advertisement


Jim Rogers JR149 DIY build

martinclan said:
ATM still struggeling a bit with building the JR149 like tubes but I am getting closer to something that would be acceptable to go in my lounge!
Click to expand...

I’d follow the original design and have sculpted foam grilles. They look great IMHO. JR149s were always really ugly with the grilles removed, the alloy tube shows the effects of the rollers that formed it and no real attempt was made to clean-up the sealing compound. Unquestionably a grilles on speaker. You’ll obviously need to form a perforated metal front grille, but just bending a bit of chicken wire fencing would likely work fine and have a more open weave than the original.
 
davidsrsb said:
See my comments in https://pinkfishmedia.net/forum/threads/tuning-my-tdl-t-line-3s.273251/
REW measurement is showing me that grilles do a lot more than a simple frequency tilt. I have a feeling that Jim Rogers would have been delighted to have something like REW in his time.
Click to expand...
The JR149 foams' effect on frequency response is mild and consistent, as you would expect from a relatively thin and edgeless open-cell foam.

I haven't measured the effect of the JR149's perforated metal grille but I expect will have some effect on the HF response because it does in the JR150. At farfield distance the effect of the JR150 mesh is pretty mild but at nearfield distance it becomes more pronounced (unfortunately I cannot locate the nearfield measurement).

I've often wondered why JR chose to include a small cutout in the JR150 grille in front of the tweeter. It's not clear to me whether this would do less or more harm to the tweeter's dispersion than leaving the mesh uncut. Any thoughts?

PS - If you want an example of how badly a grille can mess up a response, see from 2:10 - 2:50 in the following video!
 
martinclan said:
Progress is made. I have built the crossovers. A lightly modified version of that used in the Monacor LS 5/19 as previously mentioned. And before anyone comments I know the enclosure I am building is different to the Monacor LS 5/19 and will therefore have an effect on the crossover behavior. But to be honest my hearing is not that accute!
And I have obtained the driver units. Most of the crossover components and drivers came from Wilmslow Audio. Strangly enough I used Wilmslow when I built my previous speakers some 25 years ago!
ATM still struggeling a bit with building the JR149 like tubes but I am getting closer to something that would be acceptable to go in my lounge!

speaker components by Robin Martin, on Flickr

Robin
Click to expand...

Must admit I'm rather tempted to build a pair of LS5/19s myself, although I would use a wooden box.
From the info I've read they should out-perform LS3/5As (they fill in the gap around 300Hz).

It might be worthwhile to move the inductors in your build in case they interact:
http://www.troelsgravesen.dk/coils.htm

I would also try to minimise the amount of mesh in front on the drivers. Some listening tests I was involved with indicated that the mesh affects the sound appreciably and negatively.
 
So, its easier to bend and form, and then drill the driver holes? I’d have expected holes first!
 
Tony L said:
So, its easier to bend and form, and then drill the driver holes? I’d have expected holes first!
Click to expand...

Six of one and half a dozen of the other as the say. The main problem being that the hole for the woofer is (relatively) very big and I was worried it would weaken the sheet of alumnium too much before I bent it. The bending required a considerable amount of force!
As it happens probably the right decision. The front is bowed a little and I have decided to bond a piece of MDF behind it in order to flatten it and not stress the driver chassis. Also means there is something to fix the drivers into without resorting to things like rivet nuts. More pictures soon!
 
I’d personally go with good plywood rather than MDF, just a nicer sounding and far more stable material and it would be fine to screw the driver mountings straight into. I guess bonding that on first might even make drilling the holes easier, i.e. drill/route through both layers at once.

FWIW I don’t think the 149 baffles are 100% flat, I think the foam driver seals do quite a bit of heavy lifting there. The drivers are mainly just screwed into the alloy, so really fragile, the ‘Linn tight’ crowd destroy them in seconds. I assume you are going for a more robust mounting, but anyone with real 149s needs to be *very* careful here. I’m fine as I’ve come to the firm conclusion that ‘tight’ = ‘bad sound’ in hi-fi, so I haven’t stressed mine at all. They are tight enough not to come loose, nothing more. Just as JR did them originally.
 
I have thought about taking a section of plastic drain pipe (available in 200, 250 and 315mm diameter) and cutting vertically.
Soften in hot water and spread around a flat wood baffle for the drivers
 
I would be afraid there would still be tension in the aluminium shown previously which can lead to ringing at certain frequencies.

Regarding plastic pipe: At one stage I had the same thought of using plastic pipe for the JR149 but gave up on the idea.

I would make some JR149 replica's (there's a difference between replica and clone) with thin bendy ply (plywood with grooves on one side designed to be bend) and glue several layers together. It will be next to indestructable. I would also varnish the result, leave it exposed and make a shaped grille to complete the circle.

Here is a link to some thicker ply that I looked at (I know there is also some 3mm (1/8") around for even smaller curves):

https://plymasters.co.nz/static/digest/products/bendy-ply/spec-sheet-bendy-ply.26da6.pdf

Soaking it in hot (boiling) water allows you to make tighter bends too: let it dry on a form before making the next layer, let it dry and then glue. I've used this for making ribs in boats.
 
