100ft run or more, 48 v power supply location

Discussion in 'Location Recording' started by audiokid, Nov 9, 2011.

  1. audiokid

    audiokid Staff

    Another first for me here.

    I have 100 ft run to the piano. Using Mojave MA100's SP for the first time. They come with its own PS. The micpre's have 48v too. Do I use one or the other or both in a long run or longer runs? Do you ever need to use both to ramp anything up lada lada lada?
     
  2. RemyRAD

    RemyRAD Well-Known Member

    Most 48 V phantom should be able to travel up to 500 feet through most microphones snakes. At least I haven't had much trouble at that extreme distance. But it can become a problem since DC loses a lot more energy over greater cable distances than AC. Some phantom power at 48 V on some consoles don't have enough current to drive a Neumann U 87, which requires greater current than most other microphones even if the console is supplying 48 V. In the past I had considered building my stage box input to allow for an external 48 V supply into the stage box. But even at 500 feet with multiple 87's and others, it wasn't necessary with my AudiTronics 501's, Sphere Eclipse C or, the Neve, since they all had beefy phantom supplies. So I never bothered adding remote phantom to the stage box. I would say at more than 500 feet however, it may become rather iffy? So for over 20 years this is not been an issue for me.

    Some engineers like the Long and Thin
    Mx. Remy Ann David
     
  3. Boswell

    Boswell Moderator Distinguished Member

    You have to use the MA100's power supply. These are valve (tube) mics and do not need 48V phantom power.

    You will be fine on a 100 ft cable run from the power supply audio output back to the pre-amps. For minimum cable noise, turn off the PP output on the pre-amps.
     
  4. audiokid

    audiokid Staff

    Bos, Value Tube, what does this mean exactly?
     
  5. BobRogers

    BobRogers Well-Known Member

    "Valve", not "value." I would think that Canadians would speak British better than Yanks. Must be all those Agatha Christie novels I read as a kid.

    Added: and Tom Sharpe, Terry Pratchett, Graham Greene, Evelyn Waugh, J.R.R. Tolkein, (does Conrad count?),...
     
  6. BobRogers

    BobRogers Well-Known Member

    And what Bos said about the power supply is right. No phantom power. Let the power supply in the room with the mics do the work. Should have a nice strong signal for a 100 ft. run.
     
  7. audiokid

    audiokid Staff

    lol, reading without glasses. erk. such a pain. Valve, Got it.
     
  8. BobRogers

    BobRogers Well-Known Member

    But without the glasses Remy thought you were too young to have a daughter who could play like that...
     
  9. audiokid

    audiokid Staff

    Too funny. Remy should see me when I need a hair cut lol.

    What amount of power goes thru it? 110v? For variable pattern mics, I imagine its the same PS, yes, but instead the pot adjusts a phase which changes the pattern, yes? Are those the same, you shouldn't use Phantom with them? I'm sure I have done this though, but you say it will produce noise in the line? There is no manual with this to read on it more. Glad I asked.
     
  10. audiokid

    audiokid Staff

    before you answer, the PS must do a bit more than this. For you in 220v , what goes on there?
     
  11. Boswell

    Boswell Moderator Distinguished Member

    Sorry I was a bit terse in my previous post. The MA100SPs can only be powered from their custom power supply using the 7-pin XLR cable. The 3-pin XLR outputs are the audio to run off to your pre-amps. I have never used these mics, and the data sheet is no help on this point, but on other valve/tube mics I have used, there have been two different cable types. The first has a breakout in the cable itself, where one tail goes to the power supply and the other has the 3-pin XLR cable-mounted plug for taking to the pre-amps. The other type has the 7-pin lead all the way back to the chassis of the power supply unit and the 3-pin XLR is mounted there.

