Doc modding Marantz imperial 7

[Geoff.R.G]

Fixing the rubber to the sphere is a bit more problematic. Although inner tube rubber is thin there will be a lip where the rubber ends. I need to think about that a bit more especially as the enclosure is not a flat surface (spherical).

You could smooth the lip with Milliput epoxy putty, it can be shaped by using water and your fingers before it hardens.

[karatestu]

Fixing the rubber to the sphere is a bit more problematic. Although inner tube rubber is thin there will be a lip where the rubber ends. I need to think about that a bit more especially as the enclosure is not a flat surface (spherical).

You could smooth the lip with Milliput epoxy putty, it can be shaped by using water and your fingers before it hardens.

Thanks for the suggestion Geoff. I'll try anything as long as it looks good.

[Geoff.R.G]

Fixing the rubber to the sphere is a bit more problematic. Although inner tube rubber is thin there will be a lip where the rubber ends. I need to think about that a bit more especially as the enclosure is not a flat surface (spherical).

You could smooth the lip with Milliput epoxy putty, it can be shaped by using water and your fingers before it hardens.

Thanks for the suggestion Geoff. I'll try anything as long as it looks good.

Milliput is used by model makers and, with some care, can be unnoticeable. I think you will find it ideal.

[karatestu]

Been a bit busy.

[karatestu]

I took these next pics after I had only done one of them. I listened with one sphere speaker and the other still a cube whilst checking the enclosures and stand for vibrations (hands only, no accelerometer here).





They are just cobbled together of course to see what spheres and isolating drivers sound like. That stand will never look right until the top 5cm is chopped off and the straight lip at the bottom.

[karatestu]

The spheres are too big for this driver so I poured three litres of grit sand in the bottom and levelled it out. On top of that was placed a piece of open cell foam slightly larger than the magnet. This took the driver's mounting flange to exactly that same level as the cut out in the steel sphere. A 2mm gap all the way round was sealed with the trusty gaffa tape - temporary bodge.

So I have an isolated mid bass as well as an isolated tweeter. And the mid bass is finally bodged in to a sphere I temporarily joined the two hemispheres with silicon and hoped for the best. Stand isn't bonded to the circular base yet but the inner tubes are on the edge of the base plate like a bumper car With the extra weight of the sand the isolation has an even lower resonant frequency- I would say under one Hertz now.

I have listened to it of course and was nearly late picking my daughter up from karate

Must get those stands modified next.

[Toontrev]

So vibrational comparison with one spherical speaker with both drivers isolated and one chipboard cube with coupled mid bass and isolated tweeter.

The Cube with coupled mid bass was very clearly vibrating along to the music. The plastic sphere above it was also vibrating. Moving over to the steel sphere with isolated mid bass the cabinet vibration had almost completely gone (using hand). There was a miniscule amount that I had trouble detecting. I couldn't detect any vibration with hand on the plastic hourglass stand

The three legs holding up the tweeter sphere are now connected tk the steel sphere which has much reduced vibration. The plastic sphere was still vibrating but much less than the one on top of the cube with coupled mid bass. I expect this is because it is right in the up firing mid bass' firing line.

[karatestu]

The driver on top of open cell foam and grit sand didn't work out for a number of reasons. The bass was super tight but not high enough in level compared to the rest. It was rather enlightening about how much bass information I had been missing. It didn't really bother me that the day after the driver had sunk into the foam and the seal I had made with gaffa tape had come unstuck.

I was going to start again anyway because I think the springiness of the foam and the thinness of the gaffa tape were robbing some bass by letting the driver move too much and I don't think the seal around the driver was very good.

So moving on the next idea involved a long length of 60mm round brass bar hot glued centrally on to the back of the mid bass' magnet. On the other end of the bar I put a 12cm diameter circular piece of aluminum plate. The three litres of grit sand in the bottom of the sphere was retained. Once I had put in enough to find where the aluminum plate should go I poured the rest of the sand in so it went part way up the brass bar.

I never took any pics of that but here is a diagram that explains it better.



Instead of the gaffa tape to seal around the gap between the driver's mounting flange and the steel sphere enclosure I used some bitumen flashing tape and cut a ring out of that. This is still just temporary so I can prove if there is any mileage in this design. It looked like this.



My thoughts about how it sounds (if subjective bollox can mean anything to readers) to follow along with a further revision. Stay tuned folks.

[karatestu]

So, goodbye crappy chipboard cubes with steel lining and hello steel sphere I am glad to get rid of them and one step closer to a design that works visually and sonically.

The brass bar with aluminum plate idea has worked a treat, I'm well pleased with it. It obviously makes the magnet seem heavier and the plate on the end further prevents movement . The driver doesn't touch the enclosure at it's mounting flange (only via the bitumen flahing tape) and the brass bar with aluminum foot plate doesn't touch the enclosure either due to it resting on sand.

The sand is there for several other reasons. This driver doesn't work in 13.5 litres even with a worm of blutack lowering Fs and increasing Q. So the sand eats up the volume required to get the bass response where it needs to be. I think it also provides some vibration control. With the sand being in the bottom of the sphere it helps keep the centre of gravity low.

Furthermore, the sand makes the inside of the enclosure not spherical so helps avoid the possible strong internal resonance from every dimension being the same. The sand also helps to support the brass bar with driver glued on the top. So this sand is a quite essential part of this design.

There are a couple of drawbacks. The speaker must never be allowed to go on it's side otherwise I will be tearing it apart to reset everything. Don't want sand in the drivers workings so these things must always stay upright.

[karatestu]

Some observations on vibration. There is still very little I can feel in the steel sphere enclosures. I am over the moon with how well the sand is damping it. It is an order of magnitude less than the vibration I felt when using the steel lined chipboard cubes. It's ridiculous really how much of a difference there is.

I have listened to a lot of music - can't stop . The main thing I can say with where I have got to now is the amazing clarity across the board. The bass is now so detailed and clear that I realise how much mud is produced by an enclosure no matter what it is made of. I am hearing parts to bass lines I never knew existed - previously masked by floor and enclosure mush (no better word for it.)

I can't be sure how much this is due to the much reduced cabinet vibration because I have changed to steel spheres at the same time but my hand on the enclosure detects a massive difference so it must be partly due to that. A 3mm steel sphere is a very strong shape so there will be little flexing going on and for a given volume a sphere has the lowest surface area.

What effect the spherical enclosure shape has had on things I am not completely sure but I can say the sound staging and imaging is better than anything I have ever heard. There will be less ripple in the baffle step transition region as was proven by harry Olson inhis experiments with different shaped enclosures.

I am still experiencing all the dynamics I was previously. These latest changes have only added positively to everything. Transparency has got to a stage where instead of cleaning the window somebody has bloody removed it Cleaning up the bass and removal of reradiation from sharp 90 degree corners has unearthed a bloody treasure trove of extra detail in the music which doesn't sound false.

There is an ease to the speakers that is just wonderful. Everything has now fallen in to place. Yes it's highly detailed but I am enjoying the music even more now, the dynamic and emotion is all still there. No added music robbing filters were needed to do this or new devilry (dsp) - just attention to cabinet shape, positioning or drivers and vibration control. I know there are no 12" drivers or high sensitivity drivers and what they bring to the party but I can live with that. I feel like I am listening to monitors but ones that have loads of life (no BBC here) and which sound bigger than they have any right to without throwing it all at me (point & squirt).

Laters.

I'm over the fecking moon. My speaker design journey has ended, just the aesthetics to sort out and then move on to the 12" driver version