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understanding headroom -- dBv, dBu, VU primer ??

Discussion in 'Preamps / Channel Strips' started by expatCanuck, Jan 7, 2010.

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  1. expatCanuck

    expatCanuck Guest

    Reasonably new to this. This question arises from my search for a new/new-to-me preamp to replace my mobilePre
    (which, not surprisingly, doesn't provide tracks which stack particularly quietly).

    I believe that I understand the concept of headroom (i.e. - how hot a (mic) signal can the preamp handle?),
    but not the measurement units.

    Units that I've seen include dBv, dBu and VU.
    (Were I to guess, I'd assume that dB is decibel, v is volt and u unit.
    But that still doesn't get me grokking statements such as:
    "at unity gain, the RNP 8380 has a clip point of +27.5dBu (almost 25Volts peak-to-peak)"

    Are dBv and dBu synonymous?
    How (if at all) do dBv, dBu and VU relate?

    References to any good online primer(s) welcome.

    Thanks kindly.

    - Richard
    http://oldWithoutMoney.com

    P.S. - Like the new forum layout.
    P.P.S. - And no, I'm not choosing a preamp solely (or even mostly) by comparing specs. But it is a part of the process.
     
  2. Link555

    Link555 Well-Known Member

    Are dBv and dBu synonymous?
    How (if at all) do dBv, dBu and VU relate?

    No- Do a quick serch for more....

    http://www.sengpielaudio.com/calculator-db-volt.htm
     
  3. Boswell

    Boswell Moderator Distinguished Member

    I hope the search engine on the new site will make a better job of finding the many threads on this topic than the old one did.

    In brief:

    DeciBels (dB) are RATIOS and not absolute values. The ratio of the output to input amplitudes for a x1000 gain amplifier is 60dB (20 * log (1000)).

    dBu, dBm, dBV etc are ABSOLUTE levels measured as the ratio of a signal to a specified standard level. For V the standard level is 1volt, for u (= unterminated) and m it is 0.775 volts (the voltage that would dissipate 1 milliwatt in 600 Ohms)

    Headroom is ALWAYS measured in plain dB. It's the number of dB between the nominal level (e.g.+4dBu at the output) and the clipping level at the same point for the particular piece of gear. The clip level can also be specified as full scale or 0dBFS. As an example, the Yamaha 01V96 mixer with a 0dBFS clip level at unity gain of +24dBu has a headroom of 20dB for its balanced inputs and outputs. Its unbalanced inserts have a nominal level of -2dBu and a full scale of +18dBu, so still have 20dB headroom.

    "at unity gain, the RNP 8380 has a clip point of +27.5dBu (almost 25Volts peak-to-peak)"
    27.5dB = a factor of x23.71. Ratio this to 0.775V and you get 18.378 Vrms. This has a peak value (root(2) x rms) of 25.98V and a peak-peak of 51.97V. So your quote was slightly wrong in the "almost 25Volts" but very wrong in saying this was the peak-peak value and not the peak.
     
  4. dvdhawk

    dvdhawk Well-Known Member

    No, dBv and dBu are not one in the same.

    A quote from this technical conversion website
    , might help.

    "The origin of the index of dBu comes from "u = unloaded" and of dBV is from "V = 1 volt". Some say:
    The "u" in dBu implies that the load impedance is unspecified, unterminated, and is likely to be high.

    What is dBu? A logarithmic voltage ratio with a reference voltage of V0 = 0.7746 volt ≡ 0 dBu
    What is dBV? A logarithmic voltage ratio with a reference voltage of V0 = 1.0000 volt ≡ 0 dBV
    The home recording level (consumer audio) of −10 dBV means 0.3162 volts, that is −7.78 dBu.
    The studio recording level (pro audio) of +4 dBu means 1.228 volts."

    The site gets technical quick and the relationships are based on logarithms, so unless you're looking for the highly technical definition - just know they're apples and oranges. Maybe wiki has a more user-friendly explanation.

    ( I see Link555 beat me to it )

    and Boswell: dBv and dBu were too vague to generate search results.

    and a moving target too, I see the OP has added some info. - anyway best of luck.
     
  5. expatCanuck

    expatCanuck Guest

    search engine

    Thanks for the feedback thus far.

    Regarding the search engine -- I don't know if it's better, but (like many/most forum search engines) it just won't search for terms less than 4 characters (e.g. - dbu, dbv). And I'm not certain that I can require that the search results include *all of* (as opposed to *any of*) multiple search terms.

    A search for 'headroom' limited to the thread title didn't yield terrific results.
    And a search for headroom not so limited yielded a rather low 'signal to noise ratio'.

    Other keyword suggestions welcome.

    - Richard
     
  6. Kev

    Kev Well-Known Member

    this can be helpful

    http://designtools.analog.com/dt/dbconvert/dbconvert.html

    there is a lot going on here
    it can be a simple concept
    but one question at a time and be specific

    we can then take that one situation and discuss the concepts and issues

    " But that still doesn't get me grokking statements such as:
    "at unity gain, the RNP 8380 has a clip point of +27.5dBu (almost 25Volts peak-to-peak)"
    "

    yep

    unity gain
    is when, what goes in comes out at the same level
    1 in 1 out
    so for 25pp in ... 25pp out
    and at this level the stages of the unit are not clipping
    one or all stages may be just below clipping
    most probably the maximum level the unit can handle

    ... typical for items that have higher voltage rails
    and explains why items with 15 volt rails only do 20 to 22 ... perhaps only 18dbm
    and not easy to smack >500mW into a 600 ohm load

    SO
    with respect to headroom
    max level is +27dbu and 25Vpeak
    then
    for a nominal +4dbu and 1.7Vpeak
    you would have 23dbu of headroom

    hope that makes a little sense
     
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