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Author Topic: Phantom Power - When Do You Turn It On?  (Read 9439 times)

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Offline achalsey

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Phantom Power - When Do You Turn It On?
« on: July 29, 2013, 03:40:57 AM »
This came up this evening, and I know I've seen it discussed but can't remember the consensus on what was best/safest.

Is it safest to engage the phantom power after you power on your device, or can you just leave the phantom engaged all the time?

I'm know you want to hook your mics up first and foremost, with nothing on down the line, but after that, what is the proper procedure?

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Re: Phantom Power - When Do You Turn It On?
« Reply #1 on: July 29, 2013, 09:46:56 AM »
I'm with Jon; given proper professional design, hot plugging is possible but not advised. I've done it a couple of time when I needed to swap or move a pro mic right then and didn't have the time to run back to the recorder, but otherwise I kill p48 first.

I can't find the post at the moment, but I think dsatz explained part of this once.
"This is a common practice we have on the bus; debating facts that we could easily find through printed material. It's like, how far is it today? I think it's four hours, and someone else comes in at 11 hours, and well, then we'll... just... talk about it..." - Jeb Puryear

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Offline audBall

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Re: Phantom Power - When Do You Turn It On?
« Reply #2 on: July 29, 2013, 03:50:11 PM »
My Aerco is always on when using external power. The on/off switch only functions with internal batteries. Should I have my mics plugged in prior to connecting the battery or is it going to matter in this case?
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Offline dnsacks

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Re: Phantom Power - When Do You Turn It On?
« Reply #3 on: July 29, 2013, 04:21:16 PM »
yes, always (imo) preferable to plug in (and disconnect) phantom powered mics when the mic pre providing phantom power is turned off.

Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #4 on: July 29, 2013, 09:53:21 PM »
Never hot patch phantom power mic inputs if you can avoid it. This can over time damage the front end of a preamp. It can damage transistorized preamps and cause a gradual breakdown of the transistors and some types of opamp input stages especially ones using a fet topology.. I have fixed hundreds of recording and live concert consoles from hot patch issues. one of the main symptoms is a gradual increase in noise floor. So if you have a switch for phantom use it. if you don't make sure the preamp is 100% powered down and then connect mics. One last thing the transformer coupled preamps can also be damaged by hot patching. The transient spikes can't be blocked effectively by coupling caps for example because they take too long to charge therefor you can get long term damage by repeated hot patching. This is also the number one cause of input transformer failure. So ever properly designed gear can be damaged long term by hot patching....Everything from Neve consoles to Midas and even digital consoles.
« Last Edit: July 29, 2013, 10:03:03 PM by Church-Audio »
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Offline achalsey

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Re: Phantom Power - When Do You Turn It On?
« Reply #5 on: August 01, 2013, 03:35:24 PM »
Thanks for all the responses. 

I was actually really just asking about when to engage the phantom relative to powering on the preamp, not about when attaching the mics, but I think the question has been answered.

I personally avoid attaching/detaching mics when phantom is on as much as possible, but certainly have made the mistake in the past.

The question that came up on Sunday was about the proper procedure in turning on the preamp and phantom after the mics were connected.  If turning the pre on with phantom power already engaged was just as safe as turning on the pre first, waiting a second, then engaging the phantom power.

Sounds like the later is "safest" but the former should be fine as well.

Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #6 on: August 01, 2013, 04:57:39 PM »
Thanks for all the responses. 

I was actually really just asking about when to engage the phantom relative to powering on the preamp, not about when attaching the mics, but I think the question has been answered.

I personally avoid attaching/detaching mics when phantom is on as much as possible, but certainly have made the mistake in the past.

The question that came up on Sunday was about the proper procedure in turning on the preamp and phantom after the mics were connected.  If turning the pre on with uphantom power already engaged was just as safe as turning on the pre first, waiting a second, then engaging the phantom power.

Sounds like the later is "safest" but the former should be fine as well.
On a device like a console I would power the console on first then engage phantom on the channels I needed it on. The reason for this is the large inrush current on a large console particularly an analog one is very high. For a stand alone preamp or recorder we are talking miliamps of current for a large console we are talking several amps if current. It will take much longer to damage a stand alone preamp for such surges. So if you can turn phantom off and leave mics disconnected, then power up your preamp turn the gains to zero and or pad the inputs plug the mics in then turn phantom on you should never have an issue.
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Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #7 on: August 01, 2013, 09:10:38 PM »
Sounds like the later is "safest" but the former should be fine as well.

I don't think the latter is safer because neither should be dangerous.  There shouldn't be any difference really.  A power supply design has to be able to cope with inrush currents or it's not going to last, irrespective of the power-on sequence.
Its not the psu that is the issue.. Its the front end of the preamp that gets whacked ( when Phantom is already on and a mic gets plugged in. V Voltage regulators also fail due to inrush currents exceeding the capability of the regulator.  ;) But again this is mostly on large format consoles. Not on small preamps with small power supplies. So its really a non issue.. Except for when phantom is on and the mic is hot patched then you have an issue of a failing front end regardless of design.
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Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #8 on: August 01, 2013, 09:37:18 PM »
In most cases just using Zener diodes and a few resistors does little to protect an ic chip from damage. There are many papers on the subject good reading!  ;)
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Re: Phantom Power - When Do You Turn It On?
« Reply #9 on: August 02, 2013, 08:19:29 AM »
In most cases just using Zener diodes and a few resistors does little to protect an ic chip from damage. There are many papers on the subject good reading!  ;)

