[steve f]

I am of two minds about AMT drivers. I enjoy their clarity, and their very extended frequency range. I love the way they seem to "throw" their output a bit like a small horn. (Of course that is a bit subjective on my part) The vertical dispersion has always been a deal breaker for me.  My last AMT's were by Heil, and used with Horn Shoppe horns in my 2nd system. At distances of about 8 feet away, they were quite good. A few feet closer, and seating position became restrictive. I had a Bohlander-Graebner tweeter & lens on hand so I knocked off the plastic rear chamber, mounted it on a board, hooked it up and gave it a listen. To my surprise, I preferred it to the Heil. The clarity was there. I also got a bit more vertical dispersion while still being well controlled in that area.
This story is anecdotal because I didn't have the opportunity to take measurements. My room was pretty live, and volume levels were moderate. I didn't get to explore further because I had just put the house up for sale. Real estate agents cringe when audiophiles turn living rooms into laboratories. After moving almost cross country, and very recently buying another house, I look forward to building stuff again. I shall enjoy reading about your work. Thank you for sharing with us.

Steve

[matevana]

Good stuff. I too like the BG drivers I have heard.

I think Dayton did a lot of good things with their new AMT offerings. The build quality on the AMTPRO is impressive. I also like the amount of surface area relegated to the pleated diaphragm; about twice that of the more expensive Aurum Cantus AST2560. On two occasions PE has offered the AMTPRO as a "deal of the day" for $99.  I'm only sorry I didn't snatch-up a few more at those prices. 

[matevana]

After allowing the drivers to break-in another two hundred hours, I believe the crossover design is now final. Two changes were made which further simplify the design and affect the mid coupler only (*)

High Frequency Driver (Dayton AMTPRO-4)

C1, Mundorf Supreme Capacitor, 1.5 uf

Mid Coupler (Seas FU10, 8 ohm)  (inverse polarity for flattest response @ x/o)

Inductor Removed (*)
C2, Mundorf MCap, EVO Oil cap, 5.6 uF
R1, 15 ohm Mundorf MOX Resistors (*)

Lower Midrange (Eminence B102)

L2, Madisound "Sledgehammer" 15AWG Inductor, 8.0 mH



Change log:

The first change adds a bit more padding on the mid coupler. The new value is 15 ohms which yields an additional 1/2 dB attenuation and makes up for some lost resistance as a result of removing the series inductor from the circuit.

The second change negates the series inductor on the FU10. The Seas driver is marketed as a full range unit and quite frankly sounds better without the choke in the circuit. Since the AMT is crossed high, it acts more like a super tweeter in this design, rounding out the FU10 nicely.  Also note the inverse polarity on the mid coupler, which flattens the response at the crossover points.

This reduces the overall crossover component count to 4.  I am amazed by the simplicity of the design and how well it functions.  There is something to be said for drivers that are easy to cross and function well within their comfort zone.       

[matevana]

Here's some pics of the new U-frame/base. The baffle is 19mm ply with a 3/8" round over and features a rear mounted Usher 1001 woofer. I'm continuing to use the "Way Basics" pressed board cubes. Very light anti-resonant material... and they go together in about 10 minutes!


;




And the rear wire harness made from 13 AWG 4 conductor copper.

[matevana]

Tuning Out Dipole Bloom with the AMTPro

The Dayton AMTPro is a really interesting driver. For those who might not be familiar, the driver is delivered with two thick felt pads suspended between the neo magnet frame at the terminus. This allows the AMTPro to be either configured with a primary cardoid pattern (rear wave attenuated) or as a true dipole source (felt pads removed). 

Initially I was frustrated by the in room measurements of the AMTPro. The response was acceptably flat with the double felt pads in place and the rear wave attenuated. Removing the felt pads really opened up the sound, as expected, at the expense of a less than ideal response curve. In fairness, the less than perfect curve with the felt removed is partially a result of dipole bloom heard at the listening position courtesy of the front wall reflection.   

In the beginning I alternated between the more flat, more constrained cardoid pattern and the nicely diffuse but sometimes glaring figure eight pattern. At some point I got the idea to try using the felt pads to "tune" the driver's measurements from the listening position. It worked well.

