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Old 6th August 2012, 09:53 PM   #1
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Default The Hestia Open Back - Ridiculously Affordable Chewy Goodness

The Hestia Open Back Project:

Background:

I’ve spent the past few months actively auditioning drivers that might be well suited for this project. An emphasis was placed on auditioning several mid bass/HF driver combinations that would become the cornerstone of the design. All were auditioned in the same test baffle and listening environment. So as not to overburden anyone with a gigantic post, I will be breaking this project into several installments, complete with pictures, links and other information. Please feel free to subscribe and follow along

Project Design Goals:

Well integrated driver selection
Shallow slope crossover w/low component count
High efficiency
Neutral vocal presentation
Small 3-way all inclusive design
Refined “no shout” voicing characteristics
Extended horizontal dispersion
Low cost (no single driver > $50 US)
Consistency; ability to replicate the design for the DIY community

A good part of the success in an OB design can be attributed to driver selection. In addition to choosing drivers that perform well in OB, they must also integrate properly with themselves. It seems that many amateur projects consider the former but not the later. Selecting drivers with well mannered roll-off characteristics will minimize the component count and preserve efficiency. Drivers with relatively smooth curves will allow for less complex crossovers. And perhaps most important in OB is the actual voicing characteristics of the driver itself. This can be a factor of the cone’s material and thickness, VC former construction, the baskets ability to suppress reflections, and a number of generally accepted TS parameters that are conducive to OB. The fact is that many acclaimed (and often expensive) drivers typically used in conventional box designs make poor choices in OB. Conversely there are a myriad of lower priced drivers that make excellent candidates for this project.

One of the primary design goals of the Hestia was to address vocal shout that seems to be present in many amateur and professional designs alike. For the purpose of this project, shout will be addressed primarily at the driver selection stage and secondarily in the crossover design. Many hours have been spent listening to raw drivers and driver combinations in the 1k to 5k Hz frequency range. I truly believe this one factor can make or break a design.

As if the above mentioned design goals aren’t lofty enough, cost should also be a big factor in the DIY arena. For this reason I decided that no single driver should cost more than $50 US, and where possible, much less.

From previous OB projects I have concluded that I really enjoy the presentation that a well suited 10” pro sound driver (or guitar/bass driver) provides in OB for the critical mid range segment. Unlike traditional 10” woofers, they are often voiced to provide stronger midrange up through 4-5k Hz. They also provide a full lower midrange on an open panel, where otherwise two smaller drivers might be used. Crossing relatively low will also offset some issues that can be associated with larger mids.

I spent a few months listening to eight 10” driver candidates made up of pro sound, guitar and bass guitar drivers One had an aluminum cone, another two were made of hemp and fiber and the rest were paper or paper blends. One driver had an alnico magnet, two had smallish neo magnets and the rest were ceramic. One driver even had an all paper former; the rest were poly. Most were in the .5 to .9 Qts range and all had an efficiency of at least 92dB at 1w/1m.

Based on a variety of listening tests, two drivers emerged at the top of the list, but one was cut due to cost (considering the mantra of no single driver > $50). The winner in this impromptu shoot-out was the MCM audio select 10” pro sound cast frame driver, # 55-2981. Every time I considered this driver I could not believe how ridiculously low priced this driver is positioned. It is basically a very well made cast frame driver with a 2”voice coil, a treated paper cone and a very useable excursion of 6.5mm and Qts of .54.with an average sensitivity of 93dB. It is often seen on sale by the manufacturer for around $25 US. Truly unbelievable! More important, it has a very nice low mid presentation with a rapidly descending roll off above 2k and no shout when crossed properly. A perfect choice for the Hestia project.

MCM Audio Select 10'' Die Cast Professional Woofer - 250W RMS | 55-2981 (552981) | MCM Audio Select

Next up, conquering the high-end.
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File Type: jpg Hestia 1.jpg (269.6 KB, 781 views)
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Old 7th August 2012, 11:28 AM   #2
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Default Attacking the top end

Tweeter selection was almost as daunting. I know from previous use that I really enjoy the neutral sound of the Usher 9950-20. But in keeping with the design goal of < $50 per driver, I set out to test a bunch of lower cost HF drivers that exhibited similar neutrality with a nice airy top end. In a two week period I borrowed as many tweeters as I could get my hands on. One aspect that became apparent was the enhanced dispersion characteristics of a ¾ dome vs. a one inch or larger diameter. But in a world of trade-offs, I wondered if it would be possible to meet the mid driver at a relatively low frequency w/o high distortion. In short, the answer was maybe…

