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Heil Air motion transformer for Cornwalls


jwgorman

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Been reading with interest and took the plunge.  UPS lost them for five days, but finally got them hooked up.  And first listen has been encouraging.  Couple of questions if you don't mind.  Then what are placement suggestions.  I saw turning the cab upside down but was just wondering.  And then I'm using an ashly analog crossover at 500.  I think I read thats the right setting.  I do have to say it's impressive so far.

MVIMG_20181222_212044_exported_8650005304902159290.jpg

Edited by tom1066
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1 hour ago, tom1066 said:

Been reading with interest and took the plunge.  UPS lost them for five days, but finally got them hooked up.  And first listen has been encouraging.  Couple of questions if you don't mind.  This may be a dumb question but is there any break in?  I'm always worried I may not five something the proper amount of time.  Then what are placement suggestions.  I saw turning the cab upside down but was just wondering.  And then I'm using an ashly analog crossover at 500.  I think I read thats the right setting.  I do have to say it's impressive so far.

MVIMG_20181222_212044_exported_8650005304902159290.jpg

 

Yeah, the upside down arrangement is to get the woofer closer to the AMT if you're doing a 2 way.  Chris was suggesting setting the Heil about 6 inches back from the front edge for time alignment with your analog xover or passive.

 

Curious, where did you read 500 Hz xover?  I'm starting with 800 first order (6 db/octave)

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7 hours ago, Delicious2 said:

 

Yeah, the upside down arrangement is to get the woofer closer to the AMT if you're doing a 2 way.  Chris was suggesting setting the Heil about 6 inches back from the front edge for time alignment with your analog xover or passive.

 

Curious, where did you read 500 Hz xover?  I'm starting with 800 first order (6 db/octave)

 

Sorry, I meant 800 Hz.  Thanks for the correction.  

 

Are people using tubes or SS to power these things?  I think today I am going to switch to SS and see how they do.  Just curious about other peoples experiences.

 

Tom

 

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12 hours ago, tom1066 said:

This may be a dumb question but is there any break in?

Yes.  The AMT has a definite break-in period and a significant change in its acoustic performance:

 

12 hours ago, tom1066 said:

Then what are placement suggestions.  I saw turning the cab upside down but was just wondering.  And then I'm using an ashly analog crossover at 500.  I think I read thats the right setting.

You wind up having to place the Cornwall cabinets somewhat away from the walls due to the dipole nature of the AMTs.  I'd recommend at least a couple of feet of distance from the back of the AMT-1 rear face to the rear wall to maximize the AMT's backwave effect on the speciousness of sound.  Otherwise, I'd recommend placing absorption panel(s) directly behind the AMT-1 on the closest wall surfaces to absorb the backwave.

 

If you're using first order passive crossover (a single capacitor for the AMT-1 and a single choke for the Cornwall woofer) or an analog active crossover with first order filters, then I'd recommend moving the AMT-1 back from the front face of the Cornwall about 6 inches, as Mark discussed.  This will align the AMT-1's impulse response much better to the Cornwall's K-33 woofer.  If you instead want to use second order filters, then I'd recommend moving the AMT-1 back another 6 inches from the front face of the Cornwall, for a total of 12 inches--or in other words, at the back of the Cornwall cabinet.

 

I recommend using a calibrated microphone and REW to see what you're actually doing to the output of the loudspeaker if you introduce a SET into your system. 

 

Chris

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16 hours ago, Delicious2 said:

With a basic first order HP/LP set on the Xilica I'll break these in as mains for a while.  Initial impression?  They don't give as pleasing a rendition of Seiji Osawa and the BSO performing The Nutcracker in Boston's Symphony Hall as do the K402/TAD/Khorn blend, but, OTOH they don't sound bad even switching back and forth.  Checkout the backlit AMT.  Perhaps one reason they say "sound as clear as light"😎

Break In As Mains.jpg

I see you have an auditor with very keen hearing helping you. Mine assist me as well, although they tend to fall asleep at random times.

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

Actually the AMTs are just as much horns as the K-400 series of midrange horns on Heritage loudspeakers (i.e., K-260, K-500, K-600, K-700) except as they perform below about 2.2--3 kHz, where the horns lose vertical pattern control and start to paint your floor and ceiling with excess acoustic energy.  The AMT has increasing vertical directivity below about 2 kHz, too, and relies on the length of the AMT diaphragm area to control it's vertical directivity above that frequency--just like a ribbon driver does. The horizontal direction is controlled via the "waveguide" straight-sided horn. 

 

I need to do a polar coverage plot of the AMT-1, but that's going to be delayed until after the holidays, I'm afraid.  In the meantime, here's a polar directivity plot of a Beyma CP25 tweeter for reference, first in the "vertical" direction (spheres top and bottom):

 

1135817266_CP25verticalpolarspectrogram.thumb.JPG.ea0d1b3448ea880d2cf3a588433e96ae.JPG

 

and then "horizontally", with the horn spheres side by side:

1067070587_CP25horizontalpolarspectrogram.thumb.JPG.7d2fa70b47055e220410ca67be30bfb0.JPG

 

All this says that you need to crossover the CP25 above about 5 kHz in order to avoid the polar narrowing in the preferred axis (called "vertical" but is actually the horizontal direction with the horn spheres top and bottom).  Just like other HF horns, the directivity seems to widen or wander around a bit in the other direction.

