You might also consider a Sound Devices MixPre (the 2 channel mixer, not the current recorders) or Shure FP-24. It's not much larger than the PS-2, runs on AA batteries, provides phantom, and has good preamps. Its 1/8" tape out will easily feed your A7R. A used one will cost a bit more than the PS-2, but the Mixpre is far more versatile.