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Author Topic: Phantom Power - When Do You Turn It On?  (Read 8181 times)

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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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