OBL-9
Copyright 2009 Troels Gravesen

   THE DRIVERS     CROSSOVER      BAFFLE     MEASUREMENTS      CROSSOVER KIT


    Open Baffle 2009 Construction from JA8008/TW034 drivers + Eminence Deltalite-II 2515
Read also about OBL-7 experiments and OBL-11, the final OBL

If I had a second livingroom - or a dedicated hifi-room, these panels would be there. View pics below. The JA8008/TW034 combo helped by the 15" Eminence bass drivers will probably never come closer to projecting a live feeling. In that respect they even beat the DTQWT by a small margin. But... it comes at a price, a serious price: Panel size, a suitable room, an electronic crossover + extra amps - and not least ultra-low WAF.
Whether your room can accommodate these panels is the first thing to consider. 20 square meters is probably minimum for these speakers to breathe properly and if your room does not allow for experimentation with placement, look elsewhere. Our livingroom is 24 m^2 and the panels were placed on the longest wall. As discussed in the OB7 file, a more optimal placement would probably be like this:

What's seen above to the right is well on accordance with Gilbert Briggs' recommendation for the SFB - way back in the Fifties.

What I almost hate to admit is the quality of the sound coming from the JA8008/TW034 combo after passing countless operational amplifires in the dbx electronic crossover + not to forget some off-the-shelf interconnects. No wonder professionals sometimes shake their heads hearing NFB triode nerds praising their trade.

Before getting into details, here's the recipe:

- 1 x JA8008/TW034 kit (= a pair), available from Jantzen Audio, mail for quotation here.
- 2 x Eminence Deltalite II 2515 bass drivers, available from
here.
- 1 x electronic crossover, 24 dB L/R with variable point of crossover
- 2 x stereo power amps, anything from 20 watts and up will do. Small SET amps can easily run the mid/tweeter section.
- A decent clone of the beautiful vintage Wharfedale SFB speaker.
- Liniarise Deltalite II impedance from 15 uF + 10 ohms across terminals.
- Add baffle step loss compensation to the bass drivers from 8.2 mH bypassed by 4.7 ohms.
- Hook it all up, set point of crossover between 80 Hz and 160 Hz to your liking.
- Make coffee from freshly ground Arabica beans, pour a decent cognac, sit down in your favourite chair and enjoy the concert!

After numerous experiments with passive crossovers between bass and mid drivers, I gave it up. Having a pro dbx 223XL, Stereo 2-way/Mono 3-way 24 dB, L/R electronic crossover, I inserted this between bass and mid and set the point of crossover at 80-120 Hz.
The 4th order electronic crossover makes a smooth transition between bass and mid and I suggest point of crossover at 120 Hz, but take your time and try vocal recordings @ 80-100-120-140 Hz and make your choice. Going higher draws vocals towards the floor and it starts sounding a bit funny.
Some other time I'll get the passive crossover in place. I want it simple, but this ECN set-up works darn well.

The current set-up makes a really big speaker and and it can play immensely loud with low distortion, so take care of your hearing!
There's an excellent live feeling to the presentation and I won't hesitate to recommend the current system for main speakers in recording studios, built into the walls in front of the all the mixing gear. With 220 liter cabs to the Eminence drivers and a port tuning of 32-35 Hz we will have response down to around 30 Hz. The 8008/TW034 could do with 25-30 liters. Well, another project....

Those who do open baffle studies may wonder why a relatively low-Qt (0.38) bass driver was chosen here. The Eminence 2515 will produce some 97-98 dB/2.8 volts and I simply wanted some extra sensitivity to play around with, and it's easy to tune the bass driver down to 94-95 dB, which is needed to match the 8008 driver. The Deltalite has a speedy and punchy bass response and from my OB7 baffle the main problem is identifying all the things in your room that starts rattling! What the Deltalite does on this baffle is making the music not only emotional, but also physical. Upright bass players will love the sound from these panels. Two 15" drivers hardly move driven even to very loud levels, i.e. distortion is as low as can be.


The Drivers
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Left: The JA8008 and TW034 drivers. Right: Eminence Deltalite II 2515 bass driver.
Click Deltalite for large image.


The Baffle
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The finished speaker will resemble the legendary
Wharfedale SFB. But until then, the OB7 baffles will be used.
Tilt baffle 5-7 deg. depending on listenning position.


The Crossover
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OB9 crossover. Yes, the bass driver needs a passive crossover before the ECN.
The C-coil by-passed by 5 ohms takes the sensitivity down to 8008 level and provides baffle step compensation as well.
Use 24 dB L/R ECN between bass and mid-tweeter section.


CROSSOVER KIT
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Drivers and complete crossover kit available from Jantzen Audio: Mail: contact@jantzen-audio.com


SET-UP


OB9 set-up.



OB9 in our livingroom, hard to overlook!


Rotel amp in control of the bass drivers and Audio Mirror amps for the mid and tweeter.
dbx electronic crossover hanging on the right speaker during fine-tuning.


Measurements
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There won't be a whole lot of measurements on this construction as the upper part very much can be seen in TQWT and DTQWT files. What's interesting here is the performance of mid and bass driver around point of crossover - and unfortunately it's pretty hopeless to present frequency response graphs in the bass region as I do not have an 8,000 cubic meter anechoic room like B&O. And even if I had, it wouldn't represent what's going on in our living rooms. Open baffles and the room they are placed in are so much an integrated system that any frequency response graphs will only reflect what's going in that particular room - at that particular location - and from that particular measuring point.


Left: Impedance of Eminence driver on baffle. Driver not broken in.
Right: Red = SPL @ 2.8 volts, 1 meter. Blue is from filter applied (1/3 octave smoothing).
This is an excellent response from a 15" driver. Smooth response up to 3-4 kHz from where it rapidly declines.


Left: Response of all driver on baffle.
Right: 8008 response @ 2.8 volts. Around 94 dB in the midrange, which is slightly less compared to measured
response in TQWT and DTQWT, but here we have a dipole, thus we have to take rear radiation into account.


Workshop images


"Standard" JA8008/TW034 set-up. Huge Eminence Deltalite II 2515 below.


Grounded Grid preamp and dbx 223XL electronic crossover.


Rear view with JA8008/TW034 crossover and equaliser on the bass driver. These 15" bass drivers barely move, even at loud playback levels.