Question about impedance matching mic & pre

Discussion in 'Pro Audio' started by muzician21, Dec 7, 2011.

  1. muzician21

    muzician21 Guest

    Just got an MXL R40 ribbon mic. Never had a ribbon before. I see the
    mic specs say 250 ohms. My VTB-1 pre has settings of 50 & 200.

    I'm not versed in electronics, does this mean it's not optimally
    suited for this mic? I get sound out of it, I understand that a ribbon
    sounds much different than a condenser and needs more boost, I don't
    have a basis for comparison as to whether it sounds the way it's
    supposed to.

    Thanks
     
    muzician21, Dec 7, 2011
    #1
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  2. muzician21

    PStamler Guest

    Mics aren't operated into matching impedances. They're typically
    designed to operate into an impedance about 10 times their own.

    Your preamp has impedance settings of 50 and 200 ohms? Is there a
    third setting, unlabeled or with some other kind of name on it? That's
    probably something like 1500 ohms, and that's what you want for this
    mic (and most others).

    Peace,
    Paul
     
    PStamler, Dec 7, 2011
    #2
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  3. muzician21

    Audio1 Guest

    Check your manual,
    http://www.studioprojects.com/pdf/vtb1_manual.pdf

    Page 4, 'Feature Control Description' mentions you should have the
    50/200 switch set to 50 for 'low impedance ribbon mics.'

    Myself, I'd try it in both positions and use the one *I* like the sound of.
     
    Audio1, Dec 7, 2011
    #3
  4. muzician21

    hank alrich Guest

    Maybe those numbers apply to the impedance of the source, instead of to
    the input?
     
    hank alrich, Dec 7, 2011
    #4
  5. Many ribbon and moving coil mics do need to be operated into
    approximately the correct impedance (capacitor mics, in general do not).

    In the case of moving coil mics, the terminating impdance damps down
    some unavoidable resonances in the audio spectrum. If your termination
    impedance is a long way off the manufacturer's recomendations, you could
    get a slightly wonky frequency response.

    In the case of ribbon mics, the sensitivity is low and the correct
    matching is needed to transfer as much power as possible from the ribbon
    to the input stage of the pre-amp. If the matching is badly wrong, you
    could finish up with a less-than-optimum signal-to-noise ratio.

    Having said that, the matching isn't critical and a moderate error will
    make very little audible difference. In your case, the 200-ohm
    termination is as close as you need for a nominally 250-ohm microphone -
    in all probability the normal factory variations in the construction of
    the mic and ribbon will cause at least as much error as that.

    If this is your first experience of using a ribbon microphone, read the
    AES paper "The Bidirectional Microphone: A Forgotten Patriarch" by
    Ron Streicher & Wes Dooley, it is an excellent guide to the
    understanding and the correct use of ribbons:

    http://www.wesdooley.com/pdf/Bidirectional Microphones.pdf
     
    Adrian Tuddenham, Dec 7, 2011
    #5
  6. muzician21

    Mike Rivers Guest

    They do that just to confuse you. The microphone probably
    has an output impedance of somewhere around 250 ohms. As far
    as the preamp goes, the specs read:

    Mic Input (Rear switch in 200 ohm position): 2000 ohms
    Mic Input (Rear switch in 50 ohm position): 300 ohms

    Now how is anyone supposed to make sense of that? What
    Studio Projects means is that in the 200 ohm position, it's
    suitable for connecting to a mic with an impedance of about
    200 ohms (150 is more typical, yours at 250 is close enough)
    BECAUSE it has an input impedance of 2000 ohms.

    So misleading or "seems right, but for the wrong reason" as
    it may be, the 200 ohm position is correct for your microphone.
    Oh, poobah! It's true that classic ribbon mics do have a
    somewhat lower output level for a given sound pressure level
    than classic condenser mics, but because people like you
    (nothing personal) come up with questions like this that
    most dealers aren't smart enough to answer correctly,
    they've sort of dumbed down most mics so modern condenser
    mics are a little less sensitive than they used to be, and
    with modern magnet materials, they're able to make ribbon
    mics a little more sensitive than they used to be.

    If you're recording a quiet source, you'll probably need to
    run the gain pretty close to full up, maybe add some gain
    with the Output Level control. Don't worry about it. You're
    getting sound out of it, now go record something!
     
