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Pros and Cons of USB Bus Powered vs Power Adapter Audio Interfaces
Old 12th August 2013
  #1
Gear addict
 

Smile Pros and Cons of USB Bus Powered vs Power Adapter

Complete newbie question here.

Was just wondering if each of these had pros and cons on a audio interface:

USB 2.0 bus powered

vs

USB 2.0 run by a power supply adapter


Just curious.

I heard power adapter ones can better power headphone pres to better run higher ohm cans. And that they can run more of a load?
But I hear there are loopback and ground problems, what exactly are those?

Bus powered is probably more portable for laptops on the road, and less clutter from not having a big plug power adapter.
But I suppose it can handle larger/more demanding/task driving loads? Really a issue for the average engineer or musician?

What else?
Old 12th August 2013
  #2
Gear Head
 

This is personal experience, and not meant to be taken as fact.

Using BUS powered I have had better experiences. I attribute it to the fact thaty cans don't have proper shielding, no grounds with BUS powered but AC powered they have been.

Then again, I have 1/4" to XLR adapters now and the problem is gone.
Old 12th August 2013
  #3
Gear addict
 

Quote:
Originally Posted by Lehsyrus View Post
Then again, I have 1/4" to XLR adapters now and the problem is gone.
1/4" to XLR adpaters on what? Headphones? Hahahaha
Old 12th August 2013
  #4
Quote:
Originally Posted by Just Started View Post
Complete newbie question here.

Was just wondering if each of these had pros and cons on a audio interface:

USB 2.0 bus powered

vs

USB 2.0 run by a power supply adapter


Just curious.

I heard power adapter ones can better power headphone pres to better run higher ohm cans. And that they can run more of a load?
But I hear there are loopback and ground problems, what exactly are those?

Bus powered is probably more portable for laptops on the road, and less clutter from not having a big plug power adapter.
But I suppose it can handle larger/more demanding/task driving loads? Really a issue for the average engineer or musician?

What else?
There ARE major differences between USB bus-powered audio interfaces and AC-mains powered interfaces.
Warning: Long, slightly technical post.

A computer USB port is strictly "power limited" by the USB standard. USB ports supply 5.0 volts nominal (5.2 volts maximum) at up to 500 mA. Not more! The computer will switch to current-limiting if the external device (the interface) tries to draw more than 500 mA. That's only 2.5 watts of power (total) available for all functions of the interface.

An AC mains (wall outlet) powered device (an interface) is not "power-limited". The AC mains power supply can be as large as the interface designer wants it to be or can afford within his/her budget. It's not uncommon to find power supplies that can supply 15 volts at 1.5 amperes or more powering small (2-in/2-out interfaces (that's over 22 watts of power).

With the severe power constraints imposed by the 2.5 watt power limit of USB powering, designs must limit certain interface performance specifications.

Power is first required for the A/D and D/A converters, data buffers, and the actual USB connection chips. Power is then required for the analog circuitry: mic pre-amplifiers, line-level analog input amplifiers.

Then, power is required for the DC voltage multiplier that provides +48 volts for the phantom power that almost all small interfaces are expected to have to power capacitor mics that need it.

Finally, power is required for the analog line-level output drivers (amplifiers) and the headphone amplifiers.

Because of the severe power constraints of USB bus powering, the headphone outputs, and often the phantom mic power (mic current capability) are limited to power levels that are far less than ideal.

There is a very popular 2-in/2-out interface with a headphone output that is limited to only 6 mW per channel. Most USB interface headphone output ports won't produce more than 30 to 50 mW per channel. Without getting into the technical details, the output impedance and the voltage swing limits of these "wimpy" headphone driver circuits are also poor (high output impedance and low voltage swing) because of the limited USB power. That makes the phones output perform poorly when driving both low (less than 50 ohms) and high (more than 100 ohms) impedance headphones. Most are optimized for driving headphones around 65 ohms.

With an AC mains-powered interface, there are no real power constraints, so it's easy to include a headphone amp that can provide 100 mW or even more per channel and can drive very low as well as higher impedance headphones.

With line-powered devices, designers can use better, lower-noise mic pres, better output line drivers, and better (higher current capability) phantom power supplies in their designs.

The downside is that the interface must have an additional AC line connection and also include the cost of the internal or external power supply itself.

