Sorry Sky I haven't researched the V4's specs recently...maybe FlySweep will answer the output impedance question you have. There really is no battery issue. Because the unit is a dual mono, at the end of a long listening session or two lasting probably 10 hours total, one of the channels poops out. This is, for me, usually the right channel. No worries say I and Neco and others. It's just another way to tell that it's time to recharge this fine lil amp. Most of the time I run my HD420s (1981!) at my computer through my Aesus sound card into the V4, playing whatever cd I want to hear. Occasionally I'll substitute my HD650s. Tho that's a bit of a bother because I have a 10' Cardas Cable. I save my 650s most of the time for main music salon, using them with Lil Dot 1+ (rolling Russian, Mullard, and USA military tubes) and Meridian 208.24 cd player. I am old and creaky so, as yet, I haven't taken the time and expense to go the FLAC, WAV, lossless dance with some form of dac/player. But I can tell you that the V4 just lights up both these Sennies. Old as it is the 420 is a great hp for my acoustic jazz preferences. Yes I am teetering at the edge of HeadFi madness wondering what life would be like with a pair of Orthos...i.e. HiFi Man HP500. But I think I deserve it to get a Nano and learn how to use it lossless with my V4....get a Slappa case and a Zu Mobius or Canare something of 5' length so I can trot around with a shorter cable in me backpack. You can see I'm getting the symptoms of audiophillias headphonus maximus tho' to some it may seem to be a mild case. Shorter answers: batteries no issue, charged it will go 10-14 hours I reckon. (Did you know that the 420s are rated at 600 ohms and the 650s at 300 ohms? That help?) Cheers, Alejandro
