[tfoust]

Acquired a vintage Heil AMT for a DIY project.  I have no previous experience with these drivers.  I was anxious to hear it so I connected it to a 10 wpc tube amp fed by a Nuforce dac/ipod interface with an ipod classic.  The output was like a cheap tinny radio.  I knew it wouldn't have bass below 750 hz but it was far from the range I expected from what I read about them. 

Anyone have experience with these drivers and maybe tell me if this is normal or do I need to replace the diaphragm?

[Quiet Earth]

You know that it's a tweeter, right?

[tfoust]

Yes, I understand it's a tweeter but it has a published bottom end of 750 hz.

[S Clark]

Just because it claims to have output at 750 Hz, it doesn't tell you the amount of output.  It might be 6 or more dB down and not even discernible.  I wouldn't be surprised if the actual useable output is above 1200 Hz, depending on impedance/  resonance.
Looking over at Parts Express, most have crossed them from 1500 to 2000 Hz.

[Quiet Earth]

That's a good point. And where is it crossed over right now, if at all?

Yes, I understand it's a tweeter but it has a published bottom end of 750 hz.

I think you might be overestimating just how low 750Hz is, as far as musical content is concerned. I think it's mostly upper midrange territory, bordering on treble. The frequency numbers are not linear or intuitive.

[*Scotty*]

 tfoust, here is your problem in a nutshell, this driver is a "tweeter". See the frequency response graph below.

Link to website for graph. http://www.moxtone.com/Slavooy.htm
 As you can see, it probably should be crossed over to a woofer no lower than 1500Hz.
Its 20kHz output is 20dB down relative to its 2kHz level.
Not the best high frequency extension.
Scotty

[tfoust]

You're probably right about my interpretation of 750 hz.  I didn't really think that through.

Thank you, men, for your comments.

[S Clark]

Depending on your level of experience and the sophistication of your measuring equipment, the Fs (resonance frequency) of your tweeter might be a better guide for crossover determination than the frequency response curve.
 
http://www.madisoundspeakerstore.com/vifa-soft-dome-tweeters/vifa-d27tg35-06-1-dome-tweeter/

This is a link to the data for a work horse Vifa tweeter, a variation of the D27.  Note that the Fs is at 720 Hz.  A fair rule of thumb is to crossover no lower than 2x the resonance frequency. 

[sebrof]

I use a pair in a 2-way, crossed in at 1200 Hz 3rd order. I think it's a fantastic driver (the "Great Heil" is the one I have and I believe you have if off the ESS speakers).
It may or may not work for you, you may or may not like it, but if you listen to it by itself it sounds exactly like a cheap tinny radio.

[tfoust]

Will you tell me what "3rd order" means?

[jcotner]

Let me see if I remember this stuff right 30 years after Univeristy...
All filters have transfer functions (equations that describe their behavior).
So filter order is also the order of denominator polynomials in the
transfer function.
More generally, without getting bogged down in the math, the higher
the order the more rapid the filter roll off and more components
required to do it. The term pole is also used to describe filter
topology.
So if you have a highpass filter with a cutoff of 1800Hz and say put
a 1700Hz signal through it, the 1st order filter will pass the most
of that signal, followed by the second order and then third passing
the least amount of the 1700Hz signal.
Maybe better explanations but that should get you in the ball park.

[sebrof]

Will you tell me what "3rd order" means?

I'll explain in layman's terms because that's how I understand it best, and probably most useful to you.

1st order attenuates at 6db per octave. I have a 1st order filter on the bass/mid driver, a coil in line and the value dictates the attenuation begins at 500Hz.
3rd order on the tweeter attenuates 18db per octave (6db x 3rd order), and this is on my Heil. 2 caps and a coil.

so for the woofer/mid: the 6db per octave LOW PASS FILTER means that at 500hz there is 0db attenuation, at 1000 Hz there is 6db attenuation, at 2000 Hz there is 12db attenuation, etc. This gives a nice gentle rolloff of the woofer/mid because the driver I used (Altec 416) has an OK natural rolloff and when I was playing with the crossover a 1st order sounded best.
For the Heil: 18 db HI PASS FILTER means that at 1200Hz there is 0db attenuation, at 600Hz there is 18db attenuation. This give a steeper/sharper rolloff of the Heil because it sounds edgy at lower frequencies, and also because it cannot take much power at those low frequencies and I don't want to blow it up.

Here's a good calculator that might give some more info (and may contradict me slightly if I have the finer points wrong): http://www.apicsllc.com/apics/Misc/filter2.html

Hope this helps

[S Clark]

I'll explain in layman's terms because that's how I understand it best, and probably most useful to you.

