fohman
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Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 3:03 am

I want to run a Raspberry Pi using the "phantom power" from an audio mixer to process a microphone signal in a certain way. To the best of my knowledge, phantom power is +48V and about 14mA on most professional devices, although I haven't looked into exactly how the voltage is distributed over the three pins on an audio XLR cable. Does anybody know if/how this would work? Would I have to have a power regulator circuit of some kind? Also, I would need to power the unit from the same cable that I need to send the processed audio down through to the audio mixer, and I also need to prevent the phantom power from going to the microphone (maybe I'm using a ribbon microphone).

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 5:44 am

fohman wrote:
Sun Feb 04, 2018 3:03 am
I want to run a Raspberry Pi using the "phantom power" from an audio mixer to process a microphone signal in a certain way. To the best of my knowledge, phantom power is +48V and about 14mA on most professional devices, although I haven't looked into exactly how the voltage is distributed over the three pins on an audio XLR cable. Does anybody know if/how this would work? Would I have to have a power regulator circuit of some kind? Also, I would need to power the unit from the same cable that I need to send the processed audio down through to the audio mixer, and I also need to prevent the phantom power from going to the microphone (maybe I'm using a ribbon microphone).

As you provided little information, is this the Device ??

https://www.ebay.co.uk/itm/48V-Phantom- ... 2622701954
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 6:01 am

If you were able to perfectly convert 48v at 0.014A to 5v, you'd get 0.134A. A Pi0 would run on that. A B+ *might* run on it. A Pi3B? Extremely unlikely. Indeed, I'd be surprised if it even booted. Even if you don't have any USB devices at all, a Pi3B can--potentially--require 1.2A, or nearly 10 times what you are talking about trying to run it. In actual practice, you'd be very lucky to get more than about 0.1A at 5v.

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 7:07 am

Sorry, here is the equipment I am using, it is professional audio stuff, I have also provided a link to the user manual for the piece which supplies the phantom power itself...

http://www.yamahaproaudio.com/global/en ... lineup.jsp

On page 20 it shows some relevant information:
https://ca.yamaha.com/files/download/ot ... _om_f0.pdf

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 9:20 am

Remember that no current Pi has a native audio input, so you will need a USB soundcard. For the Pi0 same applies for output. As indicated above the power available will be marginal at best. All you can do is try it, but I would expect it not to work.
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 9:28 am

Regards Sound card see: http://www.audioinjector.net/rpi-hat
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 10:14 am

It will not work.

Phantom power is delivered to microphones starting with 48V. It goes through a resistor of 6810 Ohms. The power deriverad this way will not be sufficient to power a Raspberry.

https://en.wikipedia.org/wiki/Phantom_power

/Gunnar

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 2:19 pm

W. H. Heydt wrote:
Sun Feb 04, 2018 6:01 am
If you were able to perfectly convert 48v at 0.014A to 5v, you'd get 0.134A. A Pi0 would run on that.
According to the FAQ the Pi Zero needs up to 0.2A during boot and up to 0.35A during stress testing.
https://www.raspberrypi.org/help/faqs/#power

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 6:52 pm

davidcoton wrote:
Sun Feb 04, 2018 9:20 am
Remember that no current Pi has a native audio input, so you will need a USB soundcard. For the Pi0 same applies for output.
I can't use the GPIO's? I'm not much of a computer person... are the GPIO's only digital, or can they do analog signals like the Arduino boards?

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 6:57 pm

fohman wrote:
Sun Feb 04, 2018 6:52 pm
are the GPIO's only digital, or can they do analog signals like the Arduino boards?
Digital only.

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 6:58 pm

Paul Hutch wrote:
Sun Feb 04, 2018 2:19 pm
W. H. Heydt wrote:
Sun Feb 04, 2018 6:01 am
If you were able to perfectly convert 48v at 0.014A to 5v, you'd get 0.134A. A Pi0 would run on that.
According to the FAQ the Pi Zero needs up to 0.2A during boot and up to 0.35A during stress testing.
https://www.raspberrypi.org/help/faqs/#power
I spoke with the Yamaha engineering team and they explained that there is more power available if needed... I don't fully understand, but it seems that there is a central powering circuit for all the inputs (which may or may not be requiring phantom power), and that the circuit distributes the power to the various inputs as needed, with an average of 14mA per input safely accounted for if every input was to demand phantom power at once. So theoretically, if only one input out of the 64 total needed phantom power, all of the available power from the central circuit would be directed to that input, but they've never tested it. So I'm assuming that if the Pi 0 momentarily needed more than 14mA, it would most likely be fine?

