Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

At one point I wondered if the shape of a cone had an impact on driver dispersion. It has a small effect but mainly everything hinges on the size of the radiator. I had hoped at one point that doping a 5" almost up to the surround so that it is flat on top would help with dispersion. It may do a little to help but not a great deal unfortunately.

25 mm and 20 mm tweeters beam at much higher frequencies than mid bass drivers because of their small size. It doesn't have much to do with the fact that it is a convex dome. Visaton do a very small 13 mm soft dome tweeter with tiny neo magnet that was produced mainly for in cars. Not sure if it is any good but that must have excellent dispersion of sound although it's only 86 dB. Would be good for getting in tight spaces.
DIY inspired by Richard "The Doc" Dunn RIP

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Today's revelation.

I took away the big bass cubes from under the little upper cabs and put them on some stands that don't impede the down firing 5 inchers. About 65 cm from bottom of the cabs to the floor and back of cabs are 5 mm from the wall.

It still sounds good but something is missing :think: The sound is not projected out in to the room like it was, it is a slightly less exciting listen and bass is not quite as strong.

This is what I think is going on here, could be all bollox mind. There was the baffle of the bass cube 20 cm under the down firing 5 incher. It has now gone and the driver is now 65 cm from the floor. The 20 cm wide slot that was created between the bottom of the upper cab and the top of the lower cab must have been reinforcing the bass and projecting the sound out in to the room.

I removed the lower cab because I thought it would be an improvement to prevent the arse end of the unused 12 incher diffracting the sound. I plan to flip the down firing mid bass so the magnet is no longer facing out as the basket and magnet could be messing with the sound but that is for another day.

It is said that in a slot you get resonances but they are usually talking about a slightly different thing to what I had - much less distance between the panels and usually open only one one side (slot loaded open baffle's). My slot was open on all four sides and was 20 cm wide.

Maybe the panel that was 20 cm below the 5 incher was simulating an additional near boundary like when you move a speaker up against a wall in to a corner. Or was it loading the 5 incher in some - adding virtual mass to the cone. Maybe both.

So maybe it was a very crude sort of horn that projected the sound further in to the room? Well what ever it was, I am missing it :cry:
:cry: :cry: :cry:

So how am I going to get it back? I need to try a quick experiment by putting a panel right under the 5 incher with four little legs and sit this on the stands. See what that does. Runs off to find some scraps of wood ....................
DIY inspired by Richard "The Doc" Dunn RIP

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Found two 40 cm2 pieces of 6 mm aluminum which I have put on the stands and a few feet to raise the speaker up but only just enough for the magnet to clear the aluminum sheet (8 cm).

Well I don't think I was imagining things, this has restored the sound to more or less what it was previously. The gap is now 8 cm so considerably less than the 20 cm it used to be and there isn't the mess of the basket and magnet of the 12 incher to fire in to.

This is changing my design yet again :roll: I just have to listen for a while and work out if there any resonances from this smaller gap. The final design will be only 30 cm (width) x 30 cm (depth) x 85-90cm (height) so there the 5 incher will be closer to all the outlets of the gap than it is now (box is currently 40 x40).

I am thinking about making the back panel of this speaker go all the way to the floor which will act as the support to the rear of the speaker. If I add another 30 cm3 box with two 5 inchers in (up and down firing) then there will be a small gap between the two cabs and the same gap between the lower cab's down firing 5 incher and the carpeted floor. Some small lengths of round brass bar could then be used as support between the cabs and below the lower one.

The only reservation I have about this is the four mid bass drivers being so spaced vertically. I will never know until I try it though.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I decided to have a play with blocking one side of the gap off - the outer facing side. A couple of fence post blocks and a length of brass bar on the inner facing side. As the speaker is 5 mm from the back wall i thought that closing another side would simulate the speaker being in a corner to some degree. I don't think it does entirely but the bass is indeed louder.

Image

You can see I used a pair of speaker stands for each speaker. Aluminum plate is blutacked on to them. :lol:

Image

Listening now to my box set of remastered Police Cd's. Ver nice :guiness;
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savvypaul (Wed Oct 21, 2020 8:03 pm)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I moved that length of fence post as close to the driver as I could :shock: Very interesting. These are now like a double cube3 on steroids. Bass goes deeper as well as being higher in volume.

Gets me thinking about putting another length of fence post tight up at the back of the mid bass to create an L with the driver right in the corner. That might be taking things too far but I won't know til I try it.

:dance:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Next thing on my to do list is to invert that down firing mid bass so the cone faces out. Then when it's wiring is inside the enclosure and out of the way I can put another length of fence post right up to the back of it.

After that then more prototypes :roll: with 30 cm3 cabs, a back panel that goes from the floor all the way to the top of the speaker. If i decide I like the down firer in the corner of an L in the slot then i might make the outer side panel full height as well. This will require a bit of filling in so the L is right up to the driver.

A lower enclosure of 30 cm3 with up and down firing mid bass will obviously be closer to the floor with a gap between the two cabs and a gap at the bottom of the lower one. No tweeters in the lower box of course but probably a similar thing done with it's down firing mid bass (tight in the corner of an L).

That will mean two 5 inchers will be firing at one another with a gap of 10-20 cm between them. There will be some effect from this arrangement which I am yet to fathom. Mid bass firing at the carpet will obviously have it's top end attenuated. Might try so e felt in the gap between the two cabs to do a similar thing.
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Re: Doc modding Marantz imperial 7

Unread post by Latteman »

I did a similar thing a ur ‘L’ shape except it was a ‘U’ and up firing
It was interesting but in the end became boxy when compared to being removed- didn’t notice the boxy sound until removed then replaced- could have been the materials I used but didn’t investigate
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karatestu (Thu Oct 22, 2020 5:45 am)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Hello Peter. Thanks for your comment. Yes you have to be on your guard with these things and always keep both ears fully opened :grin: A boxy sound is probably due to the resonances at frequencies with a wavelegth equal to the dimensions of the gap. Imagine those sound waves bouncing around in there.

I'm trying to imagine the up firing U you that you tried. What were it's dimensions ? If you make it too long then it becomes a badly designed horn. Gaps, slots or what ever they are are tricky things to "guess" and of course doing it all by ear with no measurements is hard and basically a shot in the dark. If you read about transmission line speakers (MLTL) there are many resonances that need to be tamed with tapering, stuffing part of the line etc. Those pesky resonances are a real pain in the arse. :grin:

Of course there is nothing new under the sun. I often wonder why some of the concepts I try are not more popular and then remember that it is probably because they don't work (especially for the measurement brigade). Ears are all that matter here but as you say Peter, you don't often realise what is really going on until you reverse a modification. That is unless you are one of the self professed golden eared who make up most of forum land. :roll:
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Re: Doc modding Marantz imperial 7

Unread post by Latteman »

Page 71-72
Analogue- DIY turntable- donor Thorens TD321, DIY Tonearm; Nagaoka SB mp200- Nva Phono 1 (Modded)
Digital Sources-Teufel Connector Streaming DAC; Little Dot Dac1; Linn Mimik cdp; iPhone 6s/base;
Amplification KHOZMO Acoustic Passive Pre - & Nva A20LE (👍🏼)
Speakers- Diy ‘Isobaric Cubes’; Ls6 bi-wired
Dc blocker to mini BMU

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Oh I see what you did there Peter. The little U that you put on the top has no ends on it so the driver with it's arse in the air has no enclosure. I am not sure what you would call it ? Kind of open baffle but not. A closed box with two of the panels missing.
DIY inspired by Richard "The Doc" Dunn RIP

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