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I am talking about the typical multimedia speakers that are generally connected to the computer’s audio line-out point.

For example, my power adapter for Altec Lansing multimedia speakers says Input 230 V AC Output 9 V AC.

So 9V AC is fed into the device. What happens thereafter is a black box for me. If I correctly remember during college studies I had seen audio amplifier circuits for which power input was DC. Even I had assembled one audio amplifier using some Philips TDA chip using a ready to solder hobby kit that would take DC input to power it. I would give the stereo output of my Walkman to it and it would drive the speakers with amplified output.

Is it that the 9V AC is then internally converted to 9V DC using bridge rectifier which is inside the speaker cabinet or does the entire amplifier circuit operates on 9V AC?

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  • \$\begingroup\$ Bridge rectifier is used. \$\endgroup\$ – Andy aka Dec 15 '19 at 12:51
  • \$\begingroup\$ Note that the 9V AC may be because the speaker is actually running on +-12ish volts as a split rail done by using two half wave rectifiers and two caps to produce the power. A fairly common trick in cheap audio devices. If so the things will NOT run on 9V DC, it must be AC. \$\endgroup\$ – Dan Mills Dec 16 '19 at 14:05
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The answer to "why is it made this way?" is quite often "it was five cents cheaper to make."

The audio amps always run on DC.

Modern multimedia loudspeakers would probably use a Class D chip requiring a single positive supply, powered by a switching wall wart.

Older chips require a symmetric power supply (like +15V and -15V) or a single supply if they are bridged.

This presents a problem...

Getting a product certified for safety is expensive if it has a mains input and mains voltage inside. It is cheaper to use an AC adapter which is already certified. The same adapter can be used for lots of different products. If the AC adapter is made by a specialized manufacturer, certification costs will be spread even more. When using an AC adapter, what is removed from the product enclosure itself is cost, and this cost lies on the mains voltage AC side.

An adapter also removes heat from the enclosure, and .. heat also makes the device cost more, makes it bigger, etc. An ugly wall wart is the match made in heaven for every modern sleek and thin device ...

While it would be possible to use an AC adapter providing a symmetric +/- supply, this would make the adapter more specialized, thus harder to sell for the company which manufactures it. It would need a 3 pin connector, again more expensive and less standard than a DC barrel connector.

All this means that, if you want a +/- symmetric supply, an AC adapter which outputs AC is a good solution. The device being powered will contain a rectifier (two half-wave rectifiers) and capacitors to make positive and negative DC. This has more ripple than a transformer with proper full-wave rectification, but for cheapo multimedia speakers, no-one cares.

Even if the amplifier only requires one positive supply, you want low impedance between the supply caps and the amplifier circuit, which means the caps should be in the device, not in the adapter. And you don't want to pay for caps in two different places when you can pay for just one! Every cent counts.

Switching supplies make up for this by being cheaper and lighter (thus cheaper to transport and sell) than transformers...

...but they are a specialist job, and EMI certification is expensive, so your average tech equipment manufacturer will either outsource or purchase from a specialized manufacturer. And this is how you get a super thin sleek flat TV with a big fat power brick.

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    \$\begingroup\$ I suspect that heat isn't much of an issue for computer speakers; they are already bigger than the power adapter is. \$\endgroup\$ – user253751 Dec 16 '19 at 10:51
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    \$\begingroup\$ heat is always a factor in any powered (commercial) product, gotta do the math at some point. \$\endgroup\$ – old_timer Dec 16 '19 at 13:05
  • \$\begingroup\$ I'd like to add that it's quite simple to create a symmetric +/- power supply internally from a 9V DC supply. I did this recently in a bluetooth speaker that I made. \$\endgroup\$ – Persistence Dec 16 '19 at 14:58
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    \$\begingroup\$ The arrangement was common a few years ago when switching AC-DC supplies were more expensive than they are now. These days, not so popular anymore... \$\endgroup\$ – peufeu Dec 16 '19 at 17:12
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    \$\begingroup\$ It is interesting to note that these power bricks rarely have an on/off switch. So they use power all the time* and are thus more wasteful then the old systems with their on/off switch. And there are millions of them. In think in my house alone I have 20 or more. We see again that one type of regulation (safety) can have far reaching consequences: rising see levels. *yes they also use power if the connected system is off, especially the small cheap ones. \$\endgroup\$ – Oldfart Dec 16 '19 at 19:07
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or does the entire amplifier circuit operates on 9V AC?

If you think about this a bit more it should be reasonably obvious that an amplifier running directly on 50 or 60 Hz AC would lose power on every zero-cross (100 or 120 times a second) and the sound level would drop to zero with horrible distortion when the sinusoidal voltage dropped below the desired output voltage.

Amplifiers are DC powered and because audio is AC then some means of generating a split-rail supply is required so that the output polarity can alternate.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. (a) A single-ended single-rail amplifier. (b) A bridge-mode amplifier.

  • Figure 1a shows a single ended amplifier. The output is biased to mid-supply (so 6 V for a 12 V supply) and can then swing towards 0 V for negative audio signals and towards +12 V for positive audio signals. C2 blocks the DC from reaching the speaker. If it wasn't there the speaker would be biased off-centre on its suspension, would distort as it has less travel available in that direction and would run hot.
  • Figure 1b solves this problem by driving AMP2 with an inverted version of the signal driving AMP1. Both are biased to half-supply as in 1a and as a result no current flows in SPKR2. As AMP1 goes up, AMP2 goes down and vice versa. The result is a higher powered amplifier for a given voltage and speaker. This configuration is standard in car audio systems where the voltage is fixed at 12 V. The bridge also eliminates the capacitor which reduces the bass response of 1a.
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