Here is my inelegant solution for powering my Grundig Satallit 750 receiver.
Background: This radio is notorious for being susceptable to wall wart noise. It can be powered by batteries or a wall wart. The D cell batteries it uses are said to last a long time, but not rechargeable in the radio. Regular switching wall wart power creates hum or whiney noises throughout the bands. I'd prefer not to run off of the internal Ds because sooner or later they will leak or run down. I just don't trust them.
I tried a number of "better" $15 and $20 power supplies - they all produce noise. I wanted to power the Grundig from my Jetstream 12 volt supply - which is very quiet on my ham rig - but my Grundig is 6 volts, not 12. Marketplace (12 volt to 6 volt) converters either won't be quiet, or will be pricey. Homebrew is out of my skill set.
My inelegant solution: Use a 6 volt AGM battery ($10 to $20 for one that is 4.5 amp hours or a bit bigger). Get a cheap trickle charger ($10). Yes, they produce noise, too, but stand by.
I already have a plug in count down timer (1 to 6 hours). I connect the radio to the battery (I can't forget that the Grundig center pin is negative) so I need a polarity reversing adapter. I plug the battery to the charger, plug the charger into the countdown timer. The charge on that battery would probably last several weeks. Once every several weeks I hit the button on the countdown timer to charge up the battery for a few hours.
No more D cells, no more wall warts. No more noise.
Because trickle chargers themselves can produce noise in the radio, the battery would be charging only when I am NOT using the radio - thus the reason for the countdown timer for the charger.
My only concern is if I DO happen to power up the charger to the battery while the radio is "on", aside from likely noise from the charger, will it do anything adverse to the radio? I don't like magic smoke.