AudioCircle
Audio/Video Gear and Systems => Open Baffle Speakers => Topic started by: matevana on 11 Jun 2014, 09:25 pm
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(http://www.audiocircle.com/image.php?id=100895); (http://www.audiocircle.com/image.php?id=100901)
The Hestia V represents my 5th documented OB design on AudioCircle and includes many of the design choices I have found to be successful. It is true-to-form regarding my own design philosophies and musical tastes.
Major design goals include:
• A passive crossover/compensation network coupled with specific driver selection negating the need for active line level digital or analog filtering and more than two sources of amplification.
• Availability of CNC baffle milling service for those wishing to build the design but avoid the complexity of milling one’s own baffle. I have provided the shop with the .DXF file (Autocad) for use by AudioCircle members. Baffles can be reproduced quickly and shipped to your location. Please PM me for contact information.
• Dipole operation throughout the audible frequency range
• A 3.5 way system including a separate H-Frame woofer system plus a main baffle supporting a lower midrange, midrange coupler and high frequency drivers.
• General Baffle Shape in homage to Siegfried Linkwitz and his LX521 design.
• Rear mounting of the Lower Midrange and Coupling drivers to help facilitate an unencumbered rear wave
• Integral baffle round-overs help promote off axis radiation.
(http://www.audiocircle.com/image.php?id=100896)
Drivers:
Please note that driver selection is a major aspect of the design and integration. It is generally not possible to make substitutions without significant changes to the crossover components, baffle dimensions, or both.
High Frequency: This is my first design to make use of a true dipole HF radiator. The Dayton AMTPRO-4 air motion transformer tweeter comes equipped with two thick felt pads that can be removed (friction fit) for open back radiation. The pleated diaphragm squeezes air which results in a very open and airy presentation. It is known to sound very “fast” and articulate and compliments dipole bass nicely. The radiation pattern (both front and rear) differs from that of a typical dome radiator.
(http://www.audiocircle.com/image.php?id=100897); (http://www.audiocircle.com/image.php?id=100898)
<continued>
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Mid Coupler: The Seas FU10RB H1600-08 was selected as the 4” mid coupler in this design for its acclaimed “Curv Cone” woven polypropylene properties. This driver is very similar to the mid coupler used in the Linkwitz LX521 but with an 8 ohm impedance rating. The higher impedance lowers the cost of the associated passive crossover components in this design and the higher Qts of the mid coupler facilitates a passive compensation approach.
(http://www.audiocircle.com/image.php?id=100902); (http://www.audiocircle.com/image.php?id=100903)
Lower Midrange: In keeping with the lessons learned from the Hestia Forte design, Drivers manufactured for guitar/bass amplification can at times make great candidates for OB. These drivers are often voiced for guitar players who favor a mid-scoop in their presentation. If selected carefully, this particular voicing can be useful in compensating for dipole roll-off. By targeting the roll-off correctly within the scoop, you are essentially left with a beneficial low end tilt without active compensation or additional passive components. Contrary to the mid coupler above, I favor this approach to selecting a lower midrange driver with a high Qts. The driver’s motor keeps tight control over the cone, and unlike some other high Q drivers, the upper bass and lower mids are never sloppy.
These drivers are also typically about 5-10 dB more sensitive than their home hi-fi counterparts. The real challenge is finding a driver with the right combination of the above characteristics, along with a workably flat response, a reasonable excursion, and a slow or delayed breakup mode.
The Eminence Legend B102 is one such driver that incorporates a well vented cast frame and comes in at 93 dB 1w/1m. It has reasonable excursion before damage with XLim topping out at 8mm. It also happens to sound very nice as it handshakes with the Seas coupling driver due in part to the whizzer cone design. While the crossover starts attenuating this driver’s high frequencies early (down 6 dB at 280Hz and 12 dB at 560 Hz) the gradual first order slope still presents significant amplitude in the area where the secondary cone has some impact on clarity and detail. And unlike some guitar speakers that are designed to be overdriven easily, the breakup mode on the B102 is more akin to a pro sound driver than a guitar speaker.
