Powering an In-wall Amp

Sounds like a typical coaxial power plug. Is the metal tip about 1/2" long with a hollow center? Does it look like any of these:

Might work, might not. The voltage may drop too much after a long cable run to be useful (which is why AC and not DC is running throughout most of the world's homes - AC voltage doesn't drop the way DC does with long wire runs).

You can find panel mount jacks for your power supply's plug - but be aware that there are at least 20 very similar looking plugs - you have to get an exact fit. That's determined by the inner and outer diameters of the power plug and to a lesser extent, the length as well. If you have a Radio Shack nearby, they would have such panel jacks. It would be a simple matter to buy a blank wall plate, drill a hole for the jack and a retrofit wall box and power it through that. This is such a jack:

When they say "with switch" it means that there is a connection to break an internal circuit when the plug is inserted. This is often used when a device has both internal batteries AND a power input jack. The jack responds to the insertion of a plug by disconnecting the batteries so they aren't getting voltage from the power supply and will not be likely to explode as a result.

HTH,

-- Bobby G.

I should probably clarify my original description.

The cable itself is attached to an adapter, not dissimilar to a typical laptop PSU e.g

However at the other end (the bit which goes to the amplifier), there is currently no plug on the end if it, just the two bare cables.

I'm not following, I'm afraid. Does the amplifier have a power jack or does it just connect to the power supply with screw terminals? I suppose you should start again from the beginning just so that we are clear. Where will you be mounting this device? Are you trying to conceal the power supply, connect the power supply or both?

-- Bobby G.

That's OK I should probably learn how to describe things better!

To answer your question directly, the amp has screw terminals for power connection.

Therefore the supplied adapter has a long cable on it which has bare wires at the end (for connecting to the amp).

The adapter has an output rating of +24v (1.75A)

What I don't want, is to connect this cable to the amp and then have the cable (with adapter hanging off it) poking out of a wall somewhere.

Following your previous post I am coming round to the following idea..

Purchase a DC socket (one of these)...

Anda plug (one of these)..

Because the existing cable (attached to the adapter) is longer than I require, I could make a cut in it and put as much as I need in the wall, connected to the amp at one end, and my DC socket at the other, then pop the plug on the end of the remaining cable (from the adapter) and plug this into the socket.

I hope that's cleared things up a bit. My mention of a laptop adapter probably confused things, all I meant to point out was that the adapter itself has a socket for an IEC (Kettle) lead

Hope that's clear! Thanks for your patience and persistance!

By George, I think you've got it. Those two adapters should work nicely, especially if you're shortening, not lengthening the power cable.

-- Bobby G.

| Might work, might not. The voltage may drop too much after a long cable run | to be useful (which is why AC and not DC is running throughout most of the | world's homes - AC voltage doesn't drop the way DC does with long wire | runs).

To a first approximation (good for anything you might be doing in a home with 50/60Hz AC and DC power distribution) AC voltage "drops" exactly the way DC does with long wire runs. For power transmission on the level of grid interconnects the losses with DC are sufficiently _less_ than with AC that it becomes economical to use DC even after you factor in the cost and losses of conversion equipment on each end.

Dan Lanciani ddl@danlan.*com

So, realistically, could I run 12V DC (or so) through my 2500 sq ft house to get rid of some wall warts? (Possibly driving it off a solar panel/battery, for that matter).

Absolutely. I did the same thing in my CT home many years ago. Run the wire to mud rings (look like junction boxes only there's no back) in each location. Over the mud rings place single-gang plates with appropriate jacks mounted. Home run 16/2 from each location and you'll be fine as long as the power supply is of adequate size. If the DC current requirements are small, run

18/2.

Altronix and ELK Products both make reliable DC power supplies in various capacities.

|So, realistically, could I run 12V DC (or so) through my 2500 sq ft house |to get rid of some wall warts? (Possibly driving it off a solar panel/battery, |for that matter).

The problem I usually encounter with such schemes is that many of the little gadgets that are powered by wall warts depend on those wall warts for isolation. That is, the negative power supply input may well not be the same as the negative/ground connection of any i/o, audio in/out, etc. of the device. Some devices that don't otherwise depend on the isolation try to help you by putting a bridge rectifier at the supply input. That means that the internal ground/common is going to be one diode drop away from the power input negative.

If you power two or more devices that need isolation from a common supply and if those devices are directly or indirectly interconnected then you can get some pretty strange behavior or even damage. If the devices have no external connections or if you can be sure that the connections are either isolated or referenced to the same ground then you should be ok. In general, I find it sufficiently difficult to be sure of this that I don't replace multiple wall warts with a single supply even when they are all on the same power strip...

Dan Lanciani ddl@danlan.*com

Thanks.

I thought the payback came from HVDC having a lower overall voltage compared to AC's peak-to-peak and that allowed using existing transmission cables designed for AC to transmit more power without overloading the wire insulation and standoffs.

says: "Above a certain break-even distance (about 50 km for submarine cables, and perhaps 600-800 km for overhead cables), the lower cost of the HVDC electrical conductors outweighs the cost of the electronics."

HVDC has a lot of other important benefits that will insure its commercial success. It can tie two unsynchronized AC systems together and it's better at powering long cable runs like undersea cables where the line capacitance becomes a serious issue for AC power transmission.

I should have worded my comment about voltage dropoff differently. AC has been the traditional method of power distribution because when Tesla/Westinghouse and Edison were fighting over AC v. DC for the national power grid, there were no DC voltage convertors. AC at that time could be easily transformed to higher voltages where the voltage drop is not as significant. Use of a higher voltage leads to more efficient transmission of power which is one reason why large appliances are powered by 220VAC. The same amount of power can be transmitted with a lower current by increasing the voltage. Edison power stations had to be local - the DC systems couldn't power anything much beyond a mile from the source. Westinghouse's AC system could send HVAC for hundreds of miles and transform it down to a safe voltage at local substations.

But this all has little to do with creating a home-brew 12VDC distribution system. My limited experience with running long DC wires is that the voltage drop-off for 12VDC is greater than 110VAC simply because the voltage is lower. Some of the DC powered cameras I use come with variable voltage PS's that can be spun up to about 18VDC. By the time it reaches the camera, it's back down to 12VDC. Without that voltage boost the attenuation is so great that the cameras won't operate reliably. I suppose I could run 12/2 or heavier cable and cut down on the voltage drop, but that wouldn't be practical.

Perhaps the EE's out there can tell us what the voltage drop would be for both 110VAC and 12VDC over a 250' length of 12/2 wire.

All the above is of little consequence since as you point out, few wall wart powered devices can be counted on to have no interaction with other devices on the same power bus. I suspect that electronics designed for auto or RV use can be attached to a 12VDC bus without serious interaction because that's how they are connected to the car/RV battery, but I haven't checked to make sure.

-- Bobby G.