eliminating hiss from carputer?

Alright, this is my first post here, although I've been using the 12Volt for a while on other projects... Anyway, I recently picked up a little netbook that's going to serve double duty as a cheap school laptop and an audio source for my car... The car it's destined for is far from being complete (it's in the process of a complete teardown and rebuild), but I'm aiming to use the computer in my daily for the time being. In order to prevent draining the laptop battery, I bought a little cigarette-lighter plug-in charger... The quality isn't exceptional, but it was $25. Now, here's my issue: When I'm running the computer off of the battery, the sound quality is fine (either using a 3.5mm headphone output or an external DAC)... however, as soon as I plug in the charger, I get a high-pitched hum. Using the DAC decreases the volume of the hum, but doesn't eliminate it completely. My suspicion is that it has to do with the power supply to the computer itself, not some sort of interference between the cigarette lighter leads and the car's audio system... I've used the cigarette lighter on other occasions to charge my cellphone and such, and I haven't had the issue before. I have, however, had the same issue in my house a few years back with a similar setup... using a laptop as an audio source, plugged in, I was getting a hum through my stereo. A power conditioner fixed that. So, what I'm wondering is: Does anyone know if it's possible to buy an inexpensive, compact 12 VDC power filter/conditioner, or if it's possible to make one myself? My assumption is that perhaps the voltage (I think) is fluctuating rapidly, and somehow the variation is getting passed to the computer audio circuits at that frequency, but I'm not an expert at these things. Perhaps a capacitor in-line with the positive feed to the computer would fix this by "smoothing out" the power supply? For those of you wondering, I'm using a Gateway netbook, powered off a Scosche car charger (NetVOLT, I believe), in a '93 Volvo 850. The audio is mostly being run to an external USB DAC (USB-powered) and then through the analogue outputs into an AUX 3.5mm input on the headunit. I know the headunit and AUX input are fine, as my primary source of music is an iPod (through an external interface) into the AUX jack of the HU. I get no hum using the iPod and interface. I also believe that the Scosche charger "senses" the required output voltage and adjusts automatically... it doesn't have a manual voltage selector. My apologies for the long post... it's probably better than not including enough info though. Thanks, Fil

I can't edit my post, so here's something I forgot: I did try one other laptop car charger, a Targus brand, with the same effect. It was, however, over twice the cost, so I decided to stick with the inexpensive one if they both give me the same results.

Those supplies are switchedmode. It's likely that it uses low frequency switching (hence audible) as few use or require output filtering. See if capacitors across the output dc supply help. Try small ceramic or greencap capacitors, say 0.01uF - 0.1uF of at least 25V rating. After that - maybe a big cap or inline filter/choke.

instead of using a straight 12v car charger go get a 100W power inverter and use the charging cable that came with your laptop, id get an extra one unless you plane on unplugging it everytime but thats just me. the little brick thats hooked to the factory power cord has all sorts of neat little goodies to give your laptop clean power to charge with and should stop the hum.

Thanks for the quick replies... I was hoping someone would be able to help. Oldspark: Thanks for the suggestion... I thought a cap might help, but I had no idea on the specs required. I'll look into the ceramic/greencap capacitors, as there's a couple shops around here that should stock them. If I understand correctly, you're suggesting I wire them in parallel with the computer, right? Does the same go for the big cap? And finally (my apologies for the newbie questions), what is an inline filter/choke? I'm assuming it'd do what I need, but I'm not sure if the people at the store will understand what I'm after if I walk in and just as for an "inline filter". SnS: I'd considered that route, but decided against it for a couple reasons... first, I don't want to buy a second AC adapter for my laptop, as I'll be using it outside of my car as well, and I really don't want to have to take out all the wiring every time. The cost of an inverter and second AC adapter is a bit high... if I were to go this route, I'd much rather just buy the manufacturer's car adapter and be done with it. Secondly, it seems terribly inefficient to convert from 12VDC, to 120AC, and back to 19VDC to run a single 30W laptop (AC adapter only provides 19VDC at 1.58A). In this case, purchasing an external DC-DC converter and wiring it in permanently makes more sense to me... I haven't decided how I'm going to run the power in the vehicle I'm rebuilding, so I may decide to go with a hardwired DC-DC converter and keep the cigarette lighter adapter as a spare... at the moment, the computer's found a temporary home in my daily driver, so I'm really just after a quick and dirty fix that'll solve the problem...

