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Phasing problems with active/passive crossovers

Discussion in 'Hobby Electronics' started by Andy, Apr 4, 2004.

  1. Andy

    Andy Guest

    I'm trying to build myself a decent sounding stereo system. What I have at
    the moment is a pre-amp feeding into an active crossover which then feeds
    into two power amps, one for the woofers and the other for the tweeters and
    mid ranges, with a passive crossover onthe tweets/mids. The problem is that
    the passive crossover for the tweets/mids shifts the phasing and sounds
    awful.

    The stop gap solution I have at present is to use a simple 1st order
    (passive) crossover on the tweets/mids, and placing a coil in series with
    the woofers simply to shift the phase 90 degrees to match the other drivers.
    I've used a coil value that SHOULDN'T affect the lineararity (is that a
    word?) of the woofs - but my (untrained) ears tell me otherwise. I seem to
    be getting a slight roll off on the upper end of the freq. range. Unless I
    have other problems with my box design, which is very possible, but I want
    to eliminate other possibilities first.

    All passive components are supposed to be of suitable quality for
    crossovers (from Jaycar).

    I built the active crossover myself with a board I purchased from a bloke in
    Sydney who makes them. See - http://sound.westhost.com/project09.htm

    My question is;
    Could I build a 2nd order passive crossover for the tweets/mids and reverse
    the polarity on the woofs (removing the in-series coil of course)? Would
    that make them in phase or 360 degrees out, and would that matter?

    Would I be better off constructing a circuit to go between the active
    crossover and the woofer amp to shift the phase?

    I'm doing it this way because I was able to get the (reasonable quality) pre
    and power amps at a really good price.

    Pls note - I have no electronic qualifications but I am fairly technically
    minded so simple(ish) answers are appreciated.

    Thanks
    Andy
     
  2. Phil Allison

    Phil Allison Guest


    ** While the sound may be poor to your ears saying it is due to x-over
    phase shifts is a **very wild** assumption. Your choice of drivers and their
    mounting methods or enclosures etc is far more likely to be the problem.


    ** All x-over filters have phase shift accompanying any change in their
    frequency response, both active and passive. All drivers have phase shifts
    too - accompanying any response peaks /dips. The effect of reversing the
    polarity of a mid or tweeter in a 3 way system is normally quite small and
    hard to pick.

    If you want help then you MUST supply details of your drivers, enclosures
    and x-over designs.




    ........... Phil
     
  3. Johnny

    Johnny Guest

    Phase shift is an important consideration in the design of any
    crossover. This is one of the things that is neglected when a very
    simplistic crossover design is used such as the one on the web site
    you mention. The drivers themselves will have phase shift of their
    own, and also due to the fact that cone and dome drive units radiate
    sound from different points when typically mounted on a flat baffle.

    I suggest you try swapping the connection polarity of mids and
    tweeters, which might improve the situation if you are lucky. But to
    do the job properly you will need to measure the speakers, each driver
    individually and as a system. That is the only way to get an
    understanding of what is really happening in a particular speaker
    design. You can download freeware for speaker measurement at
    www.speakerworkshop.com . You will need to use the gated measurement
    option otherwise room reflections make it impossible to get meaningful
    measurements.

    best regards,
    Johnny.
     
  4. Andy

    Andy Guest

    Thanks Phil, I didn't mention it in my post, but I wasn't, altogether,
    assuming. I mucked around with different things for quite a while and tried
    reversing polarities all over the place but could only make it worse, or no
    different. As soon as I placed the coil in series with the woofer the sound
    kind of "opened up" and became much clearer.
    Drivers;

    Tweeter - Vifa D19
    Mid - Response 5" poly
    Woofer - Response 12" Carbon Fibre

    All enclosures are sealed and I followed the manufacturers recomendations as
    to box volume. Tweeters have an "L pad" attenuating circuit to knock of
    about 3db. I used the formula for linkwitz-riley alignment to work out
    component values for the passive crossover. Can't remember the values right
    now but I will pull the box apart and let you know if this is crucial.
    Formulas came from the Loud Speaker Design Cookbook by Vance Dickason.
    Enclosures are built so the voice coils of all 3 drivers are on the same
    plane.
     
  5. Andy

    Andy Guest

    Yep, I tried to take that into consideration when I built the boxes - the
    tweeter and mid are on the same baffle and the woofer has it's own which
    "juts out" so the voice coils are aligned vertically (they look a bit ugly).

    The designer of the crossover circuit seems to believe that it is reasonably
    phase coherent though. Do you know of any reason it might not be? As I
    mentioned I'm not trained in electronics so I believed what he claimed.
    Crossover Freq is 300Hz BTW.
    Thanks for the link. That's exactly what I've been looking for. I've been
    wanting to do an impedance response test to look for any obvious dips or
    spikes - or is that barking up the wrong tree?

    I've already tried reversing the mids and tweets, but I'm not that lucky :)

    Thanks Johnny
    Andy
     
  6. Johnny

    Johnny Guest

    Depending on your definition, it might be phase coherrant if the drive
    units have didn't have any phase shift of there own. Since that is
    an unrealistic assumption, the complete system is extremely unlikely
    to have in-phase summation between the drive units as required for a
    Linkwitz-Riley response.
    Impedance measurement won't tell you about the acoustic response of
    the system. You need to use a microphone. Decent measurement
    microphones will cost upwards of a couple of hundred dollars, but
    think you will probably get some useful clues by using a cheap PC
    microphone. Gated measurements will help mostly for the crossover
    between mid and tweeter. For the crossover at 300Hz, you will find it
    quite difficult to make measurements using the gated method. Pink
    noise and a real-time analyser (RTA) is probably more useful in this
    frequency range. There are some RTA software available for demo on
    the net, but I am unsure if they are able to playback a pink noise
    test signal while measuring.

