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Author Topic: Phantom Power - When Do You Turn It On?  (Read 9459 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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"The ear is much more than a mere appendage on the side of the head." - Catherine Parker Anthony, Structure and Function of the Human Body (1972)

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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?




Offline achalsey

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Re: Phantom Power - When Do You Turn It On?
« Reply #15 on: August 12, 2013, 10:38:37 AM »
According to Jon you'll be fine just leaving the phantom engaged.  My original question and dogmusic's above were both the same as you're asking and both answers (from Jon at least) were the same.

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.

Chris's response was more dealing with professional boards that have large power supplies.  Apparently the safest way to protect those kind of things is to have a power on/off procedure similar to what you are asking about.  According to Jon though, our small portable machines are built to easily withstand the normal power surge of phantom being engaged as the unit is turned on.

Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #16 on: August 12, 2013, 01:03:37 PM »
Let's say you have a 100 mic input channel console (just for grins).  Maximum theoretical phantom current is 1.4A, but the inrush current to the phantom capacitors will be more than that.  If that inrush current is a problem for any devices in the power supply, then there has be be inrush current limiting.  There are a number of different ways of doing that, but let's focus on the switching sequence:

power source --> master power switch --> power rail --> phantom switch --> phantom rail

There are going to be large capacitors at the main power rail and the phantom rail, but the phantom supply is *usually* somewhat independent of the main rail (either a separate tap on the transformer or a voltage multiplier off the main rail).  So at some point there has to be inrush current to the phantom rail; it doesn't matter whether or not the phantom switch is on when the master power is turned on, there will be inrush current if not limited or managed (if it indeed needs to be).

Of course, there will always be inrush current on the main power rail, it doesn't matter whether or not the phantom switch is on.  If the phantom switch is on, then there *may* be more total inrush current.  So as a power supply designer you have to anticipate the total maximum possible inrush current.  If you build a PSU that will die at less than that maximum, then it will die sooner or later, probably sooner.  So the supposition that phantom inrush can be damaging but the master inrush is not requires a PSU designer that was smart one day and an idiot the next.  Or two different designers, one of whom is an idiot.

Consider that many large format consoles never have the phantom supply turned off and are used every day with no ill effects.  Even the lowly Mackie at my church is switched on every day (twice a day on weekends) with the phantom switch always on and six condenser mics always attached.  That's been installed for 12 years now--about 5,000 power cycles.

How stupid do you think SD would have to be to design a box that would die if the phantom wasn't switched after the unit was on, and yet they made the phantom switch a DIP?  If it was an actual problem, their devices would be so unreliable as to be worthless for field use.

Remember that a lot of large consoles are used in the field too . . . probably more than in studios.

Chris said this:

Quote
Voltage regulators also fail due to inrush currents exceeding the capability of the regulator.  ;) But again this is mostly on large format consoles.

If a vreg fails because of inrush current (note that a lot of vregs have built-in current limiting) then the designer is incompetent or the manufacturer was too cheap to install a few pennies worth of inrush current limiting.  If that's the case, then the main power rail is going to fail irrespective of when you switch on phantom.  That gear should be considered what it is, crap.

More on the topic here:

http://prorecordingworkshop.lefora.com/2011/04/03/why-cant-you-turn-lfacs-off/

http://www.searsound.com/pdf/leaveiton.pdf

I love theory, but in reality I have repaired many consoles that the front end preamps were damaged by phantom power spikes. Midas, Soundcraft, Neve. I could go on. Its not just dc voltage we are talking here Its also ac voltage. The bottom line is phantom power spike damage from hot patching does exist and over time can and does damage preamp circuits. Its been very well documented and I have personally fixed many consoles that were damaged by it.

Here is a link to some useful information.
http://www.benchmarkmedia.com/wiki/index.php/Phantom_Power_Damage
Also THAT corporation also has some interesting advice and views on the subject.

Mackie designs also mentions the problem and have come up with a solution. The gear that does not have this protection is NOT crap.
Slow degradation (and eventual failure) of mic preamps resulting from hot patching is one of the mixing console manufacturing community's dirty little secrets. If you route a phantom-powered mic through a patchbay, it's roughly equivalent to shorting out a cable every time you patch it. The mic preamp's input transistors progressively break down (called zenering), while the mixer channel gets noisy and can eventually totally croak.

Mackie engineers weren't interested in any "solution" that even slightly affected the preamp's sound. A lot of research and considerable, old-fashioned trial and error listening resulted in the addition of ultra-high-speed, large-geometry input diodes to the front end of our input circuit. It completely protects XDR mic preamplifiers from the consequences of hot-patching and direct short circuits in cables carrying phantom power. Yet it has no affect on sound quality.


