Doc modding Marantz imperial 7

[karatestu]

It seems as i add more foam the bass becomes more well behaved and more subtle low frequency information is evident.

I haven't a ffing clue what is going on really and i can't give a technical reason why what is happening is happening I only know that with my gear, ears and room i like it more.

I will have to live with the room modes, bass peaks and nulls i have as the room can't change. Doc always said up firing drivers work with the room rather than fighting it like point & squirt. It would be silly of me to say that i have a perfect room because i don't but i have witnessed the difference between front firing bass and up firing. The bass is much more well behaved with up firing and more consistent across the room. It may also have something to do with which direction the foam back of the speaker is facing.

I don't know what is going on with allowing the back wave to exit the rear of the cabinet. It is attenuated and delayed (i think) by the foam so may be there is some cardioid things going on or maybe it's just aperiodic. Without measuring i will never know....... so i will never know All i can do is keep tweaking things until my ears tell me i have the best sound for me.

[karatestu]

Next up on my list of things to try / mess about with is to move the open part of the enclosure to the bottom of the rear panel but still keep the bass up firing. I will take the little feet out from under the bass cab and have it sat directly on the concrete slab. A bit of sealant and the box will only be open at the back.

I look forward to hearing what that does, for better or worse. It will involve butchering the chipboard panel but no great loss there I will try and make a way of experimenting with the size of the opening. All the foam will remain inside and i might move it right up to the magnet. There will be less distance for the sound waves to travel between the front of the outer bass driver and the back of the inner bass driver. This may have an effect on the bass response (like open baffle) but the foam might muddy things there.

We shall see. Hope nothing weird happens but i can always back track if needed.

[Alfi]

Time to build some aesthetically pleasing speakers now Stu, having learned what you have learned and enjoy the benefits of your work whilst relaxing to music, if you can relax of course?

Alfi.

[karatestu]

Thanks john.

I won't be relaxing Although watching polystyrene beads dancing up and down is quite hypnotic When the speakers are done i have amplifer, turntable and equipment housing projects to finish, although they do progress slowly alongside the speakers which seem to take precedent (not sure why ).

I am going to build a bass cab out of 2" thick concrete slabs when i have finished messing with the bass cab (won't be long now ) and that will be the final build for the bass cab. I will then set about plastering it and eventually painting and lacquering - that's the plan.

Then i can finish playing with the top cab - need to try one or two more 5" drivers in that and buy the visaton tweeters. Then i can build the final top enclosure out of baltic birch ply. That will leave putting it together with some nice legs for the top enclosure to sit on, some speaker grille cloth to hide all the up firing bass and down firing mid bass drivers in the gap between the cabs and some grilles for the tweeters and top firing mid bass. Oh and not forgetting the wiring, binding posts and FFS badge for the front in honour of the Doc

Not much to do eh ? It would be nice to have the reward of all the work i have done and have something that sounds and looks killer. It would be a shame to build something that sounds pants at the end. Not going to let that happen

[karatestu]

Has anyone here ever made a diy coaxial driver ? And does it work ? It's just a thought about getting a tweeter on the same baffle as a 12 incher where there is no room.

I could use one of my doped 12 inchers and made a frame to mount a Visaton CP13 or my little B&W neodymium tweeter right above the 12 inchers dust cap. Seems a bit crazy.

There would be a hole in the frequency response but that doesn't matter as i am not using it as the sole driver and it would be up firing. Vibration might be a problem and diffraction . It doesn't matter about blocking the dust cap area as we don't want the higher frequencies produced from there. Could fill the 12 inchers cone with polystyrene beads and put some fine grille cloth over the top.

With a 12 " driver mounted magnet out then the only way would be to plonk the tweeter on top of the magnet firing up. Only works if there is no other cab above the bass driver. Something to think about anyway.

[karatestu]

Still watching the polystyrene beads jumping up and down on top of my up firing 5 " mid bass drivers

The only draw back is if you put your head an inch or two from it then you can hear a very quiet rustling noise when there are bass guitar notes (meaty ones). But then who listens with their ears right up to a driver ? At four inches away you can no longer hear it

It is hard to do comparisons of with and without as the beads take a bit of removal ie they are small, weigh bugger all and are hard to get hold of. I am going to carry on with using it and try and do a subjective comparison when i can.

I currently have the gap where the up firing 12" and down firing 5" meet enclosed on all sides (between the four legs) with open cell foam. I am convinced it attenuates some upper mid range and treble, but need to do far more listening and with and without comparisons.

[karatestu]

Decided i am going to add more 5" drivers to the top baffle of the upper enclosure. How about four in a square pattern as close together as possible with a tweeter (visaton CP13 which is 40 mm across) sat in the centre. The enclosure volume is just about big enough for five of them.

I gather the mutual coupling of the drivers will get me an increase in dB below a certain frequency depending on how far the centre of the drivers are apart. This could upset the balance of the speakers but i have ways of balancing things up again

[karatestu]

This is a new one for me. The Bessel array. Can be done with as few as five identical drivers. Two of them are out of phase with the others. It is said to act as a virtual driver without the comb filtering found with some array's.





Probably the biggest feature of the Bessel array is it doesn't have to be laid out vertically like an ordinary array.It can be horizontal like a sound bar.

[karatestu]

Just worked out a way of hooking my speakers up to only one pair of monoblocs. It's worth having a go and seeing what they sound like that way.

Only alteration i have to make is wire the isobaric pair of 12 inchers in series rather than in parallel. That will give me approximately a 5 ohm load overall. This will affect the bass output i should think so will have to see if i can live with it (it might be better but i doubt it).

[karatestu]

Moved the tweeter high pass filter capacitors in to the tweeter power amps. I have two 3.3uf K75-10 PIO caps in series to give 1.65 uf. This is too high for the amount of tweeters i have wired in series.



Yes they are massive especially with two per channel. I have ordered some 1 uf K75-10 to use instead. That is close to the capacitance required for the impedance of the tweeters wired in series. It will have to do as i can't be arsed wiring two in series to get a non standard value. Next value down i could find was 0.68 uf which is too low. Will just have to live with it

On that note, i have added another tweeter rear firing So five wired in series now - up, forward, in, out & rear firing The only place i can now put a tweeter without doubling up on each baffle is down firing on the upper cab. Not sure about that. I didn't know what to expect with a rear firing tweeter (if anything) but i like it