Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

The leg(s) of this thing have always caused a problem or two. Keeping the centre of gravity low and the speakers stable was always on my mind. That meant that either the legs had to be heavy or heavily splayed or if using a single leg it would require a large base. If I am having a solid 30cm diameter sphere under the driver enclosure sphere then I can just join the two spheres with very short standoffs. The lower spheres can then have very short spayed legs.

I need to draw this to see exactly how it looks but it solves the legs, centre of gravity and stability issue. :dance:
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Here's a crap photo of a crap drawing of what it looks like. It is to scale. Dashed line horizontally is where the plywood divider will go - I forgot to include it in the drawing and couldn't be arsed to do it all again.

Image

The speaker is 30cm wide and 75cm tall. Doesn't look as imposing as a boring rectangular monkey coffin would. I have shown the 4mm threaded rods used to clamp the hemispheres to the dividing plywood and a solid metal round bar which mechanically connects the back of the opposing driver's magnets. All for force cancellation of course - this requires the opposing drivers to be identical.

If I go three way then the solid round bar won't be happening and the up firing mid will need a small back box inside the main enclosure - possibly a stuffed cone shape..
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CycleCoach (Thu Sep 30, 2021 12:04 pm) • antonio66 (Fri Oct 01, 2021 12:48 am)
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

It does seem a waste of space to have a 30cm sphere at the bottom doing nothing. I could just fill it with sand to try and "mop up " the vibrations.

Other idea was to put another driver in it but then I will have two drivers almost kissing. Would have to be a low passed bass driver of course. Would require some thinking to make sure the balance of the whole speaker was not upset. Four way speaker is out of the question, no point separating the frequencies any further and I would need another two amps :roll:

Last idea was using it to house the nva amplifier boards. Whilst an attractive proposition it does throw up some problems like how do you attach flat output transistors to a curved surface. Need to think about this some more as there are positives that can't be ignored.

Less cabling is one - if the power supplies are remote (like I have now) then this would mean I could reduce from three pairs of LS6 speaker cables down to a pair of interconnects. DC power lines would be required but if I am lucky I could possibly find a connector that will provide enough terminals (seven).

The other plus is having the speaker drivers very close to the amplifier boards. The down side is having to have a long interconnect - swings and roundabouts especially when using a passive preamp (volume control and source selector in a box).
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I replaced my crappy tweeter sphere support with three 35mm bolts hot glued at the edge of the cut out. That has dropped it much closer to the up firing mid bass. Not listened enough to form an opinion on this tweak.

Latest picture with lowered tweeter sphere, shortened legs and foam around the down firing mid bass. Looks total crap but that's no different to anything I have cobbled together in the past :grin:

Image

I wish all my speakers had the same baffle width then I could do this passive line level xover and be able to swap around. Thinking of clamping 5cm of wood either side of my B&WP4's just to enable this.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Don't know about you but I'm getting sick of this thread :roll: And the possibility that I am chasing something that can't be achieved.

Anyway, further messing about. I replaced the HDPE balls with the steel spheres. I did not put the foam back as I thought it was choking the openness of the speakers. The bass was also too low in level for my liking too. I did it because thought the midrange was too prominent. I have however put some 10mm thick fluffy stuff that came out of an old car seat over the spheres.

A change in presentation has obviously occurred. Two things have changed - steel spheres instead of plastic and much less stuffing between the mid bass cube and the sphere. Well, bass levels have increased I am happy to say and the mid range isn't headache inducing like it was. :dance:

The steel spheres are obviously not elastic like the plastic ones so that may be responsible for bringing up the bass. The open cell foam was 75mm (3") thick and whilst reducing the midrange it was strangling the vibrancy of the speaker.

I may be able to get away with no stuffing material over the steel sphere and will try it in due course. It seems like the tweeter pod lowered down closer to the up firing mid bass is still sounding good although issues sometimes arise later on after much more listening or something else changes.

I expect things will all change when I get the mid bass drivers in a sphere. At the moment the crude horn shape is flat on the underneath of the mid bass enclosure and curved on the top of the steel sphere resting on the floor. Like this

Image

There will be some reflections going on which is what the issue with horn resonances is all about.

When the mid bass enclosure changes to a sphere then it will be curved on top and bottom with the only difference being one has a driver in it and one doesn't. Like this

Image

There will be much less reflection going on with both sides being curved . I expect that to sound better

So it looks like if this design stays the same going forward then I need to buy another four steel hemispheres. I think the rigidity of the lowest sphere is going to be just as important as the rigidity of the mid bass enclosure.

The placing of the steel sphere under the down firing mid bass has had a positive effect in that the bass is stronger, more tuneful and punchier than the plastic sphere and the evening out of the bass frequencies is still occurring, probably even more so.

Well chuffed so far.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Now I get to thinking do I actually need a full sphere at the bottom ? I could try just a hemisphere with the flat side on the ground. Not sure what effect that would have on things. The bass with a wavelength longer than 30cm probably won't care as it just wraps around it anyway.

What will be changed is the height of the speaker and the proximity of the the mid bass (the down firing one especially) to the floor boundary and the corner where the floor meets the wall. As these are positioned right against the wall the corner loading will become more pronounced.

