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CIRCUIT NOTEBOOK Interesting circuit ideas which we have checked but not built and tested. Contributions will be paid for at standard rates. All submissions should include full name, address & phone number. Portable amplifier built from modules One of the things I like about Silicon Chip is that some of the articles published give me ideas for future applications. A good example of this is the El Cheapo article on Class D amplifier modules (May 2019; siliconchip.com. au/Article/11614). Several problems about the particular amplifier described were identified in that article. I searched online for a better unit, and found a TPA3116D2-based 2x50W + 1x100W power amplifier for less than $20, including delivery. The layout of the components on the board was different from the one shown in the article; I suspect it is an improved version. It seems to have none of the problems described in the article. The specifications for this amp quote a working voltage of 12-24V, but the data sheet for the TPA3116D2 IC shows that it will work down to 4.5V. I could 80  Silicon Chip not find a voltage regulator chip on the board, so I tested the amplifier at lower voltages and found that it worked without any apparent problems. With that in mind, I used this amplifier board, in conjunction with two other modules, to make up a radio, Bluetooth and MP3 player that is powered from a nominally 11.1V Li-ion battery pack. The pack I used contains nine 18650 cells arranged in a 3S3P configuration (three sets of three paralleled cells connected in series). I bought this second-hand, and it has built-in cell protection and equalisation modules. It can be charged using a standard 12V DC, 1A regulated plugpack. That charges each cell to 4.0V, which is close enough to full charge voltage (4.2V) for my needs. Australia’s electronics magazine The other two modules I used are a 12V-powered MP3 decoder board with Bluetooth, USB, SD card and FM radio support. It also came with a remote control, and cost just $6. The remote control allows for input selection and volume control. Earlier versions of these MP3 decoder boards were sold as running from 5V, so would need a regulator in my circuit. But later ones are rated to run from a voltage as high as 12V, and still work well at lower voltages. I also bought an 18650/26650 Li-ion battery tester, which shows the battery voltage and load current, for just $10. I decided to incorporate it in my unit to make checking the status of the battery pack easy. I mounted all these components (battery, amplifier, MP3/Bluetooth/FM module and voltage display) inside a large ABS plastic box I bought from Jaycar. I added speaker connectors stripped from a dead amplifier so that external speakers could be easily connected. I sourced some speakers from my local op shop for $2 each, and the sound isn’t too bad. With the battery between 9-11V most of the time, it can be wound up far too loud for an average room. Larger (and better) speakers can be connected to improve the sound quality, making it into a portable outdoor unit with a lot of grunt. I sourced the on/off switch and 5.5mm DC charging socket from Jaycar. The internal wiring is quite straightforward, as you can see from the circuit diagram. I used hot-melt glue and a mixture of nuts and machine screws for a quick and straightforward build. I gave the finished device to my granddaughter as a Christmas present in 2019. It has worked flawlessly for the last eight months, and she has only had to charge the battery four times! She tells me she charges the battery when it stops working. She usually connects her mobile phone using Bluetooth, but occasionally uses other input methods. Sid Lonsdale, Whitfield, Qld. ($100) siliconchip.com.au The MP3 decoder/FM module ► incorporates the display. While most current versions of this module are powered from 12V, earlier versions used 5V and so a separate regulator is needed. The amplifier controls (below the display) are from left-right: left/right channel volume; subwoofer adjustment; total volume control. The amplifier is the smaller device sitting below the twin JVC speakers. Larger speakers (up to 50W) can easily be used if needed. ► A close-up of the display of the ► 18650/26650 Li-ion battery tester module. It displays the battery voltage and load current. The internals of the portable amplifier are shown above at larger than actual size. The power amplifier module measures approximately 100 x 85mm and is sold for about $20 including postage online. Note that this TPA3116D2-based module has a power-on artifact which induces a pop noise on all speakers even when muted. siliconchip.com.au Australia’s electronics magazine August 2021  81