### Author Topic: Blocking Phantom Power 48V from Audio output  (Read 512 times)

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#### Saimoun

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##### Blocking Phantom Power 48V from Audio output
« on: April 27, 2024, 11:20:08 am »
Hi

I have a simple audio output circuit, the audio comes from an opamp with no negative rail, so the signal is from 0 to 3.3V (centered around 1.65V), so I have a coupling cap to bring it to zero volt.
How could I make the output blocking +48V phantom power? Would it be enough to add a 63V cap (in the opposite polarity as the current caps) just before the output?

What about if the plug is a TRS instead of XLR, thinking that pluging the cable in and out will momentarily create some shorts?

Thank you!

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #1 on: April 27, 2024, 11:40:55 am »
How could I make the output blocking +48V phantom power?

What do you mean with "phantom power"?

#### BennoG

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #2 on: April 27, 2024, 12:15:39 pm »
yes you can, I would also add a extra resistor of 100k or 220k to the ground after that to discharge the capacitor.

However you need to double the capacity of the 22uF to 47uF or something like that, because capacitors in series act the same as resistors in parallel.

Benno

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#### Saimoun

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #3 on: April 27, 2024, 01:33:52 pm »
What do you mean with "phantom power"?
Like the 48V that an audio mixer can send over an XLR cable for example. As I understood it is typically raising the DC voltage from 0 to +48V on both pins 2 and 3 through a 6.8k resistor.

However you need to double the capacity of the 22uF to 47uF or something like that, because capacitors in series act the same as resistors in parallel.
Very good point I did not think about that! Actually what about the discharge - would that not damage the first cap and/or the opamp behind it?

#### donlisms

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #4 on: April 27, 2024, 07:47:52 pm »
A very thorough analysis is here, in AES Convention Paper 5335:

https://www.thatcorp.com/datashts/AES5335_48V_Phantom_Menace.pdf

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#### Terry Bites

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #5 on: April 28, 2024, 12:22:26 pm »
See page 11.

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#### Saimoun

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##### Re: Blocking Phantom Power 48V from Audio output
« Reply #6 on: April 29, 2024, 08:14:12 am »
A very thorough analysis is here, in AES Convention Paper 5335:

https://www.thatcorp.com/datashts/AES5335_48V_Phantom_Menace.pdf
See page 11.

Both very good read thank you! Though they only seem to cover the Phantom Power source protection, i.e. the mixer/audio input side.

I'm on the other side, I have an audio output which may or may not receive phantom power. Are you saying I should simply implement their proposed protection on my opamp's outputs?

I understand the 1N4004 diodes to the rails, though I am not sure I get what R4, R5, and RG are (I believe they are just part of the audio input - my design is an opamp output). And what about the capacitor, would my little 6.3V cap not be in risk of damage while the plug is connected/disconnected?

Thank you!

Smf