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The Modern Device LM4889 audio amplifier breakout board uses one of National Semiconductor’s “Boomer” amps. It uses what in the industry is referred to as a “bridge-tied load”. This is an efficient system that increases the total amount of power available to the speaker. It also eliminates the need for an output capacitor (like in the LM386), which tends to be bulky and can limit the low-end response. It has a shutdown pin to put the chip to sleep – just in case you’re trying to build a super low-power microcontroller application, that spends most of its time sleeping.
For an illustration of the why a bridge-tied load eliminates the output capacitor take a look at the schematics.
LM386
LM4889
The LM386 needs the capacitor in series with the speaker to block DC offset voltages from appearing at the speaker. This also allows it to use a single-ended power supply. When powered with five volts, the LM386’s output will float at 2.5 volts, with no audio input. So at a quiescent (no) input 2.5 volts would normally appear across the input. The capacitor prevents this from appearing at the speaker (which would quickly damage the speaker as well as wasting power). The capacitor also acts as a high pass filter, so it needs to be large enough to pass the lowest frequencies one might require.
The LM4889’s output also floats at 2.5 volts, but by using another amplifier, the LM4889 generates an equal and opposite voltage to drive the speaker, thus neatly solving the single-ended power supply issue. It also insures a frequency response down to DC – perhaps raising the question of what small speaker is going to reproduce the bass.
The LM4669 has a shutdown pin and we pinned that out in our implementation. In some appliances it is feasible that the audio amp might have one of the higher current draws (the datasheet says is draws 5mA typical quiescent [no input]), so we provided a pin for interfacing to a microcontroller. We also pulled the pin up with a 100K resistor in series with a normally closed solder jumper, so you can ignore the shutdown pin if you want. If you do choose to tie the shutdown pin to a microcontroller to shut down the amp, you can also unsolder the jumper to save the tiny current of 10 microAmps through the resistor. I would advise against unsoldering the jumper unless the project is really engineered for extreme low-powered operation, such as some of the projects at JebLabs that run on 10 microAmps average current draw.
Modern Device MD0105 Audio Amplifier schematic
Here’s a picture of the board without the pot soldered on:
We put a volume control on it because having a potentiometer with a knob makes it easy to adjust to volume without fishing for a screwdriver. One application with which I am familiar is making audio pieces for art galleries. The opening at an art gallery (when it’s full of people who are often drinking) is always a power of magnitude louder than everyday browsing when the gallery is almost silent. This makes it imperative to be able to adjust the volume on the piece if you it to work in both situations.
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