new Heil AMT type driver

[Santiago53]

This may be useful to those looking for good AMT type drivers

For a long time I've been looking to get a reasonably priced AMT type driver, and never felt excited about buying used Heil AMTs with cracked and aging diaphragms for 350+ a pair!


So afterMUCH looking, I was able to get and test the dm2a driver from Mark & Daniel. Their on axis response was pretty scary, but if loaded with a diffusor to get an Omni type response they did great.

So yes, a good AMT at a fair price BUT....be careful, they require some special acoustical loading.
You can get them from Jason Terpstra, contact him at:

jterpstra@markdanielofamerica.com

About the tests:

I measured with a calibrated Mic using a Behringer DEQ2496 pink noise... SPL isn't shown correctly, but I referenced to a known 88dB source, so the levels can be extrapolated fairly easily.
The divisions between dotted lines are 5dB.

1) Direct Radiation Response: see picture dm2a on axis direct radiation

the dm2a has an average sensitivity of about 82 dB followed by a "tilted shelf" response starting at 7KHz. The 10KHz level is at about 88dB and the 20KHz level is about 95dB and appears to continue on a strong rising trend.

Comments:
The dm2a response makes it a little tricky to integrate without resorting to lots of equalization. If used in a system at 82dB it will be smooth from 1KHz to 5KHz with a 4 dB dip at 6Khz, but will then have a MAJOR high frequency emphasis and sound quite sizzly.


One could put in a first order roll-off on the top end but this would aggravate the dip at 6KHz, make it very inductive.. and still end up with only 81 to 2dB on axis sensitivity.

Also note, even though the level at 450Hz is about 84 dB, that is due to a major mechanical resonance. The crossover should be at least 3 to 4 times higher to avoid exciting this resonance. Don't even THINK about a 900 Hz crossing unless you like a very resonant vocal band!

Not so good if you want to use these with even a medium sensitivity midrange or woofer. But read on, there IS a way to make this work and work WELL.

2) Where one could say that the dm2a is not a great choice for direct radiation, it can be SPECTACULAR if used as shown in the picture entitled "dm2a omni mount with deflector"

If you look at the series of pictures of the Omni w. deflector response, you will see that the use of a properly proportioned and spaced deflector adds an acoustic impedance load which brings the response of the combination to a level that is much more than just acceptable, it is very, very good.

Because of the improved acoustic loading (and dig that fancy cut glass deflector!), the average sensitivity goes up to near 88dB across the band and is now very flat, within a +/- 2dB band from 1.5KHz to 20KHz. See the pics of response for the Omni array at 5 degrees below horizontal as well as the responses at 10 degrees above and below.

This array puts out very FLAT response well beyond 20Kz


PLEASE NOTE: 88dB at 2.83V is decent efficiency, but because this response is nearly identical at ALL of the 360 degree circle (It's omni ! It's OMNI !!), the acoustic power output is equivalent to that of a ScanSpeak AirCirc D3004/6600 direct radiator at 94dB. This level of output is about the same as that of the AirCirc … and the impedance is also about the same at 4 Ohms for the dm2a vs 3 Ohm for the AirCirc


I really like the AirCirc... but I may like the dm2a even more...need more time to tell for sure, but this omni array really works wonders.


But please note! the deflector shape and distance play a CRITICAL role in smoothing out the dm2a response. IT WILL NOT BE THE SAME UNLESS YOU USE THE DEFLECTOR ARRAY!! What you see in these pictures is the result of very careful adjustment of the distance between driver and deflector (23mm). other shapes and sizes give different (and mostly inferior) results. I tried other deflectors, this one worked the best... there may be others that work better, so please let me know what you find.


So how does the Omni array sound? Crossed over at 2KHz with a simple 1st order slope, and mid bass drivers at 88dB, the sound is spectacular: open, clear, quick, tremendous impact (impact? From a tweeter? YES!!) and uncompressed clarity that is very hard to beat, even at very high SPL.

