From: wkb@cbnews.cb.att.com (wm.keith.brummett)
Subject: Re: Another Sub Woofer question
Organization: AT&T
Distribution: usa
Date: Fri, 1 Jan 1993 15:35:35 GMT


    In article <1992Dec31.170723.12609@colorado.edu>,
    baldwin@frodo.colorado.edu (Dan "Viper" Baldwin) writes:
    > 
    >    I have a Yamaha RXV-850 reciever and an Infinity 10" subwoofer
    > powered with 100 Watts.
    
    Since I believe the RX-V850 is rated at 80wpc, are you saying that your
    subwoofer has a built-in (or external) 100W amplifier?

    > From answers to a previous post, I have gathered that to reap the
    > maximum benifits from the sub-woofer, I need to include a crossover
    > so that signals going to the main speakers are devoid of lower
    > frequencies ( say < 200 HZ ).

    You gather correctly, although with a single subwoofer you should be
    looking at a crossover frequency below 100 Hz.  200 is high enough that
    your ears will be able to locate the sound source (the subwoofer).
    Below 100 Hz, you'll probably get the impression that the deep bass is
    coming from the main speakers.  80 Hz would be even better, but your
    main speakers should have good response down to an octave below that
    (40 Hz).

    > Does the crossover connect to the high level main speaker terminals
    > of the reciever and then divide the signal to the mains and the
    > subwoofer?

    Well, I'm not up on these fancy A/V receivers, so I don't know what the
    Yamaha does.  It may have a built-in crossover.  (Well, it certainly
    has a low-pass section for the subwoofer out.  The question is: does it
    have a similar high-pass section for the main speakers and, if so, how
    does one invoke it?).

    Subwoofer crossovers can be passive or active.  A passive crossover is
    almost always included in a non-powered subwoofer.  The implementation
    is to run speaker wires from the receiver's main outputs to the input
    terminals on the sub.  Low signals are routed to the sub's driver(s),
    and high signals are sent to a pair of output terminals.  The output
    terminals are then connected to your main speakers with more speaker
    wire.
                              __________________
       passive crossover:    |                  |
       (inside subwoofer)    |     receiver     |
                             |__________________|
          ________                   |  | spkr outputs      ________
         |        |                  |  |                  |        |
         |   L    |               ___|__|___               |   R    |
         |  spkr  |--------------|   sub-   |--------------|  spkr  |
         |        |              |  woofer  |              |        |
         |________|              |__________|              |________|


    Active (or line-level) crossovers require that the subwoofer be powered
    by a different amp than the receiver itself.  This amp can be external
    or built into the subwoofer cabinet.  In use, a line-level pre-amp-out
    signal from the receiver is fed to the active crossover.  The crossover
    splits the signal, and sends the lows to the sub amp, and the highs back
    to the receiver via line-level main-in jacks.  The main speakers are
    connected to the receiver in a normal manner.

       active crossover:      __________________
                             |                  |
              ,--------------|     receiver     |--------------,
              |              |__________________|              |
          ____|___       pre-out |  |    |  | main-in       ___|____
         |        |              |  |    |  |              |        |
         |   L    |           ___|__|____|__|___           |   R    |
         |  spkr  |          |                  |          |  spkr  |
         |        |          |  active x-over   |          |        |
         |________|          |__________________|          |________|
                               |  | low-out
                          _____|__|_      __________
                         |          |----|   sub-   |
                         | sub amp  |----|  woofer  |
                         |__________|    |__________|


    Your receiver has at least half of the active system.  It is supplying
    the low-out signal to be fed to a sub amp.  But, how does one limit the
    main speaker outputs to just the high frequencies?  I don't know -- does
    the manual say anything about it?  Is a high-pass section invoked when-
    ever you plug something into the sub-out jack(s)?  Is there a switch?

    If there's no provision for limiting the main speakers, then you're
    going to get a big, ugly hump in the total response as the main speakers
    overlap with the sub's range (and, if this is the case, I don't think
    much of Yamaha's design engineering).

    > How much is a good crossover?  Where does one get one?

    Well, you don't need the low-pass sections; your receiver supplies that.
    If you need the high-pass sections (to limit the main speakers), the
    easiest thing to do would be to put some passive filters in the main
    speaker wire connections -- some big non-polar capacitors (with,
    optionally, some wire coils, depending on the crossover slope you need).
    There are formulae to calculate the values needed to get you into the
    ballpark.  A more expensive option would be to get an active crossover
    and not use the sub-out connection of your receiver.  Marchand
    Electronics has some good x-overs available for about a hundred bucks.
    (You have to supply an enclosure.)  You'll need two XM1 modules and a
    PS10 power supply.  If you're handy with a soldering iron, they offer
    kits that will save a few bucks (quite a few if you can design and build
    your own power supply).

          Marchand Electronics, Inc.
	  1334 Robin Hood Lane
	  Webster, NY 14580
	  (716) 872-5578

    > If there is a choice of roll over frequencies, where is a good cut
    > off ? My main speakers are Klipsh Forte I .

    See above.  Klipsch claims the Forte II -3dB point is 32 Hz.  If the
    Forte I is similar, 80 Hz should work fine.  If you use the receiver's
    sub-out jack, you're stuck with whatever they chose, unless it's
    variable.

    -- Keith

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