OBL-15
Copyright 2015-16 © Troels Gravesen

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DRIVERS   CROSSOVER    BAFFLE    MEASUREMENTS    SPEAKER-KIT     CROSSOVER LAYOUT     SOUND

Since the introduction of the new JA8008-HMQ driver, there have been requests for an update on the OBL-11. The new 8008 driver has a different frequency response and can't just replace the old driver in the OBL-11.
Doing my fourth open baffle, I wanted to try out a different bass driver as well, and my eyes fell on the Ciare NDH 15-4S. Another candidate is the Faital 15PR400 (300 £/pair!). See Faital section below.
Out of the box I measure Fs = 34 Hz on the Ciare, but Ciare measures at 18V input (!), hence lower Fs. The Ciare features a massive 4" voice coil and a 1000 Watt (AES) power handling. I guess this will do for the OBL-15 to put it mildly! With its 400 watts power rating (AES), the Faital is way  more than adequate for the OBL-15 and with a system sensitivity around 93-94 dB we'll never feed this speaker any serious power. The Faital measures Fs = 32 Hz out of the box. Quite impressive! I'm used to PA driver showing much higher Fs compared to specifications.
Point of crossover between bass and mid has been lowered a bit, now around 160 Hz leaving all of the midrange to the JA8008-HMQ. Tweeters can be Audax TW034 or SEAS T35, please read below.

     
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DRIVERS
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Specs: Ciare NDH 15-4 S     JA8008-HMQ     Waveguide TW034     Audax TW034XOP47N

I bought the Ciare from LEAN, UK who has the best price I've seen (230 £/each)
Same place for the Faital 15PR400.
For Germany the Faital is available here.
For the US here.

Like the Ciare the Faital must be 8 Ohms.

The 8008+tweeter+crossover you get from Jantzen Audio, go to kit on this page.

The TW034 waveguide is mounted with four screws. See here how.

SEAS T35C002 tweeter can be used in replacement of the Audax TW034 without changes to crossover.
Special waveguide with threads is needed for T35. See here how to mount waveguide.

General advise: Never route for drivers before you have them at hand! Diameter can always vary a little if you want a tight fit.

FAQ:
- The SEAS T35 DOES NOT  sound better than the Audax TW034, only slightly different as the two domes will have slightly different frequency response, but nothing that necessitates different crossovers. I that respect they are too close. Only your ears can tell if you prefer one to the other.
- I can't tell if any other bass driver will work, so please do not ask, but my guess is there will be a range of suitable bass drivers. Based on modeling the Eminence Deltalite 2515 can be used by removing R12 and C9 and as said, the Faital 15PR400 for sure. The Eminence and Faital are so alike, I'm sure the Deltalite 2515 is OK.

Basics
3-way open baffle speaker from 15 inch bass, 8 inch midrange and 34 mm dome tweeter with waveguide.
Points of crossover 180 Hz (6-12dB) and 2 kHz (24 dB).
Sensitivity: 93-94 dB/2.8V, 1 meter
Impedance: 4-8 Ohm, minimum 4 Ohms @ 70 Hz.
Power requirement: Minimum 50 wpc if you run it from one amp. If you bi-amp, you can run the mid and tweeter from a smaller tube amp.


Faital 15PR-400, 8 Ohm


Faital 15PR-400. Click image to view large.

Download date sheet here.

Right after publishing the OBL-15/Ciare version, Eighteen Sound, another Italian speaker manufacturer, acquired the Ciare company and it will probably be some time before the full integration is in place and it is decided which Ciare products to continue and which not. The long and the short of it is that there's a shortage of NDH 15-4S drivers and I bought a pair of Faital 15PR-400 drivers for implementation. According to Murphy's Law, the Faital is 3 mm wider than the Ciare, so I had to route the black part of the front panels. The good news is that that's it! No changes to crossover or anything. Plug'n Play.

The Faital 15PR-400 is an extraordinary linear driver! This driver goes flat up to 4 kHz; a 15" driver! Sensitivity is the same as the Ciare, so no worries there. The 300£/pair price tag from Lean makes it a bargain compared to the 460£/pair for the Ciares.


