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Bi-wiring Mission 775 with bridged rotel power amps

Discussion in 'AV Pre-Amp/Processors & Power Amps' started by CIMPSA, May 25, 2002.

  1. CIMPSA

    CIMPSA
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    Hi there
    I have been running a pair of Mission 775 with 2 Rotel RB971 in bridged mode, one for each channel, for a while now, and because I am also using Qed Silver Ann Biwire, this morning I just realized that:

    1. According to the Rotel´s manual, when in bridged mode, the impedance of the speakers should be at least 8 ohms, because the amps will be effectively seing the impedance halved, this is, 4 ohms. So far, no problem, the 775 are rated 8 ohms.

    2. But, because also biwiring (, I understand this also "halves" again the impedance?), am I then taking down the impedance to 2 Ohms? Because according to the manual, I should´t been doing that!

    Anybody can confirm this interpretation is correct or not? If so, should I stop doing this, or there is no real harm to the amps?

    By the way, I use a Marantz SR5200 as a preamp, and to power the rears (another bridged 971 for the center).

    Thanks in Advance

    Carlos
     
  2. baileych

    baileych
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    Not true, bi-wiring will make no difference to the impedance of the speakers as 'seen' by the amp.

    --

    If you bi-amp, because you are driving just part of an 8 Ohm speaker the impedance of just part of the speaker will be greater than 8 Ohm.

    If it's a two driver model and your driving each driver from a separate amp note that they won't split into 2 x 16 Ohm drivers as the bass driver will usually be of lower impedance than ithe high frequency driver. They will definitely both be greater than the original 8 Ohm though.

    Charles.
     
  3. buns

    buns
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    forgive me if im incorrect......but is impedence not basically a measure of the resistance to current flow (with complex aditions)?

    In which case the impedence of the speakers wont change no matter what amp you are powering it with be it a cambridge or a levinson.

    In bridging.....impedence stay the same, current ability doubles therefore the power doubles. If you biwire, apart from any additional resistances due to extra cable, there will be no difference. I think.

    Suffice to say im all but certain you arent harming your amps! Though to be perfectly honest......i think i biamped unbridged system would be better than a biwired bridged one!

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  4. lynx

    lynx
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    CIMPSA, You are correct in thinking that the amp will ''see'' the impedance as being halved. Luckily you have been using 8 ohm speakers so have ''got away with it''.
    I use bridged amps in my car and they can give of extreme amounts of heat,so hot that they would burn skin.I assume the Rotel hasn't got that hot.
     
  5. buns

    buns
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    OK......theoretically what you are saying might be correct from an absract view point.......but abstract is the word! In saying the impedence halves......you are saying that the amplifier has remained the same despite you having effectively double the current available to each channel which means that the amplifier must always output exactly the same......hence it always outputs its full volume......hence again to control volume you are effectively varying the impedence......

    if someone want to explain why im wrong i would appreciate it!

    The heat is not necessarily a sign of problems.......class A amps will run at 60 degrees at standard......

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  6. baileych

    baileych
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    The reason that everyone is using the word "see" in inverted commas is that the impedance of the speaker of course doesn't change.

    By changing the amplifier to bridged mode its the amplifiers that changes. Now how this happens internally is probably not trivial but the effect is having the two halves of the amp wired in series driving the same single load. (I'm kind of making a d.c. analogy here, like when - if you remember physics lessons - if you wire to 1.5V batteries together and connect a bulb, it has 3V across it and glows brighter. Things are a bit more complicated in a.c. world and lots more when you are amplifying a signal as opposed to just using a.c. power supplies.)

    If the unbridged amplifier was rated at 4Ohm then to get the same load on the bridged amplifiers you would need two 4Ohm speakers in series or an 8Ohm speaker.

    [ Note that with the bridged amplifiers doubling the output p. d. you would only need the preamp at half the volume to get the same output level as an unbridged amplifier which would mean the same current as before going through the bridged amplifiers so you should be fine using the same volumes as in unbridged mode. However, if you want to use the extra power available :D then yes, the current will be larger, meaning hotter running power amps. ]
     

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