LS bass Mod - Neil

[Marvel]

Bruce,

I look forward to hearing your impressions of the La Scala bass reflex mod.

As you may know, it was my original plan to make boxes for
under the La Scalas to provide the additional volume and to locate the
ports. I planned to make them rough out of MDF just to see if I liked
the results. After realizing that elevating the schools La Scalas
another 10" from their already high perches atop the instrument lockers
was unacceptable, I decided to seal the tops, open the bass bins at the
tops and port out the backs of the tops.

If I were doing it for home use, Id do the boxes underneath. Its much more easily reversible. Feel free to contact me for info and/or encouragement. The mods to the balancing networks seemed perfect. I can help you avoid re-inventing the wheel.

Neil,
I seriously contemplated doing the boxes underneath, and they would be easier to implement. But I will go the other route. I have been thinking over (for months now) just how I want to seal the back, open the doghouse, etc. I have some ideas, and some would take me a bit more work, but make it far easier if I want to reverse it.

I just don't want these things taller than they already are. The mid/tweeter horns are just the right height for me in my space (maybe I slouch too much).

Problem one, is that the K55 comes further back than I would like. I want the back to be flush. Still can be done, but a little more work. On and on it goes. I am trying to get a room rearranged tonight, and I still want to finish wiring up the new crossovers so I can get John Albright's loaners back to him. It never stops. We are at our busiest at school right now, getting everything ready for the fall semester. One of my co-workers just resigned to take a job at another school. He has one week left. Just too much to do, at work and home, so the mods will be in another month or so. I am sure I will be in touch.

Bruce

[DizRotus]

Bruce, <?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

I agree with you about the undesirability of making them taller. But, even a single rough box for underneath would let you determine if you like the modification before doing any surgery on your La Scala cabinets. A test box could be made in such a way that it could be disassembled and you could then incorporate the ported side into the final modification.

If you like the change, then go at the cabinets to seal the tops and port out the back of the tops. What you describe could be done, but it would be much more difficult than attaching a ported panel to the back of the top. I had the luxury of modifying La Scalas that had been ridden hard and put away wet and that are not seen from the backs.

The simplest way to have the ports clear the K55 and the network was to mount the network upside down to the underside of the top. Then the ports were located so as to pass next to the K55 and beneath the inverted network.

So far the band director is thrilled with the modified La Scalas. Some of the perceived improvement to the bass is probably attributable to the newly added braces to the bass bins. Nonetheless, he likes how they sound and so do I. Id do it to any La Scalas that I owned.

[DizRotus]

The fact that there is a lip on the top of the opening necessitated the inversion of the networks. If the networks were left in the original location, one port tube would interfere with the network. The lip prevented raising the tube to clear the network. Inverting the network behind the lip left enough space in the opening for the two 4" x 7" port tubes to straddle the K55.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

Try it . . . youll like it.

[Guest " "]

I am going to try a variation of this using a passive radiator.

A 12 inch passive will fit in the dog house opening.

Will probally fold it out.

The specs and computer modeling for 3.0 cubic feet (Jubilee projections for 2.98 cubic feet) (this will require an additional bass bin) look very promising.

May also do a push-pull senerio using a K-42 driver.

Impedance will be managed using an atlas AF140 autoformer.

[Marvel]

Neil,

I'll send an email to you one of these days and explain what I am going to try. Thanks for the input. I've been pushing Dennis' mod for so long, without even having a pair of LS. Now that I have them there is certainly the fear of mesing them up, but I am game for most anything.

Been working on the new crossovers tonight (Modified DH2). Almost no parts, which I like since I'm using the Moondogs now.

Bruce

[Klewless]

Of all the bass mods for the LaScala that I have seen, this one makes the most sense to me!

I may give it a try. My idea would be to put it under the LaScala because:

1. You would not have to raise it more than 3 to 4 inches,

2. I think the "sealed compression chamber" is better preserved with the passive rather than the port (ala the Jubilee).

3. The new "base" could be made to vent all around, the front, or even the back.

Just my thoughts on the subject.

[Duke Spinner]

. I think the "sealed compression chamber" is better preserved with the passive rather than the port (ala the Jubilee)..

What should the weight of the PR be ...??

[Guest " "]

Duke Spinner

How about using this one, add an additional cubic foot to the internal volume via a bass bin which should bring it to 3.0 cubic feet, assume tuning frequency target of 45hz, no additional weight needs to be added to the radiator.

http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=295-192

[Duke Spinner]

have you modeled that ..??

curious as to the increase in extension you would get using the PR

[Marvel]

Of all the bass mods for the LaScala that I have seen, this one makes the most sense to me!

I may give it a try. My idea would be to put it under the LaScala because:

2. I think the "sealed compression chamber" is better preserved with the passive rather than the port (ala the Jubilee).

A passive radiator would act just like a port so there is really no difference.

[DrWho]

A passive radiator would act just like a port so there is really no difference.

