Doc modding Marantz imperial 7

[karatestu]

Good morning Peter.

I think any effect depends on whether you use the extra magnet as piggyback or bucking .

Piggyback the magnetic fields are aligned with the fields running in the same direction. With bucking the fields are reversed. Adding a magnet of the same polarity you will feel the magnets repelling each other. This can increase Spl and decrease Qts . Also this will dramatically increase stray fields which may be a bad thing if you have crossover near.

If using the magnet bucking style this can also increase Spl and decrease Qes and therefore Qts. It has a bit of a shielding effect - the opposite of piggyback as it reflects stray field back. Bl parameter of the driver can be increased slightly

In both cases the effects are said to be minimal because the extra magnet is not in the magnet circuit. At most 1dB increase in spl . The benefit in both cases is said to be minor to minute.

If you get it wrong you may actually lose spl. Bucking magnets is the way to go and the ones seen on commercial drivers mostly look to be smaller than the main magnet.

Not sure if any of this is of use to you Peter. What are you trying to achieve ?

[karatestu]

Quote of the day

"Sometimes, we forget the real reason we got into this hobby in the first place ... to learn, experiment, and build our own great sounding speaker systems. Too many people on this forum are expecting instant gratification.

Yes it is fun do mess about with things and learn from your mistakes. Beats reading stuff on the internet and taking it as gospel

[karatestu]

When buggering about with speaker design we must not forget Hoffman's Iron Law

"Hofmann argued that the designer had "...three parameters that cannot all be had at the same time": good, deep low-frequency sound, a small cabinet size, and high sensitivity. Hofmann stated that designers could pick two of these three parameters, but in doing so, it would compromise the third parameter.[4] For example, a designer who wants good, deep low-frequency sound and high sensitivity can obtain these goals, but they will have to use a large speaker enclosure. Similarly, if a designer is forced by space constraints to use a very small cabinet, and they aim to get good, deep low-frequency sound, the sensitivity will be compromised. A small cabinet with deep bass would need a very powerful amplifier."

As i scrabble about trying to lower box volume i must not forget it comes at the cost of lower efficiency .

[Latteman]

Just experimenting- it’s all fun

[karatestu]

Just experimenting- it’s all fun

[karatestu]

Revisited down firing 12" cube because i so wanted it to work with the clamshell isobaric arrangement. After two songs i have concluded that it still sucks. Bass is overblown and i swear the efficiency increased a little. Definitely not trying it again.

So as i was moving things about anyway i turned them upfiring (again) Small cuboid situated right behind the big cube and slightly elevated. This is not bad at all, bass is good but i still don't think it is as good as the 12" front firing and at floor level (the Allison effect). I need to try it with all the mid's and tweeters in the same enclosure before totally ruling it out. Mid's would have to be side firing though as no room on the top baffle. No room for upfiring tweeter either

[karatestu]

This is breaking news. Just to show you the lengths i go to for highly scientific experimentation using state of the art materials and techniques.
I wanted to retry up firing bass drivers now that i have the drivers in clamshell configuration with a much larger volume behind the rear driver. Obviously the air volume between the two driver's cones is very little with only 19mm chipboard between them. It is hard to arrange the little cuboid next to the big cab when doing up firing bass drivers. So i came up with this little elegant solution which has me thinking now




I really have surpassed myself this time. Using an ikea lack platform i spanned the space between the window ledge and the top of the driver's magnet but the heights were slightly out. This was solved with a little length of wood between the driver and the ikea lack panel. A big weight on the back (56 lbs) and little cuboid speaker plonked on top and we were ready to rock.




There is quite some weight pressing down on the driver magnet which i guess can only be a good thing. Although there might be more vibration passed to the upper cab. I tried to bring the tweeter's dome to the centre of the 12 incher's cone for some crude time aligning.

This works way better than the last time i tried up firing bass drivers. I am listening to it now and it is one of the best configurations i have come up with yet. I am still going to try putting all the drivers in the big bass cube and not using the smaller cab. But i will live with this for a while and think about how it can be refined for the final build.............if that ever happens

[karatestu]

The problem bass tracks on Portishead Dummy are completely ok now All i can hear untoward now are things rattling in the room Must be the weight on top of the driver magnets or the fact the room is loaded differently now and driver distances to walls, floor and ceiling have changed.

The bass driver is only partially covered by the platform on top, both bass and mid are up firing. In a way i wish i had not done this as it has me thinking about all sorts of different ways of arranging things that i had never thought of before

[karatestu]

Some more buggering about with speakers......when will it end ?

I moved the platform with little cuboid on right over the top of the up firing clamshell isobaric bass driver, resting on four wooden legs. Time aligned it best i could by having the front facing tweeter dome above the middle of the bass driver's magnet.



I really like this. Having the magnet facing out on the bass driver and facing upwards seems to be a winner. The basket doesn't hinder output and the spread of sound is more room filling than having the cone facing outwards. There is maybe slightly less mid range coming from the bass driver with magnet out as it blocks the centre of the cone and dust cap area.



Tweeter is now 100cm from the floor which is probably about 5cm too high. No strange rattles or booming bass so far but i haven't played Portishead dummy yet (that is my deep bass test CD). Currently listening to Air -Moon Safari. Another of my test CD's. The bass is lovely and full sounding and the rich texture of it on this CD is gorgeous.

Finally found my scribbled notes about the 12" driver's T&S parameters from a site which is no longer on the net. Vas is quoted as 183.6 litres. They recommended a sealed enclosure of 70 litres with 470uf ballast capacitorfor Qtc of 0.71. I expect this ballast capacitor is a filter to roll off the bottom end . Not going there thank you. Qts is quoted at 0.4 which i don't believe, i think it is higher than that especially with the added mass from cone doping. EBP with the quoted figures (which i don't really trust) works out at approx 50 which definitely means the 12" drivers are more suited for closed box loading.

I remember way back running the figures through winISD speaker design software (it's a free download ) and it came up with 84 litres for qtc of 0.7. Having tried that i think it is bollocks. Something doesn't quite add up as the bass was too much meaning a too small box volume. Currently with 84 litres behind the inner driver in isobaric that corresponds to a Vas of 168 litres - not far off the 183.6 litres i got from the net. I am going to add things inside the cab to reduce the volume as i think it could do with it, bass is just a bit too tight and lean now. This all leads me to distrust the TSP i have

I have just had another idea......back soon.

[karatestu]

I have just come up with the design i have drawn below. Hope it makes sense, it's just a rough sketch.



Up firing clamshell isobaric 12 inchers in a floor standing box. The dotted lines represent four legs, one in each corner. Then the top enclosure which holds the 5" mid bass and tweeters. I thought it would be interesting to try one 5" down firing magnet out into the same space between the two cabs as the 12" . Neither of those two drivers would be seen as i would cover the gap on each side with grille material.

Another 5" up firing on top along with a tweeter and additional tweeters firing forward and to both sides. The two 5" would be in a push/push configuration and wired in parallel and out of phase with each other. If i am thinking correctly this push / push arrangement will cancel out a lot of the cabinet vibrations . Wired in parallel to increase efficiency as these 5" drivers are the most inefficient out of the three. I don't think there is room to add side firing 5" drivers but i will have a think about it

Thoughts turn to how would the 12" and 5" drivers perform facing each other in a gap in the enclosure ? Is it some kind of slot loading or what ? I like how the 12" is performing at the moment firing in to a panel above. Only trying it will tell, the devil is in the detail as always