Comparing against the ScanSpeak AirCirc revelator, the dm2a in the omni array will give similar levels of neutrality, MUCH wider dispersion at high frequencies, and apparently higher levels of low distortion output with what seems like more impact. And the fact that it can be had for USD100 each instead of USD250+ for the Revelators, well that makes up for the shiny plastic faceplate which covers the (adequately substantial) painted metal housing.


More later, maybe test em with controlled directivity waveguides and the foam bung like GedLee... but that will come MUCH later, if at all. I like this in omni so much that I may never get around to it.

So, anyway, looks like great choice when used in this way.

[Santiago53]

Could only get one pic in at a time, here is the on axis Frequency Response for the Mark & Daniel dm2a.

[Santiago53]

Hi, here is the picture for the dm2a Omni array at 5 degrees below horizontal axis.

[Santiago53]

another picture of response at above horiz axis for the dm2a mounted in omni array with diffuser

[c2cthomas]

Hi Santiago53 - and Welcome to diyAudio!

Impressive work you are doing and I am interested in your findings and progress. I already have AMT1's and have loved them from the 1st time I listened to them back in '75. I lived in Sacramento California where the ESS plant was at and had a couple of friends that worked there and introduced me to the AMT's. I would need to come up with a different diffuser from the one you are using because if the wife sees that thing she will want one too!

Your AMT's from M&D cost $100 USD each? Did I read this correctly???
If this is correct it is one heck of a deal!!!

[Santiago53]

yes, Jason is making them available at $100USD each...

I think it's a fairly good price given the performance these are capable of. But I do advise users to respect this one warning: they really sound excellent, but again ONLY if used with an Omni diffuser similar in distance, size and angle to the one I used...

and please no jokes about "crystal clear highs" ....

and , no, I don't really believe the diffuser has to be made of crystal

[c2cthomas]

M&D use their Omni-Harmonizer from 7K on up - and it is made out of marble (they like to use marble don't they). Something a little bigger along the same design lines should work out. If I used 2.5 KHz for the targeted lowest fs then the diameter of the cone should work out to 5.4 inches and 8.76 inches high (golden ratio). So - make a little "snow cone" shaped mold out of heavy paper and mix up a little acrylic plastic and "presto" - I have a diffuser. Or I could turn one out of a nice hard wood - that might have some WAF to it. I would finish things off with BudP's EnABL pattern just for S&G's. Hmmmm - currently have two speaker projects in the Q - plus a couple of class A mono blocks in assembly - so that would make the ETA on this thing - ????? Damn - I just luv this little hobby!!!

[oon_the_kid]

Dear Santiago,

Won't the usage of diamond deflector, won't the sound be a bit bright

Did you by any chance have any measurements of the DM4. I bought a pair, wondering if anyone has measurements for that...

Oon

[Santiago53]

Sorry, I haven't been able to measure or hear the DM4 yet, although i am quite interested as they may be able to be crossed in lower than the DM2a

I believe there is a newer version in the works, the DM4a.

Still, with the deflector, the DM2a gives good measured performance and subjectively a highly musical and transparent sound, coupled with tremendous dynamics, rhythm and pace. Superb for crossing in over 2KHz

So, i guess you could say I like em!

[Santiago53]

Hi, I just found this series ofDM4 measurements at ZaphAudio, check his site at

http://zaphaudio.com/tidbits/

A few comments:

1) the freq response looks fairly similar to that of the DM2a, meaning that this driver will likely benefit from a similar omni deflector to what I described previously for the dm2a...
I would echo Zaph's comment about the FR seems unusable without major eq'ing aqnd DEFINITELY echo his comments on the midrange honk if crossed in too low

BUT...
the omni deflector, as used for the dm2a may reslove many of these issues (not the honk at 400Hz though)

AND... the deflector probably has to be bigger in order to extend the acoustic loading to lower frequencies

2) The distortion figures look fairly good, even very good in the lower ranges

SO, despite looking like a difficult driver, similar to the dm2a, a properly conceived omni load/deflector may turn this into an excellent performer

enjoy!