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Sun Feb 04, 2018 7:53 pm

The professional mixers use the XLR connector and balanced signal lines and a ground tor the audio connection. The balanced lines are use to remove any noise added to the signal during transmission from the microphones to the mixer. Phantom power is introduced by additional circuitry the isolates the added DC power from the mixer/amp by use of a capacitor. This is same technology for Power of Ethernet, POE, except the voltage is 48 VDC and not the 5VDC used by many computer devices. Not only will you need a device to extract the voltage from the signal but you will also need circuity to reduce the voltage to 5 VDC.
f u cn rd ths, u cn gt a gd jb n cmptr prgrmmng.

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 5:52 am

So I'll need to build a power regulator that extracts the power from the XLR cable and knocks it down to 5V to power the Pi but also allows audio signal to be sent back down the line from the Pi in a balanced format? Any ideas on how to do this? Where do I find someone who can design and build such a circuit?

Also, as far as audio in and out, can the Pi be programmed to recognize changes in voltage on the digital ins from the microphone signal, and can PWM on the outputs be filtered to sound halfway decent? (I don't care that much about the sound quality, as long as it is intelligible for vocals, this is merely for pitch practice). Or, must I use a sound card of some kind? Where do I find people whom I can pay to solve these issues for me and complete my project? I have a budget, I just don't know where and how to find people...

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 6:50 am

Where do I find people I can pay to figure these things out and then build it for me?

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 8:14 am

It seems to depend on aht phsntom poawer system you use.
Digital microphones complying with the AES 42 standard may be provided with phantom power at 10 volts, impressed on both audio leads and ground. This supply can furnish up to 250 mA to digital microphones. A keyed variation of the usual XLR connector, the XLD connector, may be used to prevent accidental interchange of analog and digital devices.[11]

https://en.wikipedia.org/wiki/Phantom_p ... ntom_power
That seems to imply 2.5W of usable power, which would be enough to run a PiZero. You will need a switch mode buck convertor to get a stable 5V for the Pi

Consider what additional power the mic and audio input require.

A system that can only supply 4mA won't do. Even at 48V that's only 192mW whihc in not enough for any Pi.

Maybe it's possible to find XLR to Cat5 adaptors and use them with a Power over Ethernet(POE) system. POE can provide more power than phantom power.

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 8:34 am

The idea behind powering this unit via Phantom Power (from the Yamaha Rio32 I/O device) is to simplify the mechanism for possible mass production. Sound engineers know how to plug in an XLR cable and turn on the Phantom Power, and if it's that simple, I've got a marketable product. Having additional cables or power supplies or dongles or whatever decreases the marketability of the final product, whatever it may end up being.

The basic function of the project is to take a microphone signal and do some basic audio processing (EQ, compression, gates, etc.) and also provide a pitch recognition system, possibly in the form of LED lights indicating how precise one's tuning is, and pump out the processed audio to the XLR cable (which also supplies the phantom power) for input to the audio mixer where it can be routed to headphones for real-time monitoring during practice for vocalists or instrumentalists.

I have a decent budget worked up to pay someone to make this work... I'm just trying to get an idea of what's possible and what isn't. So far, this discussion has been really useful and informative. Thanks to all!

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 8:38 am

Also, let's say we got this to work... what happens when one plugs in the unit? Does everything boot up ready to go? How much time would it take? In the event of a power loss or a disconnected power cable, how much time is there between reacquisition of power and system functionality?

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 8:44 am

It won't work.
You would need a device that can supply >1000 mW (200mA @5V)
The equipment you want to use only provides 672mW (14mA @ 48V)

Use equipment that supports digital phantom power designed to power electronics or forget it.

You almost certainly don't need or want Linux and you definitely need analogue inputs, low power and low latency so a DSP is probably a better option, but it won't cost $5 / £4.

fohman
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:00 am

What hardware would you suggest?

mfa298
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:08 am

fohman wrote:
Mon Feb 05, 2018 8:34 am
... I've got a marketable product. Having additional cables or power supplies or dongles or whatever decreases the marketability of the final product, whatever it may end up being.
If this is based around the pi zero then it might not really be a marketable product. You cannot get the pi zero at the $5 price point needed for a production run (they might be available in bulk at a higher price)

The other concerns about power budget and processing latency just add to making this an unlikely route to having a viable product.

I would also wonder how much demand there is for such a device, most mixers allow for adding in effects processors via an insert Jack per channel, this puts the control over that processing at the sound engineers fingertips where they can adjust it to fit in with the overall mix. With the growth of digital mixers a lot of that type of processing is also now being included on board the mixer.

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:24 am

fohman wrote:
Mon Feb 05, 2018 9:00 am
What hardware would you suggest?
I don't know. It's a big topic You haven't defined the requirements that well. Can't existing equipment do everything you want to do (EQ etc)?
If someone buys a 32 channel RIO why wouldn't they get one EQ etc in the rack?
This is not the place to discuss such open questions. It isn't a Raspberry Pi project. You could probably do the audio processing, there are projects that do that sort of thing, but with suitable power supplies, audio interfaces etc and probably more latency and boot delay that a commercial market would stand.

