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Thread: Filtering the facts

  1. #1
    John Y.
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    Filtering the facts

    Let's say, for the sake of argument, that one were to take a 3115 crossover and separate the HF section from the LF section to yield passive filters.

    Further suppose that one were to have two amplifiers, each fed a full range signal, with one amp connected to the HF (high pass) filter and HF driver, the other connected to the LF (low pass) filter and woofer.

    Without getting into an active vs. passive filter discussion, is there anything wrong with doing as I described? I'll tell you why I am asking later, but I don't want the discussion to deviate from the fundamentals.

    Thanks for your input.

    John Y.

  2. #2
    Dang. Amateur speakerdave's Avatar
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    I believe what you describe is called "biwire" and is built into some speakers, like my LSR 32's.

    David

  3. #3
    John Y.
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    Originally posted by speakerdave
    I believe what you describe is called "biwire" and is built into some speakers, like my LSR 32's.

    David
    Doesn't biwire utilize the same amplifier channel to drive both sections over different wires? I'm asking about using two completely separate amplifiers, each of which has an input fed with the same full range signal.

    John

  4. #4
    Dang. Amateur speakerdave's Avatar
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    My LSR 32's have two sets of connectors which are normally strapped. When the straps are removed an amp channel can be connected to each set. One would power the woofer the other would power the midrange/tweeter. Between the woofer and its connectors is a low pass network; between the mid/tweeter and its set of connectors is a high pass network which also divides frequencies for the midrange and tweeter. Both networks may also provide other response tailoring compensation. I believe this is called biwire to distinguish it from biamping which requires external crossovers.

    David

  5. #5
    Dang. Amateur speakerdave's Avatar
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    Some speakers use a combination of the methods. My 4333A's have two sets of terminals and a switch. For use with a single amp channel the switch is set in one position which connects the drivers through the full network, providing frequency division and compensation; only one set of terminals is used. In the second switch position the terminals normally used connect only to the woofer, and the network to the woofer is truncated; the low pass filter and response tailoring are removed, but there remains a zoebel across the woofer to smooth impedance variations. This setup eliminates the big inductor from the woofer circuit, thereby improving transient response in the low frequencies. An external low pass network would be used between the preamp and the power amp.

    An amplifier for the mid and high frequencies would then be connected to the other set of terminals. The internal high pass network for those drivers remains in the circuit.

    In the literature these speakers are called "biampable", but it is a mixture of biamping and what came to be called biwiring as I described in my earlier post.

    The advantage both of this method of biamping and biwiring is to be able to choose different amps for high and low frequencies, as is often desireable.

    David

  6. #6
    John Y.
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    Originally posted by speakerdave
    My LSR 32's have two sets of connectors which are normally strapped. When the straps are removed an amp channel can be connected to each set. I believe this is called biwire to distinguish it from biamping which requires external crossovers.

    David
    David,

    When you say "an amp channel can be connected," are you saying separate amplifier channels, or the same amp channel connected to each of the two terminals? The LSR32 manual (pdf from the JBL site) talks about "passive bi-amping", which I take to mean separate amplifier channels - one to each terminal.

    Do you have access to the LSR32 schematic?

    John

  7. #7
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    Hi John

    is there anything wrong with doing as I described? I'll tell you why I am asking later,
    I don't see anything wrong with this approach - after all - it's just taking advantage of the inherent "parralleled circuit topology for this sort of passive crossover .

    You could/might run into some unprediticable phase issues if the two amps are of different makes ( or types ). I speculate that direct coupled amps vs transformer coupled may not be in relative phase to each other . On the input side ; transformer balanced vs capacitor isolated/active could easily be in a 90¡ phase quadrature. A few extra 90¡ phase rotations, unknowingly induced by different manufacturers' amps could break apart any "designed-in" cohesion ( in the crosssover/speaker system ). A person should check out ( think/follow through ) all these phase issues, if going this route .

    Why do you ask ?

    regards <> Earl K
    Last edited by Earl K; 10-11-2004 at 07:06 AM.

