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Dc power supply array - rack mounted

Discussion in 'Electronic Repair Modifications DIY' started by bksanders, Feb 13, 2013.

  1. bksanders

    bksanders Active Member

    Hello All,

    New here but been doing this since the 300 Baud days of BBS'es
    (So probably know some of you cats!)

    Soliciting help with my new project.
    Replacing wall-warts with arrays
    Currently have multiple (10+) Presonus 120VAC 23W - 16v 1000 mA wall-warts
    Envision rack-mounted arrays of four (4) or six (6) units per tier / rack
    Is heat my nemesis?
    Anyone care to eSSIST me in the design?
  2. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    Why not just get a large current capacity 16 VAC transformer and run them all off one transformer??? I personally hate Wall Warts because there is no standardization in the connectors or voltage and you wind up with 20 or more Wall Warts taking over your power strips in the studio. Someone, a few years ago, made a universal power supply that you could tap off any voltage from 3 to 24 VDC, I looked at it but it was just too expensive so I just keep using and hating the Wall Warts I have.

    Best of luck!
  3. bksanders

    bksanders Active Member


    Thx for the response, this is precisely the kind of info I am soliciting. I did take the AC/DC transformer 101 class last night :wink:
    Can you give me more info so that I can look further into the matter; keyword search(es) and the like?
    If I elect to go the 6 unit array with each unit requiring 23W, 1000mA @ 16v would the transformer / components conceivable fit a 2U rack?
    I suppose I will need the tolerance(s) from Presonus or just spec it overkill.

    What are my worst enemies aside from cost?
    • Heat
    • Ripple
    • Noise
    • EMI
    • Availability

    Should I be looking at SMPS?

    Any feedback I can get will be greatly appreciated!
  4. RemyRAD

    RemyRAD Guest

    You're going to have to be careful here because a lot of these wall warts provide for a 9 V AC output and not a DC output. Most others have a DC output. Obviously with a large frame audio console, it is supplied from a high current single supply. And in doing so, you also need to provide resistor/capacitor power decoupling between each device and/or channel on the console. So if you're trying to our multiple units from a single power supply that can deliver more than adequate current and a ± bipolar 15 V output, you're playing with the wrong power supply LOL. You just need a large capacity high current, single ended power supply. And you will need to provide for some kind of decoupling between each device. And the values of which I cannot possibly determine? 100 ohms, one watt & 100 micro farads at 25 V might be a good starting point? Because audio ends up on those power supply rails inside the devices and you don't want them all to be glommed together willy-nilly. You may end up with crosstalk the likes of which have never been heard with severity of that extreme? So sometimes it makes perfect sense to have separate power supplies for separate pieces of equipment. You mount those wall warts on to multiple AC outlet strips and you put nylon tie wraps over and around them to hold them in place.

    Just some other things to consider.
    Mx. Remy Ann David
  5. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    These are all 16 VAC devices so no decoupling needed. As to where to find a large transformer places like Home, Electronic Parts, Components and Suppliers | DigiKey, 16 volt AC | Mouser Electronics, Inc., or Search results for "16 vac transformer" - Allied Electronics. There maybe other and a short search on the internet should give you more results.

    Add up all the currents of all the 16 VAC units, add 25% for inrush current and you should have how big a transformer you need. Example 10 units 1000 ma each =10000 ma or 10 amps + 2.5 amps for inrush and you will need a 16 volt 12.5 amp transformer. Knowing PreSonus they already provide a fudge factor in their equipment so a 10 amp transfomer should be all you need. If you can find a toroidal transformer it would be best as there will be less shielding required. If you can only find an open frame transformer then mount it away from your audio gear in its own metal box.

    Hope this helps.
  6. RemyRAD

    RemyRAD Guest

    Yeah, I wasn't sure if he was talking about AC or DC wall warts? Yup no power decoupling for the AC. An open frame standard power supply transformer will likely also cost less money than the toroidal? And many of us run the audio consoles where the power supply is separate and you place it as far away as practically possible. No problems with hum or noise. The toroidal seems to be the transformer du jour, of today. They offer a quieter power. Less extraneous electromagnetic fields. Meaning less hum when it's closer. I've just never had any problems with regular transformers. And I wouldn't be placing it right next to the equipment. Even the API 3124 mixer has a toroidal power transformer, right next to Channel 4 microphone and it's still induces hum. It can be rotated to reduce the hum but it never has eliminated the hum from Channel 4 microphone preamp. So I always make certain that I never plug a ribbon into channel 4. If there was ever a nice high class, top shelf microphone preamp/mixer that should've had an external power supply, the API 3124 is it. I've actually considered ripping out the internal power supply and stick it in a metal project box to get it away from my mixers/preamps. But then I would be fixing something that ain't exactly broke. So I'm just careful what I stick into channel 4. No low output microphones for sure. No problems with 1-2-3 but 4, whoops. On both of my 3124 mixers. And those ain't cheap. So that toroidal didn't accomplish what it was intended to do. So that can still occur when in close proximity. Just perhaps not as badly?

