Doc modding Marantz imperial 7

A place for DIY project discussions.
User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Found another quote from Roy Johnson (Green Mountain Audio). Parked it here for anybody interested.

"If a speaker uses a 4th-order crossover at 3,000Hz (common for a 1" tweeter), what will we actually hear from this system?

*

Down at 1,500Hz, the tweeter isn't really contributing. Thus, the image is formed by the woofer. However, the woofer moves back by one inch because the circuit imposes 75 degrees of shift at 1,500Hz. But the dimensional shift is always relative to that 3,000Hz wavelength reference point, where (75 degrees/360) x (13,500ips/3000Hz) = 1 inch.
*

At 2,000Hz, the tweeter output is now audible (12dB softer then the woofer). We hear the image from the woofer move back by a little more, to 1.1" (90-degrees of 3,000Hz equals ¼ wavelength). Yet the tweeter is actually forward by 3/4ths of a wavelength of 3,000Hz, or 3.4". As a soprano's harmonics move through this region, we'll hear the image begin to split by 4.5" total. The image will begin to diffuse from front-to-rear, and harmonics structures will begin to fuzz over -- the sound will become grainy.
*

At 3,000Hz, the woofer and tweeter contribute equally. However, the woofer image has moved backwards by 2.25", for the same total split of 4.5". At this point, the image is very confusing and textures are at maximum graininess. The voice is split into two parts -- 'esses' and 'tees' from the tweeter arrive first, stretched out from the sound of the throat as delivered by the woofer -- an unnatural occurrence. The voice hisses and spits, and strings are edgy*. There'll be comments such as "the transients are etched..." "...detail thrust at the listener..." "...this speaker seems very fast..." "...analytical..." and "...very sensitive to electronics...." Of course, these comments would be expected because the tweeter arrived first!

* Not all higher-order designs sound like this, because there are ways to disguise the problem. Again, if the designer tunes the "Q" of the circuit (computer-optimized, or by ear), or otherwise misaligns the actual crossover point, the graininess will be reduced, along with transient response. The spatial distortion remains, however, and even a solo flute can be heard to wander.
*

At 5,100Hz, the woofer is now 12dB softer than the tweeter -- just audible. It's now 3.4" behind the tweeter's highest frequencies, and the tweeter is still 1.1" ahead -- a 4.5" total split.
*

At 12,000Hz, two octaves above the crossover point, the woofer output is non-existent, but the tweeter output will still arrive 30 degrees ahead (0.4 inches) of its very top end at 24,000Hz, where phase error returns to zero. This will disturb the timing of a wooden stick striking a bell, blurring its image. A musician striking the bell .4" late is definitely behind the beat as well."

and

"So, why use a first-order filter?
That Phase Difference remains Constant for a first-order filter: The output from the two halves of a first-order filter are always 90 degrees apart, on every frequency, not just at their crossover frequency. So their Relative Change in Phase Difference, at every frequency, is zero."

and

"You're right. I oversimplified. Actually the two drivers are 90 degrees out of phase on every tone (at least that's the goal), so the summation at every frequency has a constant relationship- there is no relative phase shift between the two drivers. The group delay is constant.
These users thanked the author karatestu for the post:
CycleCoach (Fri Jun 04, 2021 10:24 am)
DIY FREE ZONE

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Concerning spherical speakers.

From a diffraction point of view a sphere is not quite the best shape for the exterior of an enclosure. That award goes to an ellipsoid apparently. Won't be making one of those.

Some cut and paste bits and bobs......

When the sound from your speaker hits anything, it creates a reflection. These reflections are perceived by your ears as a secondary sound source. What's interesting about this phenomenon is that it's VERY time dependent. A reflection that occurs an inch or two from your speaker is 10x worse than a reflection which occurs a foot away.

Subjectively, the speaker becomes more difficult to localize when it's in a spherical enclosure. The soundstage becomes more diffuse, and seems to float "around" the speakers instead of being tied to them. The sound is also sweeter and less fatiguing. And this is only common sense of course. Imagine a sound wave striking a sharp edge, diffracting violently.

