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La Scala Woes


Desert Noises
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On 12/8/2021 at 11:04 PM, Deang said:

Sure they do. 

 

You would know better than me that's for sure.

 

I had an old drawing of an A network with the 2uF crossed out and 4uF written in for a value. I was always under the impression this was kinda the original mod when upgrading the tweeter with the eminence driver found in CT125 and now the CT120, which will handle lower frequencies better so the larger cap drops the filter to lower cutoff. As a bonus the next step was to clean up the mid-horn by lowering the filter down to 4500 since the tweeter is now covering this range and does a better job at it so the inductor was added.

 

It's been so long and the internet now makes everything so much easier to keep track of. I know now just about everyone adds the inductor and of course Crites sells them with the inductor.

 

Have you heard the type A with just a larger value tweeter cap and no mid-horn inductor?

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5 hours ago, Deang said:

 

Nope.

 

As for the schematic, it's been too long ago. Just because I don't remember one with no bandpass coil doesn't mean it doesn't exist. 

 

It was so long ago I can't remember fully either. I do remember some guys putting larger tweeters on top and time aligning a little better, this was the first I heard about increasing the 2uF cap to let lower frequencies into the tweeter since the K-77 is pushed up pretty high at 6kHz. From memory with just a broader bandwidth on the bigger tweeters the sound was an improvement. Kinda like why we like the less steep, more "blended" simpler A and AA crossovers. They say music lives in the mid-range and now the tweeter and mid-horn are presenting the upper mid-range frequencies in a more blended fashion between two drivers. I think Bob was the first to add the inductor and drop the mid-horn down after doing measurments of it and seeing higher distortion up there from it and since the tweeter is passing down near 4kHz now the speaker can be cleaned up by getting rid of the distortions from the mid-horn in these upper mid-frequencies. But some of us like non-linear distortion as it sums to even harmonics.

 

I wish Bob was around to make a call and ask him. I don't know his son that well but Bob would chew my ear off back in the day.

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When it comes to their business, there isn't much that Michael doesn't know about.

 

No one thought much about lowering the crossover point until Roy did it in the AK-4, but he mostly did it to even out the power response and to not force the PD5 to work in an area that it really wasn't meant for. 

 

6000 to 4500 doesn't sound like much, but you're moving 25% of the sound to the tweeter. In a low order filter, well, all I can say is I hope you like the sound of the tweeter you're using, because you are going to hear a lot of it. It's a drastic shift in sonic signature, and I don't think it coalesces all that well. I tried several things early on and never did get to the point of where I cared for it. 

 

18 hours ago, captainbeefheart said:

But some of us like non-linear distortion as it sums to even harmonics.

 

An interesting observation. I've thought about this much over the years.

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6 hours ago, Deang said:

An interesting observation. I've thought about this much over the years.

 

I too have put much thought into this.

 

If you look into the equations of fluid dynamics you will notice they are nonlinear, these equations are used for sound waves through liquid or gases. Because of this nonlinearity, sound waves are being distorted as they travel. For audio acoustics it's a gas form in the air and localized pressure changes, basically the waveform will travel faster during the high pressure portion of the waveform and slower during the low pressure part of the waveform, this is the nonlinearity. For a typical sine wave the peaks will be the high pressure portion and the troughs the low pressure portion. Because of this nonlinear distortion other frequencies are introduced shown by a Fourier analysis which are even harmonics, mainly second order harmonic distortion.

 

This is how our hearing has evolved, we obviously naturally hear in nature a single ended, nonlinear distorted wave form and this has always been natural to us. I 100% believe this is why people tend to prefer single ended amplifiers that produce nonlinear distortions and the subsequent even harmonics added. This is because it is natural to us, we are use to processing this type of information because that is how it is propagated in nature. Push pull amplifiers are more efficient and look great on paper BUT cancel even harmonics and so a Fourier analysis shows odd harmonic dominance which isn't present in nature and so can sound out of place, mostly fatiguing I presume because since it isn't natural our subconscious is analyzing and being worked harder to process the information. It's like doing a particular physical workout for millions of years and then changing to a different workout program, the next day those new muscles will be sore that haven't been worked out.

