OK so I got a 2nd unit delivered and it has the same problem. Thus I have come to the following conclusion(s): 1) The RH10 is earthed to its own casing. This is how it is designed, but to avoid problems with external power you MUST have a power supply with an earth pin. If you do not then recording with a microphone (well, the one I have anyway) while plugged in to ac power gives a lot of hum, rendering it useless (because the power running thru the casing seems to also connect to the mic input). Also, if you are playing through a Hi-Fi using the headphone socket as a line out then touching metal parts of the casing produces very loud and noticeable hum, which could be very annoying in particular situations. 2) Sony have overlooked this AT LEAST in the UK, where they have distributed the units with power supplies that have a plastic earth pin, i.e. no earthing. I tried the unit with the power supply from my old MD player (the Sony MZ-R55 ), which has an earth pin, and this solved ALL the problems. 3) The same problems occur when the USB cable is connected up, and the unit is running from the USB bus power, BUT this doesn't matter so much because you cannot record with a mic now anyway, and there is no need to listen through line out or whatever, you know what I'm saying.... 4) When USB or AC external power are connected to the RH10 unit, touching the metal casing can result in small electric shocks! (But don't worry because it is only 3V, 1A. So it won't kill you. However it is noticeable). My friend verified this. If you don't believe me, rub the hairs of your fore-arm across the top-left corner of the metal side-casing! So, perhaps someone else in the UK can verify this for me. Like I said, I have tested this on two separate RH10 units/power supplies. And people elsewhere in the world, try what I said with the USB cable attached... it should do the same thing as mine! Please try it and see. Connect USB with the line out attached and touch the casing (obviously the volume on your hi-fi needs to be moderate). So to clarify, the problem is not that anything is broken. The problem is that, because the RH10 casing is connected to earth, it NEEDS an AC power adapter with an earth pin, which Sony have not supplied in the UK. Surely they could have tested these things together first!!! BTW, Amaz0n sent me a new one straight away, and I sent them back the first one yesterday in the post. They said they will refund the cost of that. Hope this helps someone! T&C