Doc modding Marantz imperial 7

[karatestu]

I am currently weighing the options up and one of the things I have to consider is cost. The cost of the acrylic globes (clear and opal, 3mm wall thickness) was one I could live with, just. The cost of black 6mm thick hemispheres which need to be acrylic welded together is getting pricey (£240 + vat). Then there are the 15cm spheres on top of that (£50 + vat). Total = £360 Inc vat

I found some black stainless steel mirror spheres in exactly the right size (35cm & 15cm diameters) which work out much cheaper but the wall thickness is 1.2mm which is probably not be enough although stainless is very strong. The thin wall also causes complications with mounting drivers. The 35cm SS black spheres are £45 each and the 15cm ones are £13 each. Total = £116 Inc vat

Using baltic birch ply in a translam construction is very wasteful and so ends up being quite pricey compared to just making monkey coffins. A 35cm sphere requires 18 rings to be cut out of 18mm ply. That's going to require two 8' x 4' sheets at £70 each. Then I have to buy a router and bits. Timber total = £140 Inc vat. Router etc will add a fair bit to that

The Black 6mm acrylic option is the best but the most expensive. The least amount of work but I have to live with a visible join of two hemispheres. SS and ply options have scope for things going horribly wrong. It's not like I can knock up a cheap quick prototype.

[karatestu]

Gosh what a long thread. It's going to take a while it catch up, but in the meantime...

Objects much smaller than the wavelength are ignored by a propagating wave, immovable objects much larger than the wavelength cause a simple reflection. It's where the object size is similar to the wavelength that funky stuff happens. Now an outside corner is small compared to most audio wavelengths and typically causes re-radiation (NOT diffraction). Diffraction happens when the corner size is similar to the wavelength and causes the wave propagation to bend around the corner smoothly. A roundover aids diffraction and minimises re-radiation. Re-radiation causes wave interference with the original source and thus amplitude ripples. The haas effect prevents us from hearing this re-radiation as a separate event. Wilson audio use surface mount felt to absorb waves propagating along the front of a loudspeaker, as does a few BBC speaker, Spica and other spring to mind too. Of course nothing is 100% effective and you are best spreading out the edge distances, so the tweeter should be 1 arbitrary unit the closest edge, 1.61 units from the next edge, 1.61^2 from the next .... This spreads out the interference 'eigenvalues', minimising ripple.

Is this any help?

Cheers, Rex

Dear Rex, welcome to the forum What inspired you to seek out this crazy place ?

Thanks for your input, you certainly show extensive knowledge on the subject. It is helpful, thanks.

I wouldn't bother reading this whole thread if iIwere you, unless you want multiple laugh out loud moments combined with head shaking and plenty of FFS. It is full of horribly amateur purely subjective experiments thrown together with a chainsaw, hammer & nails and a large dose of wishful thinking. You can see me stumble blindly through failed experiments and overly excessive enthusiasm when something seems to go in the right direction. Oh, and don't forget the multiple instances where I can't help myself but go completely off topic - I am the king of that apparently. I can't back up any of my thoughts or subjective opinion throughout this with any hard evidence apart from "it's fucking ace". Every incredible improvement when all added together must produce a speaker that is so good it's like has never been heard of before. Either that or the standard I started from was truly so horrible it would make your ears bleed and ruin all music played through it.

I tend to embrace certain parts of speaker design and physics whilst totally ignoring or dismissing others for no other reason that I got it in my head that it is either a good thing to try or I don't like the sound of it. No measurements taken or pretty graphs produced here.

Anyway, your post is welcome but it won't really change what I intend to do with spherical enclosures and minimising edges. Keep your views coming though if you have any more but please don't be offended if I totally fail to act on them (in the best possible taste ofc).

Regards, Stu

[r3xj0hn570n]

What inspired you to seek out this crazy place ?

Good question. I guess taking an interest in what is happening with NVA and interacting with people to are prepared to listen to music and talk about what they hear.

Anyway, your post is welcome but it won't really change what I intend to do with spherical enclosures and minimising edges. Keep your views coming though if you have any more but please don't be offended if I totally fail to act on them (in the best possible taste ofc).

Hell Stu, there is no way i'd take offense at anything you try or have tried. I encourage you to experiment as much as you can in the time you can, everyone has a different set of priorities, you will hit on what most floats your boat. I was only trying to promote a bit of clarity in thinking about sound wave propagation and standardizing descriptions of wave phenomena. It's a complex field.

[karatestu]

Good question. I guess taking an interest in what is happening with NVA and interacting with people to are prepared to listen to music and talk about what

Hell Stu, there is no way i'd take offense at anything you try or have tried. I encourage you to experiment as much as you can in the time you can, everyone has a different set of priorities, you will hit on what most floats your boat. I was only trying to promote a bit of clarity in thinking about sound wave propagation and standardizing descriptions of wave phenomena. It's a complex field.

