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Phantom Power is a mystery

Discussion in 'Pro Audio Equipment' started by bahed, Mar 5, 2007.

  1. bahed

    bahed Active Member

    I was hoping someone could explain why a large diaphragm condsenser mic needing 48v, worked just fine today when I plugged it into an UltraSound AG-50DS4 acoustic amplifier at a music store when the amp is rated as only driving 15v for the XLR input ?

    What is going on here ? Is there a problem hidden around the corner ?

    Anybody ?

    Bret
     
  2. RemyRAD

    RemyRAD Well-Known Member

    Ahhhhhhhhhhh grasshopper you have found the hidden power within the Phantom.

    What's really happening here is that the original true condenser microphone required 48 volts to polarize the diaphragm. These were the original type microphones that Neumann and other manufacturers originally made. They would not work on any less than 48 volts.

    Enter the lower-cost, permanently polarized "back electret" condenser microphones. Originally these were not of the quality of the non-polarized 48 volt requirements of the non-polarized diaphragms. Over the years, technology has improved greatly on these permanently polarized "back electret" condenser microphones that can operate on voltages as low as 1 1/2 volts or a single AA battery, that operates the little single transistor impedance converter as opposed to having to polarize the diaphragm.

    So if you had tried to plug in a higher-quality original Neumann that required 48 volts to polarize the diaphragm, it would not have worked with that 15 volts supply as most inexpensive condenser microphones can. So there is no problems around the corner if you do not purchase a microphone that requires a true 48 volt full phantom supply. So if a condenser microphone happens not to work, such as the very inexpensive Nady, Samson and others, it is because they have a non-polarized diaphragm that requires 48 volts, to polarize the diaphragm.

    So you must always pay close attention to what the power requirements are for any condenser microphone you purchase.

    So you should not necessarily be intimidated by a back electret condenser microphone since one of the finest test and calibration microphones made by (formerly) "B&K" (Danish names too difficult to pronounce for Americans Bruel & Kajier or something like that) now known as "DPA" for Danish Pro Audio are very high in cost, quality, response and specifications and are in fact, back electret condenser microphones. Same as what you get at Radio Shaft but not really! A beautiful example of cost versus quality. You can actually get exceptionally good sounding condenser microphones from Radio Shaft, since their Taiwanese supplier of back electret capsules is the same supplier that makes the capsules for the superb sounding Crown series of condenser microphones. It's a crapshoot from Radio Shaft. You might get a good sounding one? You might not. Back in the early 1990s, I purchased 10 PZM's and out of that 10, 2 of them sounded like crap. So I gave those two, to an amateur recording enthusiast friend, who loved them.

    You have to play to win.
    Ms. Remy Ann David
     
  3. ghellquist

    ghellquist Member

    Actually,
    I think that ms Remy is off on this one. Even "true" condensor mics (whatever that is, here I mean externally polarized) can work at or even below 15Volts. It all depends on the exact setup of the electronics inside the mic.

    The capsule will work fine with only 15V of external polarization. It will still create an output signal. And this signal will be amplified by the internal amplifier. Many of the chinese mics use a similar circuit where a 12V zener diode is used to create the "mid-point". This means that they will work down to about 13 or 14V somewhere before finally cutting out.

    There is a penalty to pay though, or to be exact two. Firstly the output signal will be sligthly lowered due to the the lower polarizing voltage. Secondly maximum SPL before clipping will be lower, mostly due to the electronics.

    There are top quality condensor mics specially made to the rather unusual 12V phantom standard, Schoeps CMC6 amplifier comes to my mind as a splendid example. Avoid feeding these anything not following either the 12V or the 48V phantom standard (they autoswitch, but do not like things in between).

    Small diameter Neumanns (say KM184) are not happy and simply stops working (they are not harmed in my experience) below about 40V. The KM184 actually has a voltage converter creating about 60V as polarizing voltage for the capsule and I think it stops running.

    Other mics may be damaged by voltages not following specs, you have to check your mic manufacturer. All mics I know of dislike too high phantom voltage (max is 52V without mic connected) so donĀ“t go that road.

    Gunnar
     
  4. ghellquist

    ghellquist Member

    B&K still exists. They spun off their professional (as in studio use) mic division into a separate company DPA a number of years ago. I think that the markets where too different to work on. DPA mics are very good, I love my pair. But if you think they are expensive, look at what some certified measurement mics costs!

    Gunnar
     
  5. dpd

    dpd Active Member

    I'd add to that the linearity of the preamp will likely be different - especially if it uses a FET, since the biasing will change the transfer characteristics of the device. If the biasing is significantly different, the even harmonic distortion content may increase dramatically - even at moderate SPL. You may or may not like the sonic difference.
     
