Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Right, what to use for bracing ? Wood or metal ? If metal - steel, aluminum, brass ? If wood - soft or hard wood ? I have bits of all these ready to use.

Oh and i have some stainless M12 threaded rod, washers and nylon lock nuts. Quick and dirty method would be to just drill 12mm holes and put the threaded rod through. Washers and nuts internally and externally and tighten the buggers up. Nuts and washers internally so i don't try and crank it up too much :roll: . A few of them between opposite panels should do the trick. Won't look good but hey, they can't get much worse.

Other alternative is wood glued and screwed from the outside.

I should be charging pay per view for this thread :lol:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Another idea borrowed from elsewhere :grin:

Use threaded rod (6mm) and have eight of them through the front baffle and right through the back panel. These can be used to tension both bass drivers to the front baffle. A nut and washer on the rear of the back panel and crank her up.

For the sides and top and bottom panels i would use 12mm rod and join them to other rods going in different directions. There must be a way to do it without passing the rods through the panels (i don't mind so much about the back panel as who looks at it ????? You can get some decorative covers for the nuts anyway.

It's either that or build the bloody things out of concrete.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Will the rooms at Kegworth support concrete speakers with 5cm thick walls and a box about 37cm x 37cm x 90cm ?
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Re: Doc modding Marantz imperial 7

Unread post by savvypaul »

Request ground floor, mate...
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

savvypaul wrote: Fri Dec 06, 2019 10:00 pm Request ground floor, mate...
Not sure i can go or that i will have anything remotely resembling finished by then.

Will need a sack barrow :lol:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

The stuffing i had in my speakers was a couple of old quilts. It made the music go to sleep :lol: :lol: :lol: Much better without it but you know i had to find out the hard way be trying it for myself. Any sound from the back wave will find it hard to come through two doped and plastidipped 12" woofers.

I decided to drape these quilts over the front of my big cubes to see what that does. Well i like it. The music is not snoozing like before. All the lower frequencies go straight through it of course but i think the higher frequencies that these filterless drivers produce have been attenuated somewhat.

I think whatever grille goes over these bass drivers in the final build will use some of this material :dance:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Thread about subsonic noise on some cd's. I think this may be what happens on Portishead Dummy track no. 8

https://www.diyaudio.com/forums/multi- ... .html
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

About tuning the bass to your room by use of driver Qts and box volume.

A Qtc (driver qts and box volume combined) of 0.7 would give the flattest frequency response in an anechoic chamber but our rooms are not anechoic. Room gain will increase low bass and baffle step will reduce it. So with a driver of given Qts and a fixed baffle size, the variables are the room, speaker placement and box volume. Baffle step correction is not an option - no yucky inductors thank you.

Seems like experiment is the only thing to get the best balance and bass response. You could say baffle size is adjustable at the design stage which will affect the frequency that baffle step losses start. But i don't want a huge baffle, just enough to house the driver and also to keep a cube shape the depth also increases if you increase baffle width.

In my room placement is pretty fixed with only about 30cm of movement possible in any direction. That leaves me with only box volume to play with. Should i make it slightly too large and have some sand bags inside so Vb can be changed if i move rooms ? But who wants a cabinet bigger than it has to be. But then if i end up going true infinite baffle then baffle step is irrelevant and bass driver enclosures not needed. Seems like using the suspended floor as the baffle solves a lot of problems.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Why put bass drivers under the floor in a manifold ? And why a manifold ?

To save space and to use the underfloor void as the enclosure. At over 10 x the combined drivers Vas (i think) we will have infinite baffle bass loading. The Qtc of the system will be the same as the Qts of the driver. I don't know exactly what that is so it is a gamble on what the final Qtc will be. Between 0.5 and 0.7 would be nice. I don't know exactly what the Vas is either but i can make a rough guess at the moment of 120 to 150 Litres. When my experiments are done with adding known volumes inside the big cubes i will have a better idea.

No baffle step - nice

Extended bass - could be a bad thing as well. Increased driver excursion may mean more distortion and the possibility of reaching xmax and destroying the drivers. That's why i will use two in series or parallel to divide the work out (probably series connection).

Relieve the sound of its box like qualities as in effect there is no box. Not quite like open baffle as there is still an enclosure just very, very big. Anyway, the main advantage of open baffle seems to be the dipole effect where the front and back waves meet each other and severely limit the excitation of room modes and side wall reflections. i don't have the room to have the speakers way out in to the listening space.

Why a manifold, why not just mount the drivers in to the floor boards between the joists ?

Several reasons. A manifold is just a structure that arranges several drivers firing at each other. In my case two with the cones firing directly at each other. You could put four in though and have two pairs using all the sides of a square. The space between the drivers only needs to be about the same as the diameter of said drivers. So just over 12" square in my case. That will fit nicely between the floor joists and have room for the thickness of the plywood. This space that the drivers fire in to is sealed from the back wave completely and it's only opening (about 14" square) is to the listening room. Put some stuffing in there and a grille over the top and job done.

So having two drivers firing at each other parallel to the floor only takes up the floor space of one driver. Also two drivers firing at each other cancels out most of the vibration. We don't want the floor vibrating and singing along do we. Being underfloor the drivers will stay nice and cool. Just need to make sure that they have some mesh over the back to stop anything eating them (mice can get in all sorts of places you would not expect). Don't know if they like eating kevlar and contact adhesive :grin:

That about sums it up i think. I haven't tested any of this yet so it may be partial bollox but plenty of people especially on the cult of the infinitely baffled forum do say exactly what i have above.

There is a good chance it won't work out so i will need to test it at the farm first. Will the bass be strong enough if the qtc ends up being be 0.5 or below ? I strongly suspect that these drivers have a Qts above 0.5 so that won't happen if so. They will most likely play lower frequencies than what i have currently with a big box. The Fs of these drivers is quoted as 30Hz but you know these figures can't be trusted. And doping lowers resonant frequency so who knows what the Fs is without measuring them.

I could end up with too much low end, drivers that reach their xmax and self destruct. And the amplifiers will be asked to do more work which may end up in catastrophe. The only saving grace is i don't listen to movies with lots of very low frequencies through the sound system and i only listen at relatively low volumes. If i pick the resistor values for the stepped attenuator cleverly i can limit the maximum volume to a level way below where any problems might start.

Thats the theory anyway. Maybe i need a sacrificial amplifier to test this out. If it goes up in smoke then we can have a good laugh. Any ideas for which brand of amplifier to purposefully blow up ? (gotta be reasonably cheap though).
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Re: Doc modding Marantz imperial 7

Unread post by Latteman »

Have u tried additional magnets to the drivers- I have tried to read forums for info but to no avail.
I’ve just sucked it up and stuck some on for interest- any opinions?
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