Why not more USB-3+ power for HP amp section?

[L5730]

Hi all,
It seems that USB 2.0 spec. of 500mA current limits the possibility for a DAC/HP amp combi unit to output more power into low impendence headphones. Any one know a reason why devices aren't taking advantage of the USB-3+ specifications or USB-C where higher currents can be drawn?
I appreciate that some users would moan and grumble about a device not working as expected due to plugging it in and finding not enough current is supplied, but that already happened with external storage devices and older laptops not quite delivering the 500mA from their USB 2.0 ports and wasn't really that big of a deal (powered hub / USB Y-cable).

Is it more a case of actual voltage supplied to the device that is required to produce the current for driving harder to power headphones/IEMs? If that would be the case, then I can see pulling more current from USB would require generating more voltage and probably introducing a load of unwanted noise, so a no go.

 

[mike7877]

Yeah, it's the voltage.

Average music power of 30mW requires peaks of about 375mW. This would be up to the level of uncomfortable with high impedance / lower sensitivity headphones.

For that, you can see that 15V is needed - more than USB is capable of.

If cost was no object (or a small consideration anyway...) I'd have the headphone amp capable of at least 500mW RMS (so the transistors capable of enough current for that) and the transistors supplied with a good 5-8V more than required for that into the intended impedance (this case 600 ohms). This is to ensure proper behaviour near the crests (peaks of sound waves).

For power I'd use 3.0 and a boost converter (DC-DC increase). Half a watt x2 is a watt (stereo channels) x2 is two watts (amplifier efficiency) and boost converters worst case are 80% efficient, so here, USB 2.0 would provide enough current. If a DAC was included with the headphone amp, you might need to pull 900mA. I'd design my device to try to pull if whether it was connected to 2.0 or 3.0 because as far back as I can remember for USB, if a device took seriously high amounts of current, the user is warned, and if the usage persists, power to the port is cut. Safety feature which also saves devices. Most ports I've come across, even ones in budget systems, are able to provide an amp or more without voltage dropping much below 5 (like 4.8, 4.7 which is fine).
Intentionally pulling more than 500mA is against regulation though, so maybe it wouldn't be allowed to be sold? I don't know.
People could just bundle a cable like what comes (came?) with external optical drives. If that's before your time, it was: instead of one type A from the PC to the drive, two type As in parallel (with only one of them having data pins) going to a micro USB at the drive's rear

 

[solderdude]

I would say that it has to do with backward compatibility.
The dongle manufacturer can not know their device would be plugged into USB, USB2 or USB3 port.

Of course, one day a manufacturer may make a dongle exclusive for USB3 that can deliver a lot more power into high and low impedance headphones but it would be a niche product. Besides, the source (phone for instance) then also has to supply that kind of power to a dongle.
It is one thing to charge a phone with a higher voltage/current but another thing to have a phone supply a lot of power through its USB port.

The easiest way would be for a dongle to draw a constant max current the source can deliver, store the (not demanded) energy in a small battery. Add a DCDC converter to the dongle (it is getting physically bigger now) and let it draw power from the battery that is constantly charged so power peaks can be drawn without overloading the USB port.

 

[voodooless]

For that, you can see that 15V is needed - more than USB is capable of.

USB-C power delivery is up-to 20V 5A. I don’t see issues here. Besides, buck converters are a thing.

 

[IAtaman]

There are non dongle devices that get their power from USB bus, Motu M2 comes to mind. I don't think voltage would be an issue as boost converters are dirt cheap these days. I am also of the opinion that the issue is more of a compatibility across various devices, including mobile devices, otherwise believe it is perfectly possible to build a DAC/HPA combo powered from USB.

 

[wwenze]

I'm pretty sure my computer's USB-C doesn't give anything higher than 5V.

Laptops may come with one port if lucky.

 

[notsodeadlizard]

Hi all,
It seems that USB 2.0 spec. of 500mA current limits the possibility for a DAC/HP amp combi unit to output more power into low impendence headphones. Any one know a reason why devices aren't taking advantage of the USB-3+ specifications or USB-C where higher currents can be drawn?
I appreciate that some users would moan and grumble about a device not working as expected due to plugging it in and finding not enough current is supplied, but that already happened with external storage devices and older laptops not quite delivering the 500mA from their USB 2.0 ports and wasn't really that big of a deal (powered hub / USB Y-cable).

Is it more a case of actual voltage supplied to the device that is required to produce the current for driving harder to power headphones/IEMs? If that would be the case, then I can see pulling more current from USB would require generating more voltage and probably introducing a load of unwanted noise, so a no go.

You are not forbidden to make your own USB PD power for your headphone amplifier.
It will be one-IC (keywords are "USB PD sink controller") design if you are not worried about the post-impulse specificity of the output voltage.
Or 2...N (N can be as big as the imagination allows) if you want good filtering.