    Those little AC wall warts are not as difficult to deal with as the DC versions supplied with many other pieces of equipment. So the AC units are simply feeding a bridge rectifier and a voltage regulator, or two, in the units themselves.Not including the filter capacitors. They're also inside the units.

    AC/DC it's all electricity. You just have to handle them differently.
    Mx. Remy Ann David
  7. Boswell

    Boswell Moderator Distinguished Member

    You have to be really careful here. Pedals and other boxes that take low-voltage AC input often have different internal rectifier arrangements. For example, some derive bipolar d.c. supplies using full-wave rectification, whereas others may use a bridge rectifier and a single-rail d.c. supply. The signal ground is at the centre potential of the first type and at one of the a.c. input lines of the other. There would be black smoke when you connect audio signals from one unit to the next. I have seen this happen on stage when commoning up power supplies!

    One way round this would be to examine and test each box carefully and then divide the a.c. input ones into three types: bridge, full-wave+ and full-wave- (rare). You would then stand a chance if you used a separate transformer for each of the three types, but you would still be in line for heavy mains buzz in the audio due to supply current flowing in the ground loops you have created.
  8. Thomas W. Bethel

    Thomas W. Bethel Well-Known Member

    All good ideas but unless I am misreading the original poster's message they are all the same unit??? I guess we need to know what these are all powering.
  9. Boswell

    Boswell Moderator Distinguished Member

    Yes, agreed, one interpretation of the original post is that the devices are identical since the OP specifies the same type of mains wall wart for each. If indeed they are identical, it would remove the danger of short-circuiting the rectifiers when paralleling-up the transformers, but the problem of hum due to circulating ground currents is very real.
  10. rmburrow

    rmburrow Active Member

    Wall warts may be a pain in the b***, but there are other considerations....like the equipment warranty. Some warranties are void if the equipment is modified (i.e. substitution of other power for the wall wart). Read your warranty. Also, a ground loop problem may be introduced by operating multiple items of wall-wart powered equipment off a single supply. One writer mentioned that some wall warts furnish DC power while others furnish AC to rectifiers, etc. in the equipment.

    One other mention? Any one had a wall wart smoke or catch fire? This was a problem with early wall wart equipment! Fuses on wall wart powered equipment? The non-user servicable wall wart may be the device's overcurrent protection!
  11. RemyRAD

    RemyRAD Guest

    These wall warts have always been a mixed blessing. They really came about because of strict UL and governmental guidelines for any piece of equipment that had a power supply within that piece of equipment. No power supply in the equipment meant wall warts. And the huge amount of requirements therein made this a nonissue with wall warts. And most wall warts do not have any fusing on them. It's kind of like a Microsoft deal.

    However the blessings were and would be quite obvious, when your power supply is not directly near your audio circuitry because Hum and other noise can be easily induced. Making the power supply remote from the device itself ensures lower noise. And all major and even smaller consoles, mostly had their power supplies mounted elsewhere in a rack. And the proliferation of smaller less expensive devices defines the power supply inside those devices/mixers. I mean they work but it's not an ideal way to go. It's just convenient. And more bad things go with that than good. Especially upon a power supply failure. Then only a small remote piece of your console has smoked and not the entire console. And true of other pieces.

    So there are actually good reasons to have separate power supplies. And if you do have a single supply from which you are feeding multiple devices, you might want to consider a secondary dual set of power supplies with diode isolation, with both of them on. This is a redundant power supply and so if one should fail, you wouldn't even know it had failed. Because if it fails, you lose everything. So while it's convenient it may not necessarily be smart?

    Companies have gone as far as having dual power supplies in a single speaker power amplifier for instance. A power supply for each channel of the unit. This was done to provide a full independent current delivery, to each channel individually. This was done in case one channel was under a heavier load than the other channel. So no current delivery would be lost to the second channel. And that's in a single power amplifier of two channels. My 36 input Neve could have been powered virtually from just two power supplies supply different voltages. But there are actually eight separate power supplies for the single console. And it is a redundant configuration because you wouldn't want your console to fail during a Presidential speech, nationwide on the television network and around the world. So there was dual redundancy plus an emergency button. So a single collective power supply is not necessarily the best way to go although it certainly is a convenient way to go. But you could end up with all or nothing? And that's really not a practical engineering plan. You're just being lazy LOL like we all are. And why not? This stuff is already heavy enough as is. The large heavy power supply while it might be convenient? It may actually end up being heavier than those multiple wall warts? Not completely a big deal if it's rackmounted. What do you do on location? You drag out the wall warts. Also because you already have enough junk shoved into your portable rack boxes. You need a bulky power supply in there also? I think not.

    That scenario about the warranty/guarantee is unfortunately, accurate. But if your stuff is already out of warranty? No worries unless it blows up. And generally it shouldn't blow up. But that's also what Apollo 13 thought. And their budget was a little higher than yours. So was their quality control. And they were rocket scientists. We're just Rock It, Scientists. And while that actually sounds about the same, it's worlds away from each other LOL.

    I stopped by this planet about 57 years ago and rather liked it.
    Mx. Remy Ann David

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