    Since swapping the polarity did not help, you might try bypassing some
    of the circuitry on the 4th order crossover board. If you by-pass one
    of the 2nd order sections (on one of either hi-pass or low pass
    secion) you will shift the phase by 90 degress at the crossover
    frequency. Then by swapping the polarity you get another +/- 180
    degrees shift, and you might have better luck!

    best regards,
    Johnny.
     
  7. Rob Judd

    Rob Judd Guest

    Andy,

    Either use active crossovers or passive, not both.

    Rob
     
  8. Phil Allison

    Phil Allison Guest


    ** Which clearly demonstrates that x-over phase shift is NOT your problem.

    Phase shift at or near x-over frequencies causes either a modest peak or
    narrow dip in the combined response - the narrow dip is *inaudible* and
    hence the preferred error.

    BTW Where did you get the idea putting an inductor in series with a woofer
    shifts the phase by 90 degrees ????????


    ** That electronic x-over you made likely has a serious fault.




    ** You have so far failed to explain what the ***PROBLEM ** is -
    except that YOU do not like the sound.

    It is impossible to make any analysis with something that subjective as the
    problem.





    .................. Phil
     
  9. What Active crossover circuit are you using ? I see in another message you
    say you are using a Linkwitz riley Passive but what is the active crossover
    ? I have had problems with 3rd order butterworth active crossovers (as you
    say you can get some weird phase cancellation between the high and low
    speakers) whereas when I changed over to a Linkwitz riley Active crossover
    everything just sounded a heck of a lot better.
    As for Passive crossovers being any good .... It actually takes quite a lot
    of work and usually necessitates taking the impedance curve of the
    drivers/cabinets into account to get a half decent passive crossover - If
    you can, save yourself a lot of drama and try running your speakers three
    way active this will at least let you listen to the speaker system on its
    own and make sure that it sounds O.K to start with (no use mucking about
    with your crossover if your speakers arent up to the job). Other than that I
    have found the second order type passive crossovers are probably a better
    starting point than most .......

    Regards
    Richard Freeman
     
  10. Yeah sorry I missed this - my other advoce still applies though try running
    the speaker three way active and see what it sounds like ....
     
  11. Andy

    Andy Guest

    Geez Phil ....I tried to be polite and supply the data you said you needed
    and you seem to take it as a personal insult. I came here to see if I could
    learn something not for an argument. Sorry mate, but you are on my ignore
    list. Go ahead, flame away - I won't see it :)
     
  12. Phil Allison

    Phil Allison Guest


    ** You are an utter idiot - piss off.



    ** You are such a pathetic jerk you are completely unable to define the
    problem.





    ............. Phil
     
  13. Andy

    Andy Guest

    Thanks Johnny, I'm pretty sure I can borrow a stage microphone.

    I have a program called NCH Tone Generator that can generate white/pink
    noise or square/sawtooth and sine waves. If the RTA software can't play and
    listen at the same time I can run the generator on my lap top and use my
    desktop to measure.(yes?)

    Just done a Google search for RTA software and there are dozens! What
    features should I be looking for? Do you have any specific recomendations
    perhaps? Both my lap top and desktop can boot to Linux BTW.

    Sorry Johnny, it was late when I posted and my brain was already in bed. The
    reason I wanted to measure impedance response was to detect any resonance
    that may be compounding the problem/s. From what I have read almost every
    part of the system from source to driver, and even the room can resonate.
    Thanks
    Andy
     
  14. Phil Allison

    Phil Allison Guest


    ** Andy is fucking nut case.




    ........... Phil
     
  15. Andy

    Andy Guest

    I'd like to be able to. But I'd have to build another (3 way) active
    crossover and buy another amp. If I had the bucks that is definitely what I
    would do but I have teenagers who keep me poor :)

    What I have tried, however, is disconnecting the tweeters and running as a
    straight 2 way active, and the sound was good (albeit missing the highs)
    which leads me to believe the active crossover is working OK.

    Having said that though - if this gets much more complicated I'm thinking of
    starting over by keeping the tweeters, buying some 6 1/2 inch carbon fibre
    drivers for woofers and recycling my 12 inchers into an isobaric sub-woofer
    using a Subwoofer Amp Module from Jaycar. So I would have a straight 2 way
    active, with a subby. I'd have to build another active crossover of course,
    as the crossover freq would be much higher. Also I'd have to make some new
    enclosures - luckily my brother is a cabinet maker, but I still won't get
    out of it for less than about $500 or $600. Although I could convert the old
    3way enclosures into a realy cool dog kennel :)


    Thanks
    Andy
     
  16. Johnny

    Johnny Guest

    I don't have much of an idea whats available now, but I would bet that
    there is much better freeware around that when I looked a couple of
    years ago!
    Impedance data is useful for checking the tuning frequency of vented
    boxes and sometimes severe cone resonances can show up also.

    Good luck with your quest!

    regards,
    Johnny.
     
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