Technology

Also, regarding the power argument -- USB power can be a source of noise leaking into what you hear.

USB power can be incredibly, mind-bogglingly dirty. I couldn't believe it the first time I watched some video of some guy with an oscilloscope showing it. I guess it makes sense -- I mean, keeps USB controllers and hub prices cheap -- but there's all kinds of electrical devices that have to deal with it. Anyway, point is, it's the responsibility of the USB device containing the DAC to have a power supply that cleans that up sufficiently before feeding the DAC. It turns out that...they don't necessarily do that. I have one USB-powered (not using a USB audio interface, or switching away from my computer's USB bus wouldn't be an option) mixer with 1/4" TRS output where using the USB power bus off my computer for power resulted in perceptible audio artifacts, humming and such.

This appears to be something of a not-uncommon problem, as I see various references to it online for other devices:

https://www.audiosciencereview.com/forum/index.php?threads/cleaning-usb-for-bus-powered-audio-devices-discuss.5899/

Some of you guys may be aware of my posts and other's in the Topping D10 review thread. It seems that this DAC, like many audio devices that get their [power from the USB Bus, suffer from some noise coming from the USB port itself.

From my own experience, plugging the DAC into a Raspberry Pi 3B (+5v PSU and Ethernet connected) dropped the noise considerably compared to any port on the PC.

And if I can hear it, then I guarantee that there are USB audio devices that are inserting all kinds of garbage into the signal going out the output that are maybe less-egregious.

I wound up avoiding the problem with my mixer (well, at least to the point where I couldn't hear it) by sticking the mixer onto an isolated USB charger, not on my PC's USB tree. Now, yes, you can make a fancy power supply that avoids that, and it's fair to say that the guys that engineered the mixer should have used a better power supply if they were gonna use USB power. But if you've got some guys engineering headphones and are under pressure to try to make the things as cheap as possible, because headphones are a disposable item, not to mention as light as possible because they're gonna sit on your head, I'm not sure I'd bet on how much expense and weight they're gonna put into the power supply feeding the DAC.

I haven't tried quantifying how the power supplies on various USB DACs perform, though I would suggest that in a world where people are using USB audio rather than 1/8" TRS, given that you have headphone reviewers that cover things like frequency response, it'd be interesting to have a device that intentionally screws with the USB input power voltage and then have an oscilloscope or something attached to the leads coming off the magnet driving the speaker's membrane and see just exactly how much glop from USB power is leaking through to the membrane at various dick-with-the-voltage patterns.

EDIT: Oh, and I forgot one other point. Cable length. 1/8" TRS cares very little about cable length. If you want a 200 foot cable, sure, go for it. USB, especially newer and faster forms, is pretty restrictive on cable length. I decided, a few years back, to move my PC to the other side of the room to reduce noise at my chair and had fun discovering that a number of current PC cable standards are not incredibly friendly to long runs. USB couldn't communicate without repeaters or an optical bridge, DisplayPort had visible artifacts and occasionally saw the screen go black and need to re-handshake, etc.

https://www.cablematters.com/Blog/USB-C/usb-cable-max-length

How long can a USB 2.0 Cable be?

The maximum recommended cable length for USB 2.0, is five meters, or around 16 feet. That’s actually the longest maximum length of any standard, passive USB cable specification, with USB 1.0 cables restricted to just three meters.

You may find some USB 2.0 cables that run longer than 5m, but they’ll need to be made with a thick wire gauge to ward off signal loss and interference at anything much past that maximum. Alternatively, you can run longer USB 2.0 connections by bridging two USB 2.0 cables with a powered USB hub.

How long can a USB 3.0/3.1 Cable be?

USB 3.0 and 3.1 Gen 1 cables don’t have an official maximum length, but their recommended maximum is around 2-3 meters in length or around nine feet. Like USB 2.0, you can extend this with a powered USB hub, potentially linking a few together to extend your run, but there are far better solutions for longer USB 3.0 cable runs.

The maximum length for USB 3.0 and 3.1 was maintained into the final USB 3.0 Type-A specification, known as USB 3.2 Gen 2. It was also mirrored in the higher-performing USB 3.2 Gen 2x2 Type-C connections, which also had a nine-foot maximum cable length.

However, USB4 cables, which leverage the USB Type-C connector but can transmit data at up to 40 Gbps, only have a maximum recommended cable length of 0.8 meters or around 2.6 feet. That goes for both the existing 40 Gbps cables and the USB4 80Gbps spec cables which will see greater use in the coming years. 

These cables can transmit data at a much greater bandwidth than their USB 3.2 Gen 2x2, or earlier counterparts, which means the cables need to be made of a higher quality to ensure the data is fully transmitted. Unfortunately, that also means signal attenuation is a bigger problem, hence the shorter maximum cable runs for USB4 cables.