Wim, I don’t know if your Marantz accepts external USB power. If it does you 
could buy a high capacity powerbank (say, 20000mAh or more), connecting it with 
a USB cable, and that would probably solve both problems: autonomy and noise. 

all the best,

Magnus Robb

> On 20 Sep 2018, at 00:13:39, Wim van Dam <wim.van....@gmail.com> wrote:
> [I'm not sure if this newsgroup is still alive, but here goes.] 
> This week I finally installed my OldBird 21c microphone and I'm having some 
> noise issues. In my setting I use a Marantz solid state recorder for my 
> recordings. When I power this recorder with its standard AA batteries, things 
> are fine but it does not allow me to record for more than a few hours. When I 
> use AC power for the recorder instead, a lot of noise shows up at 1500Hz, 
> 1620Hz, 1740Hz, 1860Hz, etc. Likely this has to do with the fact that the 
> recorder and the OB21c microphone use the same AC outlet. 
> What is the best solution to this problem?
> Thanks
> Wim van Dam
> Solvang, CA, USA
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