Rechargable Battery Add-On for Mini Amplifier

Upgrade a mini stereo amplifier kit with rechargeable battery power.

This project adds a rechargeable Li-ion battery and charging module to an old Dick Smith mini amplifier.  The original was powered with a 9v plug pack, but works just fine with the 3.7v Li-ion.  Recharging is though a USB port from any 5v USB source. 

DS Kit

  The original kit is model K-5008 based on a Silicon Chip project. It is built around the STMicro dual low-voltage amplifier IC. The stereo signal is supplied at a 3.5mm stereo socket and the output is at two 3.5mm sockets. As supplied, it is designed for a 9v plug-pack or battery, but the IC works to about 1.8v, so the 3.7 to 4.2v from a Li-ion battery is quite suitable.   Output is a nominal 1W per channel, but that is at the maximum supply voltage - it will be less with the battery power. The device as constructed is used as a personal portable MP3 player, in conjunction with a pair of miniature speakers and a MP3 player.


Components

The components used in the upgrade are a Samsung 25R 18650-format Li-ion battery, a matching battery holder, a charging module based on the TP4056 IC, and a USB connector board.  Note that the charging module is the type that allows for using the device while it is being charged.  This type has battery 'B' outputs that are separate from the outputs to the load (the amplifier in this case).   This meas that the module can switch the supply from the battery to the USB 5v input when it is being charged, and switch back to the battery when there is no charge supply.   This ensures that the load does not affect the measured battery voltage while charging, so there is no risk of reading an incorrect battery voltage and overcharging the battery.   Modules that do not have this capability should not have the load applied when they are being charged.  See below for an adjustment to the kit construction than ensures that charging can only be done when the amplifier is switched off.

The components include a separate USB connector board, even though the charging module is supplied with a connector..   This is because there is no simple way to mount the charging module in the existing case securely enough to ensure reliable connection of an external USB cable.   If the charging module was being used in a custom-designed case then it would be possible to provide a secure mount for the module, and the additional USB connector would not be required. 

Construction

The original kit was constructed with the power switch, external connectors and volume control mounted on the case front, and the circuit board on the back.  For the upgrade the battery holder has been glued to the inside of the front cover, alongside the power switch and the volume control, and the USB port and charger module mounted on the back, alongside the circuit board.   The USB port is wired through to the inputs of the charger module, the battery ("B") terminals of the charger are wired through to the battery holder, and the outputs from the charger module are wired through to the amplifier, via the power switch for the positive lead.
The side of the case front is cut away as required to allow for the insertion of the USB cable.   The original external power connector is blanked off.

A worthwhile modification that has not been implemented in this example is to insert a small 'window' in the case front to allow the light from the charger LEDs to be visible. A small piece of clear plastic, lightly roughened to disperse the light, could be mounted just above the volume control.


Usage

The amplifier is used with a MP3 player and  pair of speakers salvaged from some old headphones and mounted into 3D-printed cases.










Alternate Charger

This wiring shows how the kit can be configured with a two-pole two-position (DPDT) switch so that it will only charge when the amplifier is switched off.  This form of connection is required if the charger module does not provide separate load and battery outputs, such as those chargers that are designed for use only in dedicated battery charging devices.

 

This site was last updated 27th January 2022.