OB Mk 3

Dedicated to those large boxes at one end of the room
Post Reply
steve s
Shed dweller
Posts: 2105
Joined: Wed May 30, 2007 6:19 pm
Location: east yorks

OB Mk 3

Post by steve s » Sun Jun 03, 2007 10:53 pm

Well
an interesting weekend..

thurs & friday evenings I was covered in sawdust.. got them screwed togther on sat night.. baffles are so easy to make

the axiom 61's blend in with the 301's so well...

and the lowthers virtually used as a tweeter matches in nearly as well..
i can hear the cross over where the 301 and lowther meet.. i wonder if any one else will notice next week end..

sound wise..
the lowther is 4' in the air now.. so the result as a wall of sound.. different to the lower baffles i've been using..

the sound is so much fuller at the bottom end, and the top end blends in better, a much better combination than the 'old baffles'..they where little peaky...

no crossover.. my logic was this..
lowther top end needed no reinforcement.. but no top end from the 301

lower mids supported by the inner cone of the 301= +3db ( two speakers working together

bass the same..(the 61 and the main cone of the 301) +3dbs as two speakers working together.. but a reduction in baffle size has reduced this a little..

placed touching a side or back wall, the bass thunders, but moved away a few inches they sound better balanced..

loads of low bass detail..mids and the top as before..phew..

I'm very impressed at this point...

I've have a few more mods in mind..i want the lowther to go a little lower (as the cross over point!! is a little high).. its an easy mod.. baffle extensions..

The Mk 3 are 1" chipboard.. doubled up around the speakers and bolted together
i will have to measure them to see how they really are in comparison to the old baffles.

i never thought i say this.. but they sound so natural and uncoloured

thanks nick..for inspiring me!

steve

simon
Eternally single
Posts: 4395
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Post by simon » Mon Jun 11, 2007 10:03 pm

Hi Steve.

It was a treat to hear your mk 3 OBs this weekend. I was surprised how different, and how much better, they sounded than the mk 2s. I thought you were a worthy winner on Saturday simply for the new approach that we haven't seen before at a Fest. I'm quite taken with the approach and I fancy having a go. Just need to find a suitable driver to go with the 208Es. James' Supravox would do it but I'd rather pay Emminence money...

So I'm marooned in a rather shabby hotel with Paul Merton in China on the box for company (it's called a Quality Hotel, it really, really isn't) but I've just had a browse through a reprint I have of Gilbert Briggs' "Loudspeakers" 4th edition. It's more of a pamphlet than a book and it only briefly touches on OBs.

But what's interesting is a table indicating baffle diameter for the frequencies of the keys on a piano keyboard. For 40Hz (an E) a diameter of 14 feet is required. A diameter of 7 feet gives a drop off of 6dB. I believe you've made a trade off using the floor and side wall(s)? Briggs doesn't go into this, perhaps you could give some advice on how you approached this?

Simon.

steve s
Shed dweller
Posts: 2105
Joined: Wed May 30, 2007 6:19 pm
Location: east yorks

Post by steve s » Tue Jun 12, 2007 11:47 am

Hi Simon, you have one of the many speaker building 8 page or so leaflets

i have a couple of those but there is no background to the designs.

the book i have has 330 pages and 15 of those are on ob's and alot of the book is relevant to obs and the drivers.

I will lend you it.. if you don't find a copy before i see you next..
s/h book shops for around a fiver or so....! i would pay a tenner for one?

There is too much to sum up in a couple of lines..and you will enjoy the whole book i should think, ..its not a technically written, but it covers technical basics as they where 40 years ago....
i thought it worked for me...

steve

simon
Eternally single
Posts: 4395
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Post by simon » Tue Jun 12, 2007 6:16 pm

Okay Steve, I'll keep an eye out though I've never had much luck finding old audio books. Would love to borrow your book though if I can't find one.

Simon.

User avatar
andrew Ivimey
No idea why I do this anymore
Posts: 6800
Joined: Mon Jun 11, 2007 8:33 am
Location: Bedford

Post by andrew Ivimey » Tue Jun 12, 2007 6:41 pm

Simon, if you go to abebooks.com and type in G.A.Briggs and Loudspeakers you will find the book is available from Graham's Books in Wootton Bassett.

