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Zobel networks and Bode equalizer

Discussion in 'General Electronics Chat' started by Frosty_47, Jul 7, 2009.

  1. Frosty_47

    Frosty_47 New Member

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    Hello,

    According to this Wikipedia article: Zobel network - Wikipedia, the free encyclopedia

    "Zobel networks can be used to make the impedance a loudspeaker presents to its amplifier output appear as a steady resistance"

    I want to learn Zobel and Bode equalizer networks that were omitted in my school curriculum. If anyone knows of a good article on this topic, please share it with me.

    Thanks
     
    Last edited: Jul 7, 2009
  2. Soundguy

    Soundguy New Member

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    I would suggest that you look up articles on speaker system design, there is a huge amount of them available on the web. One good source for speaker building information, articles, components, and test equipment is "Parts Express." Parts-Express.com - Speakers, Speaker Building, Home Audio and Video, Pro Audio, Electronic Parts & Accessories PARTS EXPRESS, Speakers, Speaker Parts, Guitar speakers, Bass speakers, Woofers, Drivers, speaker upgrades and replacement speakers. Emine

    The premise of Zoebel networks with regards to speaker systems is that the individual drivers in a multi-driver speaker system, i.e. 2-way, 3-way, or 4-way, have varying impedance curves depending on the applied frequency.

    The woofers in most systems have the most pronounced variations, and as such are the drivers that receive the most attention for correction methods.
    Some manufacturers also use the networks to compensate for deficiencies in the other drivers in the speaker system, to the point of silliness. I believe that, if you have to compensate so heavily with regards to a driver in your speaker system, maybe that driver is not the best choice to use.:rolleyes:

    The problem in desgning passive crossover networks for speakers is that even when you have all of the parameters in hand about the particular drivers that you intend to use, the drivers do not reflect a constant impedance to the crossover network, yet the design formulas that you typically use expect an 8 ohm or 4 ohm constant impedance from each driver.

    The principle behind including a Zoebel network is that when it is placed in parallel, series-parallel, or a more complex arrangement as needed, its reflected impedance drops in frequency opposite to the drivers raising reflected impedance at the same frequencies, thereby counteracting the varying impedance reflected from the driver, and presenting a constant 8 ohm ( or 4 ohm, if that is what you are aiming for ) impedance to the crossover network.

    Another place that you will find Zoebel networks is in the speaker output circuit of most audio amplifiers. The circuit is in fact a high pass filter in parallel with the loudspeaker load that again, compensates for the rising impedance of a typical loudspeaker with frequency, and as such, maintains a stable impedance load to the amplifier as the frequency of the signal passing through the amp rises above and beyond the expected frequencies that the amp is supposed to be dealiing with. In other words, R.F.:eek:

    Since solid state amplifiers are often very capable of amplifying frequencies well beyond the audio range, it is not uncommon to find these amps going into oscillation due to R.F. signals in the immediate area, or some other outside interference that is above audio frequencies.

    As the Zoebel network decreases the reflected impedance with rising frequency, it provides more and more attenuation to the signal, the higher it gets. This helps to maintain stability in the amp circuit.

    There are a few different ways that people use to spell and pronounce "Zoebel", ( that is the way I learned), but you can find a lot of info about them on the web.
    One such source is: Zobel network - Wikipedia, the free encyclopedia

    I hope that this helps you, but be warned, if you are going to design crossover networks, make sure that you have an understanding wife, girlfriend, boss, etc. Once you start, it can become an obsession if you are an audiophile, or a serious music listener/experimenter.;)

    Have fun, and remember, you have been warned.:D

    Cheers,

    Andy
     
  3. Frosty_47

    Frosty_47 New Member

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    Ah they already know I am crazy so that should not shock their image of me :D

    Thanks a lot for your feedback. I plan to design active 3 way crossover system using multiple-order Low Pass, Bandpass, and High Pass filter. I don't mind designing a 10th order filter if necessary (I've done it before). Oh and all-pass filter for phase compensation. Each driver will have a separate amplifier :D
     
    Last edited: Jul 8, 2009
  4. dave

    Dave New Member

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  5. Soundguy

    Soundguy New Member

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    HI Fosty 47,

    Trust me, you don't need to design a crossover larger than 18dB / octave, and even then, only if you plan to use it for concert sound systems.
    For home use, 6-12dB/octave will be more than sufficient. The speaker drivers themselves provide some filtering due to their characteristics.

    For a 2-way system, you could pretty much run the woofer right off of the incoming line from the amp.
    For a 3-way system, you might want to be a little more daring.

    Again, as I mentioned before, the characteristics of the individual drivers will determine if you need to include a Zoebel network, amongst other components, into your crossover network.

    When I was designing speaker systems in the past, we used POLK audio speakers as one of our references for design, only becaude they were very popular, and we had to compete with them. Those speakers had very complicated crossovers, due to the multiple networks in place across and in series with their drivers, and all to compensate for the characteristics of the individual drivers, themselves.
    In my opinion, they still sounded like "C**P". ( And I can prove it, compared to our designs.) Anyway, speakers and the way they sound, are a very subjective subject, and as such, if you are designing a speaker system for yourself, for your home, for serious music listening as opposed to "Home Theatre" use, then, the system will have to be designed accordingly, which means, give us all of your deepest, darkest, listening room, and speaker system requirement details (Muahaaa!!!!!) ( evil laugh! )
    We will CREATE!!!!!!!!!!!!!!!