In the linear ideal world []

Still in the ideal, but nonlinear world the PR will have about 1dB less

output and a slightly sharper Q (if the mass corresponds to the

equivalent mass of air needed for that surface area). The Q can be

modified slightly be changing the suspension compliance to mass ratios

(which also has an effect on the efficiency).

And getting out of the linear world...well, all heck breaks loose []

Btw, there are definetly some phase considerations that come into play

when the port/pr isn't on the same baffle as the driver. Probably not a

big deal at these frequencies, but definetly something to think about.

As far as port versus pr, less high frequency information is going to

sneak through the PR than the port. (If you think about it, even 4th

order bandpass designs will output sound through the port well above

the tuning frequency - which is why PR's can't be used in those

applications).

[Guest " "]

Duke Spinner

The DrWho just posted a graph in the below thread.

http://forums.klipsch.com/forums/1/770417/ShowThread.aspx#770417

[Klewless]

My comment #2 was based on my recollection of Paul Klipsch's writeup on the development of the Cornwall.

I think he said that below the point where the port is operating, the driver becomes unloaded, resulting in pure distortion if the driver is left to limit itself. I also think this is the reason Speakerlab recommended a high pass filter in front of the amp. (I do not know what order of system this becomes.).

So I concluded that the passive, while equivalent to the vented port at it's intended frequency, also helps to limit the driver below that point.

I could be wrong here because it has been many years since I read PWK's paper. But it does seem reasonable especially if one believes the original chamber is a bit small to begin with.

[DrWho]

The same unloading effect also happens with passive radiator systems. And it's probably a bit extreme to claim that any sound below the tuning point is 100% distortion [] But a high-pass filter to control cone-excursion below the tuning point is absolutely a good approach in situations cone excursion is an issue.

[Lascaladan]

Guys, there are many mods you can do to get rid of the vibrations and resonances in the side walls of the woofs. If you Dynamat the outsides, get good sheets of wood/lumber from the HD(they will cut to your specs), and screw these into the sides, you will not believe how much cleaner and extended the Lascalas are. You can "over damp" the situation, making them less musical. I used, besides my ears, a Crown mike, against the side walls, and, analyzer, to minimize this problem with the Lascala. It makes a larger difference when the spls are high. This is not easy to reverse. You will be making the boxes "swiss cheese". I have accomplished what I believe Klipsch has done by using the thicker wood on the newer Lascalas. Only halfway from front to back, on the side walls. I was willing to do this to my LS because I have done this to many other speakers I have owned in the passed, with similar results.

[Marvel]

I am barely flexing their muscle at 80-85db. I would probably put a lateral brace in the mouth of the horn before adding another layer to the outside, but that's just me.

[DrWho]

At the Hope Pilgrimage I made it a point to ask about the flexing of

the bass bins - seeing it as a huge design flaw. It turns out that the

initial runs of the lascala and prototypes had no flexing problems

whatsoever. It was just on the brink of being a problem which they

didn't know about until they started dealing with variations in the

wood quality from their wood vendor. And I have no doubt that as the

wood ages in some of these speakers that it becomes an increasing problem.

Using cross-bracing isn't the best approach because it just moves the

frequency of resonation up an octave which still isn't outside the pass

band of the speaker. In fact, two cross-braces probably don't get it

out of the passband either. Doubling up the panel thickness will

actually lower the resonation point below the passband of the speaker,

while also making it much more rigid. At least I think that's how the math works out.

For what it's worth, the newer lascalas have way more bass than the old

ones - regardless of what marketing driven specs claim.

[Marvel]

Wood gets harder as it ages...

I don't want to add another layer on the outside. I would still like to be able to move them someday. Plus, I am looking to have them appear as standard.

Mike,

All things being equal (which they really never are), if I wanted to replace the two 4"dia x 7" ports with a single shelf type port (like the Cornwall), would it be the volume of the two ports I would be aiming for?

I am looking at 15 inches wide by about 13 inches deep. I think one inch would be pretty close.

Bruce

[DrWho]

No, the volume of a port does not determine its tuning - it's the ratio of the length versus the cross-sectional area. I would recommend something along the lines of 13" wide by 2" tall by 7" long if you were going with a slot port. A 15"x1" mouth would need to be 3.5" long.

[djk]

" I would recommend something along the lines of 13" wide by 2" tall by 7" long if you were going with a slot port. A 15"x1" mouth would need to be 3.5" long."

Did you run that through a program?

The vented LaScala as I described runs to about 4.8 cu ft net. Two 4" ID by 7.25" long ports gives an Fb=35hz.

A 15" by 1" port would need to be about 5" long for a 35hz Fb.

A 13" by 2" port would need to be about 8-7/8" long for a 35hz Fb.

A 15" by 2" port would need to be about 10-5/8" long for a 35hz Fb

It helps to have a good software port model, the long narrow aspect ratio requires a different length than the same area in a square or round port. It also helps to start out a bit long, and trim it shorter to reach your target Fb.