I think NVA is going from strength to strength Paul and Tomasz are doing their very best to keep the brand available and giving many the chance to enjoy their music. Better transformers and psu caps can only be a good thing.

I just read one of the reviews from the 1990's that you linked to Rex (many thanks for that). It mentions the detail retrieval abilities of the nva gear and the fact that it does it in a non fatiguing way. I also concur that your source better be good as it comes through warts and all.

I have very much enjoyed the journey of experiments I have been on and things I have tried. I think it is slowly coming to an end though as I have found what floats my boat plus I need to get something finished, loking and sounding good and installed in my living room.

It's good to have you around Rex, I hope you stay

[karatestu]

On the subject of getting something finished, I got approval from the boss yesterday for my speaker design Showed her this after she had a glass of wine and my sister was here



Told her it was going to be a mix of acrylic and wood. "Will it be easy to dust ?" was one inquiry I got. I was honest and said yes but acrylic does attract dust like flies round a turd.

Next question was what colour will it be ? I replied black, cream or Ivory. We haven't decided yet but I really like Ivory even though it is only a little bit different to cream.

Any views on Ivory coloured acrylic ? The contrast with black drivers and grilles will be good imo. I plant to use real wood for the three tapered legs.

[karatestu]

So it seems the fact that I mentioned acrylic to my wife, sister and BIL means that really I have made my mind up about materials. This is going to be my final, end game pair of speakers so why not treat myself to acrylic spheres

My kids like them because it looks a bit like BB8 from star wars with legs on

[karatestu]

Cast iron point & squirt speakers with some sexy rounded cabinets



I see they have set the tweeter back to time align it with the mid bass . If I was doing a point & squirt then I would copy this but probably a 3 way with a bigger woofer at the bottom.

Or use a full ranger in a sphere

[karatestu]

It occurred to me the other day that when using spherical enclosures it is very easy to change the angle that the drivers are firing, it doesn't have to be exactly perpendicular with the floor. It would be interesting to try the up and down firing mid bass turned slightly towards the listening position. But that would muck up the mechanical force cancellation between the opposing drivers.

[karatestu]

Thought I better do some thorough volume calculations for the mid bass sphere as that is very important for bass response. For a sealed box there is some leeway unlike ported. Through experimenting I found the ideal volume for two of the doped 5.25 inchers to be 13 Litres minus the volume of driver that projects into the box. That happened to be twice the volume of a Cube 3

It turns out I seriously overestimated the volume of the sphere that has to be chopped off to accommodate two opposing mid bass. I had to chose a 35cm sphere because the next size down (30cm diameter) is too small.

35cm sphere with 5mm wall thickness

Volume of sphere with 17cm radius = 20.58 Litres
Volume of a spherical cap with radius of 7.5cm and height of 1.75cm = only 0.158 Litres

So the volume left after chopping off the two spherical caps is 20.58 - 0.316 = 20.264 Litres

WAY TOO BIG - would need alot of sand bags inside to reduce it.

30cm diameter sphere with 5mm wall thickness

It is too small. With 5mm wall thickness the volume is 12.77 Litres. Close but no cigar. Take the two spherical caps off and the volume is 12.378 Litres.

Possible solution

This popped in my head whilst watching Country File this morning

As I am having to purchase two hemispheres and glue them together why not add a ring of baltic birch plywood in between them This might look good visually especially if I have two similar rings - one at the top and one at the bottom which the drivers are mounted in . They could be shaped to follow the curve of the sphere and continue it right up to the edge of the drivers which would be rebated.

The main reason for doing this would be extra volume. If the ring of plywood is treated as a cylinder then the volume it adds is very easy to calculate. Radius = 14.5cm. Height depends on the thickness of birch used. Say using 21mm ply then the added volume is 1.387 Litres. That is enough

Volume of sphere with spherical caps cut off = 12.386 Litres
Plus volume of cylinder r=14.5, h=2.1 = 1.387 Litres
Total = 13.773 Litres

Result This way I can reduce the size of the lower sphere and use 30cm hemispheres with a 21mm ring of ply sandwiched between them. I could even use this ply ring to mount the legs rather than the ply ring that the down facing mid bass is mount in. Will have to draw it out to see if it works. The three legs would need to be perpendicular to the floor rather than splayed outwards.

What do the HFS massive think ?

[Geoff.R.G]

The three legs would need to be perpendicular to the floor rather than splayed outwards.

Why? You could have splayed legs attached to the plywood rings.