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Re: Altec 806A - First Order Crossover

Posted by Tom Dawson on May 24, 2005 at 08:45:50

In Reply to: Re: Altec 806A - First Order Crossover posted by Don M on May 23, 2005 at 18:27:51:

Hi -

The series element would be a similar value to that of a first order xover (capacitor in this case), but with a slightly increased value to compensate for the initial notch attenuation. What would be added is a series LC network across the driver vc (as you probably suspected) to produce the actual null.

Usually, I try to keep the null an octave to two octaves below the nominal -3db point - there's no problem with going a bit closer except that with this kind of filter, the response will recover to a somewhat higher amplitude in the 100 - 150 hz range (perhaps going slightly above -20 db depending on notch width) before going into its final rolloff. Even with a notch at 400 instead of 479, the notch should still contribute 12-20 db attenuation over a straight first order at 479 hz.

I use this type of xover for all the 2 way designs I've done for a while because it acts like a first order near xover but gives a stopband rolloff beyond half an octave into the stopband like a second to fourth order xover (depending on exact notch characteristics). It doesn't perfectly emulate a first order at Fxover, but the db or two or 10 or 20 degrees of phase angle deviation
are minor compared to its advantages, IMO.

When I was designing the horn xover for the Iron Lawbreakers running with the 2220J woofer, where I have a 288-16G driving a flat front horn with a nominal 400hz cutoff (xover at 550 hz), I was using this type of filter (with series 20 ohm attenuating resistor) with 10uF series and 100uF/10mH shunt (centered around 150 hz), as well as a mild HF boost network. Playing at an average level of significantly less than a watt into the 288G, I experimentally tried removing the shunt networks from both channels, effectively converting the xover to a straight first order with a 3db point of maybe 450hz. The change in frequency response and driver integration at the listening seat was minor, but there was a dramatic increase in audible distortion and a significant deterioration in image stability from the 288G's that had me rushing to put the shunt network back in.

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Kimber Kable

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