I recently built a sound card packet TNC and it is working fine under my standard laptop power supply when I hook it into the wall.

When I plug my computer in via a car inverter (12VDC to 120VAC), however, the radio will transmit hum with it.

I tried an isolation transformer through the audio out, but that minimally helped solve the problem. Other then getting a pure sine wave inverter, what other things can I try?

  • $\begingroup$ You said you used an isolation transformer. Was that in the 120 VAC circuit? I doubt that would help, and it might cause problems because it's not designed to handle strong harmonics that your inverter puts out. $\endgroup$ – Martin Ewing AA6E Jul 13 '15 at 22:49
  • $\begingroup$ Ferrite beads over the power supply and audio lines may suppress some noise too. $\endgroup$ – captcha Jul 14 '15 at 1:05
  • $\begingroup$ I used an audio isolation transformer between the sound card audio out and the radio $\endgroup$ – Skyler 440 Jul 14 '15 at 1:54

Well, you can always get a pure sine wave inverter, which -

Other then getting a pure sine wave inverter, what other things can I try?

Ah. So you understand the problem and why it's happening, and are hoping for an easier/cheaper solution.

One option is to get a laptop power supply that uses the vehicle power. These are relatively inexpensive, and may not only resolve the problem, but be more efficient overall.

If it's a low power requirement and a high power AC inverter, you could use a high power isolation transformer between the inverter and the laptop power supply. This will act as a huge filter and force the signal to be more of a sine wave, cutting out a lot of the higher frequencies. Could also damage the inverter, and this isolation transformer will heat up quite a bit as it's being abused. Follow that with a power line filter and you might resolve the problem, though at a significant cost in money, weight, and efficiency.

Beyond that, though, there aren't any magic tricks you can use to get rid of the hum without further investigation. You may be able to add ferrites along all the power lines and signal paths, isolators to avoid ground noise, etc, but the signal is very strong and won't be easily removed.

Try to characterize the noise - it is a single frequency, is it broadband, what is the signal level? If it's a typical cheap modified sinewave inverter, you're going to have a lot of high power harmonics, it won't be a simple fix or filter.

Once you have that information, ask for the specific methods to filter that type of noise.

My expectation, though, is that you'll find it's cheaper and easier to simply use the right power supply in the first place, than to use the existing equipment and stack filters, isolators, etc on top of it.


Do try grounding the laptop chassis to the radio chassis. Not via the sound card ground, find a solid earth like the screws of the VGA connector or a USB plug chassis. Use a heavy enough wire to keep the residual voltage to a minimum.

The most likely way for the noise to get in is thst the laptop chassis has some high frequency potential with respect to the car chassis. The isolation transformer is a good idea, but why force it to cope with a high voltage, high frequency noise between the windings? Any stray capacitance will cause the common mode voltage to become differential mode, unless it's a very fancy transformer you're looking for trouble.

A solid earth connection is quick to try out and might just help.

  • $\begingroup$ Additional info: Normal 120 VAC commercial power has a "hot" lead and a "neutral" lead, where the "neutral" is at ground potential. Commonly available inverters produce an output where both leads swing between ground potential and approximately 150 volts, with a rectangular wave shape. This causes most power supplies to have a significant amount of common mode AC on both output leads. Grounding as described above helps and may be enough to get you going, but I have not found a good solution to the problem. $\endgroup$ – HarveyB Jul 30 '15 at 4:01

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