Doc modding Marantz imperial 7

[karatestu]

I believe the acrylic spheres are only available (for the likes of me) in clear and opal because they are made for light fittings and so must be transparent or translucent to let light through.

[karatestu]

Well whaddya know, Plastidip primer is acrylic based and will work well on acrylic. And I've already got some

From the plastidip site

"
Our Plasti Dip Primer is an air dried, acrylic primer specifically developed to increase adhesion to surfaces by up to 400%. Do NOT use if you require a peel off coating!

Plasti Dip sticks to just about anything and on most surfaces bonds permanently. However, as with any coating, primed surfaces are always best, and for this reason we have developed our Plasti Dip Primer to increase adhesion to most surfaces by up to 400%.

Aerosol sprays ready to use. Also available as liquid PRIMER that can be sprayed, dipped or brushed and offers fast set up time with a high coverage rate. Typically applied in one coat - only extreme corrosion and abrasion require a second or third coat, eliminating extensive preparation for most applications.

Plasti Dip Primer is recommended on most non-absorbent surfaces, especially galvanized metal, to prevent peeling of Plasti Dip.

For best results apply Plasti Dip over our Primer as soon as it is dry to the touch. Plasti Dip Primer is not recommended for use as a protective coating on its own.

Our Plasti Dip Primer is now available in Clear for use with our Clear coatings.

[karatestu]

At this rate I will be spraying both mid bass and tweeter spheres in plastidip primer and then a colour topcoat. It doesn't have to be black I fancy Black Cherry colour

https://www.plastidip.co.uk/colour-swatches/

There are some special top coats as well that can make it glossy, satin, metallic, pearlescent etc. The options are endless.

As it is flexible when dry it may have a dampening effect on what ever it is sprayed on. So Geoff's suggestion of listening before spraying is a good one. Plastidip is even an electrical insulator - not that it is needed in this case.

I need some scrap bits of acyrlic to practice on first.

[Geoff.R.G]

Well, partly to get you to ask but mainly because, if you paint the spheres you won't know what they sound like unpainted. If you build them unpainted and then use them you can decide later whether painting them is/might be a good idea. Painting will make a difference but why not find out what the plain acrylic sounds like first?

Thanks for your suggestion Geoff. I will do as you suggest. I would go for the opal which is a milky white colour and often used in plastic lamp shades. It's a pity these spheres can't be done in black . You can get black hemispheres and join the two together but the join will be visible and the whole thing not as strong.

Painting acrylic is a tough job i.e you have to prepare the surface really well and use the right primer which does not react with the acrylic and which will adhere well to it. Acrylic paint seems to get recommended.

I would have wanted black too. From the sound of it, once you have painted the acrylic you are stuck with it. Definitely wouldn’t paint it not only could you affect the sound but you could end up with a less than ideal finish.

Build your speakers and test them, then if you really want black, see if you can find a supplier that can supply black acrylic spheres.

[karatestu]

Been thinking about the mechanical coupling of the two opposing mid bass to get the most effective force cancellation. What ever links them needs to be as rigid as possible so I might use some of the extensive stock of round brass bar. There needs to be no dampening between each driver magnet and the connecting bar. Therefore the bar has to be exactly the right length. Also I have to avoid blocking the vent in the back of the magnet because doing that raises the drivers resonant frequency and we don't want that.

To ensure there is good contact between the back of each driver and the connecting metal bar I will glue the bar on to the magnet of the down firing mid bass. The top one will not be glued to aid disassembly when required. Nothing worse than reading some of the horror stories on the net about speakers that could not be accessed without incurring damage. To make sure the up firing mid bass makes good contact I will probably use four 6mm threaded stainless steel rods which will go through the four screw holes (usually for screwing into wood) on both drivers. I can then tighten it up just enough for good contact without putting too much force on the driver basket or acrylic enclosure.

That should do it I think. The ends of the threaded rod can be covered by using dome nuts. Saying that I am thinking of making a circular cover which is rounded over and will go all the way around the mounting area of the drivers to hide everything and aid diffraction (blend the driver in to the sphere with no sharp edges between driver surround and spherical enclosure.I will have to remove the squishy mounting ring that is on now to do that of course and cut a thin circular channel on the back to sit the lip on the edge of the driver in.

[karatestu]

Well, partly to get you to ask but mainly because, if you paint the spheres you won't know what they sound like unpainted. If you build them unpainted and then use them you can decide later whether painting them is/might be a good idea. Painting will make a difference but why not find out what the plain acrylic sounds like first?

Thanks for your suggestion Geoff. I will do as you suggest. I would go for the opal which is a milky white colour and often used in plastic lamp shades. It's a pity these spheres can't be done in black . You can get black hemispheres and join the two together but the join will be visible and the whole thing not as strong.

Painting acrylic is a tough job i.e you have to prepare the surface really well and use the right primer which does not react with the acrylic and which will adhere well to it. Acrylic paint seems to get recommended.

I would have wanted black too. From the sound of it, once you have painted the acrylic you are stuck with it. Definitely wouldn’t paint it not only could you affect the sound but you could end up with a less than ideal finish.

Build your speakers and test them, then if you really want black, see if you can find a supplier that can supply black acrylic spheres.

All good suggestions Geoff, thanks. I will keep it all in mind. The prospect of a less than ideal finish had crossed my mind already even though I have a compressor and spray gun (and somewhere to use it).

[CycleCoach]

I am now reconsidering the hemispheres as they can be done in black and two joined together. Cost is the same. It is just the join that bothers me but these acrylic weld "glues" melt the two surfaces together which is a different prospect altogether to using glue. The websites states they can do them thicker so 5 or 6mm might be an option.

Back on the phone tomorrow

[karatestu]

How to make an accurate wooden sphere with a router. No need to buy a wood lathe.

[CycleCoach]

How to make an accurate wooden sphere with a router. No need to buy a wood lathe.

That seems like it would work, but then I started thinking about what shape you would need to put in it to start machining. Maybe something made out of hexagonal faces to give a hollow centre? But then that would add a whole extra level of complexity for the build. I've no doubts that you could do it though Stu!

[Geoff.R.G]

How to make an accurate wooden sphere with a router. No need to buy a wood lathe.

Yes, but, the inside won't be spherical and you thus retain some of the apparent complexities of flat internal surfaces.