New Open Baffle Project

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Simon Hickie
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New Open Baffle Project

Unread post by Simon Hickie » Tue Aug 13, 2019 9:26 pm

I have been running with a pair of modified Allison 6 speakers for about a year and a half in my new listening room. With many genres, the SQ is excellent. But the upward firing 8 inch bass/mid unit has a tendency to excite not only room modes in the listening room, but also sound almost as loud in adjacent room as in my listening position. The deal-breaker, however, is relatively poor performance with classical music, and large scale orchestral music in particular.

With this in mind and wishing to stick with a DIY approach, an open baffle (OB) has presented itself as a possible route. Without wishing to repeat what has been discussed in various fora and articles, here are some of the salient sources:

http://www.quarter-wave.com/OBs/OB_Design_Part_1.pdf
http://www.quarter-wave.com/OBs/OB_Design_Part_2.pdf
http://www.quarter-wave.com/OBs/U_and_H_Frames.pdf
https://www.diyaudio.com/forums/full-ra ... ticle.html
https://audiokarma.org/forums/index.php ... ny.566879/
http://www.hi-fiworld.co.uk/loudspeaker ... rt-15.html
https://www.hi-fiworld.co.uk/index.php/ ... rials.html
http://www.hi-fiworld.co.uk/loudspeaker ... onses.html
https://www.hi-fiworld.co.uk/index.php/ ... ffle-.html

The hi-fi world articles in general and those on the Wharfedale SFB3 are particularly interesting. The 12” bass and 10” bass/mid drivers are run wide open and the upward firing tweeter with a single 4uF capacitor. All this should sound very familiar to aficionados of the Doc’s speaker philosophy.

Whilst going for an SFB3 type design is tempting, we are not exactly blessed at the moment with suitable drivers. Troels Gravesen has had a go with his OBL11 http://www.troelsgravesen.dk/OBL11.htm and OBL 15 http://www.troelsgravesen.dk/OBL-15.htm designs. However, even a quick glance reveals relatively high expense and moreover a crossover with more than just a 4uF capacitor.

So, taking into account MJK’s work on open baffle, U and H frames as a starting point, I have decided to go with a design that sits somewhere between an open baffle ‘slab’ and a U frame with separate full range driver over. I’ve decided to go with the Monacor SP-382PA 15 inch bass driver with a view to rolling it off electrically from somewhere between 175hz & 200hz. Everything else will be served by the Monacor SPX-32M full range driver with the high pass set at around 450-500hz. Crossover duties will be implemented digitally with a miniDSP 2x4. The miniDSP will output to something suitable for the power section, with four discrete outputs needed for the four drivers.

I am not keen on using EQ digital or otherwise to achieve a decent bass response. I am also aware that running a small, inexpensive 3 inch full range driver for the top six or seven octaves may be a little ambitious, and that more conventional mid & tweeter setup may prove better. However, this would introduce crossover complications.

I have toyed with using a pair of 12 inch bass drivers per side in a double U frame configuration. However, they only shift a little more air than a single 15 incher (total cone area of 2 x 12 inchers of about 1060 cm2, compared with 825 cm2 for the Monacor 15 inch. There was also the small matter of finding a 12 inch driver with a high enough qts. The Monacor SP-382PA is at the lower end of what works at around 0.75. However, with an Fs of 25hz and a very smooth and steady roll off, it should work well enough.

Plywood cutting should happen in the next week or two, so I will post some pictures and first impressions then.

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Docfoster
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Re: New Open Baffle Project

Unread post by Docfoster » Tue Aug 13, 2019 10:42 pm

Great run down of your thinking process so far. Thanks.
I’ve found Monacor drivers and components to be superb, and good value. Tho I’ve never played with one of their woofers. Look forward to reading how you get on with that.
System 1: JRiver MC24 > Dell Latitude 7270 > Asus Xonar Essence One Muses Edition DAC & pre- > One4 Class A power amp (with Dynamic PSU boards) > DIY 3 ways (based around Goodmans 18P woofers)
LC Audio Technology mains DC filter > Sovereign MTBPS balanced power supply
TQ (Wonfor) Ultra Black cables.

System 2: Anything with a headphone socket > Inca Tech Claymore > Reworked Goodmans Magisters


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Ali Tait
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Re: New Open Baffle Project

Unread post by Ali Tait » Wed Aug 14, 2019 5:37 am

I had great results with one of Martin’s designs which I ran actively with miniDSP, that used a Visaton B200 and a pair of 15” Eminence Alphas per channel. Worked really well and no EQ on the bass.

Simon Hickie
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Re: New Open Baffle Project

Unread post by Simon Hickie » Wed Aug 14, 2019 11:35 am

DocFoster, on the Monacor front, I've been using their SPH-210 8 inch drivers in my Allison 6 semi-omnis. They are very good indeed, and may get repurposed in the OB project. Or I might just sell them! Monacor drivers seem to have very well behaved roll-off at both ends.

Thanks Raul-77 for the additional links. I was aware of the Wild Burro drivers, but not in these implementations. They could work very well as a mid/treble driver in an OB system.

Good feedback Ali on an MJK design. Thanks! I had considered the Eminence Alpha 15 but plumped for the Monacor in the end.