Funny, because that's how ICs are often internally protected, and it's also an industry standard for external ESD protection.  I direct you, for example, to an article like this:

http://www.eetimes.com/document.asp?doc_id=1275127

Or you could read this TI guide to their ESD products--note how the schematics all show a bunch of diodes:

http://www.ti.com/lit/ml/sszb130b/sszb130b.pdf

Or this guide from Linear Tech:

http://cds.linear.com/docs/en/quality/esdprotection.pdf

Where on pg. 5 they say:

Quote
Design for ESD Protection
ESD protection designs employed on LTC devices include:
1. Input clamp diodes
2. Input series resistors to limit ESD current in conjunc-
tion with clamp diodes
3. New ESD structures
4. Eliminating metallization runs over thin oxide regions
when they are tied directly to external pins

Here's some discussion from Analog:

http://www.analog.com/library/analogdialogue/archives/46-02/ovp.html

Quote
Figure 4 shows a typical bipolar input stage with ESD protection diodes and clamp diodes. In a buffer configuration, when VIN+ exceeds either rail, ESD and clamp diodes will be forward biased. With very low source impedance, these diodes will conduct as much current as the source will allow. Precision amplifiers, such as the AD8622, provide a modicum of differential protection by including 500-Ω resistors in series with the inputs to limit the input current when a differential voltage is applied, but they protect only as long as the maximum input current specification isn’t exceeded. If the maximum input current is 5 mA, then the maximum allowed differential voltage is 5 V. Note that these resistors are not in series with the ESD diodes, so they cannot limit current to the rails (for example, during an overvoltage condition).

Note that if you read an Analog datasheet, say for example OP2177:

http://www.analog.com/static/imported-files/data_sheets/OP1177_2177_4177.pdf

You'll see that the recommended solution for excess input current in a overvoltage condition is external series resistance:

Quote
INPUT OVERVOLTAGE PROTECTION
When input voltages exceed the positive or negative supply
voltage, most amplifiers require external resistors to protect
them from damage.
The OPx177 has internal protective circuitry that allows voltages as
high as 2.5 V beyond the supplies to be applied at the input of
either terminal without any harmful effects.
Use an additional resistor in series with the inputs if the voltage
exceeds the supplies by more than 2.5 V.

Of course, we probably don't want 5K ohm series resistance in an audio circuit, so we add external diodes with a current rating much higher than that IC's 5mA protection, which lets us use much smaller series resistance, thereby minimizing thermal noise in the input stage.

Let's also remember that we aren't necessarily talking about 8kV here (although that's good too), just a mere 48V, and we also by definition don't have a high frequency pulse.  So ordinary diodes (not just zeners, I never said that, although zeners can work if selected appropriately for the circuit) work fine so long as they are rated adequately for the series resistance and expected overvoltage.  This is also much easier for audio because it's not a big deal to have 100 ohm series resistance or so, while that could cause problems for an RF circuit.

If we apply 48V across 100 ohm at the IC, it's physically impossible for the surge current to exceed 0.5A (unless the resistor fails short, but usually they will fail open), and it's trivial to stick a diode with a 2A rating at the IC (and a RF filter cap too, just in case our diode is somehow not fast enough, because those are good to have anyway).

Now if we're getting more into the serious ESD or sustained high overvoltage category then we might consider sexier devices, but that's fairly uncommon in audio small-signal inputs.
My whole point here is that there are now much better ways of protecting  mic input. Don't  want to give away any trade secrets. We are in direct compitition After all ( respectfully ) :)
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Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #10 on: August 02, 2013, 11:11:00 AM »
I stand by these words with one clarification if not implemented correctly and that is what I should have said clamping circuits CAN be ineffective against surges and provide little of any opamp or transistor protection in the first section of a preamp. I do agree that they are used in some not all ic chips for basic input protection.
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Offline F.O.Bean

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Re: Phantom Power - When Do You Turn It On?
« Reply #11 on: August 09, 2013, 02:58:01 PM »
Ialways have my caps>kcy>littlebox connected. But i have ALWAYS followed the method of connecting the mics to the preamp, and THEN turn on Phantom Power. Just food for thought ;)
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Offline dogmusic

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Re: Phantom Power - When Do You Turn It On?
« Reply #12 on: August 11, 2013, 10:01:15 PM »
I have a setup where the mics are always plugged into a Sound Devices USBPre2. Because it is a real bother to turn phantom power on and off on that preamp (DIP switches), I just leave it, and power up the preamp with the mics already plugged in and phantom power already on.

Will this cause any damage over time?
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Offline dogmusic

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"That's metaphysically absurd, man! How can I know what you hear?" - Firesign Theatre

Offline tim in jersey

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Re: Phantom Power - When Do You Turn It On?
« Reply #14 on: August 11, 2013, 10:49:37 PM »
Great question.

I'm running Schoeps MKx>kc5>cmc6>SD722. When I pack my bag for a show I make sure the entire gear-chain is connected and I have the 722 configured to engage 48v upon boot. Should I disable 48V on the 722 and manually apply after the 722 boots? And then disengage 48v to the mics before powering off the 722?




 

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