Out of most of the electronics I use, I have two, both of them speakers, that use a power adapter that simply steps down the voltage without rectifying it. Why did the designers choose to put the rectification circuitry in the speaker instead of the power adapter?
Answer
One reason is that you usually need positive and negative voltage for audio amplifier. (Like +5, 0, -5 V).
Another reason: Some devices run directly on AC, e.g. Christmas light bulbs can do.
Yet another reason: Some devices need two different voltages, like 5 V and 12 V.
Yet another reason: An external step down transformer provides just safety. Voltages no higher than 50 V are usually accepted as safe (although not very safe with high current). So the transformer makes it safe, the rest is in the device.
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