Andrew

simon
Eternally single
Posts: 4395
Joined: Thu May 24, 2007 11:22 am
Location: People's Republic of South Yorkshire

Post by simon » Tue Jun 12, 2007 9:18 pm

Thanks Andrew, interesting site. Unfortunately I think that's an original of the reprint I have.

I've just watched one go for £8.28 on ebay. As good as my pda is for keeping me in touch Windows Mobile's IE doesn't recognise every button-type link, including Place Bid. To rub salt in to the wound the seller is located in Doncaster...

On the bright side they've moved me to a better room after I complained about the room above's shower dripping on to my floor at 3.30 this morning. Quality (Hotel) my a***!

steve s
Shed dweller
Posts: 2105
Joined: Wed May 30, 2007 6:19 pm
Location: east yorks

Post by steve s » Fri Oct 05, 2007 5:36 pm

As those at witham will have seen, my baffles have been modified again.
Peter suggested i roll off the top end of bass speakers a little, so last night i had a go.. he did say don't touch the lowthers

(and scott had been saying this for a long time !)

i had a small stock of inductors 4x 6 mh from an old decca decola stereo and a couple of 1mh from somwhere or other

i'm quite new to this lark so i calculated the the 6 mh would 'cut' at 200 hz and the 1 mh would do the same at 1khz

a quick clip lead up.. the 6 mh where no use.. they cut off far too much

the 1 mh made the mids seem a little cleaner. quite good..
using both the 1mh .. 2mh on one speaker was not that much different really.. so i'm happly using the 1 mh on each side... i will be on the lookout now for a selection of inductors or some winding wire..

...so i'm nearly using a crossover..

i don't have a meter ( and a lack of knowlege) would 2x 6mh in paralle make a 3mh any one? not sure how they work

steve

User avatar
ed
retired
Posts: 4262
Joined: Thu Jun 21, 2007 4:01 pm
Location: yorkshire
Contact:

Post by ed » Sat Oct 20, 2007 2:43 pm

Hi Steve

I found this whilst wibbling round the web....its NP at Rocky Mountain fest recently.....curiously similar setup to yours(except the amps of course), you are in truly exalted company

http://www.6moons.com/audioreviews/rethm3/4_5.jpg

Ed

User avatar
Scottmoose
Needs to get out more
Posts: 1562
Joined: Sat Sep 08, 2007 11:03 am
Contact:

Post by Scottmoose » Sun Oct 21, 2007 11:35 am

deleted
Last edited by Scottmoose on Sun Oct 09, 2011 12:35 pm, edited 1 time in total.

steve s
Shed dweller
Posts: 2105
Joined: Wed May 30, 2007 6:19 pm
Location: east yorks

Post by steve s » Tue Oct 23, 2007 10:21 pm

ed wrote:Hi Steve

I found this whilst wibbling round the web....its NP at Rocky Mountain fest recently.....curiously similar setup to yours(except the amps of course), you are in truly exalted company

http://www.6moons.com/audioreviews/rethm3/4_5.jpg

Ed
thanks Ed, just catching up with things they do look similar.. the bass drivers look very nice.. but i've not a clue what they are.. probably more expensive than mine..
my jobs seem to be sounding better since the mids have been rolled off the bass units..getting used to it.. very pleased so far..?

steve


sounds cleaner..

User avatar
Scottmoose
Needs to get out more
Posts: 1562
Joined: Sat Sep 08, 2007 11:03 am
Contact:

Post by Scottmoose » Wed Oct 24, 2007 10:18 am

deleted
Last edited by Scottmoose on Sun Oct 09, 2011 12:35 pm, edited 1 time in total.

steve s
Shed dweller
Posts: 2105
Joined: Wed May 30, 2007 6:19 pm
Location: east yorks

Post by steve s » Wed Oct 24, 2007 6:22 pm

Scottmoose wrote:They're these Seas Excel 10in drivers with a cast magnesium / aluminium cone & solid copper phase plugs: http://seas.worldomain.net/pdf/e0026.pdf They go for about £190 apiece. Pretty good, providing you XO at least an octave before they start to break up -like most good metal cones, they're very well behaved across most of their passband, with only the one break-up mode. But when they do break up, you know about it in no uncertain terms. I'd hand over to the next unit at 500Hz, assuming it can get that low.
thanks scott
they are very different drivers to the goodmans .. long throw jobs with a resonable motor power and low effiency..
i'm sure they will go low.. but i'll stick with the goodmans.. thanks

steve

Post Reply