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    2 Way/3 Way Passive Crossover Networks
    Testing the Results

    3/98 - updated 8/04 - Jens Moller

    Building your own 2 Way/3 Way Passive Crossover Networks

    Building 2 Way Passive Crossover networks were discussed in:
    • First Order(6db) Crossover
    • Second Order (12db) Crossover
    And it was mentioned that to make 3 way Passive Crossover networks, all that you needed to to was to stack another crossover stage on to the output of the first. While this is true, it may not be obvious without a few diagrams.

    The schematics on the above 2 listed web links create a 2 Way Crossover. You can use either First or Second order for crossovers, or mix First and Second order as each building block.


    		The advantage of a First Order is that it is the least expensive to build, and in general has very few phase related problems when implemented. You might have a problem with blown out Tweeters if you are running only a 6db roll off (First Order). Many small diameter voice coil Tweeters are quite sensitive to lower frequencies, and as such, you may want to consider a 12db (Second Order) crossover specifically for the Tweeter. The side effect of Second Order crossovers, is they tend to cause a phase shift in the high frequency output that might be very noticeable - in these cases, you should reverse the polarity of the high end tweeter (ie. swap the wires going to that speaker) and see if it sounds better - it might, or it might not. Choose the way that sounds the best to you at a volume level that you would normally use the speaker system at.


    		As you can see, connecting the output of the first crossovers high frequency output to the input of the second crossovers normal input is a simple task.

    Stacking First Order (6db) crossovers can be done as deep as you need them to go. Many people build up 4 and 5 way crossovers to do this.

    Second Order (12db) crossovers, because of phase shift issues, tend to either have a pronounced notch (10 to 30db loss) near the crossover frequency, or a mild (2 to 5db) boost near the crossover frequency. The mild boost is highly preferable to the loss - its easier to reduce a volume level at a frequency with an equalizer than it is to reconstruct something that is missing.

    Odd ordered crossover networks (First - 6db and Third - 18db), don't introduce phase shift (ie. Butterworth crossover networks) in either Passive or Active designs.

    This phase issue with Second Order (12db) crossovers is common among all even order crossover networks (whether Passive or Active), and in general, this is probably why you don't see Second Order crossover networks (outside of the Tweeter) in that many PA sound systems. It is imperative that the speakers and cabinets be matched if when using Second Order crossovers for 2 and 3 way systems - otherwise, the sound quality may not be very consistent all across the frequency spectrum.

    There are many quality sound systems that have well designed Second Order crossover networks in them - The best way to be sure that yours sounds good is to listen for dead spots in the frequency range. This is easy to do with most any synthesizer (don't try playing a recording thru the system when hunting for dead notes - there will be too much to listen to all at once). Dead notes (lower volume notes) will stand out as you move across the synthesizer scale.

    To test using a Synthesizer

    • Choose as clean a sine-wave patch as possible to test this with.

    • If possible, use a microphone connected to a device with a level meter on it to verify the consistency as you play individual notes (such as a component cassette deck - a real meter is better than an LED display, however, use what you have available).

      • Set the microphone 10 feet away from the PA or Guitar speaker cabinet, and aim it towards the center of the cabinet(s).

      • You should not expect same measured level at all frequencies - the meter level may diminish as the frequency increases. You are looking for major differences between individual notes.

      • Keep in mind the frequency limitations of the microphone that you are using when testing this.

        NOTE: There are software applications that will run on PC's to do a similar (and far more accurate) audio level/frequency test. The results of these are highly dependent on your sound cards capabilities

    • Listen carefully as each note is played up the scale (play both the White and Black keys individually) - try to hear if there are inconsistent tonal changes and/or level shifts.

    • Take note of anything that sounds out of place.
    By definition, you can hear a 1db shift in volume level. Depending on how tired your ears are, you may not notice a 3 to 5db peak, but you will definitely hear a 10 to 30db notch. Depending on how narrow the peak or notch is, you might miss it, but at least you looked for it.

    If you find problems

    Don't panic. Try to find out where the problem is and work back from that.

    • Consider moving the crossover frequency by approximately 10% and try again. This will mean changing Capacitors and Inductors.

    • You could also experiance problems if the second crossover networks crossover point is too close to the first one - try moving the frequency.

    • You might have a speaker with a different impedance than you thought - it could be 4 or 6 ohms when you expected 8.

    • Its possible that the speaker you are using is not appropriate for the task at hand. It may not output enough volume as compared to the other speakers/drivers within the cabinet at the frequency range you selected the crossover.

      I recall someone putting 5 inch woofers in place of 5 inch midrange speakers - the mids were pretty much non-existent as a result - they happened to already have the speakers but didn't realize that they were woofers.

    • If using L-Pads, the level may be set wrong.

    • You may have a damaged speaker/driver.

    Other testing

    You might want to compare the results of your cabinets to other commercial systems (my homemade ones often have more consistent frequency response at the same volume level), or a home audio system (move the microphone closer for smaller system testing).

    The reason that I bring this up is that people will argue about how balanced the sound is as a result of the components chosen in the speaker cabinet and the crossover network. I completely agree - It will make a large impact in what things sound like. Sound Engineers will toil over the design, testing and measurement of the results. Expect people to argue with how you arrived at your crossover points. For some reason, these same people won't argue about a production system - they assume that it was done better.

    Take a risk

    In reality, for PA systems and Guitar cabinets, crossover networks are not rocket science. You can do an excellent job by choosing the right combination of crossover frequencies for your Speakers, Horns and Tweeters. Look at the spec sheets of anything that you buy and work from there. Keep in mind that a PA speaker system or a Guitar cabinet is not an audiophile speaker system (you would not want to try to use one in place of the other, they are not the same). Also keep in mind that your Speaker system will be free-standing and won't often have a wall immediately behind it to provide additional boost to your sound (as is typically the case for home audio systems). Take your time and listen critically to the results. Once you know the quirks of your own system, you'll have a good idea how it works in a live situation.

    What happens to the Impedance? Am I putting speakers in Parallel?

    If you pick the same cross-over points for both the high-pass and the low-pass filter, then when you connect these together to the same power amp output, the impedance is the same as if it was a single speaker.

    Why is this?

    Since the speakers are now conducting on a limited frequency range, as long as those ranges do not overlap, the net effect (as far as your power amp can tell) is that its only a single speaker load. You are not putting speakers in parallel.

    Cross-overs always overlap as the power to the individual speakers roll off, however this roll off is generally equal to the gain as one speaker starts conducting more, the other conducts less. The speakers are now made frequency specific. Sound, being a complex waveform, can have components of highs and lows at the same time - cross-overs simply split those components out onto seperate speaksers without altering the load. This includes 'stacked' cross-over stages - done properly, the impedance remains consistant.

    If you use badly matched cross-over points, you can have overlap - in those cases, the overlapped frequency is like wiring the speakers in parallel, but only for that frequency range. For Low frequencies, this will be quite a strain on your power amp.

    NOTE: Pro systems use the same speaker loads for all branches of thier cross-over networks - ie. all the speakers are the same impedance (or appear the same by virtue of hooking them up in series or parallel to get the same load); you should do the same.

    Questions? Comments? .

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