Help and advice needed with 12-18V DC power supply
Jul 13, 2008 at 12:04 PM Thread Starter Post #1 of 10

ls206

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Heya,

I would like to connect my cmoy to a mains power supply as a beginner project.
the OpAmp I'm using has a nominal supply of 15V (DC).

I gather I would need a power circuit to transform the 240V AC to a ~15V DC supply.

I've found THIS circuit. Would this take a power supply straight from the mains?? Also, to get different output voltages would it simply be a matter of changing the value of the resistors?
EDIT: just noticed the 28V AC max input...

If there are any suggestions or simple schematics (with list of suggested materials) please input
smily_headphones1.gif


hope I've made sense,

thank you
 
Jul 13, 2008 at 12:35 PM Post #2 of 10

JamesL

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You'd need to step down first with some kind of transformer or wall wart.
You would probably adjust the output power through the trimpot.

Alternatively, you may want to take a look at the TREAD from tangentsoft.
 
Jul 13, 2008 at 2:39 PM Post #3 of 10

ls206

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thanks for the reply,
would this tansformer be OK to go before the power circuit?

with regards to the TREAD, does it too, take a decreased AC voltage?
I've noticed the power circuits don't "take" the ground wire from the power supply, how is this connected?

thanks, I hope again I've made sense
 
Jul 13, 2008 at 6:38 PM Post #5 of 10

tangent

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Quote:

Originally Posted by ls206 /img/forum/go_quote.gif
with regards to the TREAD, does it too, take a decreased AC voltage?


You can feed a TREAD AC or DC, with the voltage limit being mostly up to what components you use. In general, you feed it from some kind of transformer or wall wart, which does indeed bring the mains voltage down to a more useful level for headphone audio.

Quote:

I've noticed the power circuits don't "take" the ground wire from the power supply, how is this connected?


Most wall warts only have two connections on their output. If it's an isolated supply -- which many circuits discussed here require -- it's incorrect to think of either of them as earth ground, since a lack of a direct connection to anything on the wall side is what isolation means. You can use one of them as a circuit ground, but that's arbitrary. "Ground" is not an absolute thing. It means what you want it to mean.
smily_headphones1.gif


Where you get into trouble is when you have two grounds that aren't the same thing and you have to connect them. This may work, if the two circuits are designed to allow this, or it may cause problems.

If you're DIYing a complete power supply, and not using things like wall warts, and your local power system has three lines -- hot, neutral, and earth ground -- it's usually a good idea to tie earth ground to your enclosure, if it's metal. But again, you can get into trouble doing this if you then try to connect another ground to it, such as the virtual ground used in a lot of the circuits discussed here.

Ground is a complex topic. There's just no getting around that fact.
 
Jul 13, 2008 at 8:52 PM Post #6 of 10

ls206

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thanks guys, it looks like it would be best to use a wall wart.

if I got something like this (12v dc one) would it negate the need for the power circuit?
thanks for the link 00940, hadn't come across that website yet
smily_headphones1.gif

sorry if these Qs are tedious, I appreciate the help.
 
Jul 13, 2008 at 9:36 PM Post #7 of 10

JamesL

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You could use it... but for audio, you should always go for a regulated source, unless you're building a separate regulator, such as the tread, or the above mentioned circuit.
More information here
 
Jul 13, 2008 at 9:41 PM Post #8 of 10

00940

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The wallwart you link to is unregulated. It will thus be too noisy for audio uses. However, if you get the 12V AC-AC @1A and either the velleman kit you linked to earlier or the tread (a better option imo), you can quite easily get the 12VDC you want, with decent regulation.

Indeed, it seems a safe bet to expect 13VAC out of the wallwart with such a light load as a cmoy. It will thus give you around 17VDC (13V*1.414, -1.4V for the diodes drop) before regulation. The 5V of headroom left should be ok for both the regulator dropout and some mains variations. This being extremely simplified
wink.gif
 
Jul 13, 2008 at 9:46 PM Post #9 of 10

Daroid

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Hmm, something looks odd. If it's unregulated means that it puts out a highly load dependant voltage. Also, this voltage is likely just the result of a half wave rectification using two diodes and a capacitor for smoothing of the transformed AC voltage.
Depending of what you need, this might just be what you want, but not for feeding opamps directly.

The part about linear looks rubbish. A transformer is not a linear device, but it can of course be approximated as one, but it's a very rough approximation that won't hold for many frequencies or voltages.

If the equipment you want to use the transformer for does not load the transformer with the specificed amount (i.e. draw that amount of current) at a specific voltage, i'd insert a voltage regulator at the output. Like the variable LM317 that was used in the first link you posted. If the required voltage is closer to the output voltage of the power supply, use an LDO voltage regulator instead, the LM317 isn't that suitable for this (drops at least 1.25 V IIRC).
 
Jul 13, 2008 at 11:03 PM Post #10 of 10

synaesthetic

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If you want to regulate a wall-wart on the cheap, you can run it through a 78xx regulator IC of the appropriate voltage. It's not going to sound as good as using a standalone trafo with a TREAD, of course, but it'll work just fine. I've done a number of CMoy builds this way, generally when trying to build a mains power amp on the cheap. TREAD with trafo would be a little overkill for a CMoy in my opinion.
 

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