I was unable to get my hands on the newest series of neo magnet Vifa/Peerless ¾ dome tweeters that have a surprisingly low Fs for their size. I did have an XT19 ring modulator style driver, along with a classic (and still available) D19TD-05 poly dome. I used the Usher as my benchmark and compared it’s neutral sound to a few Morel models, the above mentioned Vifa’s and several small Dayton neo domes and one Hi-Vi model as well. I started by crossing each of the drivers at 2k Hz LR-2 (not too taxing) and just listened to them. It’s amazing what you can hear when you isolate a driver before attempting integration. I listened to a bunch of cymbal and click tracks from a drumworks test CD, paying particular attention to the air the tweeter produces. Right off the bat about half of the test samples seemed to produce a “tizzy” quality to their top end. The Morel drivers were nice and a little on the warm side but had the poorest horizontal dispersion of the group, possibly due to their 1 1/8” diameter dome. The Dayton drivers were nice and small but seemed to suffer from consistency among the sampled pairs. In two cases, the overall clarity was questionable; one was a 5/8” dome and integration would likely have been problematic anyway.

The Vifa XT19 was an interesting bird. It didn’t measure too well with a simple LR-2 alignment, but I understood from reading about this driver that it would likely need more attention. The best results came with a 3rd order alignment using a cap on either side of the shunted coil. It did sound nice, but the more complex x/o might present phase integration problems with the intended lower order x/o of the mid woofer. The additional parts would also add cost to the project. On the plus side, it did have excellent horizontal dispersion at 30 and even 45 degrees off axis.

Interestingly enough, the driver that I preferred the most was the Vifa D19TD-05. It’s a classic style poly dome that Vifa OEM’d for use in many boutique shop branded mini monitors. It has excellent dispersion properties and next to the Usher, shared many of the same characteristics at a much lower cost. The pair that I have are made in Denmark. Vifa/Peerless currently produces this driver in Asia and many of the reviews have been favorable regarding the current facility’s ability to reproduce a faithful copy of the original. This driver will likely be produced for the long haul since it’s a drop-in replacement for so many mini-monitor style speakers. It also plays loud and clear and seems to handle a fair amount of power. Best of all, it currently retails for around $16 US.

On the down side it has a fairly high Fs, which might otherwise be problematic for this particular project. Based on my listening impression and the bargain basement cost of this nice tweeter, I wanted to see if there was a way to make it work.

Next up, passive crossover design and combating driver “shout”
Attached Images
File Type: jpg vifa.jpg (7.0 KB, 700 views)
File Type: jpg vifa 2.jpg (43.6 KB, 687 views)
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Old 8th August 2012, 11:46 AM   #3
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Default Crossover Design

The inductors are Jantzen 18 AWG air core and the Caps are Dayton Audio Polypropylene. I use the Dayton pre etched 2-way PC board number 260-130 which makes a nice finished product. I typically mount them on the baffle back with rubber washers. I was able to get the inductors about 4" apart from one another on the board, w/o having to change their orientation.

L1 = 1.80 mh 18 AWG
C1 = 7.5 uf
L2 = 1.40 mh 18 AWG
C2 = 5.6 uf


While the LR-2 wiring scheme is standard, the piece that's a bit unusual lies in the component value selection. There is no significant overlap between the tweeter and midbass, and by design, there is a small deficit between the two, producing a very slight dip in the frequency range normally associated with loudspeaker "shout". This helps give the Hestia it's characteristic sound, and IMO makes for hours of non-fatiguing listening possible. Unlike some large woofer/small FR driver designs, the midrange is NOT lacking and comes across as very natural with the proper weight and authority.

I had the luxury of being able to design-on-the-fly using a 3 way active crossover at the same 12dB slope, which was a great help in listening to driver interaction and understanding the outer limits of each driver. Once the x/o points and levels were scrutinized using the active set-up, I swapped in the passive components and remeasured (and swapped components again) until the new curve more or less matched the original. My take-away from this exercise was that while many drivers operate properly within their stated response range, huge performance gains can be derived by avoiding the the fringes of their boundaries.
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File Type: jpg schematic.jpg (37.5 KB, 602 views)
File Type: jpg Hestia rear 1.jpg (311.0 KB, 612 views)
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Old 8th August 2012, 11:59 AM   #4
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Default The LF Driver

The new Dayton line of pro sound drivers were selected for this project to handle bass duties, in particular the model PA255-8 10” Pro Sound woofer. Essentially there were three factors that went into this decision, namely (1) a high sensitivity of 95dB; (2) an Fs in the mid 40’s which is about 10 Hz or more lower than most 10” pro sound drivers, coupled with favorable Qts, xmax and MMS measurements; and (3) low cost ($43 US).

