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A Different Approach to Series Networks

I have mentioned the Look Back Impedance concept before, and written an AES paper on the subject (AES preprint #4784, "Loudspeaker Crossover Look Back Impedance", 105th Convention, Sept. 1998).

There are some obvious, and not so obvious things to do to try and keep look back impedance (LBI) incorporated into a given crossover design.  One of those things is to try and have a shunt component across the driver terminals.

The easiest way to do this is to use 2nd order parallel networks, this has the shunt coil directly across the tweeter, and minimizes the out of passband LBI.

Another easy way to do this is to use a first order series topology, as shown in Fig. A.





However, if you need some tweeter level padding, or want to use a notch filter on the woofer, these things compromise the LBI for the respective driver involved on the series network.  Same goes for use of a classic L-pad in between the tweeter and the network for a 2nd order parallel crossover topology.

On a second order parallel network, you can place any tweeter padding before the actual network, and this keeps the shunt coil directly across the tweeter.  This kind of topology is described in my LBI paper, and has been described verbally at various web sites by me in the past as well.

For classic parallel odd order networks, and even order series networks, you can use a Zobel across the woofer.  This is not as effective as a full shunt component, but it helps considerably.  The same partial help arises from use of an L-pad on the above mentioned networks for the tweeter level padding.

What about the classic 1st order series network?  If you need to level pad the tweeter, or to EQ the woofer, then the excellent LBI of the 1st order series network is compromised, and you lose some of the out of passband protection this affords, when working with such a mild roll-off.

Keeping the LBI concept in mind, I developed an alternative method to provide tweeter padding, and/or driver EQ, without compromising the LBI.
I pointed a few of these out recently in a post to John K., see:

http://www.madisound.com/cgi-bin/discuss.cgi?read=208000

In this post, I spoke of placing any level padding or driver EQ in between the various circuit elements of a second order series, or a 2nd order parallel with a notch filter, keeping the shunt element directly across the driver.

Taking this concept to it's logical conclusion, one can also do this for the 1st order series network, and maintain the shunt element directly across the driver.

In order to fully explain the concept AND the additional refinements, I will proceed in a step-by-step manner, and line out exactly what I am doing.

I have already referred to the classic 1st order series network in Fig. A.  Primary crossover components are C1 and L1.
This is often used with a Zobel across the woofer (or should be, given today's modern high inductance woofers), as in Fig. B.  Woofer Zobel components are R1 and C2.






I want to emphasize that with most woofers, use of a Zobel is mandatory on even a classic/basic 1st order series network.  The woofer inductance is usually high enough to significantly affect the high frequencies that get to the tweeter.  This is because the woofer is carrying a portion of the HF's.  It may seem that the shunt cap would account for this, but the result of many measurements and listening tests tells me that such is not the case.

Use of conjugates on the tweeter are not nearly as critical, but will provide that last bit of refinement and help ever so slightly with the power handling.  Use of a Zobel on the tweeter is completely optional, and will depend on the individual tweeter more than anything else.

 On to page 2 of LBI Series web pages



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