  6. sheet

    sheet Well-Known Member

    Actually, the oldest Neumann mics came with power supplies that did more than phantom, and none of them had standard three pins that we have now. They had internal amplifiers that had line level outputs. For a long time in broadcast consoles, there was no in console mic preamplifier.

    You don't really need to be careful with anything. Except to make sure that if you do have a mic that needs a full 48V (and I doubt you do), that your power supply is stable.

    The standards for phantom power are this: 12V, 24V, 48V. The above mentioned DPAs do not use High Voltage supplies to power the diaphragm. The high voltage mics will not work with just any preamp either. DPA used those high voltages to power an amplifier circuit alone inside the mic, yielding better performance.

    Unless your mixer says "48V", then it is not putting out 48V. Not all manufacturers abide by the standards mentioned above either. Many MI mixers have 15V supplies. Generally speaking, the cheaper the mixer, the more unstable the phantom power is. If you have unstable phantom, them you will have a decrease in mic sensitivity, output and hence more noise. If you use any of those funky mic cables that light up, or those inline noise gates, you will need to deduct their consumption from the available voltage, to see if your mic will still work.

    98% of the condsensers made do not require 48Vs. Many will get by on 12V. All non DPA HV mics will tolerate a maximum of 48V. So you will not harm a mic that only needs 12V by giving it 48V.
     
  7. Boswell

    Boswell Moderator Distinguished Member

    This is dangerous advice.

    Microphone designers work to a specification. If in order to meet the specified performance they need 48V +/-4V phantom power, then trying to run the microphone off 12V will mean at the very least it will not meet the specification. It may appear to work, but be compromised at higher sound levels, or may not be capable of driving 100ft of cable, or whatever.

    More serious is applying a higher power supply voltage than the maximum specified. You should never do this to a microphone, pre-amp, channel strip or anything electronic. The designer has produced the design to work within the specified voltage tolerances, used components to be compatible with that voltage and made arrangements to deal with the resulting heat dissipation. Applying higher voltages may cause component breakdown and localised overheating, which, if not immediately apparent, is likely to curtail the active life of the unit. Heat dissipation rises as the square of the voltage in a resistive load.

    Sorry about the rant, but speaking as a professional electronic designer, I would be sad to have to put overvoltage crowbars in all my product designs so that the fuse in the power supply blows whenever some ill-advised user puts 4 times the rated voltage on the power rails.
     
  8. MrEase

    MrEase Active Member

    All condenser Mic's (whether electret or non polarised) work on the same very basic principle.

    Q (Charge) = C (capacitance) * V (Voltage).

    With non polarised types, the phantom power is responsible for establishing the Charge and the sound wave modulates the Capacitance which hence modulates the Voltage and gives us the Mic output. If we rearrange the formula we get

    V = Q/C.

    If we use 48 V this will establish a certain charge on the Mic. If we use 12 V then there will be 1/4 of the charge on the Mic and therefore 1/4 of the output. This does not take into account the amplifier, which will normally be worse performing at lower voltages. The net result is that at 12V the output will be 6 dB down AND the noise floor may well be worsened as too will be the dynamic range of any internal preamp.

    It is altogether NOT a good idea to run condenser Mic's at lower than their intended phantom voltage. In most cases though they will still work to an extent but certainly not to the published Spec.
     
  9. JoeH

    JoeH Well-Known Member

    well said; When it comes to phantom power & voltages, this is another case of "Just because you CAN doesn't mean you should."

    Good to remember that in these times of affordable stuff that cuts corners here and there, even in times of necessity (like portable/field use gear) it's always best to start out with the real deal.

    :cool:

    220, 221, whatever it takes.
     
  10. sheet

    sheet Well-Known Member

    First of all, you have no idea what you are talking about, Second, I would appreciate it if you would edit your message, removing the "dangerous" advice comment.

    So, you are saying that you should never use a 12V or 15V mic on a 48V source? Show me ONE mic manufacturer that specifies a cap voltage for their condensers mics. There aren't any. Show me ONE console or preamp manufacturer that offers variable phantom power ouputs. There aren't any.

    Few if any mics run completely from raw phantom. Mics use 5V or more to drive a low V zener that determines the polarization voltage and powers the electronics. They are powered from a fixed and regulated low voltage source inside the mic. Increasing the phantom power voltage is never seen by the microphone element or electronics, it only increases the voltage across the current source.
     
  11. Boswell

    Boswell Moderator Distinguished Member

    Exactly. Those microphones will say in their specification something like "phantom power 12V - 52V", because they have been designed to work over that voltage range. Some may also say that the max SPL is lower at 12V than at 48V.