My 'U' frame will basically have movable wings for both the bass and mid/treble sections. On the bass section they can be used either to extend the width of the baffle, be folded right back to form the 'U', or be somewhere in between. Listening tests should confirm the best arrangement before building a mk2 version with better materials and fixed construction.

For the mid/treble section, Troels Gravesen did a study many years ago into how to minimise peaks and ripples in the mid/treble region. I cannot find the relevant post from another forum, but essentially a 24cm wide central baffle with 9cm wings on each side folded back by 45 degrees gave a very smooth response, and much better than sticking the driver in the middle of a flat baffle. MJK offsets his full range drivers a bit in his OB design.

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Docfoster
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Re: New Open Baffle Project

Unread post by Docfoster » Wed Aug 14, 2019 1:34 pm

Simon Hickie wrote:
Wed Aug 14, 2019 11:35 am
DocFoster, on the Monacor front, I've been using their SPH-210 8 inch drivers in my Allison 6 semi-omnis. They are very good indeed, and may get repurposed in the OB project. Or I might just sell them! Monacor drivers seem to have very well behaved roll-off at both ends.
That doesn’t surprise me. Although I haven’t studied the specs as closely as you seem to have, their drivers always sound tidy to me.
I was introduced to Monacor a few years ago when Paul of RFC elected to use one of their tweeters in a speaker rebuild he did for me. I was very impressed by how smooth the tweeter sounded. When I started to mess around with speakers myself, I tried a few of their other tweeters, a mid-driver and an off-the shelf crossover, that basic-specs-wise suited what I was trying to do. Once installed and listened to, they’ve always performed as I’ve hoped.
System 1: JRiver MC24 > Dell Latitude 7270 > Asus Xonar Essence One Muses Edition DAC & pre- > One4 Class A power amp (with Dynamic PSU boards) > DIY 3 ways (based around Goodmans 18P woofers)
LC Audio Technology mains DC filter > Sovereign MTBPS balanced power supply
TQ (Wonfor) Ultra Black cables.

System 2: Anything with a headphone socket > Inca Tech Claymore > Reworked Goodmans Magisters

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Ali Tait
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Re: New Open Baffle Project

Unread post by Ali Tait » Wed Aug 14, 2019 4:07 pm

I currently have a pair of Bastanis OB’s, these use a Goldwood 18” woofer, 12” I think Eminence driver that is modded and a compression driver tweeter unknown to me. These do use some EQ on the bass but sound none the worse for it, bass being very tight, fast and detailed in the usual OB manner. They can be almost scary at times with the dynamics they are capable of. I see Vic’s latest efforts involve a pair of UM18 per side bolted together facing each other in a cradle, so no baffle. Xmax on these is huge, they must be seriously capable with enough power.

Simon Hickie
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Re: New Open Baffle Project

Unread post by Simon Hickie » Sat Aug 31, 2019 11:13 am

The development baffles are now made. They have adjustable wings to help determine whether a standard baffle or a U frame, or something in between works best. Drivers installed. The miniDSP has been configured with a 4th order LR XO @ 160hz on both drivers. I have applied a 5db cut to the full range driver, but another db or two may be needed to get the sensitivity matching right. MJK's rule of thumb is that the bass driver needs to be about 6-10db more efficient than the full range driver. The Monacor SP-382PA is around 94db, the Monacor SPX-32M full range is around 88db, so I'm there or thereabouts (I tend to be rather treble sensitive).

Overall first impressions are very positive. Soundstage is wide and high. Bass is solid and sufficiently extended. Room modes are not excited in the same way as with a box speaker, so one can go louder with less boominess. More fine tuning is obviously needed, but I suspect this is going to be the way forward from now on. Once I have settled on the right baffle configuration, the next step will be to make something that actually looks decent.

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karatestu
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Re: New Open Baffle Project

Unread post by karatestu » Sat Aug 31, 2019 11:26 am

Nice one Simon :guiness;

I take it that not all drivers are suitable for open baffle use. What special TS parameters have they got ? Don't know much about OB design :think:
DIY inspired by Richard "The Doc" Dunn RIP

Simon Hickie
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Location: East Staffordshire
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Re: New Open Baffle Project

Unread post by Simon Hickie » Sat Aug 31, 2019 12:35 pm

Stu, one of the key criteria for the bass driver is a high QTS. Over 0.7 is desirable. The Eminence Alpha 15A is often recommended with a QTS of 1.2ish, but Fs is a highish 41hz. But the magnet on those is a bit weedy. The monacor has a QTS of 0.75 and an Fs of 25hz. There are very few drivers that are really suitable. I'd considered a pair of 12 inchers per side, but finding high QTS 12 inch drivers is very hard. The other thing is a decent xmax.

Wiring a pair of bass drivers in parallel will give a 6db efficiency boost, whilst wiring them in series will keep efficiency the same, but will shift more air for any given voltage. A lot will depend on amplifier capability as to which will work best.

TS parameters for the full ranger (if 2 way) are pretty irrelevant, with efficiency & xmax being the main criteria. Steve's (the shadow) experience with the Fane 12 inch full ranger is interesting and is a possible alternative to often recommended 4 inch FR drivers. Fostex often seem to be used, but I much prefer the value you seem to get from European drivers.

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