During development I routinely alternated the positions/duties of the Dayton with the MCM driver and came to this conclusion. I definitely prefer the MCM driver for mid duties, in fact it is the cornerstone of this project with its accurate and unassuming presentation, beating out a myriad of other tested drivers. That being said, the Dayton driver only offered a slight advantage in bass duties over the MCM in its ability to play slightly lower, slightly louder and handle slightly more power. Could the project be built using two MCM drivers at a lower cost? Absolutely… just remember that the cut-out for the MCM is around 1/8” larger. Still though the Dayton offers slight advantages in the bass position and stays within my goal of sub $50. It also comes with rear gaskets that help dampen unwanted resonances down low.

Next up: Pulling it all together
Attached Images
File Type: jpg Dayton 1.JPG (32.6 KB, 199 views)
File Type: jpg Dayton 2.JPG (41.2 KB, 52 views)
File Type: jpg Dayton 3.JPG (16.0 KB, 58 views)
File Type: jpg Dayton 4.JPG (38.7 KB, 79 views)
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Old 8th August 2012, 12:05 PM   #5
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I like your ideas. I find your design goals laudable and their execution interesting. Coincidentally, I am working on something somewhat similar, but with closed boxes. I will read your future posts with much interest.
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Old 8th August 2012, 12:08 PM   #6
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Default Baffle Dimensions / MCM Cast MidBass

The baffle dimensions are 23 1/2" x 15" and are mirror images. Ikea cabinet doors sold under the name Besta Vara make great test baffles, are sealed well to prevent warping and are fairly dense. Best of all, this size sells for $5 each and comes in 3 finishes. You can even laminate two together with a constrained damping layer in between to make a great looking, permanent baffle.
Attached Images
File Type: jpg Baffle.JPG (23.3 KB, 101 views)
File Type: jpg MCM.JPG (39.8 KB, 102 views)
File Type: jpg Curves.JPG (79.5 KB, 95 views)
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Old 8th August 2012, 12:12 PM   #7
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Quote:
Originally Posted by majerjack View Post
I like your ideas. I find your design goals laudable and their execution interesting. Coincidentally, I am working on something somewhat similar, but with closed boxes. I will read your future posts with much interest.
Thank you Majerjack!
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Old 8th August 2012, 01:31 PM   #8
SoIL4x4 is offline SoIL4x4  United States
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Matevana, this looks like a great project! I am new to open baffle designs (and my knowledge of speaker design in general is limited too) but I have a few questions about this design. I know one of your design goals is to keep these small in size, but would the performance of these change if the baffle was taller? Is the placement of the tweeter essential or is its location due to the size constraint of the baffle?

Sorry for the beginner questions but I am looking for a project just like this and would like to understand the design and its parameters.
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Old 8th August 2012, 01:39 PM   #9
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Quote:
Originally Posted by SoIL4x4 View Post
Matevana, this looks like a great project! I am new to open baffle designs (and my knowledge of speaker design in general is limited too) but I have a few questions about this design. I know one of your design goals is to keep these small in size, but would the performance of these change if the baffle was taller? Is the placement of the tweeter essential or is its location due to the size constraint of the baffle?

Sorry for the beginner questions but I am looking for a project just like this and would like to understand the design and its parameters.
Not at all! The most critical location is actually the bass (LF) driver which in this design should be as close to the floor as possible. The baffle must also touch the ground. Both of these factors come together to address floor bounces issues and if changed would not work.

The Mid and Tweeter locations are somewhat less critical and as you guessed
were more a factor of size constraints on the baffle. I would strive to keep the tweeter close to the mid and the offset has also proven beneficial in measurements. Note that the left & right baffle are mirror images and the tweeter should be placed toward the center. (Left baffle should have tweeter on the right side).

You may improve upon the design slightly if you raise the mid/tweeter pair closer to the actual listening height (seated), albeit at the expense of a slightly larger baffle. Mine are tilted back slightly (5 degrees) to compensate for their short profile.
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Old 8th August 2012, 04:56 PM   #10
SoIL4x4 is offline SoIL4x4  United States
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I like that this design addresses the midrange. Both of the pairs of speakers that I have now have their strong points, but, they are both two way designs that are a little thin on the mids. A simple design like this might be just right. Also after some quick searching I found everything but the MCM at parts express for under $100. The MCM is $29 on their site or through amazon. At about $125 a side before shipping for six drivers and two complete crossovers this is a pretty good deal, provided they sound good too
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