Heil Air motion transformer for Cornwalls

[Chris A]

1 hour ago, jwgorman said:

Chris was suggesting the Cornwall as a two way, bypassing the horns and changing crossover points. Just FYI. 

 

Yes, I failed to make that point.  The Heil AMTs are good down to 800 Hz, and the stock Cornwall I's (those made before 1986) cross over at 600 Hz, so it's a no-brainer to go two way if using a DSP crossover--which is what I did.  The AMTs nominally have 90 degrees of horizontal coverage--which I assume is widening at the bottom end of its response, while a 15" (nominal) woofer will also have roughly 90-110 degrees of coverage at 800 Hz, so the woofer and the AMT matches horizontal coverage at 800 Hz.

 

So I bypassed the internal Type B crossover and hooked the woofer to the cabinet terminal block, and the other amplifier channel to the AMT-1 using an EV Dx38 crossover.  Here's a plot of its SPL and phase vs. frequency.  Pay attention to the scales of the curves (2 dB for SPL curve divisions, 45 degrees per division for phase)--the phase portion is spectacular, IME:

 

 

More importantly, here's an impulse spectrogram of the same thing.  You can see the backwave pulse of the AMT-1 reflecting off the wall and returning at about 4 milliseconds (above 800 Hz).  Again, pay attention to the horizontal scale of the peak energy time--1/2 millisecond divisions.  This is pretty spectacular in comparison to a stock Cornwall spectrogram--whose impulse spectrogram you really don't want to see:

 

 

I'm listening to these surround Cornwall/AMTs right now playing a 5.1 Yellowjackets New Morning Blu-Ray.  It's pretty spectacular.  It's also possible to move the AMT-1 around to put the HF energy where you want it in-room. As surround loudspeakers in a 5.1--this is something that I've wanted for a long time.

 

Chris

[Chris A]

Here's the step response(!) where you can also see the returning backwave at 4ms:

 

 

and the harmonic distortion plot vs. frequency:

 

 

Those are some pretty good looking plots.

 

Chris

[jwgorman]

Chris, you can’t see this, but I am bowing deeply in respect and gratitude! 

So, thinking about a 2 way now! Man, this looks awesome. 

[Chris A]

I guess I can throw in group delay and a waterfall for that attaboy:

 

 

 

Folks, these are pretty spectacular performance figures--something that you'd have trouble reproducing with just about anything else that's hybrid horn loaded-direct radiator (as the AMT+Cornwall woofer is).  I'd call it a serious upgrade. 

 

But I really don't recommend doing this unless you've got a DSP crossover, because the raw response was a little more ragged than I'd like to see, and the combination needs about 1.5 ms delay on the HF channel (assuming Bessel 12 dB/octave crossover filters are used) to achieve the above numbers. 

 

(There's an EV DC-One available on the garage sale for $350, and you can also look at a miniDSP 2x4 HD for $205 to do the same thing.  All you need is 4 channels of amplification to complete the setup.)

 

Chris

[ClaudeJ1]

Just think of how much easier it would be to do this to a Super Heresy (3.0???)

[Chris A]

Yes, I was going to do the Heresies in the garage, but I got lazy and did the Cornwall surrounds in the main rig so I didn't have to completely swap out the electronics in the garage, too.  Glad I did...I probably wouldn't have figured out how good the AMTs really are. 

 

Perhaps there's another pair of AMTs for the garage in the future.  I find it's better to employ the surplus electronics than to just let it sit around gathering dust. I've got a Yamaha SP2060 and an old AVR in storage that I can use to bi-amp the Heresy woofers and a future pair of AMTs.

 

Chris

[ClaudeJ1]

1 minute ago, Chris A said:

Yes, I was going to do the Heresies in the garage, but I got lazy and did the Cornwall surrounds in the main rig so I didn't have to completely swap out the electronics in the garage, too.  Glad I did...I probably wouldn't have figured out how good the AMTs really are. 

 

Perhaps there's another pair of AMTs for the garage in the future.

 

Chris

I have a pair already mounted on the AMT-1A's, which sound great. They are the speakers that got me thinking of dipoles and Carver Amazing Platinums, once I experienced the 3D effect of dipoles. I also two extra pair of the Big AMT's, which I got used (they never go bad) and a spare set of diaphragms. Thanks for sharing you positive experience with those, they are truly a steal new, since I paid double that for used ones.

[Khornukopia]

20 hours ago, Chris A said:

the combination needs about 1.5 ms delay on the HF channel (assuming Bessel 12 dB/octave crossover filters are used) to achieve the above numbers. 

