Doc modding Marantz imperial 7

A place for DIY project discussions.
User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1049 times
Been thanked: 617 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

The other thing I have done to these little 5 inchers is remove the three little steel tabs which were unused and there to give flexibility as to which side of the driver the tinsel leads come out. There are four of these tabs but one of them is used for attaching the terminals where you attach the external wiring.

The steel basket on these cheapies is quite substantial so there is not much open space for the back wave to make it's way into the enclosure. These unused terminal mounts just make matters worse so are best removed imo. An cast aluminum basket would have made for more open space but at this price you can't expect that.

With this being a small driver a 20mm deep baffle panel totally obscures the cone. If you think about it that leaves a very narrow space for the backwave from the back of the cone to rattle about in before it can get into the enclosure. Can't be good and apparently leads to very early reflections of sound coming back through the cone. The way to help with that is to chamfer or put a bevelled edge on the driver cut out but that reduces the thickness of wood you have available to screw the driver to.

I will be using 6mm aluminum rings to mount my drivers in which will leave the holes to the back of the cone completely unhindered. Some may dismiss the effect this has and I used to also but lately I have realised that paying attention to these little things can pay surprising dividends especially when all these "little" things are added up. The driver can breathe better to put it another way.
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1049 times
Been thanked: 617 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Last thing on the driver modification list is to smooth out the backs of the drivers like Ellipson did in the top left corner of the diagram below.

Image

I haven't decided if I should use a very stiff product to do this like car body filer or a more elastic product like blu tack. Many people dampen the back of the driver magnet and basket. I don't know if I want to do that. RD's reasons for hating dampening was that although it reduces the peak amplitude of vibration, it spreads it out over a longer time scale and we all know the level of importance Doc gave to timing.

I may look to strengthen the gap between the back of the steel basket and the magnet and round everything over like the Elipson driver. Why bother ? Well, I have read comparisons with measurements before and after and it seems to be worth doing. You may disagree and think I am trying to polish a turd here but apparently it is the cheap drivers that benefit the most.

Could be a load of bollox and I won't be doing a before and after subjective comparison, it will be what it will be.
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
TheMadMick
Posts: 335
Joined: Tue Jan 31, 2017 9:50 am
Location: SOUTHPORT, UK
Has thanked: 3 times
Been thanked: 65 times
ST_PATRICK'S_SALTIRE

Re: Doc modding Marantz imperial 7

Unread post by TheMadMick »

"I haven't decided if I should use a very stiff product to do this like car body filer or a more elastic product like blu tack. Many people dampen the back of the driver magnet and basket. "

Although it may not count for much round here Russ Andrews recommends epoxy or body filler - both of which are rigid.
These users thanked the author TheMadMick for the post (total 2):
karatestu (Tue Aug 03, 2021 5:13 pm) • antonio66 (Wed Aug 04, 2021 12:57 am)
Blusound Vault 2, P50SA, A80's, BMU, TIS, LS7, MA PL200.

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1049 times
Been thanked: 617 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

TheMadMick wrote: Tue Aug 03, 2021 9:02 am "I haven't decided if I should use a very stiff product to do this like car body filer or a more elastic product like blu tack. Many people dampen the back of the driver magnet and basket. "

Although it may not count for much round here Russ Andrews recommends epoxy or body filler - both of which are rigid.
I didn't know Russ Andrews recommended modifications like this :shock: Does he sell some very expensive car body filler ? :lol:
DIY inspired by Richard "The Doc" Dunn RIP

User avatar
TheMadMick
Posts: 335
Joined: Tue Jan 31, 2017 9:50 am
Location: SOUTHPORT, UK
Has thanked: 3 times
Been thanked: 65 times
ST_PATRICK'S_SALTIRE

Re: Doc modding Marantz imperial 7

Unread post by TheMadMick »

Don't think so but I wouldn't put it past him.

I halfinched his pamphlet on modifying speakers to sound better and it's in there. If you want a copy let me know and I can e mail it.
These users thanked the author TheMadMick for the post:
karatestu (Wed Aug 04, 2021 5:43 am)
Blusound Vault 2, P50SA, A80's, BMU, TIS, LS7, MA PL200.

Geoff.R.G
Posts: 1238
Joined: Thu Aug 11, 2016 2:46 pm
Location: Denham UK
Has thanked: 64 times
Been thanked: 248 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by Geoff.R.G »

TheMadMick wrote: Tue Aug 03, 2021 9:02 am Although it may not count for much round here Russ Andrews recommends epoxy or body filler - both of which are rigid.
Both of which are rigid and heavy.

