I bought a FiiO FH9 IEM. When I use it with FiiO K7 USB mode, I hear digital noise. Some very fast beeps in background, slightly audible but annoying. It's present with all power cord plugging combinations. Digital noise is even more audible when I turn the gain switch to high gain. So I bought another Topping D10s DAC and used it as DAC connected to FiiO K7 on line-in ports, using D10s as DAC, instead of built-in USB DAC of K7. This way noise is much lower on low gain but you can still hear it in a quiet place. Much more audible on high gain switch turned on but I use low gain. But is still there though I enjoy this configuration on desktop because you have to explicitly listen for it to hear it. So it's fine but not sure normal.
For portable use, I bought a FiiO BTR7 portable headphone amp for bluetooth feature. It has lower background white noise than FiiO K7 on wireless/USB cable mode. And I bought a Topping HS02 USB isolator for using another charger on power side to avoid using my phone/laptop battery when I don't move around. But this configuration brings a low frequency audible hum that I experienced first time: USB wall plug charger (Apple included, any kind of them) on power side + my phone/laptop/desktop computer etc. (signal side) -> HS02 isolator -> FiiO BTR7 is very audible annoying low frequency hum. This hum isn't present when I connect BTR7 to computers directly or on bluetooth mode with battery power. Direct connection to computer with even longer and cheapest cables don't introduce this noise. This noise is audible even when there is no ground connection as my chargers are 2 prong: Phone + wall charger -> HS02 -> BTR7 combination isn't connected to ground but there is this same level hum. I tried it with various chargers, same. So I don't use HS02 with wall chargers because it makes it unusable. When I use my laptop as power source + my phone as signal source -> HS02 -> BTR7, this noise isn't present even when I connect my laptop to mains power. So I guess chargers are introducing noise but this isolator isn't fixing an already present noise from power line.
Is this an acceptable thing with IEMs? Or is my K7/BTR7 defect? Do everyone hear this noise and ignore it? No noise on bigger headphones with absolutely any configuration on either amp, isolated or not.
For portable use, I bought a FiiO BTR7 portable headphone amp for bluetooth feature. It has lower background white noise than FiiO K7 on wireless/USB cable mode. And I bought a Topping HS02 USB isolator for using another charger on power side to avoid using my phone/laptop battery when I don't move around. But this configuration brings a low frequency audible hum that I experienced first time: USB wall plug charger (Apple included, any kind of them) on power side + my phone/laptop/desktop computer etc. (signal side) -> HS02 isolator -> FiiO BTR7 is very audible annoying low frequency hum. This hum isn't present when I connect BTR7 to computers directly or on bluetooth mode with battery power. Direct connection to computer with even longer and cheapest cables don't introduce this noise. This noise is audible even when there is no ground connection as my chargers are 2 prong: Phone + wall charger -> HS02 -> BTR7 combination isn't connected to ground but there is this same level hum. I tried it with various chargers, same. So I don't use HS02 with wall chargers because it makes it unusable. When I use my laptop as power source + my phone as signal source -> HS02 -> BTR7, this noise isn't present even when I connect my laptop to mains power. So I guess chargers are introducing noise but this isolator isn't fixing an already present noise from power line.
Is this an acceptable thing with IEMs? Or is my K7/BTR7 defect? Do everyone hear this noise and ignore it? No noise on bigger headphones with absolutely any configuration on either amp, isolated or not.
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Many of these new hybrid IEMs are very sensitive so the amp needs to have low gain. But even with low gain amps IEMs can easily accentuate noise coming from source. Mostly this sounds like hiss but depending on the quality of the source other types of noises can also be present. Over-ear headphones are much more forgiving in this respect.