Regarding the AC interface power supply creating noise and/or ground loops, that's typically a non-issue. Unless you are running your audio on a laptop computer running on its internal battery, you already have a AC power supply connected to the "system". If it's a typical "switch-mode" universal power supply, it may generate some electrical noise. Ground loops may be an issue with desktop computers using grounded mains connections (IEC 3-wire power cables), but usually are not issues with laptop power supplies with 2-wire plugs. Adding the interface power supply will usually not change any power supply noise or ground loop issues that don't already exist due to the computer's own power supply. Proper input/output connections (fully balanced are always better) are essential.

Even with the USB power limitations, there are a few bus-powered interfaces that actually work pretty well. The Sound Devices USB Pre2, the Apogee Duet-2, and the RME Babyface all sound great and the SD USB Pre2 has a decent headphone output. By using custom designed integrated circuits (instead of "off-the -shelf" parts) better engineering, and higher-quality components, those companies are able to make fairly good performing interfaces within the USB bus-power limitations.

Hope this makes sense.
Old 12th August 2013
  #5
Gear addict
 

Daaaaaannnnngggg, long post will have to come back and read it once I have more time lol. But I appreciate you taking the time to really help out. Thanks man!
Old 20th January 2017
  #6
Gear Nut
 

Quote:
Originally Posted by Lotus 7 View Post
There ARE major differences between USB bus-powered audio interfaces and AC-mains powered interfaces.
Warning: Long, slightly technical post.

A computer USB port is strictly "power limited" by the USB standard. USB ports supply 5.0 volts nominal (5.2 volts maximum) at up to 500 mA. Not more! The computer will switch to current-limiting if the external device (the interface) tries to draw more than 500 mA. That's only 2.5 watts of power (total) available for all functions of the interface.

An AC mains (wall outlet) powered device (an interface) is not "power-limited". The AC mains power supply can be as large as the interface designer wants it to be or can afford within his/her budget. It's not uncommon to find power supplies that can supply 15 volts at 1.5 amperes or more powering small (2-in/2-out interfaces (that's over 22 watts of power).

With the severe power constraints imposed by the 2.5 watt power limit of USB powering, designs must limit certain interface performance specifications.

Power is first required for the A/D and D/A converters, data buffers, and the actual USB connection chips. Power is then required for the analog circuitry: mic pre-amplifiers, line-level analog input amplifiers.

Then, power is required for the DC voltage multiplier that provides +48 volts for the phantom power that almost all small interfaces are expected to have to power capacitor mics that need it.

Finally, power is required for the analog line-level output drivers (amplifiers) and the headphone amplifiers.

Because of the severe power constraints of USB bus powering, the headphone outputs, and often the phantom mic power (mic current capability) are limited to power levels that are far less than ideal.

There is a very popular 2-in/2-out interface with a headphone output that is limited to only 6 mW per channel. Most USB interface headphone output ports won't produce more than 30 to 50 mW per channel. Without getting into the technical details, the output impedance and the voltage swing limits of these "wimpy" headphone driver circuits are also poor (high output impedance and low voltage swing) because of the limited USB power. That makes the phones output perform poorly when driving both low (less than 50 ohms) and high (more than 100 ohms) impedance headphones. Most are optimized for driving headphones around 65 ohms.

With an AC mains-powered interface, there are no real power constraints, so it's easy to include a headphone amp that can provide 100 mW or even more per channel and can drive very low as well as higher impedance headphones.

With line-powered devices, designers can use better, lower-noise mic pres, better output line drivers, and better (higher current capability) phantom power supplies in their designs.

The downside is that the interface must have an additional AC line connection and also include the cost of the internal or external power supply itself.

Regarding the AC interface power supply creating noise and/or ground loops, that's typically a non-issue. Unless you are running your audio on a laptop computer running on its internal battery, you already have a AC power supply connected to the "system". If it's a typical "switch-mode" universal power supply, it may generate some electrical noise. Ground loops may be an issue with desktop computers using grounded mains connections (IEC 3-wire power cables), but usually are not issues with laptop power supplies with 2-wire plugs. Adding the interface power supply will usually not change any power supply noise or ground loop issues that don't already exist due to the computer's own power supply. Proper input/output connections (fully balanced are always better) are essential.

Even with the USB power limitations, there are a few bus-powered interfaces that actually work pretty well. The Sound Devices USB Pre2, the Apogee Duet-2, and the RME Babyface all sound great and the SD USB Pre2 has a decent headphone output. By using custom designed integrated circuits (instead of "off-the -shelf" parts) better engineering, and higher-quality components, those companies are able to make fairly good performing interfaces within the USB bus-power limitations.