    The point about turning off phantom power if not needed is that cables form low-grade capacitor (condenser) mics by themselves. Movement of a cable by pulling, rolling or kicking can create surprisingly large low-frequency signals in the audio circuits if 48V is present, and these can upset mic pre-amps in subtle and occasionally not-so-subtle ways. This is not a heavily advertised effect, and I found it out by trying to track down some what looked like seismic signals in a recording many years back. I can imagine that the magnitude of an effect like this depends on the quality of the mic cables you use, but at the time I had only the one multi-channel snake that on this occasion, unknown to me, was being moved a small amount every time a stage door opened and shut.
     
  12. audiokid

    audiokid Staff

    Thank you Bos, that was very well received!
     
  13. RemyRAD

    RemyRAD Well-Known Member

    Yeah, I've experienced that as well Boswell but never connected phantom power causing that subsonic lump to my thinking. That was great thinking. And as we all know my thinking is largely damaged. But I did get a kick out of you saying "Sorry I was a bit terse in my previous post." I had worded that identically to you to someone else in another one of my posts, before I just read that here. I know great minds think alike even those of us that have damaged ones. So cool. Perhaps you read mine? It's a nice line isn't it? Especially when we all get so passionate about what we do and what we actually know to be right. None of us like stupid. I've enjoyed your posts so much over the years, I'm really almost embarrassed when I know you've read mine. Still, I know I am with some fabulous professionals like myself here to help those that want to make a better recording, make it happen for them. And the more different ways it's explained the better the understanding and the better the art of recording shall be.

    I'm artsy fartsy. I think it was the pizza last night?
    Mx. Remy Ann David
     
  14. Boswell

    Boswell Moderator Distinguished Member

    Yeah, funny coincidence. I must have read your post in the other thread and had not realised that I had re-amped a phrase from it in this one.
     
  15. HalS

    HalS Guest

    Hey Guys,

    A number of years ago, I was faced with a recording delimma. I was supposed to be at a rehearsal on a Sunday afternoon with a performance of Handel's Messiah in another building across the street. I decided to string a couple of microphone cables across the street between the buildings. As it turned out, I used two 1000ft reels of West Penn 452 cable and a stress wire for swinging in the air between the buildings. I soldered connectors on the ends and used two Neumann KM48i's in the auditorium end out in front of the orchestra about 10ft in the air. The other end was connected to a Tascam M15 console and recorded there on a Teac reel machine.

    The sound was great overall except on some soft vocal parts which were a little weak on the recording. In looking back, it was an experiment with both recorded sound that far away and that concert piece. I was pretty pleased with the recording and the performers were glad it was recorded.

    Hal Swinhart
    Oklahoma City, OK
    hswin@att.net
     
  16. MrEase

    MrEase Active Member

    That's not correct and somewhat misleading I'm afraid. Given similar conditions (i.e. source and load impedances) D.C. is only affected by the resistance of the cable and the inductance and capacitance is irrelevant. Given that the resistance of a few hundred feet of cable is only going to be a few tens of ohms worst case and phantom power is usually sourced through 6k8 then the phantom power will be little affected. If a particular setup does cause a problem then the phantom power will be marginal even with a short cable.

    On the other hand A.C. signals are affected by not only the resistance but also the inductance and capacitance. At the frequencies and lengths we are concerned with here we do not need to worry about transmission line impedance but the capacitance alone will increase the attenuation compared to the D.C. case due to finite source impedances. IMHO longer lines need to be used with low output impedance sources as a high output impedance could easily create a low pass filter cutting off in the higher audio band. Also it is always better to use low capacitance cable for long runs as higher capacitance always risks instability in an active source. This applies whether phantom power is used or not.
     
