Doc modding Marantz imperial 7

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I don't know how much dipole figure of 8 goodness is happening with these tweeters. It is hard to get it to go up to 20KHz because of the path length between front and back of the cone. The driver will be beaming as soon as the frequency wavelength equals the width of the cone and mounting flange. There is a dip at this point from what I gather.

I removed the little rubber ring I had which was glued on the mounting flange so the driver is a bit smaller to the eye from the front. Not sure what that has done if anything. I suspect this driver starts beaming at just under 5KHz and the front flange corresponds to about just over 4KHz.

All I know is it sounds bloody good so maybe I shouldn't care. :think:
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antonio66 (Tue Mar 15, 2022 5:46 am)
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Quote from diyaudio.com . It is somebody's opinion and shouldn't be taken as gospel.

"I have read that when an enclosure panel is in resonance, it is almost acoustically transparent. This makes sense to me.

There are two mechanisms for sound to penetrate an enclosure wall.

(1) The first is normal sound transmission. A pressure wave strikes the surface of a wall and compresses the surface of the wall. The sound moves through the thickness of the wall as a wave of elastic compressive - tension stains until it reaches the opposite surface. it then is radiated into the air. For 3/4 inch plywood, we would expect there to be a 15 - 25 dB loss, depending on frequency... about 15 dB at 250 Hz, about 20 dB at 1 kHz, and about 25 dB at 5 kHz.

(2) The second mechanism is structural resonance. The mass and stiffness of the panel together form a dynamic system with a natural frequency. If the wall is driven with a signal at its natural frequency, the wall flexes in and out of plane. It acts as a diaphragm, which is very different than mechanism (1) above. The driving signal can be either an acoustic wave or a direct force from the speaker driver, but the direct force from the driver is usually more significant.

When in resonance (case 2), the normal 15 dB to 25 dB sound loss through the wall does not apply, because the wall itself is moving back and forth.

Most enclosure designs, including very high end enclosures, basically ignore mechanism (1). With a good stuffing or lining, sound transmission through the cabinet walls is reduced by at least 20 dB, and possibly much more than that. The effect on the frequency response of the speaker system is very small. The speed of sound through plywood or MDF is much faster than air, so there is no energy storage when mechanism (1) is dominant.

Most enclosure design is focused on minimizing the effects of mechanism (2)... i.e. resonance. It is complicated because there are six walls in a box, and they mutually couple. Cabinet stiffening can transform the six cabinet walls into many more virtual surfaces. Anyone who claims that this is simple engineering or basic physics is blowing smoke. It is very difficult to model and simulate. The fixity of the edges are difficult to predict, and the effectiveness of the bracing is hard to know. Are the cabinet walls under pre-load (clamped during glue-up)? What is the effectiveness of the adhesive bond line (i.e. how stiff is the adhesive)? How much internal damping is there in the cabinet walls? All in all, a tough modelling problem."
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 »

Many twists and turns in this saga as I stumble through trying different ideas. I hesitate to call them designs as it's all mostly in my head and very amateur in nature. I haven't a bloody clue what I am doing most of the time but I think most people will have gathered that a long time ago :idea:

I love the little 2" wideband drivers but their downfall is the litre of enclosure they require. A 15cm sphere sited above the up firing mid bass does give a result but two things stop it from winning. 1) The sphere is too big and gets in the way too much 2) if the sphere is centred above the mid bass then the wideband driver used as tweeter is forward of the mid bass' surround.

I have identified these two problems through trying the visaton G20 again with faceplate removed and stuck on the end of a cymbal stand with the dome situated above the centre of the mid bass. This sounds better to me and I will put it down to time alignment and there being much less in the way of the mid bass output.

So where to go with this ? I have a couple of options. If I want to continue with the SB acoustics 2" wideband then it will require an open ended tapering pipe that is stuffed more densely as you get further away from the driver. If long enough that should soak up the backwave. It sounds best when above the centre of the up firing mid bass.

The other option is to use the visaton dome tweeter in as small a housing as possible with the dome above the centre of the mid bass.
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CycleCoach (Mon Nov 21, 2022 11:26 pm)
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Re: Doc modding Marantz imperial 7

Unread post by Latteman »

Been dormant for awhile 👋
What are u using for music reproduction at the moment Stu ?

