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DC and AC Power question

post #1 of 7
Thread Starter 

I hope this is the correct place to ask this question. I recently purchased the O2 amp and have been confused by the power supply it requires. It needs AC power at about 15v and it seems very rare to have an electronic device that requires AC rather than DC (I've never had one anyway). So, first question,

 

Why would this device be designed to use AC power input rather than DC like all my other devices?

 

I've been thinking about this, then realized that when it runs off the 9v batteries it is running of of DC power, since the batteries are DC. So second question,

 

How does it use DC power from the batteries if it requires AC for the plug?

 

I obviously don't know a lot about electronics. It could be that the amp converts the batteries to AC somehow, but that doesn't answer the first question. Any help would be appreciated as I'll be thinking about this and wondering until I find the answer bigsmile_face.gif

post #2 of 7

Electronics like this also run off of DC power.

 

Internally, the amp's power supply converts the AC into +/- 12V DC.  (Some other audio amps like the new Schiit Magni do something similar, to +/- 15V DC supposedly, using an external AC/AC adapter first.)  Running off batteries, it just runs at a lower +/- 9V DC and actually a little less as the battery drains, which means the max output power possible is lowered.

 

Some designs just use a AC/AC adapter and do the AC/DC conversion internally.  Some use an AC/DC adapter which does the AC/DC conversion with the extra electronics and parts inside the adapter.  On the other side of things, some larger devices use a normal AC power cord and do everything internally.

 

There are compromises and workarounds to avoid this, but for audio you generally want both the positive and negative voltage rails, and you want those to be relatively clean and noise-free so the power supply's not contributing noise to the output.  The designer may be able to control things better by doing the AC/DC conversion themselves on the board.  A lot of electronics just need the single positive voltage rail and don't really care much about how tightly regulated it is or if there is a little noise or ripple.  The common cheap AC/DC adapters (often to single DC voltage) are suitable there.

post #3 of 7
Thread Starter 
Quote:
Originally Posted by mikeaj View Post

Electronics like this also run off of DC power.

 

Internally, the amp's power supply converts the AC into +/- 12V DC.  (Some other audio amps like the new Schiit Magni do something similar, to +/- 15V DC supposedly, using an external AC/AC adapter first.)  Running off batteries, it just runs at a lower +/- 9V DC and actually a little less as the battery drains, which means the max output power possible is lowered.

 

Some designs just use a AC/AC adapter and do the AC/DC conversion internally.  Some use an AC/DC adapter which does the AC/DC conversion with the extra electronics and parts inside the adapter.  On the other side of things, some larger devices use a normal AC power cord and do everything internally.

 

There are compromises and workarounds to avoid this, but for audio you generally want both the positive and negative voltage rails, and you want those to be relatively clean and noise-free so the power supply's not contributing noise to the output.  The designer may be able to control things better by doing the AC/DC conversion themselves on the board.  A lot of electronics just need the single positive voltage rail and don't really care much about how tightly regulated it is or if there is a little noise or ripple.  The common cheap AC/DC adapters (often to single DC voltage) are suitable there.


Thanks for the response! So, to make sure I'm understanding, this design is better able to control the power quality by doing the AC/DC conversion internally rather than relying on the wall wart for conversion? Seems to make sense, so it is a step between a totally internal power supply that requires no wall wart, and a totally external supply that is totally reliant on the wart.

 

So what would happen if you plugged in a DC power supply? Would it try to convert it and cause problems, or would it be just fine? I actually know the answer to the second question, since the site says it won't work with DC input, I'm more looking for the "Why".

post #4 of 7
Quote:
Originally Posted by mnarwold View Post

Thanks for the response! So, to make sure I'm understanding, this design is better able to control the power quality by doing the AC/DC conversion internally rather than relying on the wall wart for conversion? Seems to make sense, so it is a step between a totally internal power supply that requires no wall wart, and a totally external supply that is totally reliant on the wart.

 

So what would happen if you plugged in a DC power supply? Would it try to convert it and cause problems, or would it be just fine? I actually know the answer to the second question, since the site says it won't work with DC input, I'm more looking for the "Why".

 

Maybe I made too much of a distinction between where the parts are placed, which is mostly about size, weight, and cost (also handling usage at ~230V parts of the world as opposed to ~115V).  The main feature we're looking at is where to put the transformer, which does the AC/AC step-down, which requires a couple of copper windings.  If you use a switched-mode power supply, complexity is higher, noise can be higher, but you can use a much smaller transformer, which saves on transformer cost / size / weight.  The purists really don't like that though, and it's not necessary.

 

If you gave the amp a DC voltage of a high enough level—maybe 15V or so?—it will just power the positive +12V fine, but you're not going to get anything for the negative rail.  A lower voltage, and the positive rail should work but at a lower voltage I think?  I'd need to check what those regulators do again, when operating under the dropout (i.e. out of spec and normal design operation).  Anyway, the lack of the negative rail is why it won't work.

 

As for why, the schematic is here (<- click).  The power supply is at the top.  Were you looking for discussion of the electronics, part-by-part?


Edited by mikeaj - 12/28/12 at 12:17pm
post #5 of 7
Thread Starter 

I'm not looking for part-by-part discussion. The schematic is a bit over my head as I have very little experience with electronics in that way. You're answering my questions well, just looking for an explanation in layman's terms. So I understand everything except what the negative rail does.
 

post #6 of 7

At a very high level, let's just say that the parts don't work unless they have both positive and negative power supply voltages.

 

The output is music, which is AC, meaning alternating.  Compared to some reference value (let's say 0), it goes back and forth between positive and negative.

 

An amp can't output a signal greater than its positive supply rail, and it can't output less than its negative supply rail.  Actually, the electronics require some margin on top of that, to operate, like a couple volts on each side or so (less for different parts).  Based on this design, the output is referenced to 0 and can't be split halfway between the positive rail and negative rail, or something like that.  Certain parts are designed internally to run just off a positive rail, but some parts need both positive and negative.  So if the negative rail is at pretty much 0, this circuit doesn't work.  Certainly you're not generating negative values on the output, and you're not getting music.

 

Somehow I don't think I got the right balance of fudging over details and making sense.

post #7 of 7
Thread Starter 
Quote:
Originally Posted by mikeaj View Post

At a very high level, let's just say that the parts don't work unless they have both positive and negative power supply voltages.

 

The output is music, which is AC, meaning alternating.  Compared to some reference value (let's say 0), it goes back and forth between positive and negative.

 

An amp can't output a signal greater than its positive supply rail, and it can't output less than its negative supply rail.  Actually, the electronics require some margin on top of that, to operate, like a couple volts on each side or so (less for different parts).  Based on this design, the output is referenced to 0 and can't be split halfway between the positive rail and negative rail, or something like that.  Certain parts are designed internally to run just off a positive rail, but some parts need both positive and negative.  So if the negative rail is at pretty much 0, this circuit doesn't work.  Certainly you're not generating negative values on the output, and you're not getting music.

 

Somehow I don't think I got the right balance of fudging over details and making sense.

Well I think you got it spot on, because I get it now.

Thanks!

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