Center speaker.......Xover filters ?

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JKnPA
JKnPA Posts: 41
edited October 2006 in Troubleshooting
I was interested in the Xover filters used in my CS-1. This is the link to the schematic.
http://www.polkaudio.com/forums/attachment.php?attachmentid=18796&d=1155785350
The specs. for the low/high pass filters are listed on the far right of the sheet. Unless I am reading the specs. wrong, shouldn't the 3db. points cross near the center of the vocal range(300-3khz.). Why is the low pass only down 2db. @ 10khz.??????
I see 4ohms listed in the specs. Are these 4ohm or 8ohm speakers?
I calculated the low pass filter to be -3db. @ 1.5khz. and the
high pass at 6kz., but this doesn't seem correct either!
Can anyone tell me or calculate the -3db. points for this Xover filter?

Thanks....
Denon 1906AVR
Polk... M40s, Cs1
JVC DVD player XV-SA600
Sharp LC-37D6U
Post edited by JKnPA on
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    The crossover frequency will be quite different than what you calculated because the drivers when connected to the network do not behave like a resistor.Voice coil inductance and variations in the drivers freqency response usually make standard crossover values calculated using an assumed 4 or 8 ohm resistor far from ideal.

    yes the sample speaker appears to be 4 ohm.
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  • JKnPA
    JKnPA Posts: 41
    edited October 2006
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    The nominal impedance of the driver is either 4 or 8 ohms, depending on the 'flat portion of the driver curve' used. I thought the nominal impedance was 8ohms, but the schematic cited 4 ohms. I calculated the 3db. points using both 4 & 8 ohms; it was basically the same due to the design of the filters!
    Can someone explain the data on the schematic & the calculation of the 3db. Xover points.
    I thought the chart would look similar to this...@ Figure 2.4.....
    http://sound.westhost.com/biamp-vs-passive.htm

    GV#27.................. thanks for the reply!
    JK
    Denon 1906AVR
    Polk... M40s, Cs1
    JVC DVD player XV-SA600
    Sharp LC-37D6U
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    Hi JK is it the specs stated in section 8 of the schematic that you are wondering about?
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  • JKnPA
    JKnPA Posts: 41
    edited October 2006
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    Yes .......... specifications stated in section 8.
    What is that ?
    Denon 1906AVR
    Polk... M40s, Cs1
    JVC DVD player XV-SA600
    Sharp LC-37D6U
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    OK I will give it shot.It looks like they have a reference standard xover that they compare every production unit too.For the tests they use a 4ohm resistor instead of a driver.The -3 points are not specified but the tolerances given are what the tested xovers response should be in direct comparison to the reference standard xover in the stated frequency ranges or it fails the test.
    Does that help?
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  • JKnPA
    JKnPA Posts: 41
    edited October 2006
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    Perhaps they do have some sort of standard...........
    Looking at this Polk link......... the CS-1 has anominal impedance of 8 ohms and is a 2nd order filter. Which means it is a low/high pass filter with a 6db/octave slope, the X-over point being 2.3khz. .......... see link.
    http://www.polkaudio.com/homeaudio/specs/cs1/
    That makes sense, because the X-over point ( I believe) s/b near the center of the vocal range(300-3.5kz.), since it is a center speaker.
    When I calcculate the 'XL' of L1(0.25mh) at 2.3khz, I get an impedance of 3.5ohms, which is in parallel with the 8ohm impedance of the speaker. The top leg of the high pass filter( R1 and C1) look like 9ohms. So it looks like the 'transfer ratio' is 2.4ohms/ 11.2ohms, which is about 21% not 50%. Did I lose you?
    ..... 0.5 .... 8.5..................
    . .
    . . 8ohms(tweeter)
    . .
    3.5 .
    ......................................... .
    * Calculate the equivalent circuit( high pass) at 2.3khz and verify the impedances. I get the above circuit, if you follow! The top (series) impedance s/b the same as the parallel impedance at 3db. point(50%)
    Jk .
    Denon 1906AVR
    Polk... M40s, Cs1
    JVC DVD player XV-SA600
    Sharp LC-37D6U
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    Both woofers and the tweeter appear to be 4 ohm units but the series connection of the woofers give the system its 8ohm nominal rating.

    A 2nd order filter will actually give you a 12db per octave attenuation in the drivers stopband not 6db.

    Because it improves the off axis response most speaker designers like to move the xover to as low a frequency as possible (usually in the 1.8-2.5k range)without putting extreme demands on and risking the long term reliability of the tweeter.

    Without knowing the actual frequency response and impedance curve of the driver you cannot accurately predict the response of the crossover /driver combination.That can only be had with measurements and computer modelling.
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  • JKnPA
    JKnPA Posts: 41
    edited October 2006
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    You are correct..... 2nd order filter is 12db./octave not 6db( typo).
    Polk states the nominal impedance is 8 ohms..........
    http://www.polkaudio.com/homeaudio/specs/cs1/
    I thought we could calculate the circuit response with the given components and the 'nominal impedance' of the drivers.
    I thought that was the intention of stating the driver impedance!

    Thanks...
    JK
    Denon 1906AVR
    Polk... M40s, Cs1
    JVC DVD player XV-SA600
    Sharp LC-37D6U
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    They state the impedance as an indicator of the load that the amplifier will see.

    Other factors other than just impedance namely the frequency response of the driver in the xover region also has to be considered.
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  • JKnPA
    JKnPA Posts: 41
    edited October 2006
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    Yes............. exactly. The amp is driving the X-over filters & the driver.
    I understood the 'nominal impedance' of the driver was taken from the flat section of the driver impedance curve. I use Fig. 3.2 (woofer) and Fig. 3.5(tweeter) as examples of typical speaker drivers. They use the flat portion( freq. range) with the X-over design to try maintain a constant load as seen by the amplier.
    This is a great site............ IMHO.............
    http://sound.westhost.com/lr-passive.htm

    JK.
    Denon 1906AVR
    Polk... M40s, Cs1
    JVC DVD player XV-SA600
    Sharp LC-37D6U
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  • Systems
    Systems Posts: 14,873
    edited October 2006
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    The site you linked to is excellent and is known to be one of the best audio related sites on the web.

    Fred
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