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Crosover frequency confusion - anyone explain?

Discussion in 'AV Receivers & Amplifiers' started by balders, Mar 22, 2004.

  1. balders

    balders
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    I thought I understood what this crossover-malarky was all about, until today when I started playing various test frequencies on my Denon 1804.

    I had assumed that if I set the amp crosover to, say, 100 Mhz, then everything below 100 Mhz would be sent to the sub, and everything over would be sent to the speakers (set to small on the amp). This made sense.

    However, I played a 80 hz test tone with a 100 Mhz crossover, and the sub turned off, and I heard sound from the main speakers. I then played 120 Mhz tone with the amp on, but speakers switched off, and heard a tone from the sub. Admittedly, the sounds were a lot quieter when coming from the 'wrong' speaker, but I hadn't expected to hear anything!

    So, is the crossover frequency just the centre-point of a crossover-slant, or is it supposed to be all or nothing as I had supposed?

    Or could it be that my 'pure' test tones actually have some other frequencies in them too (generated use a PC-based tone generator and then burnt to CD)?

    Can anyone explain as I'm a bit confused?

    Thanks,

    Balders.
     
  2. StevieDvd

    StevieDvd
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    Have you got a DVD with THX? They have a test setup which you can use to check the fade between speakers & sub. It says it should be a smooth transition so I guess one decreases as the other takes over.

    With my amp set at 100 crossover I get a smooth transition so I guess that's OK.
     
  3. balders

    balders
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    Thanks Stevie.

    It sounds fine playing the THX test sweep, and the various test sweeps that I've generated myself. There are a couple of dips now and then as the frequency increases, but I think this is more to do with the room acoustics.

    I'm just curious about how the crossover works as some of the articles I've read implied a distinct cut over point, not a gentle fade. A gentle fade sounds more sensible!

    Balders.
     
  4. davehk

    davehk
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    Crossovers are not brick walls; it's a (nore or less) gentle fade - the crossover point is the point where the signal level is 1/2 (-6dB if we're talking voltage, I think) the nominal level, and decreases at a rate dermined by the type of filter - anything from 6dB to 12dB or more per octave.
     
  5. balders

    balders
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    I see. Thanks, Dave. That makes sense now!

    So am I correct in thinking that at the crossover point, half the signal is being sent to the speakers, and half to the sub. And charting the two signals on a graph would show a kind of stretched "X" shape around the crossover point.

    Balders.
     
  6. davehk

    davehk
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    Yes, that's about the shape of it! If you get the x-over point wrong, you end up either sending mid-reange frequencies to the sub - which doesn't reproduce them well, and gives you a "hole" in the middle of the frequency reponse, or you send too low frequencies to the mains, which can'r reproduce them and give you a hole in the upper bass region.

    It's a balancing act that, IMO is best resolved by using your ears on well known source material. Other may suggest using Real TIme Analysers with pink noise sources, etc but they by no mean guarantee an optimal result.
     

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