To Heil AMT or not to Heil at $140, that is the question?

[daqvin_carter]

If you look at the replacement AMT diaphragm, you can see how easy it might be to reverse polarity during assembly. Good thing to test when building.

 

[fedde]

On YouTube an example how to remove the diaphragm:
Removing Heil Diaphragm - YouTube

But it does not appear to be easily remountable. What is this glue paste that is removed and how to reapply this?

 

[puppet]

That's just a putty fedde. It seals the top and bottom of the diaphragm channel. As long as it isn't dried out you should be able to reapply it.

 

[fedde]

Recently I did an experiment to mount the (complete) AMT in opposite way, so with frontside facing to the back wall (and reversing polarity back to original next). This gave quite an improvement in stereo imaging and clarity of HF! So it looks like indeed my AMT is not creating symmetrical sound waves. One side sounds somewhat muffled (measurable even). So it looks like I will need to open my AMT and swap the diaphragm around...

 

[fedde]

I decided to validate the DC-fix mod from the ESS modification sheet (see post #34). Indeed, I can confirm that it works, though for me especially around 4 kHz instead of 5 kHz...

I used black DC-fix velours and mounted it on both sides and around the large ESS AMT. I got help from two friends (Bart & Rob). Rob has a nice automatic measurement rotation table. We measured the frequency response for full 360 degree range at 10 degree intervals. A good measurement microphone and setup were used. Measurements were done before and after the mod.

 

[fedde]

Hereby the VituixCAD post processed results... (note that no EQ nor passive filtering was applied during measurement and no smoothing was done either).
Lowest image is after the mod. Directivity is quite a bit more constant between 3 kHz and 5 kHz, very nice!

 

[druidaudio]

So it looks like indeed my AMT is not creating symmetrical sound waves. One side sounds somewhat muffled (measurable even). So it looks like I will need to open my AMT and swap the diaphragm around...

The diaphragm is meant to go in one way only to allow the wires to exit in the correct place, so reversing is possible but not ideal. I had a similar problem with phase reversal on one AMT after a rebuild following some physical damage which resulted in damage to the plastic case. What I had inadvertently done was to mount one (or both - I am unsure) magnets the wrong way, which then caused a phase problem.
In the end I just reversed the wiring!

 

[xrk971]

Nice DIY rotation table! Is that an Arduino project?

 

[Rob Dingen]

No not Arduno.
Made this for the Clio 12 system so I can do automatic degree measurements 5, 10, 15, 20 or 30 degree steps.
Its with a PIC 18F type.

 

[fedde]

This rotation table made the full 360 measurement a matter of a few minutes, very handy. Puzzling is why there is still a 7 kHz dip in the response. Measurements with a different AMT pair did not show it, so it must be some sort of resonance in the foil. Left right matching is regrettably also not perfect, though within 1 dB or so.

I have the impression that the sound has also improved with this mod (though I can obviously not AB it directly, only can compare against 2nd unit). Still need to mod the 2nd AMT. Anyhow, this mod is already highly recommended for the small investment of 7 euro for a roll of velours...

 

[fedde]

We did also a bit of investigation on what is wrong in my left AMT. We could see that foil was mounted correctly, but magnets have wrong polarity. This is a manufacturing fault. I tried to reach ESS regarding this, but without success, quite disappointing!

As we have full 360 measurements, we could check what is the most optimal way to compensate for the manufacturing fault. So far, it looks that measurement wise, it is best to just inverse electrical polarity on left channel and mount both AMTs with label on front side...