I could not replicate your findings using iphone 4s > cable into op1 > sennheiser hd25 headphones. Notably my phone doesn’t hum when it switches off.
The i/o of the OP1 isn’t filtered very well, and this sounds like high-frequency switching noise, of the kind that dogs every computer bus and often power rails on portable devices too, and it gets into the OP1’s audio path. The source could be your phone, or it could be the OP1 having impedance issues.
I spent a good deal of time today trying to find a clean signal path with usb+input+output connected via mixer to fairly high end computer interface, and still have this sort of noise, but it’s a lot quieter. If you’re recording from Op1 to computer directly, you may need to decouple them electrically.
Standard OP-1 behaviour as far as I can tell, I have 2 and they both do it, you will notice that the position of the OP-1 volume control can reduce the whine sound. All of this was well documented on the old forum, and you might find some threads about it on this forum too.
So the good news is your OP-1 is not faulty, the bad news is you have to experiment with things to reduce the whine, unfortunately lots of people used to get this mixed up with the USB ground noise so the discussion very often ended up going off topic to the USB noise problem, which has been resolved by turning off charging while using connected to computer.
Please ignore the hum, it only occurs when using the iPhone 4 for testing. When using an iPhone 5S or just a mp3 player the hum is gone, but the whine is left (which stays there even when disconnecting the iPhone from the cable).
I tried three different pairs of headphones: AKG K44, AKG240 and Urbanears Plattan, all with the same result.
Also, when not using headphones at all, you can make the whine audible by cranking up the volume and master-compressor-drive. You can then even hear it in the onboard speaker (but it’s a lot quieter)
When connected to the PC audio interface, I found out that it’s still there, but really really quiet, on a level that wouldn’t bother me at all. The impedance-theory sounds right to me, it’s seems to depend on what’s connected. When I was sampling from a drum machine app I used on my iPhone, you could hear it on every bass drum tail, as if it was ducked by the signal. When the signal decreased, the whine came back up and got quieter when there was strong audio signal.