 

[KSTR]

USB-C is just a wrapper, a common form factor, for a variety of USB interface specs with regard to speed and bus power, and even direction (host/device). That's why every USB-C piece of gear must use negotiation to determine the capabilities of the connection and then decide if they actually "turn on" or not.

 

[voodooless]

I'm pretty sure my computer's USB-C doesn't give anything higher than 5V.

Laptops may come with one port if lucky.

Mine do. I can charge another MacBook from the USB-C ports just fine.

But in general, this is a problem. You can’t see from the outside if there is more than 5V available.

 

[KSTR]

As for power, the guaranteed 2.5W one can draw from USB 2.0 (500mA @5V) -- after enumeration -- can go a long way. With clever design about 1W average per transducer could pushed. That should suffice even for more demanding headphones. The major problem with a truly universal headphone amplifier design is the extreme range of impedances it must cater for, ranging from below 10 Ohm to above 1 Kiloohm giving large difference in voltage-based efficiency and the also large range of true power efficiencies (in dBSPL/mW). You need a large gain/attenuation range, ultra low noise, high voltage and high current, probably some safety features (DC protection, gross overload detection, etc).

 

[boXem]

USB-C power delivery is up-to 20V 5A. I don’t see issues here. Besides, buck converters are a thing.

Boost converters

 

[voodooless]

Boost converters

They are both a thing

 

[ZolaIII]

There is still no USB DAC in the world that utilities USB Audio 3.0 standard. Even standard whose finalised more than 5 years ago and based on USB 3.0 (and backwards compatible of course) including better power delivery among numerous and useful power saving futures. Sad but true if you ask me.

 

[KSTR]

Sad but true if you ask me

Standards have good intentions but often err in what consumers/markets really need.
USB1.1 (FullSpeed) over USB-C is all the common USB-Dongles (like Apple's) need.
High-Power USB bus-powered audio devices are not that attractive it seems, no reasonable market share as for now.

 

[ZolaIII]

Standards have good intentions but often err in what consumers/markets really need.
USB1.1 (FullSpeed) over USB-C is all the common USB-Dongles (like Apple's) need.
High-Power USB bus-powered audio devices are not that attractive it seems, no reasonable market share as for now.

Not quite, you need USB 2.0 for asynchronous transfer as it is in USB Audio 2.0 and with USB Audio 3.0 you can get more power and or significant operational power saving. I am certain that upgrading to newer standard is not either expensive nor difficult. We can write novels why manufacturers don't care and don't make effort but it won't get us anywhere reasonable.

 

[FrantzM]

Hi

I am with @solderdude . High Power draw from the portable devices is likely the main issue, plus the market for such is microscopic with most people using high sensitivity IEMs or "ear pods". Perhaps for those of us who care enough to plug it to our desktops, then again that is what internal or external power supplies are for...

Peace.

 

[Trell]

Not quite, you need USB 2.0 for asynchronous transfer as it is in USB Audio 2.0 and with USB Audio 3.0 you can get more power and or significant operational power saving. I am certain that upgrading to newer standard is not either expensive nor difficult. We can write novels why manufacturers don't care and don't make effort but it won't get us anywhere reasonable.

The only advantage with USB 3.0 for desktop DAC/HP amp is more available power at the expense of less compatibility with source devices, and it costs more to implement as well. The max cable length for USB 3.0 is 3 meter while for USB 2.0 is 5 meters.

For a manufacturer like RME it would be a dumb move to require USB 3.0 for their ADI-2 DAC FS that idles at 7 W with max at 18 W, as an external power supply would be needed in most cases anyway.

And for a mobile phone I don't want a DAC/HP Amp that empties the battery in no time, so max allowed power draw is worse than useless for me there.

 

[ZolaIII]

@Trell please read the specification.

USB Audio Devices Release 4.0 and Adopters Agreement | USB-IF

www.usb.org

If you don't want masiva efficiency improvement on battery powered device's that's fine with me. Main point for putting a power delivery difference and with what it's limited for me lies in thread title.

 

[Trell]

@Trell please read the specification.

USB Audio Devices Release 4.0 and Adopters Agreement | USB-IF

www.usb.org

If you don't want masiva efficiency improvement on battery powered device's that's fine with me. Main point for putting a power delivery difference and with what it's limited for me lies in thread title.

The thread title nor the OP mentioned anything about mobile usage specifically. For mobile usage the USB Audio Class 3.0 can certainly be useful for power savings, but are there any mobile that has even implemented that? Besides, power hungry headphones still draw power.

 

[ZolaIII]

The thread title nor the OP mentioned anything about mobile usage specifically. For mobile usage the USB Audio Class 3.0 can certainly be useful for power savings, but are there any mobile that has even implemented that? Besides, power hungry headphones still draw power.

Surely he ment on desktop class DAC's and amps with independent not USB power supply's. USB Audio and USB standard in generally where used by me in general to explain why it is as it is (USB 2.0 and USB Audio 2.0 disregarding what cable you use).