounted elsewhere.

Troubling frequencies
The first clue that it might not be the highest-quality product is the alignment of the components – the larger through-hole bits, like the screw terminals, were all wonky. Not a huge problem, but a sign that care hasn’t been taken in the manufacturing. There is though, a much bigger issue: the sound quality was atrocious. When working with cheap audio hardware in the past, we’ve had problems with hums, clicks, and other bits of audio errata introduced, but we didn’t have that problem with this receiver; instead, the frequency response was off completely. Low ‘bass’ sounds had almost completely disappeared from the music leaving a very tinny, high-pitched sound. Something has gone very wrong with this module to end up like this. It’s so far off that it’s not easily explainable by the components. Bluetooth comes via a CSR8635 Qualcomm module, and amplification is done with a TDA7492P. Both of these should be capable of producing reasonable quality audio, so either:

a/ one of these is a low-quality fake part, or
b/ there’s some gremlin in the way they’re set up.

It might be something as simple as the wrong value capacitor used somewhere along the line, and if it were, it would be a real tragedy that the module was rendered just another bit of e-waste by something so simple. Perhaps the manufacturers would benefit from reading Bunnie’s advice on hardware testing in issue 17 (hsmag.cc/issue17).

An unsure future
The sound problems are in both the speaker output and the audio-out jack. In principle, it might be possible to recover the bass using an equaliser, but it’ll always be very distorted, and you’re never going to get results as good as if the bass wasn’t filtered out in the first place. Also, if you need extra hardware to correct the sound, we might as well use separate modules in the first place.

MicroAmp-cutout

Unfortunately, it’s hard to see any useful future for this module. Even traditionally low-fidelity audio uses, such as spoken word, would be distorted to the point that it’s difficult to listen. There’s certainly no need to turn the thing up to 25 watts, and we wouldn’t want to be anywhere near the full 25 watts of ear-splitting, high-pitched whine that would result if you did. Perhaps the one thing slightly redeeming the module is that the large capacitors and inductors add a certain aesthetic charm and could be used if you’re looking for something that adds an ‘electronics-y’ look, without needing it to do any actual electronics.

Despite costing only £5.40, this still feels overpriced, but then it probably would at any price. If you ever find yourself in need of 25 watts of remotely controlled, high-pitched audio, then this is the module for you. Otherwise, keep looking. We’ve had much better success using a module that is just a Bluetooth receiver (such as the KRC-86B, or even a Raspberry Pi) which can then be paired with an amplifier of your choice – whether integrated into an existing sound system, or one of the many available from a range of sources. The end result will probably cost a little more, but you should be able to get a sound quality that’s actually worth listening to.

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