I asked Richard what he thought of the Leak sandwich speakers and he wasn't impressed, the sandwich cone is very rigid and light, imagine what effect a higher mass rigid cone might have. I actually quite like the Leak sandwich but having had them for around 40 years I am biased.

User avatar
r3xj0hn570n
Posts: 94
Joined: Fri Jun 25, 2021 4:54 am
Has thanked: 18 times
Been thanked: 64 times
Norfolk Island

Re: Doc modding Marantz imperial 7

Unread post by r3xj0hn570n »

TheMadMick wrote: Tue Aug 03, 2021 9:02 am "I haven't decided if I should use a very stiff product to do this like car body filer or a more elastic product like blu tack. Many people dampen the back of the driver magnet and basket. "

Although it may not count for much round here Russ Andrews recommends epoxy or body filler - both of which are rigid.
I've used knead-it to reinforce a steel framed driver in the past. It's not heavy but it does set rock hard and sticks really well. Blutack is good for sealing driver baskets to cabinets or damping the cone surface.

I'll resize an example... brb

User avatar
r3xj0hn570n
Posts: 94
Joined: Fri Jun 25, 2021 4:54 am
Has thanked: 18 times
Been thanked: 64 times
Norfolk Island

Re: Doc modding Marantz imperial 7

Unread post by r3xj0hn570n »

These users thanked the author r3xj0hn570n for the post:
karatestu (Wed Aug 04, 2021 5:42 am)

Geoff.R.G
Posts: 1238
Joined: Thu Aug 11, 2016 2:46 pm
Location: Denham UK
Has thanked: 64 times
Been thanked: 248 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by Geoff.R.G »

Geoff.R.G wrote: Tue Aug 03, 2021 9:14 pm
TheMadMick wrote: Tue Aug 03, 2021 9:02 am Although it may not count for much round here Russ Andrews recommends epoxy or body filler - both of which are rigid.
Both of which are rigid and heavy.

I asked Richard what he thought of the Leak sandwich speakers and he wasn't impressed, the sandwich cone is very rigid and light, imagine what effect a higher mass rigid cone might have. I actually quite like the Leak sandwich but having had them for around 40 years I am biased.
I miss read your post, epoxy putty to reinforce the driver chassis would work I suppose, filler too possibly but either would have to be keyed to the metal.

User avatar
karatestu
Posts: 4790
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1049 times
Been thanked: 617 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

The 30cm plastic pig balls are unfortunately going to be unsuitable for my needs. Just too flexible around the cut outs. They are not going to be wasted however.

I am going to use them as inner molds for casting hemispheres in concrete :dance: As they are 30cm diameter then I will be able to produce an enclosure with a little more volume than before. 30cm diameter sphere has a volume of 14.13 Litres which is about right once the volume of the bits cut off, driver volume and brace between the opposing driver magnets has been taken off. No wooden spacer now required between the two hemispheres to increase the volume a little.

I still need to acquire an outer mold hemisphere. It will need to be 35cm diameter or thereabouts to give a concrete wall thickness of 23mmapprox depending on the thickness of the mold. There are some suitable bowls and hemispheres available to use, just need to chose the best one which isn't too expensive.

Done some research on casting concrete. The molds need a release agent applied to the walls to stop the concrete sticking to them. WD40 works apparently. The mixture needs to be carefully thought out. Too much water in the mix weakens the product so I will probably use a water reducing additive that increases slump and fluidity with low water rates. Some people use rapid setting cement but I have read in some places that this is not a good idea and can lead to cracking. There are other additives like plastic fibres but don't think I will use those.

Need some vibration to get the air bubbles out whilst it is going off. I think I will make a platform to go on top off my 4X4 bonnet to make it level and so I can leave it on top with the engine running. Who needs to buy a vibration table :lol:

I thought about making the mold so it has a rebate for the drivers but I don't really want to create a weak point. I think if I make the mold so it leaves a cut out of 114mm then there will be a substantial flange around it. The driver is 134mm diameter and the cut out required is 114mm - only 10mm of flange is needed outside the diameter of the cut out. With the concrete being over 20mm thick then I will have enough spare to add a sliced in half donut shaped ring of wood which will continue the sphere's curve and blend it in to the surround of the driver.

It would be easier if the drivers didn't have a 3mm tall lip sticking up all the way around it's circumference. I guess that is there to add strength but as the driver is going to be attached from the outside of the enclosure and on to a hard, flat and rigid surface then I really don't think it is required. It would certainly be better if it wasn't there. I could cut it off and grind it flat but the heat produced through the steel might melt the glue attaching the cone's surround to the basket. :shock:
These users thanked the author karatestu for the post:
CycleCoach (Sat Aug 07, 2021 1:54 pm)
DIY inspired by Richard "The Doc" Dunn RIP

Post Reply