    If a piece of equipment, for example a T-power microphone, says "power source 12V" on it, you don't connect it to 48V. Your post said "you will not harm a mic that only needs 12V by giving it 48V". My comment stands.
     
  12. sheet

    sheet Well-Known Member

    Dude, you are not comparing apples to apples. The only way that a T-Power mic works is if it is connected to something that accommodates it in the field recording industry. Even a 58 would be destroyed probably being hooked up to a T-Power source.

    My comment still stands, and is supported throughout the industry. You are referencing another niche powering option for something that is beyond the scope of the OP.
     
  13. dpd

    dpd Active Member

    Interesting. A Zener isn't the greatest regulator in the world - not very low impedance. Is it standard practice to use the same voltage source point for both preamp power and polarization? I would think that could cause some nasty non-linearities if that is the case. Or, are some using a zener and others a true, low-noise, low-z regulator?
     
  14. sheet

    sheet Well-Known Member

    The zener diode is the most common. Zener diodes in reverse bias mode are used in the phantom power supply as well.
     
  15. MrEase

    MrEase Active Member

    Now I don't really want to get involved in the dispute between Boswell and Sheet but......

    OK then. I went to the first site in my favourites which just happened to be Rode and the first Mic I looked up the spec on (Rode NT2000) said Phantom Power "P48" which means 48V nominal. The second (NT1000) said P48 (35V- 53V) so it even specified the limits. I don't see much point in going further as by pure chance I have a 100% hit rate as a rebuttal of your statement.

    I have no idea where you got this statistic. Please provide a link otherwise I will have to assume it is your own assessment. The previous argument I made is already indicating that the "statistic" may well be untrue.

    The bottom line is that the physics is very much as I stated in my previous post. I see little point in going into greater detail than the earlier post. No Mic manufacturer has yet found a way round the fundamental physics. So while, for example, a Rode NT1000 may well "get by" at 12V it is being undoubtably used outside the manufacturers spec. Do not expect the sensitivity, noise figure or overload margin quoted for the mic!

    Funny, I thought I made the point earlier! :wink:

    EDIT: Add all of the AKG range to those that specify the phantom power even though most are electret types that will work from 9V - 52V, the C414 is NOT!

    EDIT2: My goodness, so do the SE electronics range (48V +/- 4V). Where DID you get your information sheet?
     
  16. sheet

    sheet Well-Known Member

    My point was fellas, that there are no manufacturers saying "don't use a 48V phantom power supply, because this mic only needs 15V and you will blow it up", nor are there danger stickers on mics, or legal disclaimers, or reports of smoldering lips or missing facial hair in the news. The circuit in the mic sees the various voltage and applies a constant charge, no matter what that incoming voltage is. That is the point.

    Yes, there are mics that will take 52V now. On average, there are fewer mics requiring a full 48V to operate. Go to Shure, AT and other websites and check it out for yourself. As stated, some will get by the 9V. This range of acceptable voltages proves my point again.
     
  17. MrEase

    MrEase Active Member

    I actually responded to (quoted) points you made and disproved them. If your argument is changing do let us know as I was responding to your argument that using 12V phantom power with virtually any Mic is OK.

    As I said in my last post it really depends if you just want to get by, or work to full spec. I also noted in my last post that many of the AKG mic's will work happily down to 9V at full spec. That is simply because they have electret capsules and NOT traditional condenser mic capsules. With the traditional condenser mic's the charge is established by the phantom power. As Q = C * V, as I pointed out before, it essential that the charge is kept constant. This is achieved by feeding the phantom power through a very high impedance (several MegOhms) which effectively creates a low pass filter cutting off BELOW the audio spectrum. As the capacitance of the capsule is determined by the physical size, plate spacing and dielectric constant then the capacitance is in the order of 100pF or so give or take. To achieve sufficient charge to give good noise and overload characteristics then a reasonably high voltage is required - hence the original established 48 V "standard".

    So how do the manufacturers who quote MINIMUMS of 35V or 44V prove your point? It does no good, as you have already pointed out, to compare apples to oranges. In this case capsules that REQUIRE a polarising voltage and those that don't (Electrets).

    For a true condenser mic (NOT electret capsule) then reduced voltage WILL compromise performance. I guess that is why, of the manufacturers I could be bothered to check in my last post, ALL (as opposed to your assertion of NONE) quote a MINIMUM phantom voltage WAY above the 12V that you claim is OK. Of course electret capsules are a completely different kettle of fish as the phantom power is ONLY required to power the buffer amp. If you disagree with this perhaps you should contact the manufacturers to tell them that they are wrong about their own products....