 

Chris

My AMTs are en route and I am starting to sketch out cabs for them. It will be interesting to see how these differ from either my Corns or the now lost DQ-10s. Thanks again for the measurements

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They are probably too close to my ears currently at 5 feet, but, it does allow 5 feet of clearance behind the AMT on the left and 3.5 feet on the right.  If I move them just outside the heating vents they'll be 6.5 feet away but only 3.5 feet to closet door on left side and 1.5 feet to sub on the right.  I could put dampening thick fuzzy fabric on those surfaces.

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I too, could not pass up the price of a pair of these. I do not own Cornwall’s so I will probably be building a cab for these. I am thinking something like with a similar volume to a Cornscala cab that houses only the woofer. I am hoping to be able to alter the shape of the cab a little to push these into a smaller footprint and bring the heil closer to ear height. (I need to do a ton of reading, I know there is more to it than just internal volume, but any advice would be appreciated.) 

 

I was lucky enough to be able to acquire some ess amt monitors several years ago that needed a few minor repairs to be perfect. That project spawned my interest in diy, and I have building ever since. These speakers have been seeing daily use in some form in our house for as long as we have owned them. The heil really is special.

 

Interesting note about the ess design. The heil is “housed” on top of a TL cabinet in a separated enclosure. Oxblood speaker fabric hides the Heils. The top enclosure is open on the sides, the front and the back. The top directly above the heil is closed off with hardboard panel. I am curious if this top panel was used to control the dispersion, or if it was used because the designer knew someone would be unable to resist placing a drink on top of these speakers while dancing the night away. 

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3 hours ago, No.4 said:

I too, could not pass up the price of a pair of these. I do not own Cornwall’s so I will probably be building a cab for these. I am thinking something like with a similar volume to a Cornscala cab that houses only the woofer. I am hoping to be able to alter the shape of the cab a little to push these into a smaller footprint and bring the heil closer to ear height. (I need to do a ton of reading, I know there is more to it than just internal volume, but any advice would be appreciated.) 

 

I was lucky enough to be able to acquire some ess amt monitors several years ago that needed a few minor repairs to be perfect. That project spawned my interest in diy, and I have building ever since. These speakers have been seeing daily use in some form in our house for as long as we have owned them. The heil really is special.

 

Interesting note about the ess design. The heil is “housed” on top of a TL cabinet in a separated enclosure. Oxblood speaker fabric hides the Heils. The top enclosure is open on the sides, the front and the back. The top directly above the heil is closed off with hardboard panel. I am curious if this top panel was used to control the dispersion, or if it was used because the designer knew someone would be unable to resist placing a drink on top of these speakers while dancing the night away. 

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.

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So I did some more measurements this morning.  Here is the on-axis frequency response taken at 1m (microphone on the AMT-1's axis, with the AMT-1 sitting about 6 inches back from the front face of the Cornwall box) of the AMT-1 + Cornwall I bass bin using first order (i.e., 6 dB/octave) crossovers, but using 603 microseconds of delay--after leveling the AMT-1's channel gain to match the Cornwall bass bin (bi-amping):

 

1247509042_CornwallBassBinAMT-1at1mOn-AxisSPLPhase.thumb.jpg.fa9177b7ad135cefb1ef16b7b167037d.jpg

 

I'd recommend moving the AMT-1 to the back of the Cornwall cabinet if using a capacitor and choke for a crossover.  This will get you time alignment to achieve the frequency and phase response shown in the plot above.

 

Here is a impulse spectrogram of that same measurement showing the flatness of the maximum energy time vs. frequency (i.e., almost linear phase all the way down to 30 Hz...!):

 

1813086115_CornwallBassBinAMT-1on-axisnoEQspectrogram.thumb.jpg.a6ebf642c69a8946af86c2829804fcee.jpg

 

and the relative harmonic distortion plot at 90 dB on-axis, 1 m:

2120930305_CornwallBassBinAMT-1at1m90dBOn-Axisharmdist.thumb.jpg.61ff8e1d7be22179f02c351dc6940fec.jpg

 

 

These are pretty spectacular performance indicators.  I'd say there is a reason why these drivers are getting such good reviews.  One caveat--make sure that the AMT-1 driver is on-axis to your ear height or within 10-20 degrees of being on a level line to your listening head height.

 

Chris

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As far as the crossover frequency--there is nothing that I've measured to date that sways me toward a higher crossover frequency, in fact, the AMT-1 rolls off at almost 24 dB/octave below 600 Hz. So unless you're starting the roll off at least a couple of octaves above 600 Hz (i.e., 2.4 kHz) using second order crossover filters, you're not going to materially change its roll-off using electrical filtering because it's natural roll-off is so steep as-is.

Additionally, I see no evidence of 3rd harmonic and higher growth at low frequencies (the ones that have a chance of not being masked by human hearing) until you below 500 Hz. Modulation distortion in direct radiators goes approximately as the harmonic distortion (proportionally) but horn-loaded radiators have approximately 20-25 dB lower modulation distortion proportionally. The AMT-1 is horn loaded horizontally, so the real story is where does its horn loading begin to drop off.  It appears that starting frequency is about 600 Hz, and the loss of SPL is significant below that point.

 

Chris

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