Best is if you can try other's chargers & compare (more later....). The caps are wired in parallel with the supply - ie, across it; between +V & 0V (GND/chassis). It's the same as an audio stiffening cap or "dip filter", but instead of using a large cap (of around a Farad), small caps of micro-Farads are used. The aim is to eliminate high-frequency noise on the power lines. (Big caps filter/eliminate/short LOW frequency AC like audio dips etc. Capacitors are like a short to high frequencies.) You could try "condensors" (caps) as used for ignition points and alternators. These are typically 0.22uF from memory (surely not 0.22mF??) Their lowest voltage is usually 50V (DC) which is fine for vehicles unless. Higher is ok too. (I prefer 400V for diodes etc since 200V spikes are not uncommon.) As to ceramic or greencap values - small. Probably 0.1uF. But 0.01uF (aka 10nF) or 1nF might also be okay. May as well by a few for future use - they are probably only 20c-50c each.    And having said that, somebody had placed a electrolytic capacitor (of maybe some hundred to 10's of thousand uF) across a friend's ignition coil (+12V to chassis) to suppress noise. Allegedly it worked, but I removed it will no ill effects. Usually those 0.22 "auto" condensors are used. The "suppressor" (eg - the cap) should be places as close to the noise as possible to minimise noise radiation etc. IE - at the charger. But suppressors can be at the equipment end to stop the noise getting in from the line (though it still acts as an antenna). The inline noise filter is any noise suppression filter that is used for (eg) audio equipment. They are usually rated in amps 5A, 10A etc and cost maybe $20. The consist mainly of in-line (series) inductors (coils/chokes) that fight current changes. Since they are designed for audio frequencies, they should also block higher frequencies (just in case it is some supersonic frequency that is causing problems albethat unlikely because PCs aren't that bad). Inductors block high frequencies (the opposite to capacitors). But if you get to that situation, I recommend a better charger. I recently bought a ~50W charger for under $25, though it has yet to be used. I suspect however that it will not have noise issues. Though voltage selectable, it uses the same SMPS chip as most phone etc chargers and USN supplies (MC34063 etc). Because of their high conversion frequency, only small filter caps are required.

Oldspark: Thanks again for the extra info... it should be a big help in sorting this out. I have tried one other charger, of a similar type, and it had the same issue. The shop where I typically purchase my electronic components has one available, so I could pick that one up and test it out as well. It uses manual voltage selection, so perhaps that would solve the issue. However, it's about $60, which is average for chargers here... as I said, mine was cheap ($25 with taxes), so tearing into it to add a cap is not an issue. As for placing a cap near the charger itself, that's not a problem... the charger is housed in a small "block" that plugs into the cigarette lighter, but the top comes off with a couple Torx screws, and I happen to have the bits required. I'm sure I can find a small greencap to fit in there. There's a USB charging port as well in the block, so if worse comes to worst, I can remove the USB charger and use the space for a cap. Having said that, I appreciate the extra info about wiring one across the ignition... it may be something for me to think about, as I occasionally get interference from my tachometer. It doesn't happen enough for me to consider it an issue (more a minor annoyance), but it'll be something to keep in mind in case the same thing happens in my project car. Unfortunately, the shops are closed for the weekend, but I'll stop by on Monday to buy the caps and then solder one into place... I'll pick up a few different ones, so if one doesn't work I can try another. I'll post the outcome as soon as I figure anything out. Fil

I really think you are creating a ground loop when connected to the Car's electrical system.  You need to go to a stereo shop and see if they will let you try a ground loop isolator to see if it helps.

And since another charger does the same, I'm betting that Idiot above is right. Again. (Assuming other connections - like audio. Maybe no hum thru headphones or PC speakers??)

IAAI (Perhaps when I've been around longer I can use Oldspark's name for you): That's another good point to consider. I think that may have been the issue with my previous laptop-to-home-audio setup, so it's entirely likely that could be the culprit here. However, plugging the previous laptop into a power conditioner helped, so my assumption was that this could be resolved the same way. As for the ground loop isolator, I have one concern: Are there any sound quality concerns when using something like this? Namely, will it filter out certain frequencies in an attempt to eliminate the hum? I should make a second note here: I know my current car audio system isn't exactly "high-fidelity", and I don't really have the money or inclination to fix that. However, the audio system in the car I'm rebuilding will be the best that I can design/afford. This is really the main reason for using a laptop as an audio source: For about the same (or even lower) cost as an iPod and appropriate stereo interface, I can get a laptop and USB DAC. Although it's a bit bulkier, the computer has far more storage capacity, can play all file formats (including FLAC), and the DAC will likely have superior analogue outputs to the iPod interface. Perhaps it's just bias, but even in my current car audio system I believe I notice a difference between my laptop/DAC than my usual iPod/HK Drive & Play. So, having said that, I'd much rather avoid trying to "fix" anything within the audio circuits, rather trying to solve issues like this in the surrounding paths. Which, I suppose, brings me to another question: Assuming this is a ground-loop issue, is there any other means of solving it? Better ground connection, direct connection to the battery ground, using a diode somewhere on the ground connection? Or modifying/running new positive and ground wires? Oldspark: I will look into that today, and let you know if I can hear anything through the PC speakers or headphones... I just rememebered, it's really only noticeable at higher headunit volumes (about 23+ on a 30-step volume control). Computer or DAC volume (it has a built-in volume control) has no effect on the hum.