Humless DC->AC Power Inverter?

November 15, 2005 6:44 PM

What DC to AC power inverter can I buy for use in a car that won't contribute hum to an audio signal from devices it powers?

I've currently got a cheap one-outlet low wattage DC -> AC converter that I use to power my laptop in the car. I use a tape adapter to play music from the laptop through my car stereo.* When I have the laptop plugged in to the power, there's a considerable hum in the audio coming through the speakers. I can almost completely eliminate it by unplugging the power. Of course, this means the laptop eventually dies.

What DC->AC power box will let me keep the laptop plugged in without the hum in the audio? I'm hoping to pick it up tomorrow, perhaps at Best Buy or similar Box Store.

*using predefined playlists with the lid closed and all that jazz - let's not discuss driving distractions and all that.

posted by odinsdream to technology (11 comments total)

I have a similar problem. The hum is greatly reduced if you connect the ground pin on the outlet to a ground in the car.
posted by pantsrobot at 6:48 PM on November 15, 2005

Ground the laptop's plug, you mean? How do you personally do this? I can imagine a few ways, but none are particularly pretty.
posted by odinsdream at 6:54 PM on November 15, 2005

I have the same solution as pantsrobot: Connect the ground, and the hum goes away (well, not entirely, but you no longer notice it).

We use my laptop for DVDs on long trips. It's better than a DVD player, and since my laptop is 5 years old, the DVD drive for it off of eBay ran me a cool 20.00.
posted by thanotopsis at 6:55 PM on November 15, 2005

go to radioshack

ask if anyone's heard of a ground loop isolator

if yes, they can help fix your problem

if no, go to another radioshack


Unfortunately, radioshack has been taken over by videogame geeks rather than tinkering geeks... it's a shame...
posted by hatsix at 8:17 PM on November 15, 2005

You could put a small isolation transformer on the output of the inverter. But why not find a DC/DC converter solution that gets rid of the inverter all together? Using an inverter is expedient, but you're taking DC, chopping it up into high frequency square waves, and then making 60Hz squarewaves, filtering the sqarewaves a smidge, calling that AC, and then plugging the PC power supply into it, which promptly rectifies those noisy squarewaves back to (noisy) DC. That's a very big hammer for a very small nail.

Because you're using a laptop, it is expecting about 14V DC (check your specifics) and contrive to feed it that. Ditch the AC power apparatus entirely. It may be as stone-ax simple as a plug that fits your machine, a foot of 2-strand wire, and a lighter-plug on the other end. More likely, you might need to do some voltage regulation for safety's sake.

Ratshack probably cannot help you with this, but your local car audio place might be helpful.
posted by Triode at 10:13 PM on November 15, 2005 [1 favorite]

On the signal side, a bit of isolation would be a fine thing. Here is where you really do want an isolation transformer. The good stuff is made by Jensen Transformers, but if you balk at $160, you can get most of the quality for half the price if you shop other brands. With audio transformers, you really do get what you pay for, so spend it if you got it.
posted by Triode at 10:20 PM on November 15, 2005

Here's the ground loop isolator that hatsix mentioned. I used one in a similar car-computer project, and it greatly reduced the hum but didn't entirely eliminate it. The only other problem was the multiple connections it took to get it into the stereo - mini-jack to RCA -> Ground Loop Isolator -> RCA to mini-jack.
posted by shinynewnick at 10:25 PM on November 15, 2005

Whoops, Markertek.com has the Jensen CI-2RR isolator for $120. Not quite as bad as I was remembering & a reputable vendor. Googling "rca audio isolation transformer" should get some alternatives.
posted by Triode at 10:28 PM on November 15, 2005

Triode; as a technical person, I do cringe at the DC-AC-DC conversion as being overly complicated, but the fact is - it's much easier to do this in order to support several different kinds of devices meant for use inside, rather than build the appropriate voltage regulators, wiring, and plugs for each device I want to use.

Is there a ground loop isolator transformer available that isn't specific to RCA jacks? Something I'd plug in between the laptop and the inverter?
posted by odinsdream at 6:43 AM on November 16, 2005

Or, better yet, someone who makes an inverter that has a built-in isolator?
posted by odinsdream at 6:45 AM on November 16, 2005

The ground loop isolation will only partially fix your problem.

AC power that comes from the electrical outlet in your home produces current that is in the form of a (somewhat) pure sine wave.

AC Inverters made for cars produce a crappy static-y "stair-step" approximation that causes noise in electonic devices that produce sound.

You have to solve both problems in order to get perfectly clean sound from your device.

In many cases, the AC adapter that comes with your computer can do a decent job of making clean DC current from the inverted current. You might find the noise from the AC inversion process to be subtle enough not to notice, depending on your laptop.

But if you want to cause yourself the least amount of frustration, the optimal solution is to do direct DC-DC converstion for things that produce sound, and use the inverter for any other devices you want to plug into your car.
posted by helios at 10:21 AM on November 22, 2005