LS Bass Reflex Mod Passive EQ Details

[sootshe]

HI all,

Does anyone know the full detail for adding the passive EQ filter for the ported LS mod?

I'd really like to add this to get the max out of the bass reflex mod.

I need the values, or a circuit diagram & what does it connect to? Is it a seperate crossove or added to the existing? 

 

Here's a quote from the original explanation.

 

Posted August 25, 2009

Original text from Dennis:

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

[DizRotus]

No wonder Dennis, @djk, is now a man of few words, he used his allotment in that description. 

[Marvel]

Dennis has recommended using an EV Interface A EQ between the pre-power amps. They are an active EQ, giving about a 6db boost at 39Hz. Roger Russell from McIntosh says it was/is a 6-pole Butterworth. It also has some curves for the top end, but I think the bottom end is what you need. I think this would require a petty sharp cutoff on the bottom end, but the filter looks like it already does that.

 

Bruce

 

 

[sootshe]

10 hours ago, Marvel said:

Dennis has recommended using an EV Interface A EQ between the pre-power amps. They are an active EQ, giving about a 6db boost at 39Hz. Roger Russell from McIntosh says it was/is a 6-pole Butterworth. It also has some curves for the top end, but I think the bottom end is what you need. I think this would require a petty sharp cutoff on the bottom end, but the filter looks like it already does that.

 

Bruce

 

 

Thanks for the info Marvel.

I don't run an active set up & have an integrated & cannot seperate the pre & power. 

Just wondering if there is a schematic somewhere to build a passive that will do the job. 

 

[sootshe]

I just occurred to me.......why can't I use digital EQ to achieve the boost & cut needed?

I have digital parametric EQ with the Amarra software playback that I use & can easily set that for the boost or cut that is needed.

So....6db of boost at 39Hz  & is there any cut required to protect the woofer at low frequencies?

 

[Marvel]

I looks like a very sharp rolloff on the bottom end. It's a boost at the port tuning freq., but would need a subsonic filter of some kind. More from Dennis is attached.

 

 

Just some notes.pdf

[sootshe]

34 minutes ago, Marvel said:

I looks like a very sharp rolloff on the bottom end. It's a boost at the port tuning freq., but would need a subsonic filter of some kind. More from Dennis is attached.

 

 

Just some notes.pdf

Great stuff.....really appreciate your input.....that sort of info is invaluable. I will experiment with all these options when my cabs are complete. 

Will be posting a Belle build sometime in the new year & will post pics of my crossover build in the next few days (AP12-500 / AP15-6000 with custom components).

 

Thanks again.