Now imagine the same sound wave in a spherical enclosure. The energy is the same, but the sound is diffused gently across the smooth surface. The improvement in the polar response is measurable, and the subjective improvement is obvious.

The closer to a tweeter you get the more important it is to have a smooth surface and no sharp edges.

A picture of what diffraction looks like http://www.silcom.com/~aludwig/images/diffdem.gif

Let's say we wanted to diffuse sound down to 100hz. The speed of sound is 13500in/second. So to diffuse sound down to 100hz, we'd need a spherical enclosure that's 42.98" in diameter. Here's the equation:

required sphere radius = 13500 / lowest frequency / 2 / pi
required sphere radius = 13500 / 100 / 2 / 3.14159

Required sphere is very large. Psychoacoustic studies have shown that matching frequency response above 1khz is more important than *below* 1khz. Based on that we could reduce our sphere from 43" to something much more manageable.

End of cut and paste...

It seems that I will need to blend in the edge of the tweeter face plates to do the best job with this. Also a sphere which has a diameter larger than that of the faceplate is desirable so that the round over is smoother. Plonking a tweeter in a sphere of the same diameter won't achieve much as there is still a 90 degree corner.

It won't be quite as good for me as I have three other tweeters in the spherical pod which means it doesn't quite turn out like a sphere having four flat sides. Still worth doing though. I don't want to start with a sphere any bigger than 15 - 16cm as that pushes the tweeter domes further apart which is not good.
DIY FREE ZONE

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I looked into ways of making the tweeter face plate smaller. With some tweeters the dome and voice coil (and sometimes tinsel wires) are attached to the faceplate which makes things difficult. It turned out the G20SC faceplate comes off very easily and doesn't have anything attached :dance:

Here is what it looks like with the faceplate removed

Image

A comparison with one with faceplate still on

Image

Round the back. Faceplate and shield removed from one

Image

I can reduce the faceplate from 94mm to 70mm which will make the sphere pods more effective at reducing diffraction and hopefully improve the sound. I will probably chamfer the edge of the face plate too so it blends in better with the curve of the sphere. I don't have to worry about several things like being airtight as the pods only house the sealed back tweeters.
These users thanked the author karatestu for the post:
CycleCoach (Mon Jun 07, 2021 6:27 pm)
DIY FREE ZONE

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Moving on to a possible spherical mid bass enclosure. It seems a good idea as I can help smooth diffraction down to way below 1Khz using the equation posted above (the mid bass sphere will be 35cm diameter). With the drivers only being 134mm diameter then the angle where they meet the enclosure will be nice and shallow which will be ideal.

The behaviour inside a sphere (mid bass' use the interior volume) is a much argued about subject with differing view points. That being said, lots of people say cubes are the worst shape for a box but any problems do not seem to be noticeable subjectively.

Legs become a problem with a spherical bipolar mid bass enclosure. Much thinking to do there as it is not straight forward. i have a few ideas to plan out.

An added bonus of having a spherical mid bass enclosure as well as tweeter pod would be the reduction in diffraction caused by the tweeter sound waves hitting the mid bass enclosure. As the most important edges are within 13 inches then the mid bass box comes in to play as well. Brucey bonus :dance:

I am still intending to try the tweeter pod directly above the up firing mid bass as a kind of diffuser. It may not work though with the sphere not being quite a sphere any more :(
DIY FREE ZONE

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

DIY FREE ZONE

User avatar
Latteman
Posts: 1220
Joined: Tue Aug 14, 2018 7:20 pm
Location: S. Yorkshire
Has thanked: 553 times
Been thanked: 297 times
Contact:
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by Latteman »

Hey Stu
Can I clarify- are u looking to house the mid bass in a sphere too- if so how big and how or what do u intend to use- there isn’t much on the market- Lidl have stainless half sphere bowls for sale atm- I thought perhaps a half sphere top baffle on a cube?????
Analogue Source -
Denon DP47f- Ortofon mc20 supreme
Ifi Zen Phono
Doug Self balanced Pre amp
Akai 4000DS mk2 R2R
Digital Sources- IfI stream, Ifi Neo idsd Dac; Tidal / Radio Paradise
Amplification Nva 30vdc mono blocks
Speakers Lii Audio F-15 in Open Baffle; Ls6
Weiduka AC8.8- for digital sources
Mini BMU for analog sources

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

Latteman wrote: Tue Jun 08, 2021 8:56 am Hey Stu
Can I clarify- are u looking to house the mid bass in a sphere too- if so how big and how or what do u intend to use- there isn’t much on the market- Lidl have stainless half sphere bowls for sale atm- I thought perhaps a half sphere top baffle on a cube?????
Hi Peter.