(http://www.audiocircle.com/image.php?id=100904)
Bass Cabinet: (optional) This is the one aspect of the design that can be customized more readily. I am using the same dual H-Frame units designed for the Hestia Forte project. Contrary to the LX-521 design, I prefer the use of an H-Frame orientation that places both drivers against the floor rather than a vertical orientation. I have found the Peerless SLS drivers to provide good value. Regardless of the cabinet and drivers selected, I recommend the use of my dual slope approach in lieu of a traditional subwoofer amp with built in crossover. The dual slope topology addresses the primary low pass target with an inline passive “line level” filter. A lesser slope then attenuates separately at a rate of 6 db per octave at the speaker level with the use of a traditional inductor. It is this second slope that compensates for dipole roll-off on the bass cabinet. A subwoofer plate amp with a built-in variable crossover does not accomplish the same.
(http://www.audiocircle.com/image.php?id=100905)
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Baffle Fabrication
I have teamed-up with Melby Audio to offer first rate, CNC milled baffles for those wishing to build the design. Weston is an avid craftsman and an audio enthusiast as well. He can mill the baffles, bases, and intermediate parts and then ship them to your door as a "flat pack". He also offers a variety of finishing options. His work is truly first rate and very reasonable. PM me if you would like his contact information. (Please understand that I have arranged for this service as an enthusiast and do not profit from his work).
The standard baffle material is 19mm ply with a maple face, as seen below. Other options may also be available.
(http://www.audiocircle.com/image.php?id=100906)
Melby Audio is also a source for many of the components needed to complete the Hestia V Build. They can offer discounted pricing on the Dayton AMTPRO-4 Tweeters, along with some of the other components you may need for this (and other) projects. As a customer, my experience has been very positive and I would encourage you to give them a try. I also believe in supporting the few "mom and pop" audio businesses that remain!
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Passive Crossover
The crossover topology is similar to the Forte. I am using even higher quality components as it really makes a difference with drivers of this quality. The basic layout is as follows, along with the initial component values.
(http://www.audiocircle.com/image.php?id=100907)
High frequency section (Dayton AMT Pro)
C1, Mundorf Supreme Capacitor, 1.5 uf
Mid Coupler (Seas FU10, 8 ohm) (inverse polarity for flattest response @ x/o)
L1, Erse .27 mH, Perfect Layer Inductor, 18 AWG
C2, Mundorf MCap, EVO Oil cap, 5.6 uF
R1, 5.6 ohm + 3.9 ohm Mundorf MOX Resistors (in series)
Lower Midrange (Eminence B102)
L2, Madisound "Sledgehammer" 15AWG Inductor, 8.0 mH
Please note: While the crossover topology will not likely change, the drivers themselves are still new and are settling-in. When the drivers reach their final TS parameters some small value adjustments may be necessary. They have remained stable for the first +/- 20 hours of use.
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Here's a few more pics of the baffle/base detail, before adding drivers:
(http://www.audiocircle.com/image.php?id=100913); (http://www.audiocircle.com/image.php?id=100914)
(http://www.audiocircle.com/image.php?id=100915); (http://www.audiocircle.com/image.php?id=100916)
(http://www.audiocircle.com/image.php?id=100917)
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Cool!
You didn't need any padding on the tweeter?
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how about some measurements? :)
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I'm having trouble wrapping my head around the mid coupler part of the design since the driver is way less efficient than either the AMT or B102, how does it contribute?
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Cool!
You didn't need any padding on the tweeter?
Good question. The Dayton AMTPRO-4 is rated at 94dB. I can only assume this measurement represents it's sensitivity in cardoid mode since it is somewhat less efficient as a full dipole. The driver comes with two thick felt pads saddled between two powerful neo magnets in the back. Both pads together make up a thickness of about 1/2 inch and block nearly all of the rear wave. In this mode the driver approximates a cardoid pattern and is more efficient. With both pads removed the rear radiation pattern approximates that of the forward pattern (inverse) and the driver acts a s a dipole, albeit somewhat less efficient. BTW, using only one of the two felt pads seems to yield a hybrid mode which may be helpful for dipoles that are placed too close to the rear wall since some of the rear wave is lost.