    Mike Rivers, Dec 8, 2011
    #6
  7. muzician21

    muzician21 Guest


    You anticipated my next question on that very point.
     
    muzician21, Dec 8, 2011
    #7
  8. muzician21

    Ty Ford Guest

    They obviously need better writers,,,,,,

    Ty Ford
     
    Ty Ford, Dec 8, 2011
    #8
  9. muzician21

    Scott Dorsey Guest

    The original RCA designs basically are designed to work into an infinitely
    high impedance. You can present them with a lower than open circuit impedance
    but the lower it is, the more the top end is affected.

    Those MXLs are basically RCA copies, made by people who don't really understand
    the physics or how to make high ratio transformers, so I would expect the
    loading problems to be similar or worse. So use the highest impedance you
    can get.

    The odds are the "200 ohm" setting on the VTB-1 is an impedance of a couple
    kiloohms, designed for mikes with a 200 ohm output impedance. You really
    want more than that for a ribbon if you want good top end detail, but try
    it and listen and see for yourself.
    Same effect that you get with an SM-57.... decrease the input impedance, you
    increase the current load on the mike, so the damping on the diaphragm is
    increased. It's like reducing the tension on a string or a drum head.
    --scott
     
    Scott Dorsey, Dec 8, 2011
    #9
  10. muzician21

    Mike Rivers Guest

    They know where to find me.

    Actually, what they need is better people to design the
    labels for the controls so people won't be confused. Or just
    leave the switch off entirely and sell the preamp for $5
    less. You have more mileage than I do so you may have
    encountered more mics that really sound better with a low
    impedance load, but I've only had one here that did, a CAD
    Trion ribbon which sounded flabby (best word I could use to
    describe it) on the low end until I switched the Mackie 800R
    to 300 ohms. Then it cleaned up nicely.

    I don't know why any preamp manufacturers looking for a
    gimmick haven't included an "SM-57" switch with a 600 ohm
    load that Paul Stamler wrote about several years back. But
    they all seem to skip over that number.
     
    Mike Rivers, Dec 9, 2011
    #10
  11. muzician21

    PStamler Guest

    I think Presonus had it on the ADL-600 preamp that Anthony De Maria
    designed for them several years ago. But the input impedances were
    actually different from what the labels said.

    By the way, Mike, I had the occasion to transfer a recording you did
    from LP to the computer and thence toCD a couple of days ago -- Mike
    Seeger's "New Freedom March". Going to play it on the radio on New
    Year's Day. Nice tune, nice recording!

    Peace,
    Paul
     
    PStamler, Dec 9, 2011
    #11
  12. muzician21

    hank alrich Guest

    McQuilken built a touch of inductance into the RNP to mimic iron inputs
    just so it would work well with the 57 and the 58.

    --
    shut up and play your guitar * http://hankalrich.com/
    http://www.youtube.com/walkinaymusic
    http://www.sonicbids.com/HankandShaidri

    ..
     
    hank alrich, Dec 10, 2011
    #12
  13. muzician21

    Peter Larsen Guest

    It is an oooooooooold microphone, just like the B&O's and RCA's I've just
    been reading about how to deploy (Thanks Adrian!) and thus supposed to "go
    legacy". But perhaps asking them to label it a "legacy" switch would be
    better spin?

    Kind regards

    Peter Larsen
     
    Peter Larsen, Dec 18, 2011
    #13
  14. I've tried to remain on the sidelines for this thread. I thought it had
    run its course, but no. "Legacy"? I think Scott Dorsey made it clear that
    "oooooooooold" mics expected *NO* loading. When I climbed on the Audio
    Systems Engineering bus the rule of thumb for preamp input impedances was
    "2K ohm, or greater". As far as I can determine, that rule continues to be
    valid today. Yes, the inductive impedance of transformer-coupled inputs
    can modify performance in mysterious ways, but all-in-all, the rule stands.

    The real problem here is a matter of words. Many published input
    impedances are "Nominal". That means "Named"! *Actual* impedances tend
    to be on the order of "times ten, or greater", the standard rule for
    "bridging" A.K.A. "non-loading".

    It pushes me over the edge to see that preamp makers are offering choices.
    Oh, yeah, the SM57 sounds better with Paul Stamler's "gimmick" 600 ohm
    load. That's because it is a mediocre mic that became successful largely
    because it made an average PA system (with a 2K or greater input impedance)
    sound a little better. Decades later we have learned that it really is not
    a very good tool, and that there are many better choices in the same price
    range.

    Why, oh why, can't we overcome this "Legacy" problem and, instead, choose
    the best that modern technology has to offer? Good grief! Aren't we using
    Pro Tools because of the magic it delivers? Must we celebrate "Legacy" by
    going back to using non-magnetic surgical scissors for cutting paper-backed
    audio tape?

