Appropriate power adapter for a Roland A-33 midi controller?

February 1, 2009 5:20 PM

Help me find a power adapter for my Roland A-33 keyboard. I've tried what seemed obvious and haven't made much progress.

I recently purchased a Roland A-33 midi controller. Works great on batteries, but I would like to stop feeding it batteries and instead connect it to the wall. The power connector on the keyboard is a labelled "DC in 9v 80mA", with negative on the pin and positive on the outside. It also says, "Use ACA Adapter Only".

I picked up a 9v power adapter at Radio Shack (output: 9V DC 1.5A) and an "Adaptaplug" size "Q" that seems to fit. If I plug everything in I get bupkis.

So, can I make this adapter work for me? Or do I need to find some sort of custom power adapter? Help me, Hive Mind!

posted by larsks to computers & internet (10 comments total)

1.5A is a great deal more than 80mA. The difference in wattage alone is possibly enough to damage hardware, or just not work at all (wattage = volts * amperage).

Sorry I can't help more, I'm a little too tired. Good luck.
posted by dinx2582 at 5:24 PM on February 1

Oh, the adaptaplug, while modular, can also be switched around to reverse polarity.
posted by dinx2582 at 5:25 PM on February 1

Oh, the adaptaplug, while modular, can also be switched around to reverse polarity.

Yeah, I should have mentioned that in the original post. I'm pretty sure I have the polarity correct.

1.5A is a great deal more than 80mA. The difference in wattage alone is possibly enough to damage hardware

See, this is where my knowledge of electronics breaks down. Is the current rating indicative of how much the adapter can potentially supply (e.g., you can draw up to 1.5A), or does it mean that if you connect your equipment to it it's simply going to burn everything out?
posted by larsks at 5:30 PM on February 1

A device only draws the amps it needs- you can go over, but not under. You have to get the polarity and the volts right.
posted by gjc at 5:31 PM on February 1

The polarity and voltage are correct. I guess one possibility is that while the outside diameter of the "Q" size adaptaplug seems correct, it's possible that the inside diameter is too large. There was a bewildering array of choices; I'll have to pick up a few more.
posted by larsks at 5:35 PM on February 1

(adding to glc's correct answer...)

The adapter you bought is a lot larger than you need, but should work. (You should verify that it's REGULATED 9V, because if it isn't, and the current rating is 1.5A, the voltage will be considerably higher than 9V. Check the output with a voltmeter with the unit plugged in and not connected to the Roland. If it's regulated, it'll read about 9V. If not, it'll read a lot higher. The supply may have the word 'regulated' on it, too. I checked the Radio Shack site, and true to form, their 'tech specs' section on the 800mA brick failed to say if it were regulated or not. "You've got questions? We've got the wrong answers!" Jeez.)

If you screw up the polarity, you can kill your unit. If the unit is still working after your experiment, using batteries, you haven't killed it, but that does not mean you are making a good connection. Triple, quadruple check the polarity and make sure of the size. A good fit is one that provides resistance to insertion, but does not prevent it. If it's loose, you will have intermittent connections and be dissatisfied.
posted by FauxScot at 10:49 PM on February 1

1.5A is a great deal more than 80mA. The difference in wattage alone is possibly enough to damage hardware, or just not work at all (wattage = volts * amperage).

Ya, as Faux Scot is saying, the difference in power output isn't directly the problem. It is a kind of useful wrong idea though, sort of..

Think of voltage like water pressure, and current like the water in a pipe. Now think of the water system in a house. If you open a faucet all the way, you drop the water pressure in your house -- if somebody is having a shower, they know when you flush the toilet because it messes with the water pressure balance. If you try to pull the maximum amount of water from the pipes, pressure drops. This happens even though the city water pressure is fixed because there are all kinds of things between you and the city that limit the amount of water you can get out of your pipes.

A similar thing happens to unregulated power adapters, or line frequency transformers in general. They have a peak voltage (aka pressure), but you only see that voltage when they are unloaded. That is, when you aren't drawing any current (no water is flowing). As soon as you begin to draw current the voltage begins to drop. When you get to the rated current on the brick, the voltage will be the rated voltage. You can pull a little more than that, but the voltage will drop further (just like opening the faucet all the way and causing water pressure to drop in the rest of the house).
And, you might overheat the adapter, the analogy isn't perfect :)

Typically, unloaded voltage on a 9V adapter will be ~14V (around 1.5x the rating, or a little more). 80mA out of a 1.5A rated adapter is close enough to nothing that your midi controller will be seeing the unloaded voltage.

You probably haven't damaged the midi controller yet, but don't operate it long term from a 1.5A rated unregulated power adapter. Even if it can survive the peak voltage, it will heat up more than normal and could fail over time.

All that said, at 1.5A there is a good chance you have a switching power supply based regulated power adapter (basically all power adapters are either unregulated line transformer based, or regulated switching power supply based -- unregulated switching power supplies and regulated line frequency transformers can be made, but you won't generally find them in power bricks). A regulated power supply has electronics to control the voltage, and for practical purposes it will provide the rated voltage right from no load all the way up to the rated load.

Unregulated line frequency transformer type adapters are typically very heavy for their size. Regulated switching power supply type adapters are generally light for their size (unless it is something like a laptop adapter, those are switching power supplies that are very heavy -- everything is packed in very tight on that kind of adapter)

For further reading, Am I using the wrong power adapter?
posted by Chuckles at 11:01 AM on February 2

In closing: traded out the 1.5A adapter for the 800mA adapter, and found a plug that fits (outside diameter 5.5mm, inside diamter 2.5mm). Everything seems happy now.
posted by larsks at 12:53 AM on February 3

800mA at 12V is still very high for a device that needs 80mA at 12V. Get something rated below 200mA, or a regulated adapter.

In general, ten times different is "too much", two times different (half or double) is "quite a bit", and +/- 10% different is "a little". Most of the time "a little" different is okay (for current or voltage in an adapter, or for most anything else in electrical engineering). Devices are less sensitive when it comes to current rating, so "quite a bit" different is probably okay there (well, not likely to work if it is "quite a bit" low, but it isn't likely to harm anything to try either). Never go past "a little" different for voltage.
posted by Chuckles at 1:21 AM on February 3

Ah, sorry, replace all those 12s with 9s, but the point is the same.. The difference between 80mA and 800mA is "too much".
posted by Chuckles at 1:23 AM on February 3