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Photo of La Scala II with speaker grill removed


Tizman

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27 minutes ago, moray james said:

pull the tweeter and mount it onto a piece of wood then slide the tweeter back so that you align the diaphragm of the tweeter to the diaphragm of the mid horn that will make for a significant improvement in sound quality. This will be a much more significant difference that what you would hear between a flush mounted mid horn and a back mounted mid horn.

What exactly would i hear if i tried this with my Klipschorns?   I don't have anything to mount on, could i just rest it on the top part of the speaker or will the sound be obstructed? How high does the tweeter need to be mounted over the top plate of the Khorn? Thanks.

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1 hour ago, moray james said:

pull the tweeter and mount it onto a piece of wood then slide the tweeter back so that you align the diaphragm of the tweeter to the diaphragm of the mid horn that will make for a significant improvement in sound quality. 

Yes.  A very good idea, and not a permanent thing either.

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6 hours ago, Maximus89 said:

What exactly would i hear if i tried this with my Klipschorns?   I don't have anything to mount on, could i just rest it on the top part of the speaker or will the sound be obstructed? How high does the tweeter need to be mounted over the top plate of the Khorn? Thanks.

What you will hear will be the equivalent of focusing a projector lens. The tweeter does not have to be raised from the top of the cabinet at all. You can experiment with horizontal and vertical orientation of the tweeter. I would expect for you to prefer the vertical orientation as that is what the horn was designed for. This is not a subtle improvement it is a very dramatic improvement.

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5 minutes ago, Maximus89 said:

Interesting! I will give this a try tomorrow!


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you can ask Bruce (Marvel) to send you a photo of his LaScala with the tweeters up on top. As you approach alignment the stage and image will become more and more focused. I usually use the top plate of the driver magnet assembly as my guide and match up the centre of each of the drivers top plates. That is a great starting point and you can play fore and aft to find the sharpest focus.

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If I were to do something like that I'd surely cover the surface in front of the tweeter side-to-side and all the way forward with some fairly thick and loose-matted felt or equivalent, overhanging the edges ~1/8 inch to subdue reflections and especially diffractions from the edges.  And I'd expect the best "focus" to be found with the tweeter diaphragm slight behind that of the midrange driver due to crossover phase shift.

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On 3/2/2019 at 10:43 PM, Tizman said:

...I have been doing some reading lately about horn mouth termination and it’s impact on measured horn performance and subjective listening.  I own a pair of 1976 La Scalas, and have wondered why they have a straight cutout for the midrange and tweeter horn instead of having the transition be beveled to follow the angle of the horn’s mouth.  Given the wavelengths in each case, this would seem to be a bigger issue for the tweeter horn, but is not ideal in either case. This is based upon what I have been reading online.

 

I was wondering if the LS II corrected this.  It looks like it is made in the same way as the older LS.  The transition from horn mouth to the room is straight, and thereby interupts the expansion of the horn with a constriction and a small lip at a point where it would be best to have either a beveled motorboard matching the line of the horn, or this bevel in addition to a rounded termination of some sort.  

 

They say that a little information can be a bad thing, and I am a relative newbie with respect to horn speakers, so I may be off base about this.

 

It's not a bad thing if if leads to more interest and perhaps a little data...

 

Below you will see a spectrogram (horizontal axis is time in microseconds/milliseconds--µs, ms.  The vertical axis is frequency in Hertz--Hz, in this case, a CT120 tweeter on a 1979 Cornwall:

 

999835723_CornwallTweeter-MidrangeTimeResponse.thumb.jpg.950fc1ae82778be3eada80fe7b393a56.jpg

 

Now this plot probably doesn't immediately capture your attention until a little explanation is offered.  Note the straight vertical energy spike centered at 0 µs.  This is the CT120 tweeter's (directly radiated) initial pulse.  It's well formed and its vertical appearance indicates near perfect impulse response, which your ears will be able to hear as crisp highs.

 

But that's not the end of the story.  There's another echo at about 750 µs behind, about at 8 to 10 dB down from the initial pulse.  You may think that's too short of a time span for the ear to hear, however at 6 kHz, that's about 4 1/2 wavelengths later in time.  This is audible during music transients like cymbals and bells, etc..  We'll talk about this further in a moment.

 

You can also see some energy appearing before that major reflection at 250-350 µs on a slight diagonal between the 0 µs and 500 µs grid marks.  That corresponds to something happening at about 3.4 inches from the acoustic center of the tweeter.  That is most likely mouth bounce of the tweeter.  That's something that is clearly controlled by the mounting of the tweeter to the cabinet.  If you remove the tweeter from the cabinet and set it on top (still connected to the crossover), and take another acoustic measurement, you should see that diagonal reflection be attenuated quite a bit, making the reproduction of transients a bit more crisp.  This is the time domain behavior of the tweeter that's influenced by how it's mounted to the baffle, and you can see its "mouth bounce" energy re-radiating at a later time than the initial pulse.