    EDIT: You asked us to check out Shure as an example - KSM9 spec is 48V +/- 4V. Tell me again how this proves your point?

    EDIT2: What the hell, I've also checked the AT site -AT2020, AT3035 etc are electrets. AT 4047, AT4050 are conventional but unsurprisingly require 48V (no tolerance given unlike the 3035 at 11V - 52V.)

    I make that 5 different manufacturers including two of your own suggestion - have I proven the point yet?
     
  18. RemyRAD

    RemyRAD Well-Known Member

    This is completely awesome! You know everybody, this has been one of the best discussions I've had the pleasure of reading in the over 1 1/2 years I've been on the forum!

    Those of us with a few years under our belts/skirts, all have had interesting experiences regarding the powering of condenser microphones. KIDS, DON'T TRY THIS AT HOME. I think both right and wrong, there are many valid points here. Many people have mentioned things that I had completely forgotten about! And it brought to mind the time, I think back in 1978, I was using a rented Sennheiser MKH815 (?) the long shotgun microphone and a Nagra IV on a industrial film shoot, where Sennheiser was utilizing AB powering for that microphone, which wasn't PHANTOM. It had a external portable battery pack and I don't remember the voltage? But it was not phantom. So, does anybody remember AB?? I don't remember T powered microphones? It must be my missing brain cells?

    And along with my missing brain cells, I think that few people also know (except for a very few) that many of the Neumann microphones actually have output pads in their output circuits? Really. Check their schematics. They have, throughout time, been able to offer up more output than most preamps can deal with. I know that my old KM56, U67's, U87's are that way. I've never bothered to take the output pads, out. I even believe it's mentioned in their original supplied data sheets? I'm too lazy to go searching for mine right now. Not sure why I never bothered to try? Probably because they were/are Neumann's? You don't fix it if it ain't broke.

    I actually had/have a different kind of problem when trying to phantom power microphones. When you are doing remotes, from a truck, from 250 to 500 or more feet away, that 48 volts, even coming from the Neve gets rather anemic at that distance. In those situations, I always had planned to build a power supply to be placed down at the stage, in the transformer split box, so the power supply was closer to the microphones. DC just doesn't travel as robustly as AC. Unfortunately, the road to hell is paved with good intentions. I never got to building the power supply in the split box. Go figure?

    Consistently inconsistent
    Ms. Remy Ann David
     
  19. MrEase

    MrEase Active Member

    Hi Remy,

    I'm not at all sure this problem is due to losing voltage on your phantom power as you seem to think. As phantom power is fed on the balanced lines, it is necessary for the phantom voltage to be fed through resistors so that the input impedance of the amplifier is not seriously compromised. These resistors are usually several kOhms as they MUST be larger than the input impedance of the pre-amp. If no buffer is present in the Mic then, once the capsule is polarised, there will be virtually no current drawn and the capsule will get the full source phantom voltage. Mics that power a buffer amp from the phantom voltage have an increased current consumption and there will be a consequent drop in the output voltage from the console.

    In either case, even with a very long cable run, you should not have more than a few Ohms resistance in the cable which is effectively in series with the several kOhms of phantom source impedance. The result is the phantom voltage should not be seriously compromised by long cable runs. My guess is that if performance is compromised by a long cable run it is an effect of the cable capacitance loading the buffer and not "anaemic" phantom voltage.

    Pre-amp and console manufacturers rarely seem to specify the series resistance of their phantom power but remember it MUST be more than the quoted input impedance of the pre-amp. The only saving grace is that both lines carry the phantom power so the series resistors end up effectively in parallel.

    EDIT: I just did a quick test on my ageing Yamaha 01V. Phantom was 48.4V with no load and 33V with a 10k load (on hot input). This gives the series resistance (per side) of 4.7 kOhm, which will mean a source impedance of 2.35k when paralleled with the cold input. This is fairly typical as the specified input impedance for the board is 3kOhm. Unless there is something seriously wrong with your cable then the cable impedance, even on a long run, will have little relevance to the actual phantom power arriving at the Mic. Also note that many electret Mic's will draw around 3 mA (for the buffer amp). This is close to the 3.3mA load my test gave, so with such a Mic will only give phantom power of around 41V even with a very short lead.

    EDIT 2:
    I would have said the opposite was true. They both have to contend with the cable resistance but true DC does'nt care about capacitance and inductance like AC does! :<)
     
  20. fabian-lou

    fabian-lou Guest

    hi there folks, i'm new here. My name is Louise and i'm from Scotland.

    I recently purchased a condenser mic and found i had this phantom power problem too. I've been offered a Behringer tube ultragain t1953 mic preamp at a very reasonable price. Would that do the business?
     

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