« Last Edit: August 12, 2013, 01:26:38 PM by Church-Audio »
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Offline Church-Audio

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Re: Phantom Power - When Do You Turn It On?
« Reply #17 on: August 13, 2013, 11:17:39 AM »
We aren't talking about hotpatching at this point, we are talking about the power sequence where the phantom supply is turned on after the main supply.  In that case, the phantom surge current into the mic amp is limited by the phantom supply resistors to 7mA per leg.  I would suggest that any mic input that can't tolerate a 7mA surge is crap.

I have already said that the surge current from a hotpatch can be much higher than that.  Even so, a mic amp should be designed to tolerate a hotpatch because in real life they happen sometimes, not to mention ESD which can also be severe.

But the above discussion is about potential damage from mains power switching to the power supply itself.  Those inrush currents are far higher than any phantom inrush current should be, and yet even in that more extreme case you have two of the most respected techs in the industry not advocating leaving gear on.  ssltech also mentions that it's completely down to the designer to implement a proper circuit that can handle power cycling.

In respect of the Benchmark wiki, their problem in their described protection circuit is the desire for very low series resistance to preserve noise performance.  That can be an issue, but one solution is much more robust diodes.  If you use a diode that can tolerate 5A, you're probably better off than a diode that does 0.5A.  That requires a lot of space--too large for a tinybox, for example, so I use higher series resistance.

Then they have a long discussion about trade-offs of zener types, but the simple solution is not to use a zener at the input at all, just on the power rail, and use low-capacitance diodes on the inputs instead.  The surge current thus shunts though the input diodes to the zener, but the zener's capacitance is in series with the input diodes, and thus total capacitance remains very small.

But wait, why is input capacitance a bad thing?  In an RF circuit, it's a serious problem.  In an AF circuit, it's a good idea to shunt incoming RF.  So we want capacitance between the legs and also to ground (generally you want 10x common-mode capacitance vs. capacitance on each leg to ground), but of course it has to be balanced, otherwise you degrade CMRR at RF.  The problem with zeners, as Benchmark points out, is their capacitance has a wide tolerance.  Again, solution:  don't use zeners at the inputs.  Schottkys have something like 3pF, which is totally negligible in an audio circuit.  Heck, I have 220pF at my inputs.  A "high-speed" amp manufacturer might not like that, but they are being silly as the mic cable probably has 1nF or more.

But we don't really care too much about small amounts of series resistance with condenser mic amps because the noise floor of the mics isn't that low.  It's a bigger problem if you are trying to record quiet sources with dynamic mics.  My personal solution is don't do that, but even 150R should be enough to save a mic input without serious degradation of EIN.

As for larger input caps, that is a function of input impedance.  Crank up input impedance and you can shrink capacitor size.  The trade-off is DC offset, but modern inamps are really good at minimizing offset.  I can achieve 100mV offset in a tinybox with 60dB gain; that's not exceptional but given the size of the box it's good enough.  Headroom is compromised by . . . oh, 0.3dB.  In a design with more space, I could improve that significantly.  But almost nobody orders a tinybox with 60dB gain because they aren't using dynamic mics.

Benchmark also talks about limitations of zeners and amp headroom.  Again, use Schottkys instead of zeners.  Seems like that is mostly what Mackie figured out--not like it's any big secret, it's in the TI or Analog article I linked above, I forget which.

I can't see the THAT article, but the brief discusses damage of line outputs when hotpatched to mic inputs.  That's a different topic, but also an important one.  Many line outputs are *not* designed to tolerate phantom, but they could be.  Significantly, IC outputs are often not protected internally, unlike their inputs.  Again, the same circuits can be used to protect the ICs, which I use in my designs--all of my mics are opamp-output, capacitor- (a few transformer-)coupled, with series resistance, common mode capacitance, and clamping diodes.  I 100% guarantee my amps and mics for damage from hotpatching for the term of the five-year warranty.  To date, I've replaced two diodes and no opamps or inamps in something like 2,000 devices that I've built using ICs.
I think I made a mistake I thought we were still talking about Hot patch. And we have moved on to power sequence in this case I don't think it matters if the phantom power is on or off when the preamp is powered on. The power surge would be insignificant. However I still stand by my grounds on hot patch being totally bad for any preamp in the long run. Unless corrective measures are made to the design of the circuit. Most of the consoles I have encountered in the wild do not have such protection schemes. I apologize for not reading your post more clearly.

Chris
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