The AMT diaphragm is divided into 18 sections by a black plastic grill/guide. The felt pads are held in place by friction and can be slid up and down the length of the frame. Covering 9 sections, for example, reduces the rear surface area of the diaphragm by 50%. Doing so reduces the amplitude of the rear wave but still maintains the proper dipole presentation relative to the front wall spacing. I suppose the figure of eight pattern is now somewhat irregular in its polar response, but it is a good alternative in the overall scheme of things.

The photo below shows the AMTPro with 5 of the 18 sections exposed (yellow diaphragm showing through) which represents the flattest on axis response in my listening room. This will vary based on distance to the front wall, room geometry, absorption and reflection, and a host of other environmental issues. In my situation I chose to uncover the rear diaphragm at the top of the AMTPro, since the expanding baffle width is said to help reduce bloom by modifying the way the rear wave wraps around the front and causes cancellation.



The second photo shows the thick felt material supplied with the AMTPro.           


   

[danvprod]

Excellent design. I've followed your other ones and this seems like a good ratio of complexity/performance. Well done. I'd love to hear them.

[bladesmith]

Matevana,

the AMTpro-4's sound very versatile, allowing more flexibility in the set up and in the actual speaker placement in the listening room. (?)

[Atlplasma]

Tuning Out Dipole Bloom with the AMTPro

The Dayton AMTPro is a really interesting driver. For those who might not be familiar, the driver is delivered with two thick felt pads suspended between the neo magnet frame at the terminus. This allows the AMTPro to be either configured with a primary cardoid pattern (rear wave attenuated) or as a true dipole source (felt pads removed). 

Initially I was frustrated by the in room measurements of the AMTPro. The response was acceptably flat with the double felt pads in place and the rear wave attenuated. Removing the felt pads really opened up the sound, as expected, at the expense of a less than ideal response curve. In fairness, the less than perfect curve with the felt removed is partially a result of dipole bloom heard at the listening position courtesy of the front wall reflection.   

In the beginning I alternated between the more flat, more constrained cardoid pattern and the nicely diffuse but sometimes glaring figure eight pattern. At some point I got the idea to try using the felt pads to "tune" the driver's measurements from the listening position. It worked well.

The AMT diaphragm is divided into 18 sections by a black plastic grill/guide. The felt pads are held in place by friction and can be slid up and down the length of the frame. Covering 9 sections, for example, reduces the rear surface area of the diaphragm by 50%. Doing so reduces the amplitude of the rear wave but still maintains the proper dipole presentation relative to the front wall spacing. I suppose the figure of eight pattern is now somewhat irregular in its polar response, but it is a good alternative in the overall scheme of things.

The photo below shows the AMTPro with 5 of the 18 sections exposed (yellow diaphragm showing through) which represents the flattest on axis response in my listening room. This will vary based on distance to the front wall, room geometry, absorption and reflection, and a host of other environmental issues. In my situation I chose to uncover the rear diaphragm at the top of the AMTPro, since the expanding baffle width is said to help reduce bloom by modifying the way the rear wave wraps around the front and causes cancellation.



The second photo shows the thick felt material supplied with the AMTPro.           


   

Any updates of your build?

[matevana]

None, other then I enjoy them every single day. The AMTPro tweeter is very articulate but can also have a fairly irregular in-room response, mostly due to reflections and perhaps some design anomalies. I have found that "tuning" this tweeter for in room response is critical to their success. The amount of rear wave attenuation can be dialed-in with the felt pads. if left unchecked I have found that in my room (bright, tiled, glass, 8' ceilings) the reflective surfaces would create a timing issue that occurs at the listening position. This did not happen when the V's were moved to a "softer" room. Reducing the level of the AMT's back wave helped significantly and also changed the overall radiation pattern. Since it is crossed fairly high, this in of itself was not a problem. I currently have only about a half inch of exposed membrane at the back of the AMTs, which provides a nice reflective and airy siblence, without the driver sounding too erratic.While very few technologies come close to the clarity and speed of the AMT design, there is something to be said for the simplicity and refined behavior of a well made dome tweeter. 

[pokerbarlo]

Hi @matevana are these still your current speakers or do you have an updated version planning on doing this for my first build.

[matevana]

Hi Pokerbarlo,

While I enjoyed building the Hestia V project, I have currently turned my attention to the Hestia Hybrid series as seen here: https://www.audiocircle.com/index.php?topic=169640.0

After experimenting with several OB designs over the years, I believe the Hybrids do many things right (and fewer things wrong), especially when working with a limited budget. I have modified the design slightly since my last post, but the appearance is unchanged.