 

Now, back to that 750 µs reflection: that is mostly the midrange horn/driver (K-600/K55V) producing acoustic energy well above its nominal 4.9 kHz crossover frequency, in parallel to the tweeter.  That's because there is no low-pass filter on the midrange driver inside the crossover of this particular version of the Cornwall (which is also true of the La Scala and Khorn crossovers from that time), so the midrange driver is producing a secondary pulse of energy several wavelengths behind the initial pulse, at only 8-10 dB lower output level than the pulse from the tweeter.  Can you hear that?  Yes.  The question is: what does it sound like?  (More on that subject perhaps in a later post--but for now, consider what it would sound like if there were no pulse at 750 µs behind the tweeter's pulse...)

 

Okay, so shifting gears to the phase domain, let's look at a plot of relative phase vs. frequency of the K-55/K-600 horn vs. something that has nearly perfect phase response:

 

530539269_CornwallTweeter-MidrangeTimeResponsevs.AMT-1.thumb.jpg.3f53ee475c05c7152e72fc5d1560bd8e.jpg

 

Note the huge negative growth in phase vs. frequency above about 600-1000 Hz (the crossover frequency interference band between the Cornwall's midrange and woofer), reaching a maximum negative phase at about 6500 Hz, where the CT-120 tweeter kicks in.  This is the time misalignment of the tweeter's acoustic center to the midrange's acoustic center.  You can easily correct this by (in this particular case) moving the tweeter back 10.1 inches, so that it is time aligned with the midrange horn.  All the Klipsch Heritage tweeter/midrange models have this mismatch in time alignment, which is easily correctable by moving the tweeter backward. In the case of the K-400 horn, you will need to move it back about 15 inches (it's different for each midrange horn/driver because of the differences in midrange horn depths in the Khorn/La Scala, Belle, Cornwall, Heresy, Forte, Chorus, etc.).  The easiest way to set the tweeter in the correct position is to move it back to where it's about aligned to the midrange driver, then move it back and forth until the resulting soundstage of the loudspeaker dramatically increases/deepens.

 

So to summarize, your interest in tweeter mouth termination has led to more revelations of performance areas that can be improved easily.  Your original concerns about the mouth clamping to the front baffle is probably the least of the issues seen, but it has led to opportunities for improvement in sound that you probably weren't expecting--areas that you can control to a great degree.

 

Chris

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Yes...

1140929807_LSIIStockImpulseResponsespectrogram.thumb.jpg.9a8ff2f685a88a72e1a31c7e83041b40.jpg

 

 

Edited by Chris A
Updated the spectrogram plot with a better one...
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By the way, by my calculations the LSII tweeter needs to be moved back 23.38 inches (59.3 cm--to add the 1.72 ms delay it needs) from its stock position in order to time align it to the midrange horn/driver.  The reason for the increased distance is due partially to the higher order crossover filters used between the tweeter and midrange in the LSII than those used the Cornwall I, which I showed above. 

 

Chris

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Here's an SPL+phase plot of the same measurement of the LSII that I posted above:

 

100079139_LSIIStockSPLandPhaseResponse.thumb.jpg.f1d772a64638aab052450bb9150c27ed.jpg

 

The phase curve is zoomed all the way out within REW.

 

The impulse spectrogram above looks much cleaner than the Cornwall due to the added low pass filter and higher order crossover filters used, but the phase curve shows about twice as much phase growth (end to end) as the 1979 Cornwall.  That should perhaps encourage some of the LSII owners here to try out the tweeter movement thing to hear how it improves their soundstage/imaging and timbre.  You have to admit, it's pretty inexpensive (and easily reversible) to try it out.

 

Chris

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I may have to try this  time alignment once we finally get moved into our new house and I get things set up.  Since my Belle (clones) have separate tops and bottoms, I can tinker around with the K-500/A-55-G and CT-120 on a rudimentary mount and hear how they perform.  I know first hand it works and makes a huge difference.

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10 hours ago, moray james said:

Bruce (Marvel) did a very nice job on his LaScala but I cannot find a picture.

Here's the pix of Bruce's left La Scala with aligned tweeter.  It takes a small piece of wood with a cutout for the tweeter--suitably stained for appearance's sake.  You don't have to make it as high as this one, but the suggestion to put down a little absorption material on top of the cabinet top is a good one.  The lower that you can make it and still see the full tweeter when sitting--the better...

 

APT_baffle_04.jpg

 

Chris

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That's nice! What tweeter is that and will it work with an AA?

I've seen photos of this time aligned tweeter being mentioned, but i've never seen any placed vertically as @moray james mentioned.  I have however seen custom mid horn options with built in tweeter cutout placed vertically and iirc i've seen the old original KHorns have the tweeter mounted inside the mid horn vertically.

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9 minutes ago, Maximus89 said:

I've seen photos of this time aligned tweeter being mentioned, but i've never seen any placed vertically as @moray james mentioned.  I have however seen custom mid horn options with built in tweeter cutout placed vertically and iirc i've seen the old original KHorns have the tweeter mounted inside the mid horn vertically.

 

It turns out that the old K-77 from ElectroVoice (the "T-35A", now discontinued) has a wider horizontal beamwidth oriented with the long horn dimension aimed vertically than horizontally above 5 kHz, which is about the crossover frequency used on most Klipsch Heritage models:

 

1528799528_K-77(EVT-35A)tweeterdirectivty.JPG.a521bc96f390241d4825c8db79f0ccb8.JPG

 

I don't know the beamwidth of the current tweeters used by Klipsch in the LSII, et al.

 

Chris

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