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Toroidal Transformer secondary in parallel

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The 9 VAC devices may operate on 12 to 13 VDC without problems, because it's LIKELY the first thing that happens is the voltage is full-wave rectified. It's not always the case though. I have a modem that will and one that won't. The one that won;t also has LEDs that change color and probably uses the AC to change to yellow.

Also note that 9 VDC does not come from 9 VAC. If you DON'T regulate (not recommended) you would need about 7.5*1.4-1.2 or 7.5 VAC to get about 9.3 VDC. The 7.4 is the secondary voltage, the 1.4 is an approximation to sqrt(2) and the 1.2 is 2 diode drops from a full wave bridge. Linear regulators usually require at least 3V more DC unless they are of a low-drop out type.

What I would consider is a custom transformer or two custom transformers. One to supply the 9 VAC and one to supply the 9 VDC. They would generally be easy for a manufacturer to make because the windings are wound at the same time. I had one made once. e.g. 8 windings x 9 VAC and one to make the DC once you decided on a regulator (i,e,) ebay.

Other options include buying say a 24 VDC supply and multiple isolated buck converters. Before investing a lot of dollars, you could try some ideas out. Audio and linear supplies go together nicely. Meanwell makes some very reasonable DC supplies. Some are even adjustable somewhat (+-10% usually which won't work for you)

UNLESS you inputs are differential (e.g. XLR connections) then you may end up with way to much hum when you try to consolidate.

==

I went through an exercise where I wanted to "clean-up" my wall warts similar to you, but for my Network stuff in the ceiling. I'm in the US and the wall warts just don't get along with the receptacles.

The first thing I usually do is:

1) Label the wall wart with the device name.
2) Label the cord with the size e.g. 9 VDC 5.5/2.1 C+; The 5.5 is 5.5 mm outer / 2.1 mm inner and the Center positive etc.
3) Label the cord with the device name.
4) The device gets the same markings.
5) Add it to my spreadsheet data-base

I decided to put them on a rack shelf with "octopus cords" and my reliability increased 100 fold.

So, if I grab a random cord, I already know a lot about it and I'm not likely to plug in the wrong one.

The Adapt-a-plug, typically available at Radio Shack **broken link removed** allows polarity reversal and sizing options. I don't like them because it's too easy to reverse the leads and it's designed for low currents.

There are also adapters available that will take a 5.5/2.1 or 5.5/2.5 to other sized plugs, but will not reverse the polarity. I decided that keeping the supplies independent was a good idea.

I created a "universal" supply that I can use in an emergency. I took one of these **broken link removed** and placed it in a translucent box (Hammond enclosure) and put a 5.5/2.1 and a 5.5/2.5 connector for the input, so ALMOST any 12 V 1A wall wart will work for a lot of devices. I also have a 24 VDC supply around, but not a wall wart.

If the polarity needs reversing, it's easy to do on the terminal strip. The output is an Adapt-a-plug pigtail I usually tape or heat-shrink the polarity. So, now I have an EMERGENCY wall wart until I can get a suitable replacement.

I've used the "emergency supply" for a year 24/7 for a 6V device.

The DSL modem uses Power Over Ethernet and I also have all of these devices on a UPS.

FWIW: I would like to make a DC UPS with a particular form factor to run a repeater that would have multiple outputs, but I want a convenient way to either monitor or negotiate the voltage, current, polarity and power limit. i.e. some smarts in the connector.
e.g. EEPROM.

And for a DUMB idea. Take a 24 VDC ($40) supply and use it to make multiple 15 V isolated ($40) supplies and finally down-convert to 9 VDC with an ebay module ($15 ea). I don't like it. It's where the custom multi-winding transformer with a bridge rectifier and capacitors could work. Fuses and power indicators could be added for each supply.

DIN rail construction is easy. I've done it in a rack case. You can buy naked PCB holders for the rail too.

When you build a project, you quickly find out that the case, power supply and real estate (the PCB) takes over the cost. The wall Wart avoids UL approvals and the like. 9V is an ODD voltage. The original wall wart does offer some power protection even if it's the leads melt internally. Surge suppression is likely missing in a typical Wall Wart. Even AC can cause issues. I finally put 24 VAC surge suppression on a furnace.
 
I think secondaries were in phase
there wasn't any problem until I connected (powered) second device
 
I think secondaries were in phase
there wasn't any problem until I connected (powered) second device

There wouldn't be, which is a further indication that they are out of phase.

I'm presuming the transformers are identical?, as you can't parallel them otherwise - but even identical ones might not be phased the same.

A few years back I paralleled three 1000W isolation transformers, which were identical - didn't stop a great big BANG though, or taking the fuse out, or welding the switch contacts together :D

Bit trickier with three though, I had to apply a little thought to seeing which one was out of phase.
 
What can happen is that the 9V gets rectified in the item and this will give about as 12VDC. The box does a rail split and creates +-6V or the OP amps in the device bias the inputs to 1/2 the supply voltage. The 1/2 isn't exactly the same and you end up with an "effective ground loop" Grounds at different potentials.

In theory, you should not see a difference, unless the audio signals are connected. I would expect you to see a voltage difference between the grounds of the devices when no cables are connected.

So, I would have measured the voltages between the commons of the devices with no cables connected, just the combined power.

Case in point:
I have 3 DSL modems. 2 seem identical EXCEPT one says 12 VDC and one says 9 VAC. The major difference is an on/off switch. The other is totally different and operates on 9 VAC (actually the same model 9VAC supply).
In one case, the 9 VAC modem would operate on 12 VDC. I think the 9 VAC modem is behaving badly. It was taken out of service.

The totally different modem that used a 9V supply would not operate on 12 VDC. The indicators must use the AC polarity to change the colors of the LEDs.

The modems are effectively isolated because of the Ethernet jacks.

So, you can't apply the 9 VAC/12 DC universaly,

Your devices operate on 9 VAC and I gave you two reasons why they might not be able to be combined. They really don;t share the same reference. Now,if these were differential XLR connections, the outcome might be different.
 
I finally received trafo and ...it does not work...
it maybe what Tony Steward pointed or something else
but it is simply not working
If some of the devices are connected to each other or connected to an amplifier or your computer then you cannot use a common power supply. if you do some can be damaged.
Tomek and the experts,
Did you really believe that you can go against moty22's advice and win?! Ha!!
 
If you recall my illustration with different bridge polarities internal, you cannot parallel inputs using a common AC source that use a common ground externally connected.
Some diodes short out the AC source.

You must verify polarity of input impedance w.r.t. ground of each device input, if you really want to do this. Some may use active SMPS input devices and may not be obvious. ( e.g. synchronous rectifier bridge.)
 
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