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Heil Air motion transformer for Cornwalls


jwgorman

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3 hours ago, ClaudeJ1 said:

You can build a taller Cornwall with a smaller footprint. As long as you maintain the internal volume and port volume, it will sound the same. I would keep the port near the ground and the woofer as close to the Heil as possible. This would be an improvement all around. Especially if you used a Super Cornwall woofer.

 

Back in 2007 or so, member HarryO built some Cornwall volume cabs that he labeled Wallrus, as they were more a Chorus shape with Cornwall parts. He was temporarily in an apartmement across from the assisted living facility where his dad was living.

 

He had no room for Corns so he made 20 inch wide by 48 inch tall and depth to match the Corns volume. I used to have pics of them someplace. They aren't on the forums...

 

Might be too tall to put the Heils on, but adjust to taste.

 

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That's my plan (sort of) I have some good 12 inch Visaton woofers, and I plan on a cab 14 by 16 by 28, with the woofer at top of the motorboard and the vent at the bottom (maybe down-firing with short legs on the cab). Those dimensions don't include the height of the AMTs. My second iteration will be to raise the AMTs up on platform and put some Phillips midranges (rescued from wrecked DQ-10s) underneath with a felt pad on the back ala the DQ-10 pseudo-TL.Crossover at 250 and 1 KHz. I'm thinking of using series network for the woof and mid and then a parallel branch to the AMT, so the impedance will still be in a reasonable region.

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16 hours ago, Marvel said:

 

Back in 2007 or so, member HarryO built some Cornwall volume cabs that he labeled Wallrus, as they were more a Chorus shape with Cornwall parts. He was temporarily in an apartmement across from the assisted living facility where his dad was living.

 

He had no room for Corns so he made 20 inch wide by 48 inch tall and depth to match the Corns volume. I used to have pics of them someplace. They aren't on the forums...

 

Might be too tall to put the Heils on, but adjust to taste.

 

@Marvel Google found these for me a few days ago when I began to hatch this plan. The description is similar to what you described. They look cool, but they are tall. Either way a nice looking project.

 

6F09AD15-1577-4365-BE14-8629151C13AC.png

3768227F-8454-496F-8080-7BFF458A7076.jpeg

8B3A927A-874F-412E-B632-CAF00B66BE5F.png

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I believe that it's worthwhile to list the downside to using AMT drivers on top of Cornwalls:

 

1) The AMTs really aren't as capable in very high SPL as the original Klipsch drivers.  If you find yourself regularly listening at 105+ dB at your listening position (for instance, daily or perhaps more than once a week), I'd rethink using AMTs because their measured distortion levels will probably be higher than the Klipsch drivers/horns at those SPLs.  This isn't really that significant a factor, but it is one that you should consider for your application.

 

2) The AMTs have a real problem in the vertical direction coverage.  I'm working on dealing with this using horn closures/extensions, but so far, it's pretty complicated.  I'd strongly recommend listening on-axis or within ±10-15 degrees.  I'd be careful of putting anything acoustically reflective just above these drivers, since they will reflect a tonally imbalanced off-axis acoustic energy which could affect your listening pleasure.  I'd be in favor of adding absorption to low ceilings and/or furnishings that are in the near field of these tweeters in the vertical direction.

 

3) Setting up your DSP crossover to balance the PEQs vs. frequency will likely be a much more difficult task than your basic Cornwall due to the dipole radiation pattern of these drivers.

 

4) I would seriously think about completely absorbing the backwave if you place the AMTs within 2 feet of a wall or another acoustically reflective surface immediate to the rear of the driver.  Otherwise what you're going to hear isn't going to be nearly as pleasant to listen to in terms of overall tonal balance and/or soundstage imaging.

 

5) Remember to keep the closest crossover driver as close as possible to your AMTs.  That means the midrange horn mouth or woofer.  Do not separate the AMT far from its closest driver unless you're ready to hear issues with lobing and separation of higher frequency sources.  If using a Cornwall woofer only in a two way configuration (they way I use mine), I recommend flipping the Cornwall bass bin over 180 degrees to keep the woofer as close as possible to the AMT.

 

6) I'd also recommend keeping the AMT centered on top of the Cornwall cabinet (and moved to the back of the cabinet if using crossovers without channel delay capability) with soft absorption material on top of its top surface to acoustically absorb the nearfield reflections.  I'd recommend making sure that you use soft absorption material on top before trying to set the delay values for the AMT channels is using a DSP crossover.  Look especially at the off-axis performance of the driver with the crossing drivers (horizontally at 20-45 degrees angle from the centerline of the driver).  You might be surprised. 