 

Does this depend on the location of the tweeter sitting on top of the Cornwall?

[Chris A]

On 12/5/2018 at 2:25 PM, Chris A said:

I found that I needed about 1.5 ms of delay using Bessel 12 dB/octave filters if you reference the AMTs flush to the front of the Cornwall cabinet.

Subtract 73.6 microseconds/inch from that 1.3-1.5 millisecond figure that you move it back from the front surface of the Cornwall cabinet top. 

 

Chris

[Khornukopia]

Ok, I should have remembered that from the earlier post. 

 

Looking at internet images of this tweeter, I see a picture of the ESS AMT 10b that has an enclosed cabinet mounting. Is this detrimental to the appeal of the AMT as a dipole radiator?

[Khornukopia]

Oops, I just ordered a pair of these AMT tweeters.

[Chris A]

That's a monopole mounting for the tweeter.   You'll lose about 2.7-3 dB of SPL sensitivity from that arrangement.  I would guess that the sensitivity of the AMT-1 tweeter is something like 93 100 dB/2.83V input....depending on how you measure the sensitivity of a dipole radiator. 

 

You don't have to use that backwave--as you see in the 10b pictures--but it would be a nice addition to a surround loudspeaker to direct that energy upwards from the attached wall surface.  You could also direct the backwave to the sides to form a pseudo-dipole or -bipole surround (depending on frequency and rectification of the wall surface on the backwave), but diminishing returns will soon overtake that kind of approach.

 

Chris

[pzannucci]

Just now, Khornukopia said:

Oops, I just ordered a pair of these AMT tweeters.

Oops?

[Khornukopia]

Just trying to be funny. I have more speakers and parts than a normal person should own.

[pzannucci]

2 minutes ago, Khornukopia said:

Just trying to be funny. I have more speakers and parts than a normal person should own.

Tell me about it.  I decided to pull the trigger a few hours ago.  Not sure why because I have a pair of Aurum Cantus G1 ribbons sitting in a box and other high end drivers laying around.  The Heil's always had an allure since I heard the AMT-1s back in the mid 70's.  For the price, a no brainer (if you don't have piles of stuff) 

[Khornukopia]

 

9 minutes ago, pzannucci said:

The Heil's always had an allure since I heard the AMT-1s

 Same here. I like to think that we are also fascinated by the technology involved in the parts and the systems.

[pzannucci]

4 minutes ago, Khornukopia said:

 

 Same here. I like to think that we are also fascinated by the technology involved in the parts and the systems.

Sadly, can't leave well enough alone.  Just as jwgorman gets convinced to go two way.

[jwgorman]

15 minutes ago, pzannucci said:

Sadly, can't leave well enough alone.  Just as jwgorman gets convinced to go two way.

Well.  Maybe. I’m going to go passive. And, I already have the mids. Dang it. I’m trying to model bessel 2nd orders at 400 and 1500 and compare them to 300 and 1200. I mean, on paper, it looks good. 

But my cabinet maker can’t get to my project for another month. So I have plenty of sleepless nights ahead.

[Chris A]

Here are the critical PEQs that I used to interface the AMT-1 to the Cornwall bass bin.  (I used nine PEQs total to flatten the Cornwall/AMT-1 response, six of which went to the bass bin.)

 

<snip>

 

You could do this in a player, etc., if you had parametric EQ filters available to you, but you'd still have to move the AMT-1 back about 17.7 inches from the front edge of the Cornwall cabinet to be time-aligned using that lower order crossover filter (Bessel 12 dB/oct).  I don't know what the Klipsch "B" crossover low pass filter is using on the bass bin, but I assume it's a second order filter of the Butterworth type. Nope, the type B is first order...

 

 

Chris

[pzannucci]

4 minutes ago, Chris A said:

Here are the critical PEQs that I used to interface the AMT-1 to the Cornwall bass bin.  (I used nine PEQs total to flatten the Cornwall/AMT-1 response, six of which went to the bass bin.)

 

 

You could do this in a player, etc., if you had parametric EQ filters available to you, but you'd still have to move the AMT-1 back about 17.7 inches from the front edge of the Cornwall cabinet to be time-aligned using that lower order crossover filter (Bessel 12 dB/oct).  I don't know what the Klipsch "B" crossover low pass filter is using on the bass bin, but I assume it's a second order filter of the Butterworth type.

 

Chris

Am I reading that correctly?  You had to add 7db on the lower end of the heil but dropping the output around crossover of ~900?  Would inversing the polarity work to pick up some of the distance change?