I’ve moved on quite a bit with many changes (should do an update thread) but with Nva at the heart- including a return to point n squirt and now ‘open baffles’- 😉
Analogue Source - Sony PS-2251; Nagaoka mp200
Yaqin Les carpenter phono stage
Akai 4000DS mk2 R2R
Digital Sources- MF M1 Clic; Lenovo Laptop
Amplification Nva / Nelson Pass
Speakers- Open baffle Marsden Hall 12/8: Seas tweeter ; Ls6
Dc blocker to Nva mini BMU- analogue sources
Weiduka AC8.8- digital sources

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karatestu
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Latteman wrote: Tue Nov 22, 2022 9:39 am Been dormant for awhile 👋
What are u using for music reproduction at the moment Stu ?

I’ve moved on quite a bit with many changes (should do an update thread) but with Nva at the heart- including a return to point n squirt and now ‘open baffles’- 😉
Using a JBL charge 5 Bluetooth speaker, my tablet and YouTube music subscription. It is hassle free, portable and never sounds bad. In fact I very much enjoy listening to it.

My diy stuff is still set up and I do listen to it whilst doing my paperwork. The process is thankfully nearly done so mostly cosmetic things to think about. I am presently very pleased with the sonic outcome which is very much helped by my improved state of mind. Just goes to show that things are not what they seem when mental health gremlins are doing their worst.

I feel optimistic about it without feeling the need to be trying something else. This is it. Just need to get the bloody thing finished.
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CycleCoach (Wed Nov 23, 2022 10:28 am)
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 »

Well I did say above that I felt no need to try other things. That didn't last long as I ended up trying the SB acoustic 2" wide band drivers in a progressively stuffed tapering tube.

Looks like this...

Image

Yes it looks like shit but in my defence it is only to prove it works. And works it does. I stuffed the pipe loosely with fibreglass insulation near the driver and got denser towards the small opening at the back. Used a plastic kids chair leg. Recycling at its best.
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CycleCoach (Wed Nov 23, 2022 3:50 pm)
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

That was the first post I have ever made here with my first ever smartphone. Finally dragged into the 21st century.
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Now I remember why I was reluctant to give up on these 2" widebanders. They are certainly more detailed and clearer than the visaton tweeters, I can hear much more going in all parts of the soundstage.

Why this is so I have no idea. Just a better driver, dispersion is different, rising top end. works better at lower frequencies - less distortion with a first order filter ? I will never know for sure, the above is all speculation.

The sound is not fatiguing in any way. I have always stated that I am not an imaging or sounstaging disciple. Many things have changed over the course of this journey and I have actually realised that I do value those things after all.

I never thought semi omni could image as well as what I am experiencing now. I presume this is because of the attention I have paid to enclosure shape, getting the tweeter above the centre of the mid bass and controlling cabinet vibrations.

These speakers do not produce vast amounts of low bass. But there is zero bloat from cabinet resonance or the floor due to the inner tubes under them, the mid bass not actually touching the steel sphere and the length of 60mm diameter solid brass bar I attached to the back of the magnet and sunk into the sand in the bottom of the enclosure.
DIY inspired by Richard "The Doc" Dunn RIP

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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

That tapered tube is too long and sticks out too far at the back of the speaker. To really make this work aesthetically and so they can go right back to the wall I need a way of providing a 90 degree bend so the pipe can terminate vertically.

It may also provide a way of mounting the tweeter and I can get rid of the cymbal stand section and gaffa tape :grin:

Solving this is going to get the brain cells going.
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Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Another thing - when using the 2" wideband driver I don't need the padding resistor which is required with the visaton dome. With the B&W drivers I need about 5 ohms when using the dome tweeter.

Call me paranoid if you want but I much prefer the idea of having nothing but cable between amplifiers and both the mid bass and tweeter. That was achieved by doing the high pass filter for the tweeter passively at line level before the power amp.

The sensitivity of the 2 incher is perfect to marry with the up firing B&W mid bass so no resistor required either. It just seems like a logical extension of Richard's ethos regarding simplicity.
DIY inspired by Richard "The Doc" Dunn RIP

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