High Efficiency Speaker Asylum: Re: Is it wise to lower the lf cutoff ? by djk

The taper rate of a LaScala is 100hz,The mouth area is good for 125hz.Below this it is a big woofer in a small sealed box.If we plug the T/S parameters for the K33E into a box program we will see that the Qtc=.85, the Fc=82.5hz, and the F3=70.9hz . If we close in the back of the high frequency cabinet and open the woofer rear chamber up into this volume and fill with fiberglass we now have Qtc=.577, Fc=58.2hz, F3=73.6hz .Bessel=Qtc.577=D2 gives the best transient response and the least group delay of all the sealed boxes.At 30hz the Qtc=.577 has 4.5dB more output than the Qtc=.85 .Compared with sealed enclosures,the transient performance of the best vented enclosure is worse than the best sealed box enclosure.Since we have made great gains in performance some may elect to stop here.But please read on.The next step is to port it.The K33E is not the optimum woofer for this but it works pretty good.With the box size optimized for a D2 we can port the stock woofer to an SC4.The transient response of an SC4 while not as good as an SBB4(more on this later)is better than the more common QB3.Unequalized the F3=49hz and has 3dB more output at 30hz than the D2.The response curves are parallel with the D2 the only difference being the Fc being a half octave lower with the SC4 and the output being 3dB higher from 30~70hz.The output of the SC4 does not drop lower than the D2 until you go below 17hz.Again this may be a stopping point for some.But by adding a simple two pole high pass filter ahead of the power amp we can now have a C6 with a -3dB point of 31hz.Since this is the Fb of the system there is no increase in cone excursion or distortion.The filter consists of a cap an inductor and a pot.The pot allows adjustment at Fb of +/- 3dB.This is similar to being able to change the Qtc of a sealed box from .5~1.0 .If you think about it we have the choice between a D2,SC4,and a C6 in the same box by plugging the port and/or bypassing the eq.If you have a SET amp or simply want to get the most out of the LaScala you will want to upgrade the woofer to something with a lower Qts.The Klipsch K43E does the trick,as do the EV DL15W and the JBL 2205.The EV and JBL drivers require some minor network changes.The lower Qts drivers allow for an SBB4 alignment which has the best transient response of the vented alignments.With no eq they have 3dB more output at 30hz than the stock woofer.With eq we have a maximally flat B6 and the F3 of the system is 27~28hz.If using a solid state amp with the low Qts woofers a small resistor must be added in series with the driver to have the same Rg as the SET does.This mod can be backed out of a stock LaScala with no externally visible changes if you don't like it.No one has ever gone back to stock after hearing this mod.Paul Klipsch was violently opposed to the venting of horn speakers bassed on his experience with venting the K-horn.In retrospect it is easy to see why.The 12" Jensen field coil woofer he was using had an Fs of 60hz and a quick calculation of the vent area vs the Vb based on the photographs of this experiment looks like an Fb of ~80hz.I am sure this sounded horrible.Paul Wilbur Klipsch is a giant in audio.If I appear to see farther than PWK it is only because I am standing on his shoulders.