Left: Faital 15PR-400 impedance profile in free air, Fs = 32 Hz after a little suspension massage.
Right: Comparing the 0.25 m response of Ciare (red) and Faital green), both merged with nearfield response @ 150 Hz. The Faital is extraordinary smooth with only a minor dip at 1 kHz, no problem here.

 
Faital measured at 0.25 m distance on baffle with no crossover (red) and driven from crossover (green).

 


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CROSSOVER
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Above the final simulation. Now, the midrange usually looks anything but nice on a open baffle due to rear-wave cancellation, but driven from the very simple crossover it looks nice and smooth. Crossover between bass and mid is around 170 Hz Hz, and the crossover slopes follow an (almost) 2nd order LR topology between bass and mid and fourth order between mid and tweeter. The latter facilitates some more upper midrange presence, something that brings the music into the room rather than being a window to the music (read OBL-11 to follow the discussion). The 8008 mid has an LCR on terminals smoothing the impedance peak at 45 Hz and a resistor by-passing C3 makes a 1st order roll-off for the mid-driver. This worked better compared to a straight 2nd order filter and made a more flat response in the 100-200 Hz area - phase is not a huge issue in this area. Having the 8008 driver on a wide baffle almost leaves no baffle step loss, hence quite some response level in the 100-300 Hz range. To overcome the bump here the large chunk of series capacitors are by-passed by R14, something you will not see in any other construction on these pages.

 

Crossover schematics
 
For Eminence Deltalite 2515 bass, remove R12 and C9 (based on simulation)

 

 

BAFFLE
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Baffle size for an open baffle speaker is a matter of compromise! The sound coming from the rear of the bass driver is obviously in negative phase compared to what comes from the front. This means that rear radiation will out-cancel the front radiation. Imagine a speaker with no baffle and we have no bass; the reason we usually put a driver into a box, either closed or vented. You can make the bass from the OBL15 deeper if you use a larger baffle than seen here, but given its size we get a reasonable bass response. The baffle size is chosen to give a decent bass response on level with the target 92-93 dB/2.8V system response. A very narrow baffle will also create trouble in the midrange response.
You should not put the speaker on spikes or lift it off the floor as the sound waves then will travel below the panel and out-cancel bass response. If we used a narrow baffle we would have to equalise the bass response accordingly and we would get an overall much lower system sensitivity. A example is the JAMO R909 open baffle speaker, which uses 2 x 15" high-efficiency drivers only to reach 89 dB sensitivity on a very narrow baffle. On a suitable baffle we might easily make 96-99 dB sensitivity with two 15" drivers - price to pay is size. This doesn't mean there's anything wrong with the JAMO, only a matter of design choices.


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Use 20-22 mm panels for all parts.

 


Left: Too bad I don't route for more 15" drivers! All bass should come from a 15" driver...
Right: Testing driver and waveguide rebates.


Right: Curving rear support panels.

You can make the panels much easier than mine. One-piece front + bass and mid extra panel, two rear support panels and two rear horizontal panels to keep things in place. And you can make them even better by gluing two 22 mm basic panels together to make 44-66 mm baffle thickness. Remember to chamfer 8008 driver hole accordingly! No need to chamfer bass hole. This driver is so big it has no trouble with proper ventilation.

 


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MEASUREMENTS
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Everything that can be counted does not necessarily count; everything that counts cannot necessarily be counted". Albert Einstein.

A few comments on MEASUREMENTS before you start interpreting all the readings below.
First of all, if we think measurements will tell us how a speaker sounds, we're wrong. The perception of sound is way too subjective to be reflected in any measurements we can perform. A loudspeaker system is meant to give us a satisfying idea of an acoustic event and for some people a pair of 5 USD ear-plugs are enough, others spend 200 kUSD on a truly full-range pair of speakers - and the latter may not be happier than the former.

Above you see the same graph presented in different width. Now, we may think the graph to the right looks rather rugged, but if we stretch out the presentation (left) it suddenly look rather smooth. The left presentation is often used by Chinese manufacturers to make their frequency response graphs look better. No smoothing was used on the above measurements, but if we apply e.g. 1/6 or 1/3 octave smoothing things look even better. Just this to display how easily measurements can be manipulated to look nice.


