Teaching Old Speakers New Tricks with a Bluetooth Amplifier

Seeing a great pair of Acoustic Research Red Box speakers sit around collecting dust did not spark joy. However, I needed a more convenient solution than the inefficient, huge and connectivity-lacking class-A Technics amplifier that powered them previously.

There were a few options:

  1. Retail Bluetooth Amplifier (eg. Nakamichi MSZZ1BT)
  2. Bluetooth Audio Adapter (eg. Logitech Bluetooth Audio Receiver) + Amplifier
  3. DIY Bluetooth Amplifier (There are loads out there)

And after extensive Googling, YouTube watching and forum reading, I chose option 3, took a small chance and pulled the trigger on this one:

It goes by many names, but I will refer to this specific model as the Sanwu Audio HW-337 (Bluetooth Name + Code on board) though most places just call similar boards “TDA7492P” after the ST Microelectronics TDA7492P Class D Audio Amplifier that it is built around. There also seems to be some confusion regarding the power output of the chip with some claiming 50Wx2 and others claiming 25Wx2. Looking at the chip itself, you’ll be able to tell which variant you have by the presence of the P. If the chip just says TDA7492 then you have the 50Wx2 variant. If it says TDA7492P then you have the 25Wx2 variant.

As for my personal pros:

  1. Cheap (it’s practically a meme that this is always #1 for me)
  2. Very compact (roughly the size of a deck of cards)
  3. Efficient (Class D amplifier)
  4. Powerful (25w x2)
  5. Available locally in South Africa

Cons:

  1. Connects via Bluetooth only (no WiFi) – so no Airplay, Spotify Connect, Google Cast or other fancy methods. This is somewhat mitigated by me only using it for music and only streaming Spotify to it from my iPad.

Shopping List (with costs @ 15/06/2020)

  1. TDA7492P 25W+25W Wireless Bluetooth CSR 4.0 Digital Amplifier Board – With Jack (I bought 1 @ R299 + R69 Shipping = R368 from DIYElectronics). Awesome that I can support a shop near me in Durban.
  2. 25v >2A DC Power Supply – Although the board specs state 8-25V, the datasheet shows that the power output of the chip scales with voltage so it makes sense to use the highest voltage DC power supply you can get (up to 25V) with a power output exceeding the max power of the board (50W). I used an old Huntkey 19V 3.42A 65W Universal Laptop power supply.

Other Requirements

  1. Speaker cables
  2. Small Flat Screwdriver to tighten the terminals
  3. Enclosure (Optional)

Wiring and Setup

Regarding the setup, let me defer to the excellent DIY Perks YouTube Channel where Matt uses an almost identical board to build a fully wireless battery powered system:

Since I had no desire to make my system portable, I ignored all the battery-related information. The only issue that I had in connecting everything together was that the jack on the laptop charger was incompatible with the board so I’m temporarily using one of these hideous DC jacks to connect it up:

I need to change this to a more neat looking inline jack which I’ll order with my next shipment!

Photos

After wiring up the board and just using it bare on my desk for a while, I decided to spray paint the enclosure of an old broken router and mount the board inside for a much sleeker look. DIYElectronics sent a logo sticker with the order so I stuck it on the side.

One of the previously mentioned Acoustic Research speakers with dust cover removed

Conclusion

I am impressed by this tiny amplifier. To my un-audiophile and primarily AirPods listening ears the sound quality is great. One weird thing I noticed is that the Bluetooth audio quality on my iPad (6th Gen) seems better than the iPhone X with this single un-upvoted Reddit post mentioning similar. At least it’s proof I’m not crazy. I guess you should use your iPad for your bluetooth music consumption if you have that option.

Saying that I took a chance in buying this board in the introduction was due to some of the issues people have experienced (see search results on diyaudio.com for examples). Loud pops and speaker excursion on startup were noted and I was prepared to do some board level tweaks if necessary.

However the only issues I’ve experienced are:

  1. A fairly loud tone on startup that is (I assume) played by the Bluetooth chipset.
  2. There is an audible beep every time the volume is changed. So if you’re adjusting the volume in an “analog” manner (eg. using the control centre slider on iOS) there will be an annoying string of beeps as you make adjustments.
  3. There is an audible pop when turning off the amp while music is playing. You can easily mitigate this by pausing audio before turning off the amp.

These are small in comparison to some Bluetooth chips audibly speaking “Bluetooth on” and “Bluetooth Connected” etc. which I was intentionally trying to avoid purchasing for my sanity.

The Future

A more connected and powerful Raspberry Pi with better quality amplification from JustBoom or HifiBerry or IQaudIO will be needed to exceed the features and sound quality of this setup. Until that happens though, I will be pleased with how things currently are!

If you have any questions/would like to share your experience with DIY Bluetooth audio please leave a reply below. Thanks for reading!

Amplifier in its natural habitat with AirPods, a Raspberry Pi Zero W in official case and an AA battery for scale.

 

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