Doc modding Marantz imperial 7

[antonio66]

Done some more work on the sputnik today.

The gaffa tape holding my tweeters in their little plastic spheres was a bodge and only temporary so I could see if isolating the tweeters had any merit. Even with that far from ideal bodge it was clear from the first ten seconds that it was a winner. Since then the tape started to come unstuck and I got fed up pushing it back on.

So the next stage was considered and implemented. This is still not the final thing but gives me an idea on how suitable butyl rubber will be to attach the tweeter to the enclosure. I cut out a donut of wheelbarrow inner tube just wide enough to overlap the enclosure and with a hole in the centre that just fits around the edge of the surround.





I then proceeded to add some 3mm thick plastic to the tweeter to make it circular and give a bigger area to stick the butyl rubber to. None of this is seen as the butyl rubber ring very conveniently covers everything right up to the tweeter's surround.




So now the parts were made I had to adhere the rubber ring to the enlarged face plate and the spherical enclosure. This went rather well and looks much better than the gaffa tape effort




Well it not only looks better but sounds better too. The gaffa tape was a poor solution but this butyl rubber ring has well and truly done the business. These are the best performing speakers I have heard (not that I have heard loads).

I just need to think now of how to blend the edge of the butyly rubber smoothly into the sphere without any edges and the re radiation that brings. Geoff's miĺiput suggestion is under serious consideration but as are other options.

[antonio66]

Personally I like the short hourglass as in the drawing and the straight pipe the best. My wife likes a nice straight piping . I got a lot more replies on another forum and the voting is all over the place.

I won't have to buy any more stools if I go with the hourglass cut down at both ends.

Well it's a no brainer then Stu, don't know what I was thinking when I said it would have to cost you another £18.

[antonio66]

Personally I like the short hourglass as in the drawing and the straight pipe the best. My wife likes a nice straight piping . I got a lot more replies on another forum and the voting is all over the place.

I won't have to buy any more stools if I go with the hourglass cut down at both ends.

Well it's a no brainer then Stu, don't know what I was thinking when I said it would have to cost you another £18.

It all depends on the trimming down and how well it goes. It takes a lot of care to position the marking line and make sure it is square. I started last time with a dremmel with cut off disc and then inserted a hacksaw blade jnto thd hole. It went really well but took some sanding afterwards. Four full size sheets of sandpaper stuck to a glass table top and rotating the stool back and forth did a nice job. It's how I make sure bearing edges are square on drums.

[Latteman]

One thing that sticks out for me (OK more than one) since I started with isolating speakers from the floor and even drivers from their enclosures is the massive increase in dynamics. It's not subtle. On well recorded jazz there can be brass and woodwind instruments truly soaring. It is quite amazing. The other things being the added detail from less bass mush and the increased sense of hearing the venue or room it was recorded in.

I find the reproduction of drums to be quite amazing. These have only a 6.5" mid bass and 2" tweeter but the dynamics on well recorded jazz is quite something. Same with piano and those are the two hardest instruments to reproduce. There may be better speakers in this regard but I bet they are a lot bigger than mine with a price tag to match. The level of expression of detail in cymbal work, cross stick and brush work is spine tingling. Never heard anything like it (except a real life talented drummer un amplified of course).

The drivers are nothing special price wise. The B&W mid bass driver is very well made with cast aluminum basket and smallish magnet which really get out of the way of the back wave. I think this is very important- it's very important to chamfer cut out's especially with very thick baffles. As my enclosures are only 3mm thick they are completely out of the way and the space for the backwave could not be any more open. You can hear the reflections back through the cone if the rear of the driver is restricted.

I added blutack and bitumen flashing tape to the the back of the driver in all the right places. I don't know what difference that made if any as I did the brass bar and sand modification at the same time. Interestingly I can't detect much vibration of the mounting flange.

The SB acoustics 2" full ranger (can't really call it a FR as it rolls off at 140 Hz if run wide open) is very well regarded on diyaudio, It too has a small neo magnet and very open backside. The way I have it mounted with mounting flange inside the cut out means there is nothing to restrict and reflect the backwave. This little 2" has really good treble dispersion due to the large dustcap which is very much like a dome tweeter with the cone being a small waveguide.

The isolation I have implemented at speaker and driver level along with the very stiff 3mm steel sphere and the attention to low re radiation from sleek enclosure shape has paid off massively. They are good drivers but not major expensive and I feel I have wrung the very last drop of performance from them.

Happy days, I feel like the last five years of hands on learning has not been for nothing.

[karatestu]

Next thing ok decide is the finish for these speakers. Being spheres it makes it a bit more complicated. The only flat area on these is the wooden base which I might leave as is. I can't finish with wood veneer, leather or any flat sheet material.

That leaves spray on products like paint or plastidip. The steel sphere will be quite easy to do as paint adheres very easily to steel. The plastic tweeter sphere is another matter - it's much harder to get paint to adhere to it. I will have to flame treat it and then apply plastidip primer soon after.

I could of course use a steel tweeter sphere that would solve the issue. They are available in the exact size I need which is 15cm diameter. It may sound better too ? But I won't know until I try.

If using the hourglass stand then that will have to be left as is which is gloss black. Everything else will have to look good with that. It could be all the same colour or two tone.

Too many options

[antonio66]

I'm happy to vote on whichever colours you think fit.

I do think two tone, black with silver or grey or white?

[karatestu]

I'm happy to vote on whichever colours you think fit.

I do think two tone, black with silver or grey or white?

Thanks Dave. I can always rely on you

[karatestu]

Quote from diyaudio by an ex designer at KEF ( he wrote the white paper I quoted a page or three back

"KEF was possibly the first to introduce de-coupled drivers, with the R105.
The idea has been used by many people many times.
De-coupling is only effective at frequencies above the resonant frequency of the suspension, but can still work extremely well.
I have used it both for my KEF designs and for Pioneer and TAD.
The best solution is the dual driver bipolar type, as the reaction force should be fully cancelled at all frequencies, as long as the drivers are well matched and identically loaded. But if you roll one of them off at a lower frequency than the other (as you might be tempted to do in a bipolar design), then the attendant phase shifts will negate some of the force cancellation

The reaction force is typically 10-30 times stronger than the force generated by internal air pressure at the frequencies of the cabinet resonances, and with appropriate choice of de-coupling you can readily reduce cabinet resonances by 20db or more.
I have many times demonstrated the effect of de-coupling upon the sound quality, taking a pair of speakers matched to better than 0.5dB and then de-coupling just one of the pair. The improvements are not subtle.!"

[slinger]

Next thing ok decide is the finish for these speakers. Being spheres it makes it a bit more complicated.

[karatestu]

Next thing ok decide is the finish for these speakers. Being spheres it makes it a bit more complicated.

BB8 -I don't mind that actually My kids would bloody love it but my wife probably not.

I'm veering towards two tone just not sure on the combination. I have used a black marker on the yellow kevlar cone of one of my cheap Chinese drivers and it looks acceptable. So the B&W driver will get the same and no longer be bloody yellow. Unless there is a silver marker pen that would cover up yellow. The worm of blutack at the cone and dust cap junction will be changed for blacktack. The tweeter driver is black and silver so maybe silver would be a good option ?

Inner tube will get sprayed with black plastidip as it is flexible. I just hope it doesn't affect the isolation resonant frequency. Fake wooden base will probably stay fake wood effect but could be painted if needs be.