DT990 needs some voltage, but actual power need is just average and because of impedance current draw is low.I currently have with me some HD560s which aren't hard to drive at all. However, I do also have some DT990 250Ohm, K702 and some Beyerdynamic T1.Gen2 600Ohm headphones that I use.
AKG actually needs little more power per dB and because of lowish impedance draws it as far higher current than DT990.
T1 again has higher impedance than DT990, but its power need per dB is actually lower and it doesn't need any more voltage.
Doesn't work that way.USB 3.0 Female Splitter Cable,USB3.0 Male to Dual USB 2.0 Female & USB3.0 Female Y Splitter Charger Cable 1 Male to 2 Female Power Cord Extension Hub Adapter for Car/Laptop/LED Etc £5.50
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This is all you need to get too or use any usb hub you may have. The idea is the two usb ports one sends power and one sends data to keep the sound cleaner that way.
Using splitter from one port already allows coupling of power draw's EMI into signal wires.
But unlike analog signals, digital signal's ones and zeroes are quite robust.
And use of differential signaling further increases strength of actual signal.
So with proper circuit design use of same cable for power and digital signal has negligible effect.
And even use of completely separate power source isn't necessary better by bringing in ground loop risks.