 

Chris

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1 hour ago, Chris A said:

with soft absorption material on top of its top surface

 

I ordered ESS's AMT mounting kit... for a lockdown spot on top (haven't opened it yet) - working on my recently acquired old Stephens cabinet, which measures about 74 sq/in (28x32x12) inside . o O (close to the Cornwall enclosure volume) - and have tomorrow off for a new motorboard (saving the original) - and want to put some cabinet drawer slides under the AMT to adjust the "back and forth" eventually... would post an idea for acoustical absorption, but too embarrassed if the word fuzzy is used, based on available sources around my house lol...

edit: 1/4" rubber pad with long bolts included

 

DSC01937.JPG

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My quick & dirty test set up. Roughly 900 Hz (50 uF). Yeah, break in is key. The non-polars in my cap substitution array had never been used and the sound changed as they formed up. Still impressive in the narrow vertical window. A wild thought: if used with corner bass bin, absorb the back wave and turn the AMT vertical to have a tall spray of HF, less sidewalk anomalies,. at the expense of the middle, in which we'd have a distinct hole, without a center channel. 

AMT test 29 Dec 18.jpg

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The AMTs are an excellent match-up with the Cornwalls. Next, I am going to try these with my active crossover Klipschorns which will allow me to adjust the gain to the tweeter. I modified some 2" horn brackets given to me by @Emile. The bracket is turned down onto its face, a piece of plywood is attached and the AMT is secured with 8" TimberLOC screws. 

 

P1020870.JPG.6f56229f3e67de3412f61d20cce1a954.JPG

 

Directly behind the tweeter is a piece of #703 rigid fiberglass insulation, pushed down onto some thin nails sticking up through the plywood.

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On 12/29/2018 at 8:53 AM, Chris A said:

I believe that it's worthwhile to list the downside to using AMT drivers on top of Cornwalls:

 

1) The AMTs really aren't as capable in very high SPL as the original Klipsch drivers.  If you find yourself regularly listening at 105+ dB at your listening position (for instance, daily or perhaps more than once a week), I'd rethink using AMTs because their measured distortion levels will probably be higher than the Klipsch drivers/horns at those SPLs.  This isn't really that significant a factor, but it is one that you should consider for your application.

 

2) The AMTs have a real problem in the vertical direction coverage.  I'm working on dealing with this using horn closures/extensions, but so far, it's pretty complicated.  I'd strongly recommend listening on-axis or within ±10-15 degrees.  I'd be careful of putting anything acoustically reflective just above these drivers, since they will reflect a tonally imbalanced off-axis acoustic energy which could affect your listening pleasure.  I'd be in favor of adding absorption to low ceilings and/or furnishings that are in the near field of these tweeters in the vertical direction.

 

3) Setting up your DSP crossover to balance the PEQs vs. frequency will likely be a much more difficult task than your basic Cornwall due to the dipole radiation pattern of these drivers.

 

4) I would seriously think about completely absorbing the backwave if you place the AMTs within 2 feet of a wall or another acoustically reflective surface immediate to the rear of the driver.  Otherwise what you're going to hear isn't going to be nearly as pleasant to listen to in terms of overall tonal balance and/or soundstage imaging.

 

5) Remember to keep the closest crossover driver as close as possible to your AMTs.  That means the midrange horn mouth or woofer.  Do not separate the AMT far from its closest driver unless you're ready to hear issues with lobing and separation of higher frequency sources.  If using a Cornwall woofer only in a two way configuration (they way I use mine), I recommend flipping the Cornwall bass bin over 180 degrees to keep the woofer as close as possible to the AMT.

 

6) I'd also recommend keeping the AMT centered on top of the Cornwall cabinet (and moved to the back of the cabinet if using crossovers without channel delay capability) with soft absorption material on top of its top surface to acoustically absorb the nearfield reflections.  I'd recommend making sure that you use soft absorption material on top before trying to set the delay values for the AMT channels is using a DSP crossover.  Look especially at the off-axis performance of the driver with the crossing drivers (horizontally at 20-45 degrees angle from the centerline of the driver).  You might be surprised. 

 

Chris

Chris, is the distortion issue mitigated by say a second order filter on the AMT, crossed at say 3k? Every once in a blue moon I’ll briefly play music that loud. 

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The distortion is simply a function of the degree of horn loading of the driven element (the diaphragm) and the stiffness of its diaphragm (reportedly it's Kapton...in this instance).  It is also a function of the breath of the frequency band covered (i.e., the degree of the higher order modulation distortion sidebands from the high frequency fundamental frequencies). So you can decrease a little distortion by crossing to a midrange driver at a higher frequency, but the lion's share of the distortion is really just the construction materials of the driver diaphragm itself.  This is also true for planar and ribbon loudspeakers, which are even more severely limited in their output SPL during transients relative to stiffer diaphragm horn-loaded drivers.  The AMT is horn-loaded, but it just isn't as stiff as metal and polymer diaphragm compression drivers.

 

Chris

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Relative to a K-77, I'd agree without hesitation.  I never have really liked the sound of a K-77 over a select set of third-party tweeters.