AmadeusMozart said:
I would be afraid there would still be tension in the aluminium shown previously which can lead to ringing at certain frequencies.
Click to expand...

They are remarkably inert cabinets. Remember the bent alloy cylinder is only one part of a layered and tensioned construction, the internal layer of ‘deadsheet’ and internal foam likely do their part, as do the rod-tensioned end-caps. There is no discernible ‘note’ or ‘hollow’ sound tapping a JR149 cab. They are just dead. Any resonance in this design comes from the alloy grilles over the drivers, which are certainly imperfect and arguably the weak aspect in the whole design.
 
I made some more progress.
Finally decided to make the tops and bottoms from MDF, recycled from my defunct Daline speakers made over 20 years ago. Compared to "modern" MDF it is much denser and has a better surface so I am going to spray paint them rather than use a veneer. I have routed out a groove for the tubes to fit in. The plan is to fill this with a PU adhesive and simply drop the tubes in place. It should be much better than the JR technique of using a metal rod to hold them in place and then just sealing the edges.
Interesting discussion about the tubes ringing. I had already decided to line the tubes with deadsheet which should minimise the risk of this happening
Next job is to paint the MDF then they will be just about ready to assemble. It's getting exciting...
first_fit by Robin Martin, on Flickr
 
Tony L said:
They are remarkably inert cabinets. Remember the bent alloy cylinder is only one part of a layered and tensioned construction, the internal layer of ‘deadsheet’ and internal foam likely do their part, as do the rod-tensioned end-caps. There is no discernible ‘note’ or ‘hollow’ sound tapping a JR149 cab. They are just dead. Any resonance in this design comes from the alloy grilles over the drivers, which are certainly imperfect and arguably the weak aspect in the whole design.
Click to expand...
JR149 and JR150 enclosures are the deadest I've wrapped my nuckles against, the original Goodmans Maxim comes a close second - a surprisingly heavy little bugger for its size due to the metal baffle and huge Alnico magnets!
 
martinclan said:
I made some more progress.
Finally decided to make the tops and bottoms from MDF, recycled from my defunct Daline speakers made over 20 years ago. Compared to "modern" MDF it is much denser and has a better surface so I am going to spray paint them rather than use a veneer. I have routed out a groove for the tubes to fit in. The plan is to fill this with a PU adhesive and simply drop the tubes in place. It should be much better than the JR technique of using a metal rod to hold them in place and then just sealing the edges.
Interesting discussion about the tubes ringing. I had already decided to line the tubes with deadsheet which should minimise the risk of this happening
Next job is to paint the MDF then they will be just about ready to assemble. It's getting exciting...
first_fit by Robin Martin, on Flickr
Click to expand...
Any thoughts on dealing with the high frequency diffraction off the top end cap? ;)
 
If going for a modern grille-less look I’d probably have gone with a ‘D’ shape following the baffle shape. The top cap diffraction was certainly an issue with JR149s and was helped a lot with the foam and deadsheet found on later versions.

PS There are always things to be learned from LS/35As too, their treble is astonishingly good IMO so that chunky felt rectangle is likely doing something very right.
 
ToTo Man said:
Any thoughts on dealing with the high frequency diffraction off the top end cap? ;)
Click to expand...
To be honest I am not sure my hearing is accute enough to notice! But if it proves to be a problem I will probably do something like that which was used in the later JR149 as mentioned by Tony L.

Tony L said:
If going for a modern grille-less look I’d probably have gone with a ‘D’ shape following the baffle shape. The top cap diffraction was certainly an issue with JR149s and was helped a lot with the foam and deadsheet found on later versions.
Click to expand...
I am afraid the modern grille-less loook isn't compatible with active grandchildren. I already had one of them poking a finger in the tweeter of my of Daline speakers fascinated by its appearance I guess (who know what goes through youngsters minds....).
 
martinclan said:
To be honest I am not sure my hearing is accute enough to notice! But if it proves to be a problem I will probably do something like that which was used in the later JR149 as mentioned by Tony L.


I am afraid the modern grille-less loook isn't compatible with active grandchildren. I already had one of them poking a finger in the tweeter of my of Daline speakers fascinated by its appearance I guess (who know what goes through youngsters minds....).
Click to expand...

Is that dome not a button to switch something on or off? (in this case more like a permanently off....)

Can you redo the crossover with the coils sitting on the outside perimeter - oriented the same way they are too close to each other.
 
Helen Bach said:
According to the internet, aluminium has just less than the square root of damn all damping ability, as I found when I measured it.
Click to expand...

Interesting. Having handled sheet steel and sheet aluminium the former rings when you hit it with something suitable solid but you don't get the same effect with aluminium so I would assume the latter was more inert - looks like I'd be wrong then.

Regards

Richard
 
You must log in or register to reply here.


advertisement


Back
Top