Yes, the up and down firing mid bass in their own sphere and the four tweeters in a smaller sphere above. A bit like BB8 :lol:

The sphere needs to be 35cm diameter to get the volume I require for two mid bass. There isn't much around to use you are right. I found some 35cm acrylic spheres of 4mm wall thickness. That is on the thin side but by it's very shape a sphere is stronger than a cube. The company can make the holes for the two drivers. I only have a vague idea on cost as I have not contacted them.

The other option which I like is to make it out of Birch ply in a translam fashion but it is wasteful and I would need to buy a lathe (which I am seriously considering now).

Example of what translam construction looks like

Image

A little sketch to scale of what it would look like from the front (I have drawn where the spheres have to be cut for the drivers . I fancy three wooden legs splayed out slightly

Image

Ikea do some good looking 28cm diameter bamboo salad bowls - two could be joined together to make almost a sphere. The volume isn't enough for my needs unfortunately but there are a few examples on the net, like the one below. Looks good I think.

Image
These users thanked the author karatestu for the post:
Latteman (Tue Jun 08, 2021 6:20 pm)
DIY FREE ZONE

User avatar
karatestu
Posts: 5945
Joined: Sun Jan 08, 2017 4:40 pm
Location: North Yorkshire
Has thanked: 1865 times
Been thanked: 1378 times
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by karatestu »

I like the proportions of the big and little spheres. If I make them in wood I think the wife would quite like them. Certainly would be a talking point with visitors.

I hope I can pull it off :pray: When these are done they are going to be my forever speakers so I heed to get them right.
These users thanked the author karatestu for the post:
CycleCoach (Tue Jun 08, 2021 8:50 pm)
DIY FREE ZONE

User avatar
CycleCoach
Posts: 1072
Joined: Tue Nov 15, 2016 5:20 pm
Location: Kings Cliffe
Has thanked: 1739 times
Been thanked: 677 times
EUROPEAN_UNION

Re: Doc modding Marantz imperial 7

Unread post by CycleCoach »

Loving it Stu. So imaginative!
These users thanked the author CycleCoach for the post:
karatestu (Wed Jun 09, 2021 5:43 am)
I think I might be in the HiFi trade.
I am not currently seeking treatment for this.
Current System: Linn LP12 Lingo Ekos Troika, NVA PHONO2 with BBPSU, NVA AP20 (White), Quad ESL 57s &/or NVA Cube3s (White)

User avatar
Latteman
Posts: 1220
Joined: Tue Aug 14, 2018 7:20 pm
Location: S. Yorkshire
Has thanked: 553 times
Been thanked: 297 times
Contact:
Great Britain

Re: Doc modding Marantz imperial 7

Unread post by Latteman »

Do u remember being in Skool and playing with paper machie/ balloon - made a reasonably rigid structure- grownup version would be fibre glass-?????
These users thanked the author Latteman for the post (total 2):
antonio66 (Wed Jun 09, 2021 12:17 am) • karatestu (Wed Jun 09, 2021 5:44 am)
Analogue Source -
Denon DP47f- Ortofon mc20 supreme
Ifi Zen Phono
Doug Self balanced Pre amp
Akai 4000DS mk2 R2R
Digital Sources- IfI stream, Ifi Neo idsd Dac; Tidal / Radio Paradise
Amplification Nva 30vdc mono blocks
Speakers Lii Audio F-15 in Open Baffle; Ls6
Weiduka AC8.8- for digital sources
Mini BMU for analog sources

Post Reply