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Passive Biamping: How much improvement?

Discussion in 'AV Receivers' started by Jigesh Patel, Jun 18, 2003.

  1. PaulT

    PaulT Well-Known Member

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  2. Brett DiMichele

    Brett DiMichele Well-Known Member

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    Chung,

    So wrong you are.. But hey that's fine...

    I will stick with my "Fools" Bi Amplification rather than

    some jerry rigged setup with an active crossover that does

    not even take into account baffle step, time alignment or

    numerous other "specific" crossover issues that go into

    designing a proper network, passive or otherwise.

    As a side note... Even IF the amp is sending a full range

    signal to the crossover and then to the specific driver.

    The amp is NOT under the same loads. As soon as you remove

    those shorting straps on a properly designed crossover

    network you are now driving "seperately" the tweeter and

    midranges and the woofers. A woofer always will present a

    lower impedance than a tweeter and will require more work

    from the amplifier than the tweeter will. So it's not

    accurate at all to say that "even though" the XO is now

    seperated that the amps are still working just as hard

    because it has to send a full range signal to the crossover.

    *Shakes head* Keep reading... But perhaps you should "try"

    it.. It's NOTHING at all like Bi Wiring, not even close.
     
  3. Jigesh Patel

    Jigesh Patel Well-Known Member

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    Thanks, Paul, for the web-link.

     
  4. Chu Gai

    Chu Gai Well-Known Member

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    Well I certainly wouldn't rip out the crossovers and try an active approach especially if the speakers were designed by competent engineers. I think Paradigm is competent [​IMG]
    However, in this case, I'm assuming the poster wants to remove the strap and basically run two amps into the upper and lower binding posts. This, I assume, will be done without the amps having been preceded by any sort of active crossover (filter). If so, then both amps will certainly be presented with the same full range frequencies and they will do what amplifiers do: amplify the full range of frequencies. Hence any increase in the amp headroom would be very speculative. Were it that they were preceded by filters, then yes, I could see there being this headroom increase. The amp just doesn't know how the frequencies that it's sending out are being used. So I must politely disagree with you Brett.
    Often what occurs, is a situation where the gains of the two amp are not critically matched. In this case you've changed the frequency response of the system. Run a little more juice up top and I'll just bet things might be called sparkling and greater delineation of cymbals and such. If you like it, who am I to argue with it? Since you own the equipment already, it's not going to hurt matters for you to play around.
    However, if you carefully match, you may find that the primary benefit was to the person who sold you the amps.
    On a side note, keep in mind that most of your acoustic power is down below. Only about 10% or so exists above ~ 3.5 kHz.
     
  5. chung_sotheby

    chung_sotheby Well-Known Member

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  6. Michael R Price

    Michael R Price Well-Known Member

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    Passive bi-amping improves sound mainly by reducing the current demands on the high frequency amplifier. However, active bi-amping will improve sound even more because the amplifier only amplifies the intended signal and nothing else (that would be filtered by a passive crossover), so the intermodulation distortion and any voltage swing related compression will be reduced. But it's important that the active crossover keep the same frequency response as the passive one... which can be difficult.
     
  7. Chu Gai

    Chu Gai Well-Known Member

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    difficult? perhaps next to impossible considering the monumental task of 1st matching what the engineers made then 2nd improving upon it. IMHO, it you want to actively biamp, buy it that way.

    so Jigesh, whatcha gonna do?
     
  8. Jonathan M

    Jonathan M Well-Known Member

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    There seems to be quite a bit of mis-information here, so I'll try and help clear some things up.

    In a passive bi-amp setup, where one is using the internal (passive) crossovers on a speaker with the various sections separated (ie any plates removed) then one has one amp powering the lowpass xover inputs, and one powering the highpass xover inputs.

    The LOAD on each of these amps is LESS than the load present if one of the amps was driving both xover inputs in parallel (As is the case in a normal single amp situation.)

    WHY?

    Because the impedance of the tweeter+highpass network is high at low frequencys - there is a capacitor in series (and perhaps an inductor in parallel) with basically a resistive device (The driver) in the below 1kHz range. This presents a very high impedance which is very easy to drive - the current involved is minimal. Thus the amp is under less stress. It has higher current reserves for the highs (Which it doesn't need much for anyway as Chu indicates above).

    What about the lows?

    The impedance of the woofer plus any series inductors and parallelled caps rises at frequencies above the xover point. Thus, the load the amp must supply current to is reduced. Note that it is not reduced all that much as the power content of high frequencies is very low. Thus, one does not see the same advantages as for the tweeter amp.

    What's the benefit of active biamping (possibly using a passive xover before the amps)?

    This has the further effect in that the amp is no longer amplifying any signals outside of the small(ish) passband that the particular driver it is supplying is operating over. Thus one achieves all the benefits of the above PLUS the added benefit that there is no xover network between the amp and the speaker. This means that ALL the power that the amp is producing is used to drive the speaker. None get's wasted away through heat via the inductors and resistors etc. in the xover network. One sees benefits here for the amp supplying the lows as there is no series inductance which has an inevitable loss of efficiency due to the DCR of the inductor. Efficiency is also increased in the amp supplying the highs as there is no series resistance involved (Usually used in passive xovers to decrease the level of the tweeter to that of the woofer).