  17. RemyRAD

    RemyRAD Well-Known Member

    I don't know where you get your brilliant experience from? You might be correct as far as a book is concerned? But when dealing with real-world scenarios such as 24gauge balanced microphone snakes on a 500 foot long run, a crap load of incidences come into play. Yes, loss of high-frequency response due to the capacitance and inductance effect caused by the cabling. There is also a voltage drop of the +48 V of phantom, in the real world. Maybe not in your book world? So Mr. Knowledge I am Ms. Practical. You're lucky if +48 V of phantom can survive at 1000 feet. And while it does, it's almost like a guy without "V i a g r a" that needs to see Alice. And that ain't very fun nor very efficient. But surely you must know that since you have already run microphone lines 1000 feet? And probably why Edison lost his push for DC distribution of our power grid to Tesla/Westinghouse. You might be correct from a theoretical standpoint but we realized what worked and what didn't work, over 100 years ago. And we're not feeding DC to peoples homes. Now this might change if we all get hydrogen fuel cell technology to power our homes? But not yet. Then again, most people don't use microphone cables much longer than 15-50 feet. So no problem with that. So both energy and force up long lines are reduced unless you are using welders cables for your microphone runs? Money might get you an education but money cannot buy you experience. Theoretically we might be able to make the trip to Mars and back? Would ya like to be the first to try? Where we might find that experience might teach us something else?

    With crappy audio, nobody gets killed.
    Mx. Remy Ann David
     
  18. audiokid

    audiokid Staff

  19. MrEase

    MrEase Active Member

    Indeed you don't, for, unlike some people we know, I do not constantly bash on about it! Suffice to say I have been around at least as long as you and been similarly independent as an electronic design consultant. Even though my profession naturally required that I have an appropriate university education, I too have to live in the real world where everyone gains real world experience and education is only helpful in analysing the unexpected. Of course, neither education nor experience counts for anything if we do not learn from it.

    Your reference to DC and AC mains is just hot air trying to back your assertions. The reasons behind the ultimate choice of AC mains had nothing to do with losses but rather with transformers. They don't work at DC! The convenience of distribution where being able to step up and down between 66/132kV grid (UK) and domestic levels demanded AC (being able to distribute at tens and hundreds of kilovolts cuts resistive losses enormously). This far outweighs the drawbacks of a grid that must, despite the low frequencies, be regarded as a true transmission line system due to the vast distances involved. There is also a very basic safety issue but I'll let you guess on that one if you haven't already experienced it!

    Despite your post, which actually contains no factual information, just pure conjecture, none of us, not even Ms. Practical, can change the fundamental facts and laws of Physics.

    Phantom power is fed, by requirement, through a resistance of several kilohms (usually 6k8). Fact!

    Two of these are in series with each of the balanced lines. If each of the balanced lines has a resistance as high as 68 ohms/1000 ft (unlikely - the 24 gauge cited by Remy has 25.6 Ohms/1000ft) then the phantom power arrives at the remote end with a highest possible source resistance of 6868 ohms. Fact!

    The total feed impedance has risen by just 1% Fact!

    The tolerance of the 6k8 resistors used in the phantom power source is highly unlikely to be less than 1% and will normally be 2 or 5%. Fact!

    The phantom power voltage is highly unlikely to be regulated to better than 1%. Fact!

    Conclusion 1. The effect of 1000ft of cable has the same or normally less effect than the tolerance of the resistors feeding the phantom power. Fact!

    Conclusion 2. If you do experience problems with long cables (not unknown) then a simple resistance measurement will eliminate the phantom power as the source of the problem. Fact!

    Conclusion 3. If the resistance is not ridiculously high you should look for problems elsewhere. E.g. Driver oscillation and other ac problems. Fact!

    Rhetoric does not solve problems. Rational thinking has a chance. These words do not come from books as you assert but from me, with just as much (if not more) experience of "real" engineering as you.

    Less of the personal hyperbole and conjecture in future please, let's be professional.
     
  20. MrEase

    MrEase Active Member

    BTW, Remy you're absolutely right I've never had to run 1000 ft snakes for audio. I have, however, had to design systems to run power and audio frequency signals (but not audio) over cables measured in miles, not feet! They worked....
     

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