Hope this makes sense.
I have a Resident Audio Thunderbolt 2 interface (T4) that will not work with my ASUS Transformer 3 Pro (Microsoft Surface Pro 4 clone with Thunderbolt 3 via USB 3.1 port). ASUS says the TP3 puts out 2.5 watts. Do you think this is the problem? Thanks!
Old 22nd January 2017
  #7
P99
Gear Maniac
 

RME says a computer can give up to 900mA.
Old 17th July 2017
  #8
Here for the gear
 

Quote:
Originally Posted by P99 View Post
RME says a computer can give up to 900mA.
USB 2.0: 500 mA
USB 3.0: 900 mA
Old 19th February 2018
  #9
Here for the gear
 

Not got enuf megabytes innit
Old 19th February 2018
  #10
Here for the gear
 

Hi - that long post was a good read.
I’ve had like three or something audio interfaces ..
The best 2 have been mains powered.

I’ve got way too many questions going around my head at the mo , I dunno where to start....
Old 10th April 2018
  #11
Gear Maniac
 
exit2studios's Avatar
 

Babyface

I have a question pertaining to this topic. I have the original (blue) babyface and I have powered it via two usb 2 slots for years without issues. I recently switched to a usb A to B extension cable, and as a result am having to power it with only one usb hub. I seem to be experiencing clicks/pops and am wondering if that is the cause. Would a usb 3.0 A to B cable resolve that or is would the Babyface not be able to handle the additional current given by the usb 3.0? I'm assuming it would since it comes with a dual usb 2.0 y cable out of the box.
Old 10th April 2018
  #12
Lives for gear
 

USB powered interfaces can only boost that 5V up so far for phantom powered mics. The laws of physics say dictate when you use voltage multiplier to kick the voltage up the opposite happens to the current. Voltage goes up > current goes down.

Most of the posts people make with their phantom powered interfaces involve problems powering mics. Fact is you simply aren't going to get enough power (voltage+current) to power many mics that require 48V. Some may have preamp circuits that will allow operation down to maybe 12v which is likely all these phantom powered interfaces can provide. At least with a wall wart the current can be much higher and you can trade off allot more of that current for higher voltages many condenser mics require to operate the most efficiently.

If you only use Dynamic mics then I see no big issues using a phantom powered interface its mainly when you use a condenser mic that also has to share that current when problems crop up.

Of course the manufacturer isn't going to bother telling you their interface sucks when it comes to providing enough power to run your mic too. They go down that path and they'd have to document the power specs of all the different mics manufacturers. They don't even warn you about the differences between desktops and laptops. Laptops have settings which conserve power and things like USB port power throttling only makes the phantom power even more problematic, especially if you don't know how to optimize a computer and override those settings.
Old 11th April 2018
  #13
Gear Guru
 

I would stay away from bus-powered anything unless I am headed out to the woods with my laptop
Old 11th April 2018
  #14
Quote:
Would a usb 3.0 A to B cable resolve that or is would the Babyface not be able to handle the additional current given by the usb 3.0? I'm assuming it would since it comes with a dual usb 2.0 y cable out of the box.
Think of current as a supply. A device will only draw as much as it needs. You could plug it into a supply with 10amps of available current and not have a problem. It is most likely the extension cable causing a problem. You should keep data cables as short as possible. I have had plenty of problems with cheap/generic USB cables in general. Best to get a good quality one to keep the bits flowing.
Old 2 weeks ago
  #15
Here for the gear
 

From my personal experience, when i first started my home studio back in 2002 i started with a usb bus powered m-audio mobile pre interface (2 channel interface). I never had any issues whatsoever using the internal preamp of that interface and I would use both channels/preamps at the same time (recording 2 separate tracks) when i would record acoustic instruments. Keep in mind this was in 2002, we didn't have usb 3.0/thunderbolt/etc. Now I use an external preamp (which almost everyone that has home studios do) and send the audio signal from preamp to interface using TRS cable which doesn't require using the preamp on the interface. That's even less power being used in the interface. Usb powered interfaces are mostly advertised to use on the go but I have used it in a full studio set up and has worked flawlessly or some cases even better than my ac powered interface (presonus firestudio project).
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