Battery connection noise

[hetrazom]

I use 12V battery + 240V/220V (Pure sine wave) inverter to run sound gear. I have tried connecting 12V to 9V DC/DC converter for my 9V pedals (instead of using DC power adapter - the object being reduce my heavy load with one less transformer) straight to the same battery as inverter is connected to and I get hiss/noise. Any suggestions to eliminate this noise? Is there an electronic component I can add that will address this problem?

P.S. No I dont want to use another battery (more weight) and using the small 9V batteries is an added cost I dont want. I would be happy if I could use a DC to DC converter as I outlined above. Its light and one time cost.

 

[shrtrnd]

There are a lot of possible 'what if' potential problems here.
I'd check your grounding between the sound gear and pedals, should they share the same ground or not? Cheap fixes I'd try next are a capacitor to try to reduce the hiss, maybe a varistor.
My guess is, somebody would have to look at both circuits, and determine exact fix.
Just a couple ideas to try first.

 

[Resqueline]

The inverter may introduce a high-frequency noise (common-mode) on the AC line which then travels through audio ground and back to the 12V battery via the converter.
You have two choises; either get a galvanically isolated 12 - 9V converter, or wind the present converter's output wires many turns through a high-inductance toroid core.
The noise voltage (differential) on the battery may also pass through the 12 - 9V converter, in which case an extra capacitor on the output wires may help.