BOSE QC 25 works perfectly without a battery, and is much less bulky than the A20 and a fraction of the price.
They are pretty good passive noise reducing headsets - I used mine for a week before getting round to putting the battery in. It only takes one AAA battery, which fits into one of the earphones, so no bulky box hanging on the cable.
Yes, if required, you can disconnect the mic and do your walk around in them without having to carry a set of ear defenders too. And when you go on holiday, you can take the QC25 to listen to music or films in ANR comfort !