If this idea works out then the tweeter will be below ear height but the way things are I can angle the front tweeter up to my ear height when seated. The side firing tweeters wont be angled up however. I will also get to see more of the up firing mid bass. My speakers keep getting shorter :lol:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I removed the crappy wooden legs on each speaker in case I had to backtrack and made some more out of lengths of 19 x 6 mm solid aluminum bar. The down firing driver is now only 17cm from the floor with a 30cm diameter steel hemisphere below it.

Image

Speaker is only 61cm (2ft) tall now :shock: :shock: :shock: It's tiny :grin:

I have listened for a while and the albums I have played so far have been wonderful. The bass is further evened out, more detailed and punchier than it ever has been. I don't feel like I am looking down on the performance at all - I did worry about that being the case but my fears were unfounded. Speakers like this with up firing mid bass do tend to paint the soundstage slightly above the top of the speaker especially in this case as my tweeters are above the mid bass.

Mid range is not out of place at all and combined with the treble it is slightly better as the bass has been cleaned up . This hemisphere under the speaker works even better than with the sphere under it. I suppose I afforded myself some unique ways to experiment with up and down firing drivers. It is not really possible to play around as much when drivers are all forward facing. Nobody puts a sphere on the front of a cab :lol: I have similar things going on with the up and down firing mid bass although the sphere over the up firing driver is half the diameter of the hemisphere under the down firing driver.

Loving it so far. Sounds like a winner and being smaller it will have a lower centre of gravity and I can relax a bit with trying to keep it stable with young kids around.

More soon......
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Here is a sketch of what it will look like. You may notice I have ditched the up firing tweeter. I think it sounds better without it.

Image

I expect there to be more improvements to come as these are still a cobbled together affair. The chipboard cabs were vastly improved when I steel lined them but I expect the rigidity of the steel spheres to be even better. The diffraction will be much better as well as there will be no sharp edges.

Same with the tweeter sphere pods. The tweeters are just taped in at the moment :roll: because as usual when I removed the faceplates I was in such a rush to hear the effect of the sphere over the rounded edge foam dice I was using previously. I have cut the holes in the two new plastic tweeter spheres but I need to modify all the tweeters and I am reluctant to be in a situation where I can't listen to this system. I'm addicted.

With everything being spherical I will get lots less reflections from the tweeter on the top baffle and in the "horn" between the bottom baffle and the hemisphere base. Lots more to come, I can't wait. :dance:
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Yesterday I cobbled together a speaker using my spare pair of 17 litre cubes. I put in my pair of B&W 6.5" mid bass from my P4 transmission lines and plonked the tweeter spheres just behind the up firing mid bass. I borrowed the inductor from the P4's for the low pass and for the high pass I used a 4.7uf and 6.8uf cap in series to get an approximate 3.5kHz xover.

It was a quite rewarding process and I learned a thing or two (I think). It was a very rough cobble together. The bass was weak so I ended up putting 5 Litres of sand in the bottom of each speaker :lol: That improved the bass but wasn't enough so I suspect the Qts of the B&W drivers is too low for a sealed cabinet. A shame.

It was a bit of an eye opener as regards the difference in presentation between a pair of heavily doped 84dB 5.25" drivers and a single 88dB 6.5" non doped. There was more life for sure and it was louder but it sounded a bit rough. I don't like a xover at 3.5kHz. Also a single order low pass at that frequency is inadequate as I suspect too much cone break up is coming through.

Doping sure does smooth out the response of a suitable driver very nicely but I don't want to go down that route with these mid bass. In fact I would rather use the monacor sph-145hq with no doping at all if possible. I don't think the motor will be strong enough for the added mass.

It was also useful to compare the up and down firing bipolar design with the up firing monopole. With small 5.25" drivers I think the bipole is without a doubt superior. I tried it with the down firing driver disconnected- the lower driver was acting like a crude passive radiator and too small at that. It was interesting although not optimal as the box was now too big so bass suffered a little but the disconnected driver did make a significant contribution in the bass.

Passive radiators are not something I have really seriously considered because Doc didn't like them - probably something to do with timing as was usually the case with RD. I am now sufficiently interested in them to have a look at how to design a speaker incorporating one.

I do remember trying a bipole with 6.5" doped mid bass and whilst it was superior to the 5.25" version in some ways, the baffle step compensation was clearly too much for my room although the cabinets were crap chipboard and smaller than optimum.

That is the problem with trying to draw meaningful conclusions from crap enclosures- their contribution to the sound really us significant and so I can't be sure that the outcome would be the same with a quality enclosure of the right size.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Having now read a bit about the behaviour of passive radiators I am suitably put off the idea. They require tuning like a bass reflex and have a high mass which can be altered by adding more mass to change the Fs or Fp to be more precise :grin: You generally choose a diameter a bit wider than the active driver

It will be ouf of time, have higher group delay and possibly slow compared to the active driver. It acts similarly to a port but without the port chuffing noise. The high mass might make it slow to stop and start but saying that I am used to heavy doped drivers. After yesterday's experiment though, it would be preferable to have low mass drivers if possible- that will require a very well behaved driver if it is to be achieved without a high pass filter.

I like transmission lines so maybe the bass response with a passive radiator would not be too much of an issue for me (if designed properly) however it maybe a solution looking for a problem. Which is the more evil - a passive radiator or a phase changing filter ?
DIY inspired by Richard "The Doc" Dunn RIP

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