That being said, the drivers are still settling-in and I have tested both no attenuation and a small padding resistor (1-2 dB loss). At this point I prefer no padding at all.
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That baffle looks like it would vibrate like crazy with those large drivers. Seems like it needs more bracing in the rear.
I like them alot, just seems like they would vibrate.
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I'm having trouble wrapping my head around the mid coupler part of the design since the driver is way less efficient than either the AMT or B102, how does it contribute?
A general rule of thumb in designing a passive OB system is to start with a LF driver that is 8-10 dB more efficient than the mid driver. This is due to the 18dB/octave loss in low frequencies (12 dB/oct driver roll-off plus an additional 6 dB/oct loss due to OB). The Eminence B102 is rated around 93 dB which is close to the desired target. The large 8 mH inductor will attenuate most of this drivers upper mid and higher frequencies and we are left with a more proper response curve after roll-off. Martin King wrote about this almost a decade ago and I have found his study to be very helpful.
With regard to the AMTPRO's efficiency, note that amplitude is lost here as well when the driver functions in dipole mode and the rear wave is allowed to exit freely.
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That baffle looks like it would vibrate like crazy with those large drivers. Seems like it needs more bracing in the rear.
I like them alot, just seems like they would vibrate.
Thanks for the comment. I haven't found this to be an issue, at least at normal listening levels. Perhaps this is due to having very limited baffle real estate away from the driver cut outs. All three drivers have a rigid cast frame and really help keep the baffle well dampened and stiff. The 19mm plywood that was used in this build seems to have more alternating layers than I remember seeing at the big box stores. It is also void free. I would say this plays a role as well.
Still though, I would recommend placing the top baffle on a support that straddles the bass cabinet, similar to the LX521 design. This seems like a good way to keep vibration from the bass drivers from impacting the mids.
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Nice.... 8)
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Matevana, thank you. I had forgotten about the huge loss from the mid bass driver when used OB. I am curious about your choice of an AMT tweeter. The units I am familiar with tended to have almost no vertical dispersion. I appreciate not having an extra tweeter facing the rear, but I wonder if you tried other tweeter types before settling on the AMT.
Great design.
Steve
Edit: My concern is that the AMT tweeter will sound great in the horizontal plane, the very small vertical window will only allow for a seated sweet spot. I have Linkwitz' Orion system, and owned them since their single tweeter days, and made a few of the subsequent changes. This new project shows some interesting choices.
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Fyi. The tweeter in the "genuine" LX521 is crossed over at 8Khz, making it almost an optional component. (Linkwitz uses two dome tweeters in the LX521 to achieve near perfect polar response across the entire audible band).
It would be interesting to know what your crossover points are and to see some measurements.
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Nice.... :D
I'll second that! Nice indeed. :thumb:
One question. The "mid-coupler" sits on the back of a thick baffle. That should affect the dispersion at the front of the Seas driver as it fires into a short tunnel. I would think some off axes measurement issues or horn "cupped hands" shout would be noticeable. Is there a reason why you did not front mount this nice little driver on the baffle?
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Matevana, thank you. I had forgotten about the huge loss from the mid bass driver when used OB. I am curious about your choice of an AMT tweeter. The units I am familiar with tended to have almost no vertical dispersion. I appreciate not having an extra tweeter facing the rear, but I wonder if you tried other tweeter types before settling on the AMT.
Great design.
Steve
Steve, that’s a great question. In general I have found your comment about AMTs to be spot on. I played with this tweeter and others for months before deciding to move forward with the AMTPRO.
I have tried to mitigate the vertical dispersion issue in two ways. First please understand that in the Hestia V design the AMT is used more like a super tweeter, supporting the highest audible octaves and adding a bit of midrange transparency. The majority of the upper frequency duties are handled aptly by the Seas FU10. In fact Madisound categorizes the FU10 as a full range driver but labels the MU10 (used in the LX521) as a midrange driver. Perhaps this is just based on its intended use, but it happens to be a nice sounding FR driver as well.
