Wrong AC adapter for the job?
April 8, 2007 8:35 PM   Subscribe

I've lost the AC adapter to my Roland M-Cube amp. I have an orphan adapter who's specs are somewhat different from the original's. Can I modify it to work with the amp?

From the manual the amp wants 9VDC 185mA. From the back of the amp the diagram indicates it's center negative. I found an old AC adapter that's 9VDC 500mA but it's center positive. The plug fits and seems to be the right length.

1) Is this amperage difference in this direction - more in the adapter than the device wants - OK?

2) Can I cut the wires, flip them, and make it center negative? One wire is marked with white, so the cutting/reattaching itself won't be too hard. I will be careful and use lots of electrical tape.

I don't want to fry the amp or start a fire and I'm willing to go to Radio Shack (or any suggested online store) and spend 15 bucks, but if I can make it work with the existing adapter so much the better.
posted by JulianDay to Technology (6 answers total)
Best answer: 1) The amperage rating is how much the adaptor can safely supply. So that's OK.

2) Yes. I'd recommend getting a new plug of the same size though, one that is meant to be attached by hand (as opposed to the manufactured plugs encased in molded plastic.
You'll probably want to use some heat shrink tubing to cover the attached ends, as strain relief. You can find both items at radio shack, probably for less than $5 total.
posted by RustyBrooks at 8:52 PM on April 8, 2007

It will work. If it was my amp, I would do exactly what you are describing.

Bonus points if you are able to solder the wires after you flip them, but this is more of an aesthetic thing at this level.
posted by davey_darling at 8:54 PM on April 8, 2007

One thing to note though is that your new power supply might not be as well designed as the old one. The quality of parts matters somewhat, but design matters much more. Specifically a power supply for an amp probably has considerably more filtering to provide for a quieter amp.

FYI, it's not too hard to make your own power supply that is well filtered. Not exactly as easy as soldering a new plug though. I've built this one several times. They're pretty cheap to make, although probably not as cheap as buying one (but probably cheaper than a real Roland replacement). The biggest expense will be the case, and the tools.
posted by RustyBrooks at 8:56 PM on April 8, 2007

Response by poster: Thanks so much for the fast answers.

I can run the amp off batteries so I'll see which (rechargeable NiMh AA's or the bastard AC adapter) makes less hum, though my telecaster seems to hum so much and the power in my apt seems kind of wonky that the results won't be definitive by a long shot.

RustyBrooks, that's way beyond my soldering kung-fu, but thanks for the link.
posted by JulianDay at 9:15 PM on April 8, 2007

Assuming the 500mA replacement adapter is a line frequency transformer type.. Your voltage is right on, and the current demand is low by a factor of 2.5. I'm thinking that is just beginning to be a little marginal. Here's why..

Using traditional heavy AC adapters, with line frequency transformers inside, both the voltage and the current should match as close as practically possible. This is because line frequency transformers have poor load regulation, which means that under light load (much less current draw than the rating) the voltage can be more than 1.5x rated voltage. Basically, as more current is drawn, the voltage will drop, since the rated voltage assumes full load, the no load voltage will have to be higher to compensate for the drop.

Modern switch mode power supply based adapters are a different story - they have very good load regulation, so the issue doesn't really come up. Even with switch mode adapters, replacement isn't completely without risk, but then nothing is :P

I think you will probably be fine. Someone familiar with the product might know how robust the design is, and that might factor into the decisions.
posted by Chuckles at 9:34 PM on April 8, 2007

Your idea is reasonable.

If you, on the other hand, wished to go to Radio Shack and spend $15, you would be able to obtain an adapter with switchable voltage (3-15 V), multiple differently-shaped plugs, and the ability to swap center-positive and center-negative. I have a couple of these and I found them to be worth the money just because I screw around with a lot of different guitar effects.
posted by ikkyu2 at 12:08 AM on April 9, 2007

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