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Update:

I received a lot of good advice and ideas from you on this thread and I appreciate your feedback. I suspected my midrange to be a bit too hot. So, I ended up dropping my mids down to tap #3 on the auto transformer and replaced the 13uF capacitors with 6.8uF to attenuate the midrange an additional 3dB while maintaining that 400Hz crossover point. 
 

First impressions when I played some music were generally positive with a noted reduction in midrange “glare” and better tonal balance - perfect I would say, but that boxy “stuffy nose” bass sound was still there, obnoxious as ever. I temporarily reinforced the sidewalls of the bass horns with wood blocks and clamps to rule out resonance issues. That didn’t have any effect. I did a little more reading on here about speaker placement and interaction with rooms.

 

I came across something PWK wrote, recommending a 45 deg toe-in for the best stereo image. Looks to apply to both the La Scala and K-horn. So I tried it, ridiculous as it sounded to toe them in this extreme. Not expecting much, I listened and listened some more. They sound more natural and that dominant boxy sounding bass is reduced to a more acceptable level. I wonder if that boxy sound I’ve been hearing is related to that oft mentioned 148Hz peak inherent in the design of the La Scala.

 

To conclude, I can confidently say for sure that the mids belong on tap #3. And for my room, a 45 deg toe-in is the golden ticket to get rid of that nagging upper bass boxy sound. Not sure what science and acoustics are at play here but it works. I’m listening to Rossini overtures right now and I’m getting a great sonic image of the orchestra.

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When I first got my La Scalas. back in 2006 (!), I was initially disappointed by the "shoutiness" I was getting from the K400s and K-55s.  Being a mechanic, I went for a mechanical solution.  I stretched some stretchy black grille cloth across the inside of the cabinet, so the K400 was firing through it (having the early cabinet with removable tops made this much easier), but not the tweeters.  1 layer of cloth did nothing, 2 made a slight improvement, while 4 was too much, since it sounded muffled at that point.  3 layers of grille cloth sounded just right: not muffled and not shouty.  The upper mid/low treble details were still there, too.

 

Telling this will sometimes cause the "purists" on the Forum to declare me to be less than clever, to put it mildly, but this way of trimming down the midrange seemed to work well, with no downsides other than the typical negative comments.  As you found, lowering the squawker output really improves the sound of the La Scalas, and makes them pleasant to listen to, instead of harsh and annoying.  In my case, it also hides the insides of the horns, which  I like, since their castings were rough and the paint was very uneven.  It makes for a clean look.

 

As for toe-in, I first tried having them toed in around 40 degrees, but the sweet spot became very narrow, barely the width of my head, which made listening less than casual, requiring me to sit in a precise spot in a precise posture.  I've since reduced the toe-in, but now that I'm dealing with La Scala IIs, adjusting the toe-in is not as simple as it was with the older La Scalas, with their flat bottoms and little steel button feet.  Those speakers were easy to shift on the carpet, but they're now the Surround speakers and are behind the sofa and armchair.

 

The positions of the Surround La Scalas seem just fine, but their replacements as main Left and Right speakers are La Scala IIs, which don't have flat bottoms.  With the help of a friend, I was able to remove the rubber feet the previous owner had installed, but that didn't help, since the box-style bottoms of the LS2s have sunk into the carpet, making it a bit complicated to move them without damaging them, even with help.  One possible solution may be to install thick (3-6mm -- 1/8"-1/4") slippery plastic sheets under them, so I can continue to experiment in order to see/hear which amount of toe-in gives the best results in my room, without needing to call in a helper.  Of course, I'll need a helper or two to install the plastic sheets, since my 175-pound LS2s have ~40-pound Jubilee K-402 horns (including their bases and K-691 drivers) on top of them.  Something that should be simple isn't.

 

BTW, I use a laser level to check the "aim" of the speakers.  Holding it against the sides of the speakers, it's easy to see exactly where they're aimed, and easy to match it for both speakers.  I just place something on the sofa to represent me/my head (even a towel folded narrow is a usable equivalent) in its usual place, and then aim the speakers to focus on the sofa beside me, with varying degrees of narrowness of their targetting.  This gives easily repeatable positioning, and makes it simple to see that the toe-in of both speakers matches.

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23 hours ago, Desert Noises said:

Update:

I received a lot of good advice and ideas from you on this thread and I appreciate your feedback. I suspected my midrange to be a bit too hot. So, I ended up dropping my mids down to tap #3 on the auto transformer and replaced the 13uF capacitors with 6.8uF to attenuate the midrange an additional 3dB while maintaining that 400Hz crossover point. 
 