    Sorry, it's been a long day...

    I hope that "Audio Guru' might step in with his two pence worth, then again...

    Remember, the cabinets are required to be designed around the speaker drivers, and what you want them to do.

    The standard way to design speaker systems is to:

    1- Determine what the speaker system/cabinet is to be used for, that will tell you what kind of SPL levels that you need to achieve. Is a home system, or a Professional installation/PA system ?

    2- What frequencies are you trying to cover with the sysytem/drivers? Are you designing a Booming/Thumping system, or is this going to be a speaker system to listen to music at home, in private, undisturbed, etc.,
    Both types of speaker systems require a crossover of some type, and amplification of some type. ( Sometimes a lot of amplification )

    3.After that, it can get complicated:(
    Before I go any further, please tell me/us what exactly, you are trying to accomplish.

    Cheers,
    Andy
     
  6. audioguru

    audioguru Well-Known Member Most Helpful Member

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    My "two cents" is that most woofers have cone breakup at about 5khz and sound awful producing a shrieking resonance. They need to have a lowpass filter.
     
  7. Hero999

    Hero999 Banned

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    What does cone breakup mean?

    Will 5kHz kill the woofer?
     
  8. audioguru

    audioguru Well-Known Member Most Helpful Member

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    Cone breakup means that the entire cone is no longer a piston and the frequency response has a peak at the frequency.

    Here is the frequency response of a cheap 8" woofer at The Parts Express. it has pretty bad cone breakup at about 2.2khz so it probably cannot be used in a two-way because if the peak is rolled-off then no tweeter can go down as low as about 1khz to mate with it.
     

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  9. Hero999

    Hero999 Banned

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    Is that because the outside of the cone can't keep up with the inside?
     
  10. Frosty_47

    Frosty_47 New Member

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    Honestly I still don't know what I can do with a 3 way speaker system...
    Perhaps make 6 3 way speakers and turn them into 6 channel surround system for playing games. I wonder how scary F.E.A.R. would sound on such an audio system :rolleyes:

    Other use is for classical music listening. Yes I enjoy Vivaldi, Bach, Beethoven and Rostropovich (musician not composer). So having a decent 2 piece loudspeaker system would be nice.
     
  11. MrAl

    MrAl Well-Known Member Most Helpful Member

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    Hi, lets get physical :)


    How about using a "physical equalizer" ?

    I've often thought about trying this too actually.

    The idea is to think about what happens when the speaker can no longer follow
    the wave being fed to it. Basically, the speaker cone begins to move out of
    phase with the input signal.
    Having worked on many a control system myself, i thought about how to control
    this problem to make the speaker better follow the input signal for a wider input
    signal range.

    One way is to use a good quality microphone. The microphone provides feedback
    to a specially designed amplifier that is capable of wide dynamic range. When
    the speaker beings to drop off, the mic of course feeds back less signal, so the
    special amp provides more output until the mic is again satisfied. It's your basic
    negative feedback loop in action.

    The problem with the microphone idea is that although it might work to some
    degree, there is no guarantee that the mic will pick up sounds reliably either,
    especially if there is some background noise. That's where the 'physical'
    part comes in.
    The physical detector would be in the form of a reflective element that is
    very small and low mass, and it attached directly to the speaker cone.
    A light shines on the sensor (such as from laser or maybe even LED with optics)
    and the light that bounces off is detected with a light sensor such as a photodiode
    or maybe even a solar cell.
    The working would be such that as the cone moves in and out with the input signal,
    the light variation is picked up in the solar cell and that output is translated to
    cone position possibly with a compensation network. The electrical output of
    the network is then fed back to the specially designed amplifier and the amplifier
    adjusts the signal until the position of the cone exactly follows the input signal.

    Anyone care to try?
     
    Last edited: Jul 13, 2009
  12. Frosty_47

    Frosty_47 New Member

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    meby some other time :p

    But I like ur idea. This would be a good final project for some college students I think ;)
     
  13. Soundguy

    Soundguy New Member

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    According to Ted Nugent, that makes them perfect for "Country and Western" music.
     
  14. Soundguy

    Soundguy New Member

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  15. Soundguy

    Soundguy New Member

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    Ahhh, therein lies the problem.

    It has long been established that if you want a good sounding speaker system for "serious" music listening, get/create/buy a speaker system dedicated to that purpose.
    If you want a home theatre speaker system, or a gaming system, get a system dedicted to that purpose.
    The different applications demand different types of speaker systems with different criteria in order to sound their best in the different venues.

    Theatre type systems are typically dialogue and bass heavy, and as such do not have the strict driver and crossover requirements that music speakers typically have.

    With music dedicated speakers, subtleties are extremely important to the listening experience. This is why you hear people talking about "air" in speaker systems. That really is a valid concept and refers to the speakers and equipment being able to reproduce the harmonics in music that we humans, at least most of us, can detect in a listening experience without being aware of them, but they are important, none-the-less. Harmonics are what contribute greatly to the "timbre" of different musical instruments, as well as adding spaciousness to recordings. They are just one of the "cues" that our brains use to process sounds that we hear.

    There are good reasons that people suggest that you bring along different cd's with you when you go to shop for speakers. If the music on the cd's is varied enough, they can reveal some surprising characteristics in the speakers that you are auditioning. Some good, some not so much.
    Of course, if the cd's all sound the same, then you won't get much information from them.

    Cheers.
     

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