Measurements may give us an idea of tonal balance of a system, i.e. too much or too little energy in certain areas. Measurements may tell us about bass extension if far-field measurements are merged with near-field measurements. In addition to this, ports may contribute to bass extension. Most of us diy'ers do not have access to an anechoic room for full-range measurements from 20-20000 Hz, nor do manufacturers for that matter.
What cannot be seen is what kind of bass performance we get in a given room. Bass performance is highly dependent on in-room placement of your speaker and the same speaker can be boomy in one place and lean in another.
Actual SPL level at 1 meter distance and 2.8V input is useful for en estimate of system sensitivity and combined with the impedance profile may give an idea of how powerful an amplifier is needed to drive the speaker to adequate levels.
What measurements do not tell is the very sound of the speaker unless displaying serious linear distortion. The level of transparency, the ability to resolve micro-details, the "speed" of the bass, etc., cannot be derived from these data. Distortion measurements rarely tell much unless seriously bad, and most modern drivers display low distortion within their specified operating range. 
Many people put way too much into these graphs and my comments here are only meant as warning against over-interpretation. There are more to good sound than what can be extracted from a few graphs. Every graph needs interpretation in terms of what it means sonically and how it impacts our choice of mating drivers, cabinet and crossover design.
What measurements certainly do not tell is the sonic signature of the drivers, because cones made from polyprop, alu, Kevlar, paper, glass fiber, carbon fiber, magnesium, ceramics or even diamonds all have their way of colouring the sound.
The choice of crossover topology has a huge impact on the sound we get. We may produce the same frequency response from 1st, 2nd or 4th order filters and they may be Butterworth, Linkwitz-Riley, Bessel and others and they all sound different, very different indeed, so take care!

Actually I'm only going to show two measurements as they are pretty similar to OBL-11 and not least because measuring frequency response on an open baffle is a pain due to rear wave cancellation. What's show above is the MT @ 0.5 meter distance with 1/12 octave and 1/3 octave smoothing, red and yellow respectively. Green is nearfield response of bass driver. What we can get from this is a fairly even response vs. frequency around 93-94 dB/2.8V, 1 meter. To the right the impedance. This one we can get right. Minimum 4 Ohms @ 70 Hz.


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SPEAKER-KIT
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All questions regarding purchase of kits, please mail Jantzen Audio at contact@jantzen-audio.com

All kit and component prices may be subject to change and are always to be confirmed by Jantzen Audio Denmark.

Download Sales Presentations here

All technical questions to troels.gravesen@hotmail.com


CROSSOVER-LAYOUT
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JA8008-HMQ - Audax TW034 LAYOUT 


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CIARE BASS LAYOUT


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SOUND
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Link to youtube, click image and go to track 4, 14:09

Well, the OBL-11 was sold a few years ago (space issue) and I'd forgotten what a 15" on an open baffle does. I knew it was good, but that good? Frankly, the bass performance beats any other construction on these pages regardless of price. None of my other speakers even comes close to this. Listening to Niels Henning Ørsted Pedersen on This Is All I Ask, track four, "Just In Time", and you'll know what I mean. The articulation in lower registers is phenomenal and overall dynamics scary. This is close to what an upright bass should sound like. Even the best 10-inch driver in a reflex/closed box will never be able to deliver such detail and transient response. Never! Price to pay is size...
The sax solo by Phil Woods (track 7) has never sounded better in my living room than on these OBLs. Full-bodied and vibrant as ever.

Above my workshop set-up during crossover development. Yes, you can use these speakers in a 12 m^2 room, ½ meter to the side walls and 1 meter to the front wall and they'll blow your brains out. The 60 years old dry fillings in my workshop door were rattling like crazy. The bass can be too much of a good thing is very small rooms and bi-amping is a good thing if you can adjust level a little like I can with my 200 wpc Hypex. Alternatively add 1.0-1.5-2.2 Ohm in series with the bass driver (at the input of the bass crossover).
Pay notice these speakers can damage your hearing as they can play seriously loud without distortion and we tend to crank up the volume when we have low distortion. Tinnitus is a serious condition and I know a couple of guys having had their lives seriously impaired by this.
It's a strange thing that mid and treble is impacted so much from the quality of the bass, but it's true. Better bass and it pays off on mid and treble.