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:27 am

fohman,
I want to run a Raspberry Pi using the "phantom power" from an audio mixer to process a microphone signal in a certain way
I think we are missing something important here. As is often the case. What is it you actually want to do?

In what "certain way" do you want to process a microphone signal?

Why the need to use a Raspberry Pi? Why does the processing need to be at the microphone and use phantom power? That all seems incidental to what you actually want to do.
Memory in C++ is a leaky abstraction .

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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:32 am

Heater wrote:
Mon Feb 05, 2018 9:27 am
fohman,
I want to run a Raspberry Pi using the "phantom power" from an audio mixer to process a microphone signal in a certain way
I think we are missing something important here. As is often the case. What is it you actually want to do?

In what "certain way" do you want to process a microphone signal?

Why the need to use a Raspberry Pi? Why does the processing need to be at the microphone and use phantom power? That all seems incidental to what you actually want to do.

There was this outline:
fohman wrote:
Mon Feb 05, 2018 8:34 am
The basic function of the project is to take a microphone signal and do some basic audio processing (EQ, compression, gates, etc.) and also provide a pitch recognition system, possibly in the form of LED lights indicating how precise one's tuning is, and pump out the processed audio to the XLR cable (which also supplies the phantom power) for input to the audio mixer where it can be routed to headphones for real-time monitoring during practice for vocalists or instrumentalists.

fruitoftheloom
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 9:41 am

PiGraham wrote:
Mon Feb 05, 2018 9:32 am
Heater wrote:
Mon Feb 05, 2018 9:27 am
fohman,
I want to run a Raspberry Pi using the "phantom power" from an audio mixer to process a microphone signal in a certain way
I think we are missing something important here. As is often the case. What is it you actually want to do?

In what "certain way" do you want to process a microphone signal?

Why the need to use a Raspberry Pi? Why does the processing need to be at the microphone and use phantom power? That all seems incidental to what you actually want to do.

There was this outline:
fohman wrote:
Mon Feb 05, 2018 8:34 am
The basic function of the project is to take a microphone signal and do some basic audio processing (EQ, compression, gates, etc.) and also provide a pitch recognition system, possibly in the form of LED lights indicating how precise one's tuning is, and pump out the processed audio to the XLR cable (which also supplies the phantom power) for input to the audio mixer where it can be routed to headphones for real-time monitoring during practice for vocalists or instrumentalists.

The Raspberry Pi 3B $35 and a quality $20 Sound Card (as above) will be able to process audio.

Though you will need a quality PSU like the Official one to power the 3B:

https://www.raspberrypi.org/products/ra ... er-supply/


Anything else is not really Raspberry Pi related, unless I have missed something in the noise ?
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Re: Powered by XLR cable "Phantom Power" (+48V 14mA)?

Mon Feb 05, 2018 10:03 am

As others have said, phantom powering isn't going to give you the power that you need for powering a Pi, let alone with audio interfaces added.

- You mentioned adding LEDs - those will typically be ~1mA each out of your power budget (some lower powered versions are available if you search).
- Secondly you'd need to be extremely careful to avoid creating audible issues. Yes phantom is common mode (ie it affects both sides of the balanced signals equally), but any form of mismatch between the two sides (eg a slightly dodgy solder joint adding a few extra 10's of ohms) when you're varying the power drawn will result in differential and therefore audible differences. Switched mode power supplies aren't going to be great in that regard.
- Thirdly you've just nicked all the power available for powering condenser mics. You're therefore forcing the use of dynamic mics, with the loss in sensitivity that entails.

You need to be looking at a low power DSP to be able to do your processing efficiently. The ARM core on a Pi Zero is very slow and power inefficient at the sort of processing you're looking at. You could use the VPU or QPUs (part of the VideoCore GPU), but doing so is pretty advanced.
Using I2S (the main audio interface on the Pi) for your audio transport isn't going to be great on latency either - for reference your linked Yamaha Rio lists a latency of 1ms using Dante, when I suspect you'd be pushed to get below 5ms over I2S.

If you're looking at Yamaha Rio units, then you are already spending £4k on the audio interface side, so you'll have plenty of processing grunt in your desk for EQ and dynamics. Yes it could work on much cheaper desks, but you have then put the controls at the stage end rather than under the control of the sound engineer.

Adding a unit that only powered itself from phantom, listened to the audio, and gave a tuning indication would be plausible if you choose your hardware carefully (probably whilst still allowing the use of condenser mics). Intercepting the audio, processing, and forwarding on is unlikely to ever be plausible off phantom.
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