how can I filter noise I get laptop audio out from car inverter?

A differential amp, to get rid of a ground loop.

Or feed both devices from the same minus 12v point.

Or both.

You've tried it with good headphones, right?

Have you considered using a direct supply with an input of 12v and output to suit most laptops (19V? You didn't say which make you have). Typical example here, but many others available at all sorts of prices and quality:

Hopefully, although also a SMPS it will not produce the sort of interference you are getting from your 12/120V inverter.

Get rid of the inverter. They are all horrendous noise sources. Run on battery.

I am using a standard 400 W 12 VDC to 120 VAC inverter to power my laptop in the car. While this works well, lately I've started doing audio work with the laptop and feeding the laptop 1/8" audio jack output into an amp hooked into the car radio. While the laptop is plugged into the inverter, I get much noise over the audio system. I know the noise is coming from the inverter, so how would I negate it? I've tried about

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Well the best bet is to use the laptop battery when noise is critical.

Gareth.

Use a 12V supply for your laptop?

For my Dell Lattitude D630, I can use one like this:

RwP

I don't know the problem, but a transformer isolator may help or work.

Greg

Has the laptop & car audio got bluetooth?

or an audio transformer to break the ground loop

or see if you can find a 12 to 19V ISOLATED power supply.

Isolated means the input and output grounds are not connected, This will also break the ground loop.

As is the car itself or anything else operating near it.

Not all of them. Unfortunately price is no indication however. We've used quite a few different inverters for our in-car fox-hunting gear, and some cheapies are quiet, some $$ are noisy. You just have to suck it and see in most cases.

No, I don't know brands and models - been away from this for a few years.

Cars can produce lots of interference, or next to none. At least with a car you can usually chase down the cause and perhaps cure it.

Clifford Heath.