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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?

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    \$\begingroup\$ I have not seen it on speakers (all the powered speakers I saw were DC powered), but I did see it in a 56K US Robotics external modem of the late 1990s vintage, which was powered by a 18VAC (output) wall wart, and a pretty beefy one too, capable of 0.5A on the secondary. Looking at USR's web site, they still use AC output adapters, but the current models are of 9VAC 1A. \$\endgroup\$ – Fizz Oct 15 '14 at 23:21
  • \$\begingroup\$ Older dial-up modem needed bipolar supplies for analogue circuitry as well as RS-232 drivers and the AC supply was cheapest solution with just 2 wires. Some modems used 3 wire power connectors but part of it may have been to prevent third party transformers from causing out of tolerance supply. \$\endgroup\$ – KalleMP Jan 24 '17 at 21:37
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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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  • \$\begingroup\$ I'm curious because everything else I have has rectification circuitry built into a power adapter that outputs DC. Why do (cheap) speakers in particular have AC adapters that output AC? \$\endgroup\$ – user381521 Dec 26 '12 at 5:42
  • \$\begingroup\$ I expect there is an amplifier inside your speakers. So see the first sentence of my answer. You would need a special adapter with 3-wire power cable, but that wouldn't be cheap. Using a standard adapter is cheaper. Also, it probably provides better sound quality compared to solution with whole adapter built-in to the speaker. \$\endgroup\$ – Al Kepp Dec 26 '12 at 5:48
  • \$\begingroup\$ @AlKepp I don't get how you are supposed to have a differential power with only two wires, either AC or DC. \$\endgroup\$ – Vladimir Cravero Dec 26 '12 at 8:54
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    \$\begingroup\$ Like this: play-hookey.com/ac_theory/power_supply/images/… Two wires from the transformer go into the rectifier, three wires go out +, 0, -. \$\endgroup\$ – jippie Dec 26 '12 at 8:59
  • \$\begingroup\$ @jippie: That's exactly what I mean. \$\endgroup\$ – Al Kepp Dec 26 '12 at 17:38
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There could be any number of reasons it was done the way it was, and only the designers have those answers. Some possible scenarios:

A suitable transformer may not fit inside the device's enclosure.

In testing, the magnetic fields from the transformer may have caused unacceptable audible artifacts.

The transformer wall warts were cheaper than a chasis or PCB mount transformer.

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  • \$\begingroup\$ For a low power device, what are the advantages of placing the rectification in the device rather than in the power adapter? \$\endgroup\$ – user381521 Dec 26 '12 at 6:28
  • \$\begingroup\$ I just realized I entirely missed the question. It's still a design consideration. Without seeing the PCB or schematic, this is all speculation, but my vote goes to needing a split supply. There are ways to get the negative rail from the positive using a SMPS, but the current draw would be higher and increase the costs of the wallwart. These are all design considerations that need to be evaluated in terms of cost for mass production. \$\endgroup\$ – Matt Young Dec 26 '12 at 7:22
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It can be two things, and since it's audio, probably both. The first, most electronic device companies specialize in one thing, the main product they sell. They tend not to get into the highly annoying area of power supplies. Their engineers figure out what voltage/current/quality power supply they need, and then they buy generics bulk from a Power Supply manufacturer (often paying extra for custom labeling or tighter quality control). It's just way cheaper that way.

The second thing would be, especially in audio devices, additional quality control. Might cost 10 cents for a ac to dc power supply compared to 5 cents to put in a rectifying stage inside the speakers while providing better rectifying control (eliminating cheap wall wart rippling or ground loop issues, shorter signal/power/trace lengths to deter emi etc), they will do it. The power supply manufacturer(s) might not have a cost-effective standard part with a quality dc output.

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