On 23/02/18 15:22, JBI wrote:
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
5 snap closed ferrite cores around the power cable going into the laptop
but they haven't helped. Thanks.

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:
https://www.ebay.com/itm/Universal-DC-15-24V-Car-Auto-Charger-Adapter-Power-Supply-for-Laptop-Notebook-PC/191191789071?epid=560493099&hash=item2c83eb2e0f:g: FEwAAMXQ1ZhThFqB

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

--

Jeff