Buck Gardner's "Kryptonite" mallard call. Got one for Christmas last year and I like it alot. True sounds at both high and low volume. once you have the birds attention and they are closing I like to use a bellows type feeder call that you shake. Primos or Quaker Boy both make them. Sounds like a group of feeding mallards and the birds really seem to set their wings once they hear that.
Late season mallards can be spooky though. My neighbour Bob is a hard core waterfowler and he says the later the season gets, the less he calls, as they get more call shy. Bob also has a few Buck Gardner calls, including "Brads' Reactor". He gets really good sound out of them, much better than me, but he puts in the practice and is out there from the opener to ice up. When I hunt with him I leave the calling to him and just shut up and shoot.
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Bruce Ogilvie
"Fish Hard, Hunt Hard, Live Hard"
http://www.muskiescanada.ca/