    I know I'll take some flack from some readers. Shrug. I made my living
    using my own brain to figure out what works best, not playing "monkey see,
    monkey do" or using inferior "Legacy" tools when there were far better
    modern choices.

    Bring it on!
     
    Roy W. Rising, Dec 19, 2011
    #14
  15. [...]
    It is clear from all the literature published at the time, that most old
    designs of microphone only gave their intended performance when they
    were correctly loaded. I am not referring to sales literature, I am
    talking about the research papers published by the designers of those
    microphones.

    Good quality mixing desks, in some cases, had tapped input transformers
    to allow a range of different microphones to be use. In other cases,
    such as the BBC, the types of microphone were standardised to one
    particular impedance value and the mixers were designed accordingly.
    When a 'foreign' mic had to be used, a separate matching transformer was
    interposed to match it to the 'standard' impedance of the studio. It
    was an expensive business, but it was necessary in order to meet
    specification.

    In those days it was far easier and cheaper to obtain really high input
    impedances, straight into the grid of the first valve, than to use a
    matching transformer. So if the no-loading approach had worked, they
    certainly wouldn't have bothered with the bulk and expense of
    transformers.



    [...]
    One of the better aspects of 'legacy' technology was the necessity to
    understand what you were doing in order to use it at all.

    Modern technology does not need a deeper understanding of what you are
    doing. It gives better average results with less effort in an average
    situation when used by an average person. (It does not help much in a
    difficult situation unless it is backed by good fundamental
    understanding of the relevant disciplines: physics, electronics etc.).
    It works even better in skilled hands and can can give phenomenally
    accurate results, far better than anything legacy equipment offered, if
    that is what you want.

    Legacy equipment used properly by a skilled operator can also give
    excellent results, but it take a bit more effort on the part of the
    user. Sometimes it's shortcomings can actually be turned to advantage
    to produce a pleasing sound quality which is lost with 'better'
    equipment.

    However, legacy equipment used as part of a cult by the unskilled or
    technologically-ignorant operator gives the worst of all worlds. I
    think it is that aspect of 'Legacy' which gives it a bad name.
     
    Adrian Tuddenham, Dec 19, 2011
    #15
  16. muzician21

    Mike Rivers Guest

    I hardly consider the SM57 a "legacy" microphone, though I
    exoect that Shure would be proud of that designation. It's
    still a very popular mic, both among old and new users. RCA
    "legacy" ribbons indeed work best into a high impedance load
    but most of today's preamps that provide a choice of input
    impedance rarely go above 3k Ohms, but often go as low as
    500 or even 300 Ohms based on the "ribbon mics are very low
    impedance" myth (which is true for the element, but not of
    the microphone).
    Yes, it does. I've run across some preamps in the past year
    that have a (fixed) input impedance around 5k Ohms. This
    tends to sound pretty good with modern transformerless
    condenser mics and puts the preamp and mic on a pretty even
    price-and-quality match. It's probably a good choice for
    someone who is going out to the music store today to equip
    his first studio. But it may not be such a great choice for
    someone who has been acquiring mics over a 25 year period
    and has the itch to buy a new preamp.
    This is indeed something that confuses newcomers. Some mic
    manufacturers swing both ways and state something like
    "Impedance 200 Ohms, suitable for preamps with an input
    impedance of 2000 ohms." Most, but not all, preamp
    manufacturers usually just specify the input impedance.
    Who's this "we?" While today I would advise someone setting
    up a home studio fir the first time to look further than an
    SM57 for his first mic, I'd suggest that a band putting
    together their first PA system get a couple of SM57s because
    they're just so useful. But, on the other hand, you don't
    usually look at the impedance of the mic inputs on a PA mixer.
    What "Legacy" problem?
    We who? What magic? I don't use Pro Tools. I don't believe
    in magic when it comes to audio or software. Sure, I
    appreciate editing on a computer rather than with a razor
    blade, and hard drives save money over magnetic tape. But
    that's not magic, it's progress.

    Pro Tools is rapidly becoming the SM57 of DAWs. Let the
    clamor begin!
     
    Mike Rivers, Dec 19, 2011
    #16
  17. muzician21

    hank alrich Guest

    Spot on, Adrian. Ignorance may be bliss, right up until it plugs an SM58
    into a Mackie 1202.
     
    hank alrich, Dec 19, 2011
    #17
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