Second I was able to cheat a bit by tilting the V’s base a few degrees back with the use of sorbothane feet in the front and thinner felt pads in the back. I have found this further minimizes any delta between seated and standing listening positions. That being said, there can also be some benefit (in certain rooms) by reducing ceiling and floor reflections that a more narrow dispersion provides.
If time permits I will build a dome radiator version of the V for direct comparison. In the meanwhile I am really enjoying the presentation that the AMT provides. I have heard many comments about the “fast” nature of this tweeter and how it can “keep-up” with dipole bass. For me this benefit is dwarfed by the positive effect the AMT has on “opening-up” the midrange below.
I also noticed that my Dayton AMTPRO sample has a lens that is different from PE’s catalogue photo. My sample has elliptical ends at the top and bottom of the lens; the catalog shows horizontal lines. Perhaps this was done to address the concern as well.
Ed
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Fyi. The tweeter in the "genuine" LX521 is crossed over at 8Khz, making it almost an optional component. (Linkwitz uses two dome tweeters in the LX521 to achieve near perfect polar response across the entire audible band).
It is said that SL was shooting for near perfect polar response when he designed the LX521. This seems logical as he has accomplished nearly everything else he has set out to do. I really hope to be able to hear the LX521 soon; it truly seems like an amazing system.
The primary design goal of the Hestia V is more basic; to approximate a flat-ish response curve through passive filtering and proper driver matching, negating the need for multi-channel amplification.
I am also a proponent of passive preamps (a single Luminous Axiom attenuator feeding a B&K power amp). I did not want to introduce a MINIdsp or an FIR based device into the chain.
The AMTPRO is rolled off gradually with a single Mundorf supreme 1.5 uf capacitor. It is down around 12 dB at 13k Hz, and 24 dB at 6500 Hz. Its primary function is to add some sparkle to the FU10 while maintaining dipolar radiation. I have listened to the V while bypassing both the AMT tweeter and the inductor on the FU10. It is still quite presentable with only a slight degradation to the highest audible octaves. As mentioned previously, another benefit of the AMT tweeter is the transparency it lends to the overall midrange presentation.
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very nice thanks for sharing. :thumb:
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I'll second that! Nice indeed. :thumb:
One question. The "mid-coupler" sits on the back of a thick baffle. That should affect the dispersion at the front of the Seas driver as it fires into a short tunnel. I would think some off axes measurement issues or horn "cupped hands" shout would be noticeable. Is there a reason why you did not front mount this nice little driver on the baffle?
Nick, that too is a great question. I have spent many hours listening to the FU10 as both a front and rear mounted driver on the baffle. First off, I haven’t experienced the “cupped hands” effect in this design (but definitely have in others so I was always very conscious of listening for midrange shout). The baffle front has a rather large 3/8” round over on a fairly tight driver cut-out. I believe this helps mitigate the effect you speak of.
But I had a bigger concern with the effect of linear and non-linear distortion if the rear of the cutout was not addressed properly, considering the physical design of the FU10’s magnet and motor structure. This phenomenon was noted by Zaph when he reviewed the Scanspeak 10F which is constructed similarly. As a design choice, I chose to eliminate this concern altogether by rear mounting the FU10 for a truly unencumbered rear exit. The alternative would have been to design a large chamfer on the back of the baffle, which would have necessitated further than ideal driver-to-driver spacing and may have structurally compromised the baffle at it's narrowest point.
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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
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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.
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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.
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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!
(http://www.audiocircle.com/image.php?id=101911) ; (http://www.audiocircle.com/image.php?id=101912)
(http://www.audiocircle.com/image.php?id=101913)
And the rear wire harness made from 13 AWG 4 conductor copper.
(http://www.audiocircle.com/image.php?id=101914)
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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.
(http://www.audiocircle.com/image.php?id=101976)
The second photo shows the thick felt material supplied with the AMTPro.
(http://www.audiocircle.com/image.php?id=101977)
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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.
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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. (?)
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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.
(http://www.audiocircle.com/image.php?id=101976)
The second photo shows the thick felt material supplied with the AMTPro.
(http://www.audiocircle.com/image.php?id=101977)
Any updates of your build?
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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.
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Hi @matevana are these still your current speakers or do you have an updated version planning on doing this for my first build.
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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 (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.