    As has been indicated above by other posters, any active setup has to be well designed from the beginning. I strongly suggest you learn something about speaker design before attempting anything of this nature. There's no point throwing money at something in hopes of improving it if you don't understand how things work - this is a waste of time and money and is why so many charlatans exist in this industry.

    So what's the answer?

    YES, it IS possible to increase the quality of sound in this fashion. Any improvements will come in the highs mainly - note that you can use a lower powered amp on the highs in this case. Will you HEAR any difference? Try it and see. (Yes, I wussed out).
     
  9. Greg_R

    Greg_R Well-Known Member

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    Greg
     
  10. Michael R Price

    Michael R Price Well-Known Member

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    Thanks for clarifying Jonathan, you seem to really know what you're talking about. I think that because of the intermodulation distortion in the amplifiers (Chu rolls his eyes [​IMG]), bi-amping can help (cleaned up highs for me, except I didn't have a good enough bass amp so I had to go back). Also, it can give more headroom to bi-amp than to simply use an amplifier of twice the power rating... the sum of the wave amplitudes with momentary peaks can be double what it was before, that's in theory up to 3db more headroom than a double-power amp with 40% higher voltage rails.
    And yes, mirroring a passive network's response is really hard. I'll be trying, soon, to get it "close enough" actively with my speakers. Steep filters and nice linear/accurate drivers help, except mine are iffy.
     
  11. Craig_Kg

    Craig_Kg Well-Known Member

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  12. RichardHOS

    RichardHOS Well-Known Member

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    In some cases, others have already done the work involved with modeling the factory passive crossover network as an active network.
    And, with a little understanding, you can recognize compromises made in the factory crossover due simply to the values of components available for a reasonable price. It's not always difficult to improve on what the factory did. But, even if sticking with the factory slopes/points, going from passive to active crossovers doesn't have to be a three year government funded research project! [​IMG]
    It would help to have access to knowledgeable people on forums like this though. [​IMG]
     
  13. Chu Gai

    Chu Gai Well-Known Member

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    that'd be one hell of a challenge let alone expense to even match a good crossover using active components don't you think?
     
  14. Brett DiMichele

    Brett DiMichele Well-Known Member

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    I just personaly feel that this term "Fools BiAmping"

    should be applied to Active Bi Ampification. Because only a

    Fool would take a prebuilt speaker and throw an active

    crossover in the mix. Unless your name is Likwitz I doubt

    you have the skills to do it correctly (I am not pointing

    at anyone in particular, just a general rambling).

    And I stick by what I said Chu, sure the amp is being feed

    a full range signal and yes it is amplifying said signal

    but if the ampifier is only feeding say one midrange and one

    tweeter it has a much easier load than if it was feeding a

    woofer a midrange and a tweeter.

    Bi Amplification (call it passive or active or whatever) has

    worked wonders for my system not only due to more head room

    available but the ability to run Tubes on the Mids and Highs

    and Solid State on the lows.. Absolutely wonderful!
     
  15. Brett DiMichele

    Brett DiMichele Well-Known Member

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  16. Jigesh Patel

    Jigesh Patel Well-Known Member

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    Thanks a lot, everybody...! Too much knowledge to digest over the coming weekend! This was my first post on this forum and as you could already see, I am a novice in the HT area. I appreciate everybody's time and painstaking care for the details in replies; and a very healthy debate.
    It occurs to me that since I am already using a 200w per channel (multi-mono design) amplifier (Sherbourn 5/5210) to drive my speakers, I might get more improvement by replacing the receiver with a prepro than by adding two channels to biamp passively as I suggested earlier.
    Anyway, in case I still happen to experiment with passive biamping, I will be happy to post my results here. Have a nice weekend everybody!
    Jigesh
     
  17. RichardHOS

    RichardHOS Well-Known Member

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    Chu:
     
  18. Brett DiMichele

    Brett DiMichele Well-Known Member

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    Richard,

    Ok how to you rework an active solution that can take into

    account Baffle Step, Lobing, Comb Filtering and numerous

    other specific issues that the OEM Passives ARE engineered

    to combat?

    There is absolutely NO question that Passive networks are

    horrible by design.. Active is most definaty an "Ideal"

    setup. But I don't see how you can just rip the passive

    out and stick an Active in it's place without fudging

    the way the speakers work.. Show me a way to do it with

    my mains, I have the amps.. I will buy the active components

    tomorrow...
     
  19. Michael R Price

    Michael R Price Well-Known Member

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    Well, let's see, to replace the 4th order passive 2-way crossover in my Kit281s, I'll put a notch filter on the woofer to take out the breakup peak, put a passive zobel on the tweeter. The active section will use 4th order filters, and apply a shelving filter of 3-6db for baffle step. The response won't be the same, but it will probably be close since my speakers don't have much significant EQ going on... and I can always add filters to change the response to my liking. I may have to experiment with baffle modification to get the treble response flatter without the impedance interactions of the passive crossover. Either way it's cheap, fun to try, and I can always go back to the passive filters. Now, if only I could get these doggone amplifiers working again...
     
  20. Brett DiMichele

    Brett DiMichele Well-Known Member

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    Mike,
    That's all well and good for experiments sake.. But it
    seems like any time "I" even mention crossover mods I get
    smacked upside the head "What are you, Crazy?" [​IMG]
    Good luck.
     

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