 

Also note that the AMT-1 has about 10x the input power tolerance as a K-77 (40w vs. 4w), so you're not having to worry about blowing a phenolic diaphragm during high SPL operation.

 

Chris

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On 12/22/2018 at 7:37 AM, Chris A said:

I need to do a polar coverage plot of the AMT-1, but that's going to be delayed until after the holidays,

Well, I got the opportunity to measure the AMT-1 polars in-room...the thinking, of course, being that large amounts of absorption and/or significant distance separation from walls and other surfaces was not required due to the higher inherent acoustics absorption of those surfaces at higher frequencies.  It turns out that really isn't true, but I generated a normalized polar plot in the fairly noisy environment close to a wall.  The results are interesting:

 

1923788341_AMT-1Polarplot.thumb.jpg.e7ed64a2da762050bb1af89a6894d208.jpg

 

The vertical axis is increasing frequency, and the horizontal axis shows polar SPL.  The frequency sweeps are full range.  The on-axis response is generally down about 6 dB from the peaks measured symmetrically off-axis a few degrees (10-20 degrees), so it is okay to position this driver a bit off-axis horizontally from its 0 degree (centered) position.

 

Overall, the polar plots appear to show consistent polar coverage vs. frequency down to 700 Hz and up to 18-19 kHz.  This extended tweeter (which covers about half of the midrange, too) appears to match its horizontal polars with the 15" K-33 woofer at the crossover frequency of 800 Hz.

 

When I get clear weather to do polar plots outside, I'll put the AMT-1 in the queue.

 

Chris

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Based on the plot above, I added a fairly significant attenuating PEQ to the first order crossover below the AMT-1's crossover point of 800 Hz:

 

     frequency = 660 Hz

     Q = 10

     gain = -12 dB

 

This attenuates the large rise in response that is seen at polar angles greater than 10 degrees that can be seen at the bottom of the plot, such that the average on- and off-axis response around 660 Hz is smoothed and the Cornwall bass bin takes over the output at that point, which is just below the polar narrowing region of the 15" woofer (i.e., just below the point that one wavelength of sound equals the diameter of the woofer cone).  The above added PEQ enables the operation of the AMT-1 well below the typical 2 kHz (or higher) crossover frequency used by others.  The advantage of this approach of course is that the need for a midrange horn/driver is eliminated and near-minimum phase behavior of the Cornwall bass bin + AMT-1 is achieved.

 

This 660 Hz issue turns out was an artifact of the polar normalization process.  The 660 PEQ is not needed.

 

Cornwall Bass Bin + AMT-1 at 1m On-Axis SPL Phase.jpg

 

This is a very good sounding combination to my ears.

 

Chris

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6 hours ago, Chris A said:

Based on the plot above, I added a fairly significant attenuating PEQ to the first order crossover below the AMT-1's crossover point of 800 Hz:

 

The phase graph looks good, but I was wondering, does the PEQ applied below the crossover point introduce any phase altering characteristics that you were avoiding by using a shallow slope, lower order crossover filter? 

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The PEQ will actually correct the phase--that little wiggle from 550-850 Hz--very mildly due to the minimum phase nature of the problem.  The PEQ mentioned above is on the AMT-1 channel only, not the Cornwall bass bin channel.  So below the crossover point, its effect will be very small except to compensate for a polar bump in amplitude response at 660 Hz outside of 10 degrees off-axis alignment when both the bass bin and AMT-1 are playing.

 

Chris

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It's actually the shelving crossover filters that cause the big phase growth from high frequency to low frequency, not PEQs.  PEQs return the phase to nominal when you get outside of the PEQ's amplitude effects in terms of its frequency boosting or attenuating response. 

 

Chris

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10 hours ago, Chris A said:

Based on the plot above, I added a fairly significant attenuating PEQ to the first order crossover below the AMT-1's crossover point of 800 Hz:

 

     frequency = 660 Hz

     Q = 10

     gain = -12 dB

 

This attenuates the large rise in response that is seen at polar angles greater than 10 degrees that can be seen at the bottom of the plot, such that the average on- and off-axis response around 660 Hz is smoothed and the Cornwall bass bin takes over the output at that point, which is just below the polar narrowing region of the 15" woofer (i.e., just below the point that one wavelength of sound equals the diameter of the woofer cone).  The above added PEQ enables the operation of the AMT-1 well below the typical 2 kHz (or higher) crossover frequency used by others.  The advantage of this approach of course is that the need for a midrange horn/driver is eliminated and near-minimum phase behavior of the Cornwall bass bin + AMT-1 is achieved.

 

Cornwall Bass Bin + AMT-1 at 1m On-Axis SPL Phase.jpg

 

This is a very good sounding combination to my ears.

 

Chris

What are your other PEQs Chris?  I"d like to use your settings as a starting point for my tweaking.

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