1st order attenuates at 6db per octave. I have a 1st order filter on the bass/mid driver, a coil in line and the value dictates the attenuation begins at 1000Hz.
3rd order on the tweeter attenuates 18db per octave (6db x 3rd order), and this is on my Heil. 2 caps and a coil.

so for the woofer/mid: the 6db per octave LOW PASS FILTER means that at 1000hz there is 0db attenuation, at 2000 Hz there is 6db attenuation, at 3000 Hz there is 12db attenuation, etc. This gives a nice gentle rolloff of the woofer/mid because the driver I used (Altec 416) has an OK natural rolloff and when I was playing with the crossover a 1st order sounded best.
For the Heil: 18 db HI PASS FILTER means that at 1200Hz there is 0db attenuation, at 600Hz there is 18db attenuation. This give a steeper/sharper rolloff of the Heil because it sounds edgy at lower frequencies, and also because it cannot take much power at those low frequencies and I don't want to blow it up.

Here's a good calculator that might give some more info (and may contradict me slightly if I have the finer points wrong): http://www.apicsllc.com/apics/Misc/filter2.html

A good explaination of how frequencies are affected by crossovers.... if only it were that simple.  You can look at the FR graphs of a tweeter and woofer and get an idea of you they will sum, if every thing stays in phase.  Unfortunately, that is usually not entirely the case.  So two signals, instead of adding to each others amplitude, can subtract.  My experience is that measurement equipment is needed to really design a crossover.

[sebrof]

A good explaination of how frequencies are affected by crossovers.... if only it were that simple.  You can look at the FR graphs of a tweeter and woofer and get an idea of you they will sum, if every thing stays in phase.  Unfortunately, that is usually not entirely the case.  So two signals, instead of adding to each others amplitude, can subtract.  My experience is that measurement equipment is needed to really design a crossover.

Fair enough, but he just asked what a 3rd order XO was so I explained.
FWIW - A friend who guided me through the design of the speakers brought over his RTA after I thought I had them dialed in and the speakers measured pretty good. I do believe it was a combination of luck and tweaking and listening for many hours. So I think you can get a very satisfying sound without a lot of measuring.

[*Scotty*]

What did the final speaker look like between 10kHz and 20kHz. Was the speaker 20dB down at 20kHz relative to the 2kHz output level.
Scotty

[sebrof]

What did the final speaker look like between 10kHz and 20kHz. Was the speaker 20dB down at 20kHz relative to the 2kHz output level.
Scotty

  It was his RTA and he operated it, a model with green LEDs in the middle and yellow and red above and below for each frequency. The yellows indicated +1db or -1db and the reds +3db or -3db (or you could flip a switch to expand the scale to 3db / 6db I believe).
He measured with one speaker on, mic in the listener chair.
He said they were within 3db from 20KHz to 200Hz. Below 200Hz and the readings aren't reliable.
I didn't know what to expect, he seemed to think that was pretty good.
(A picture of them is in my gallery, the Loft)

Back to the OP's point - The Great Heils are pretty well regarded, at least in some circles. I had never heard one until I bought a used pair and made the speakers I've discussed in this thread. My opinion is that they're excellent drivers and certainly worth using if you have a pair and plan to build a pair of speakers.
And they'll sound like a cheap tinny radio if you listen to them by themselves.

[tfoust]

Follow up on my testing.  I bought an additional pair of used Heil AMTs and tried them with the same input as with the first one.  They sound like each other and sound like you would expect a tweeter to.  They are high pitched but not what one would call "tinny".  I think the first one is in need of service.

Thanks again, for your input.

[whell]

Here's what happened to my AMT's:



 

[neobop]

Acquired a vintage Heil AMT for a DIY project.  I have no previous experience with these drivers.  I was anxious to hear it so I connected it to a 10 wpc tube amp fed by a Nuforce dac/ipod interface with an ipod classic.  The output was like a cheap tinny radio.  I knew it wouldn't have bass below 750 hz but it was far from the range I expected from what I read about them. 

Anyone have experience with these drivers and maybe tell me if this is normal or do I need to replace the diaphragm?

It's not a good idea to feed a full range signal to any tweeter.  They're easily damaged. 

Minimum recommended crossover is 800Hz @ 12dB/octave.
https://www.parts-express.com/ess-heil-air-motion-transformer-amt-tweeter--264-600

That means the -3dB point should be 800Hz or higher.  If the crossover frequency is 1KHz, then that is the -3dB point and at 12dB/octave the attenuation will be 15dB at 500Hz. 
At 92dB efficiency these will need similar efficiency woofers or the tweeters will have to be attenuated.  Because the tweeters are 4 ohm, two 8 ohm woofers can be used in parallel to increase efficiency and match tweeter impedance.  Here's a handy calculator:
http://www.diyaudioandvideo.com/Calculator/XOver/

I have some ribbon tweeters, but they're not air motion folded diaphragms.  If they're fed too much power at a low frequency they break up and rattle. 
BTW if both drivers have 12dB filters the polarity is normally reversed going to the tweeter.  This prevents the output at the crossover frequency from being cancelled.
neo