  8. #8
    Administrator Robh3606's Avatar
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    Sure you can do that it's Passive Biamping like in the LSR manual. On the down side running both full range you won't get the increased headroom and lower distortion levels that an active crossover set-up would give you. Curious to know why you are asking.

    Rob
    Last edited by Robh3606; 10-11-2004 at 07:06 AM.

  9. #9
    Senior Member still4given's Avatar
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    It would seem to me that driving low frequencies with the amp that is driving the HF driver is just wasting horsepower. I doubt that driving the LF amp at full range would matter all that much but still, why make the amp work to produce frequencies that aren't going to be heard?

    Blessings, Terry

  10. #10
    Alex Lancaster
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    Smile

    It looks like You could burn up both sections of the separated xover, where is the energy that is filtered going to go?

  11. #11
    Administrator Mr. Widget's Avatar
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    John,

    As long as you fully separate the two sections, no common point (neg), there will be no problem at all. Both amps need to be inverting or non inverting. If they are different you could always flip the polarity at one crossover input.

    I could see this being useful if you were going to use a low power tube amp or a small class A solid state on the HF section and wanted a solid state brute on the woofer. You would want to remove the padding built into the crossover for the HF section.

    In this sort of set up you would gain some of the benefits of biamping but not all. It seems like a lot of trouble and expense to achieve only some of the benefits. I would build a passive network in front of the amps. That would give you all of the benefits.

    Widget

  12. #12
    Administrator Mr. Widget's Avatar
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    Originally posted by Alex Lancaster
    It looks like You could burn up both sections of the separated xover, where is the energy that is filtered going to go?
    The same place it always goes. It is dissipated as heat by the inductors and capacitors of the respective network.

    Widget

  13. #13
    Alex Lancaster
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    Smile

    Widget:

    Yes, but in a normal passive xover, the bass energy goes to the woofer, the HF to the tweeter, if You run the amps full range, You will overload the LRC components, it is not the same.

  14. #14
    John Y.
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    Originally posted by speakerdave
    Some speakers use a combination of the methods. My 4333A's have two sets of terminals and a switch.
    In the second switch position the terminals normally used connect only to the woofer, and the network to the woofer is truncated; the low pass filter and response tailoring are removed, . . . This setup eliminates the big inductor from the woofer circuit, thereby improving transient response in the low frequencies.

    An external low pass network would be used between the preamp and the power amp.

    An amplifier for the mid and high frequencies would then be connected to the other set of terminals.

    The advantage . . . is to be able to choose different amps for high and low frequencies, as is often desireable.

    David
    David,

    That answers my last post, which I wrote after you sent the above.

    I understand that you have to apply a low pass active (desirable) or passive (less desirable) filter to keep the HF out of the woofer path, unless the woofer has a natural roll off at the intended crossover.

    John

  15. #15
    RIP 2011 Zilch's Avatar
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    Ummmm,

    Originally posted by Alex Lancaster
    It looks like You could burn up both sections of the separated xover, where is the energy that is filtered going to go?
    It doesn't "Go" anywhere, it's simply blocked. It is never drawn out of the amp.

    Consider, for example, a capacitor in series with a tweeter. Is there any "dissipation" of low frequencies there? Nope, they don't pass through, is all.

    Depending upon the design of the the crossover, there may be attenuation of the high frequency leg(s) to balance the inherent higher efficiency of those drivers. There seems to be little value in retaining that function if they are to be driven by a separate amp.

    Similarly, is there any dissipation of high frequencies in an inductor in series with a woofer? Well, maybe, but minimal. The primary dissipation there is the IR drop across its impedance by the low frequency current that IS passed to the woofer.

    So, as Terry suggests, what is "wasted" is the full-range capability of each of the several amps, assuming that is what is used. Presumably, however, one can still employ different kinds and sizes of amps for low vs. mid/high drivers, as desired.

    "Biwire" to me implies separate amps or separate channels thereof, at least as much as any advantage (dubious) that might accrue from running different wires from the same source to low vs. mid/high drivers.

    Bottom line: Sure, it'll work just fine....
    Last edited by Zilch; 10-11-2004 at 10:37 AM.

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