Doc modding Marantz imperial 7

[valvesRus]

Hi Stu, I've been keeping up with your speaker madness, but don't contribute 'cos I've got nothing constructive to add.

There must be more like me who appreciate your efforts.



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[karatestu]

Thanks for popping in. Do i revel in madness ? Maybe. Honestly, I don't feel that I should have or deserve an audience as such but it is nice to know some are interested.

I have read many speaker threads at various places. A trend appeared to me that is when ever somebody proposes something mad, radical or just a bit unusual they are usually slapped down and told it won't work for what ever reason. The OP then changes their design to please others and it ends up being a boring old point & squirt two way stand mount with 6.5" mid bass, 1" tweeter and a bloody port.

That won't happen here basically because objectivists don't frequent a forum with subjectivist in the title unless they look in for a good laugh. Plus I am not sure anyone actually understands what I babble on about, even me sometimes.

I glean a lot of information from sites like diyaudio where there are some very knowledgeable people. But I only take the bits I want or am interested in , ignoring the rest. They know a lot about speaker design but they have no idea about the way I want to design one or how I want it to sound.

Maybe one day when they are finished I might pluck up the courage to let others hear them. Although I don't relish the thought of being told they are crap

[valvesRus]

Maybe one day when they are finished I might pluck up the courage to let others hear them. Although I don't relish the thought of being told they are crap

If you really like something then anyone else's negative opinions should not matter.

[Latteman]

Of course that is the right way to think about these things but I am very good at forgetting.

I have been through so many ideas in this thread that no wonder it has taken three years to get this far. I know what I don't want to try (bass reflex, TL, horns, dual concentric drivers) but have wasted so much time trying to get two isobaric 12 inchers to play nice in a box that is on the small side. This thread started off with a simple Doc mod of a knackered old pair of Marantz speakers

Well at least I have a concrete plan now and I have given up the trouser flapping qualities of the big enclosure which was hard to do. But I was kidding myself that I could ever shoe horn those 12 inchers in to a domestically acceptable enclosure. They just need more volume than I was willing to give them for use in my living room at home which is smaller than my office where I have them now.

But I haven't given up on them. I will always need a system at the farm so that is where they will reside. As I don't have to care about how big they are or what they look like then the shackles are off. I am going to make a cube out of two foot square paving slabs which are two inch thick. Not going to bother doing anything to them like making the walls thinner, just cut a hole in the top for the drivers and glue them together.

Worked out I can get 180 Litres of internal volume that way. Just hope the floorboards can take it That might end up being too much Vb but a good place to start from. Can always put things inside to get it right, I will then make a little cab to go above and then job done. Doesn't matter what it look like, within reason. There is only going to be me looking at it.

[Latteman]

Hi Stu
Question for u
Most of my tweeters are sealed units - as such is it safe & correct to say that they do not interact with the cabinet volume- only the baffle. If so why are most open to the cabinet- could the baffle in the case of a semi omni be closed and thick enough to hold your tweeters of choice?
Cheers peter

[karatestu]

Hi Peter.

You are quite correct, almost all tweeters are closed back so they don't interact with the mid bass. Cone tweeters are a different matter though on the whole.

Dome tweeters are deeper than most panels so you have to cut a hole. But say your baffle was 5 cm thick you wouldn't have to cut all the way through (most domes are about 3 cm deep) just deep enough to get the tweeter face plate flush with the baffle.

You would have to drill holes for the wires though.

[karatestu]

Been reading about Larsen model 4 speakers. Very odd looking things with drivers at 45 degrees. Designed for use right up against the wall which is what interested me the most.

[karatestu]

On diyaudio somebody asked if big woofers sounded slower and if adding mass to a woofer made it sound slower. Here is the answer.......


"Big woofers tend to sound slow because being “big” they have large Voice coils, which have a lot of inductance. It is the Rdc and Le, which form the “low pass” filter in the mechanical system.
Adding mass to a woofer, assuming it is anchored to the VC securely, does not affect the “speed of response” at all.
The reason is that woofers are “acoustically small” and so operate on the slanted portion of the radiation impedance curve (for the radiator size vs. wl).
This means to have flat response, a “small” radiator must have a velocity that falls at the inverse of the slope of the radiation resistance (6 dB /Oct).
In a woofer, this is accomplished by the action of the driver’s series Rdc, coupled through the BL conversion in the motor, to the moving mass.
The moving mass looks like (electrically) a capacitor and that forms the R/C filter that has the roll off that makes the driver measure flat.
Adding more mass, does make the capacitance larger, lowering the frequency where the transition from Velocity to Acceleration response happens, but that is at the bottom end of the response. The added mass has no impact on the driver’s “speed” or high frequency cutoff."

[karatestu]

What has become apparent is that you need a driver with a strong motor and substantial surround (high BL =big magnet) to get a successful outcome when adding mass that comes from doping. Doc chose his drivers well as they are PA speaker drivers mainly with big magnets and a cone which is already quite heavy (kevlar).

Try doping a paper cone with flimsy surround and small motor and the results are likely to be less favourable. I have experience of this as I tried doping the original Marantz imperial 7 twelve inchers. Didn't work out well. They have a relatively small motor and the surround is a very flimsy doped cloth.

[karatestu]

Well, I managed to track down four more of the 5.25" kevlar drivers at £11 each. Not an easy task I'll tell you. Most places are completely out of stock, others have a single one left which doesn't work out well with postage. I called a few stockists who showed the item as in stock on the internet only to find that they were actually out of stock. A couple said there is no date from the manufacturer for when they will become available again. A common problem it would seem in these Covid times.

So I will have four of theses 5 inchers per channel. Should raise the efficiency of the speakers quite nicely depending how I wire them up. Probably going to wire them in parallel series to give a nominal impedance of 8 Ohms. See how that matches up with the tweeters. I might have to revise the tweeter wiring arrangement. The four tweeters are currently in series but I could do parallel series if needed .

Looking at Baltic Birch plywood at the moment. It's not cheap but then for it's properties it should be worth it. I am edging towards 21 mm sheets rather than 18 mm, and 25 mm seems a bit overkill to me. I could always double up on the panels holding the mid bass drivers but we will see. Router selection is under way as well. Half inch or quarter inch ? I am leaning towards half inch. Then there are bits to select. Will have to make a jig for cutting the driver cut outs and rebates. Never had a router before so this is all new to me.

I plan on having all the drivers with cone outward facing. No more of the magnet out mid bass madness. As the 5 inchers are on opposing panels I should get some force cancellation meaning the cabinet vibration will be lower . Also baffle step compensation for free

I intend to mechanically link the magnets of the two opposing drivers in each 30 cm cube. Will probably do that with a round wooden dowel like an old garden tool handle. This will block the vent on the back of the magnet but I shouldn't think that will be a big deal as I will hardly be thrashing them and there are four of them to share the load. It will take some fannying about getting this piece of wood just the right length as to rigidly contact each magnet without causing any stress on the basket etc.

Got enough crap chipboard to make a pair of prototypes and sharpen my wood working skills before doing the final build.