First impressions when I played some music were generally positive with a noted reduction in midrange “glare” and better tonal balance - perfect I would say, but that boxy “stuffy nose” bass sound was still there, obnoxious as ever. I temporarily reinforced the sidewalls of the bass horns with wood blocks and clamps to rule out resonance issues. That didn’t have any effect. I did a little more reading on here about speaker placement and interaction with rooms.

 

I came across something PWK wrote, recommending a 45 deg toe-in for the best stereo image. Looks to apply to both the La Scala and K-horn. So I tried it, ridiculous as it sounded to toe them in this extreme. Not expecting much, I listened and listened some more. They sound more natural and that dominant boxy sounding bass is reduced to a more acceptable level. I wonder if that boxy sound I’ve been hearing is related to that oft mentioned 148Hz peak inherent in the design of the La Scala.

 

To conclude, I can confidently say for sure that the mids belong on tap #3. And for my room, a 45 deg toe-in is the golden ticket to get rid of that nagging upper bass boxy sound. Not sure what science and acoustics are at play here but it works. I’m listening to Rossini overtures right now and I’m getting a great sonic image of the orchestra.

 

The way you describe your issue to me sounds more of a problem of the amplifier just not "happy" with the tough load these speakers can present. There are ways to fix this, one is adding a bunch of parts to the crossover to power correct the reactive loading, this is the approach ALK uses with his crossover design. The amplifier gets a much easier load to drive and you get less losses in the passive networks. The other solution of course is changing amplifiers until you find one that can drive difficult loads. I know I say this quite a lot, "many people don't know" but I repeat myself again many people don't know that when companies design your amplifier and test it they do so on a very easy to drive purely resistive load. Yes some well educated designers will make sure their amps can drive difficult loads, some claim they can but they are vague and typically are talking about one variable which is can deliver extra current with dips in the impedance plot but that's only one issue.

 

To rule this out of course I would try at least 3 different amplifiers with the speakers and if the problem persists than it is within the speakers or room acoustics and a room sweep with a calibrated mic and software would be the next step to see what's going on.

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1 hour ago, captainbeefheart said:

There are ways to fix this, one is adding a bunch of parts to the crossover to power correct the reactive loading, this is the approach ALK uses with his crossover design. The amplifier gets a much easier load to drive and you get less losses in the passive networks.

 

I don’t see how less current draw at higher frequencies creates a more difficult load for an amplifier, or how adding a swamping resistor, a dozen capacitors, and 20lbs. of copper reduces loss in the filter. 
 

Below is Kerry Geist’s response to the idea of adding the swamper. 
 

195187D8-B2DF-49FB-9CA5-1945537DD288.jpeg

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Has anyone figured out a passive correction for this 6db (?) peak at 148hz?

 

...bet it’s in the latest La Scala crossovers 

 

I think I remember a post with ALK and BEC talking about how the AK-3 crossover was changed to use two 100uf capacitors paralleled across the woofer to tame it. Waaaay back. In it ALK said something to the effect that the La Scala crossover would benefit from the change as well.

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1 hour ago, Deang said:

I don’t see how less current draw at higher frequencies creates a more difficult load for an amplifier, or how adding a swamping resistor, a dozen capacitors, and 20lbs. of copper reduces loss in the filter. 

 

I am not trying to say ALK networks are the way to go, I only mentioned his method of power correction for reactive loading.

 

You cannot think in terms of solely current and voltage, there is a time vector and loss angles with reactive loads. I am not even mentioning stability and phase margin of amplifiers, that is completely different problem but ideally you want to get your power factor as close to perfect as possible and you want to make the load as linear and as close to a pure resistance as possible.

 

In case you need a brief refresher on the subject;  https://electrical-engineering-portal.com/reactive-power-power-factor-correction

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29 minutes ago, captainbeefheart said:

In case you need a brief refresher on the subject;  https://electrical-engineering-portal.com/reactive-power-power-factor-correction

Reactive power and correction. 

 

I remember this from engineering school (in the prior century).  This is going way back and I don't remember the specifics, but don't you fix a reactive load by adding a capacitor?  Something to do with the waves not aligning and it taking a lot of power to overcome this. 

 

 

 

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