Modifying CMOY to cope with switching power - diyAudio
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Old 2nd September 2008, 09:28 PM   #1
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Default Modifying CMOY to cope with switching power

Well, this is my first post here. I hope it doesn't leave me treading on thin ice. I built a CMOY for my girlfriend to use with her Denon 1001's and Ipod while she's at work. She's grown tired of forgetting to power it off, and having to replace the batteries so frequently, so she's asked for a wall-power solution. Being as cheap as I am (had enough spare parts that the cmoy only took about $10 to build), I'd like to try to get by with what I've got laying around. The closest power supply that I've got to the voltage that is needed is one of these wall warts from a project a while ago:

http://search.digikey.com/scripts/Dk...me=T987-P5P-ND

I've read all over that switchers are bad news for audio, some folks making it sound worse than others, but I'd prefer not to hock $30-50 on a linear regulated supply if I can make this one work. Building one is out of the question for this application, since it'll be in an office. I've not attempted anything yet, since the amp is not at home, but I'd like to know if anyone has done anything like this before?

Might I be able to get away with just throwing some caps on the power line to clean it up? I don't have an ocil, so I can't really test the effects of it.

--Matt
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Old 2nd September 2008, 10:16 PM   #2
Zigis is offline Zigis  Latvia
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You can buy linear unregulated ps cheaper, and made small regulation schematic yourself. Or you can use unregulated itself, I try it, not so good as battery, but not bad at all.
You need smallest one, 100-200 mA is more than enough. Voltage from 9V up to 30V, depending on el caps you use.
Zigis.
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Old 2nd September 2008, 11:04 PM   #3
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So it would not be worthwhile to try the switcher?

--Matt
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Old 3rd September 2008, 12:37 AM   #4
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Sure it's worth a try. Not all switching supplies are built the same, some are better than others. I doubt your girlfriend would even know the difference sonically, if there even is one.
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Old 3rd September 2008, 03:41 AM   #5
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Cool, I'll give it a try then. If it doesn't work out so well, I found a 12v unit laying around that is intended to power a DSL modem. It's definitely not a switching unit, unsure of the other details since I cannot find any details on the part number and don't care to crack it open.

--Matt
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Old 5th September 2008, 03:14 PM   #6
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It doesn't sound bad at all. Definitely better than direct to the IPod, also better than the headphone amp on the Pioneer receiver that I have in my work area.

--Matt
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Old 5th September 2008, 07:41 PM   #7
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Quote:
Originally posted by mattkosem
It doesn't sound bad at all. Definitely better than direct to the IPod, also better than the headphone amp on the Pioneer receiver that I have in my work area.

--Matt
See, there was nothing to be afraid of.

Just don't tell the audiophiles, they will no longer respect your opinion.
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Old 5th September 2008, 08:12 PM   #8
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Quote:
Originally posted by mattkosem
Cool, I'll give it a try then. If it doesn't work out so well, I found a 12v unit laying around that is intended to power a DSL modem. It's definitely not a switching unit, unsure of the other details since I cannot find any details on the part number and don't care to crack it open.

--Matt
That makes the DSL modem PSU seem like an unregulated wall wart, which will be very bad producing line frequency hum in a cmoy because it doesn't have an isolated ground channel.

You can use a switching supply but keep in mind that as noise frequency goes up (as it does a lot with a switcher) the PSRR of a opamp like used in a cmoy goes down. This means you need either a very strong background (engineering degree level) to design appropriate LC filtering after the switching PSU, or the easier way is to just put a linear regulator after it.

Frankly I'd just do it the easy way for a CMOY, use a typical wall wart unregulated transformer and after it put a LM317 reference circuit (see National's LM317 datasheet).
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Old 5th September 2008, 08:36 PM   #9
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Quote:
Originally posted by mattkosem
I found a 12v unit laying around that is intended to power a DSL modem.
!!!

Careful!

Look twice at the label. Some of these are 12v AC.

w
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Old 5th September 2008, 09:13 PM   #10
! is offline !  United States
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Quote:
Originally posted by wakibaki


!!!

Careful!

Look twice at the label. Some of these are 12v AC.

w

Really? I've seen quite a few that were AC for (USR/3Com for example) analog modems but never came across one for a DSL modem.
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