Originally Posted by
sgs233a
What kind of AA batteries did you use? One (surprising) thing I noted a few years ago when I tried Energizer Lithium batteries in the A20 is that they didn't work well. There was a weird hiss that's never been there with any alkaline battery. While both nominally 1.5V, the Lithiums have a higher initial voltage - unsure if there's any other characteric that might have contributed. I just shrugged and went back to alkalines.
Any power source has what is called a source impedance, and the 'wrong' impedance can adversely affect some circuits.
As has been suggested, try different makes of batteries. Even though headphones use very little power, batteries designed for high current applications - e.g. big photographic flashes or things with motors in - will have a lower source impedance and might help. If not, try some designed for low power applications such as clocks or television remote controls.
I have Bose QC 25's and they only use a single AAA battery for noise cancelling, and it lasts a long time. The QC 25 ANR does not introduce any significant noise when switched on. Significant hiss or static, sounds like a fault to me - although my highest frequency hearing is probably not as good these days, (thank you Motörhead, Black Sabbath and Van Halen concerts for doing that !).