Doc modding Marantz imperial 7

[antonio66]

It's time you had these finished Stu.

[karatestu]

Trying to get the thread to 100 pages

[karatestu]

More speaker foolery. Also known as flailing in the dark

One thing i have learned recently (well i knew it already) is how crap my big bass cube cabinets are. Recycled cheap shit ikea chipboard, no bracing, no steel lining. It appears they are a musical instrument all of their own. I didn't realise how much sound they must be emitting until my latest modification.

There are a couple of tracks on the Portishead Dummy album which really cause problems. The bass is synthesised and low. The cabs are humming away quite annoyingly. Must do something about it.

Latest experiment is a clamshell isobaric configuration with the rear driver mounted on the front baffle from the inside and the front driver mounted magnet facing out on the front of the baffle. Only 20mm of the crapest chipboard divides them. At first i had an air leak and they were farting away to themselves. Also had the front driver oscillating quite badly, was a bit scary. Cured that and things are much better but i really must brace the cabinets.

I now have 80 litres behind the rear driver which is twice as much as before. Tried some stuffing in there as well but that didn't last long Only the smallest air volume between the driver cones. Sounds good but obviously over damped (too much cab volume). The bass rolls off much earler than before when i had the box equally divided between each driver. So the optimum for me is somehere between 40 litres and 80 litres. Not a trivial amount. How doc got them to work in such a small enclosure i will never know, god knows they must have been severely under damped.

I will experiment by adding one litre bottles of water inside the enclosure until i have perfected it. Not knowing the true T&S parameters for these drivers is a real pain in the arse as i could have worked out the ideal box size mathematically. Never mind, will get there in the end. It has become evident that giving each driver the same volume behind them is unworkable as the enclosure is just going to be too big. With the clamshell arrangement i can keep box volume to it's minimum. Don't want to introduce stuffing to lower volume as the price paid is too great. Maybe i will try aperiodic for the rear chamber at some point as that also reduces volume needed.

Must buy plenty of bottles of water

[karatestu]

What a plonker i am After the latest modification i knew i had to wire the rear driver out of phase with the front one (because they are facing cone to cone) but i forgot about the phase as related to the mid and tweeter.

Something was not quite right and i only realised last night that the isobaric pair were out of phase with the mid and tweeter Corrected that today and it sounds absolutely amazing. Tightest and cleanest bass i have had from these isobaric pairs so far . Still a bit too over damped though so will start adding sealed plastic containers of known volume ie 5L, 2L, 1L. When the bass starts to get a bit too much and humpy at resonant frequency i will back off. Enclosure volume - volume of liquid added = volume for final build.

[karatestu]

A quick note about adding tweeter's to semi omni speakers. I have four tweeters at present all firing in different directions and wired in series. It dropped distortion quite considerably from using just the visaton. With my big ass isobaric bass cubes i don't need a padding resistor for the tweeters.

But the other day i was playing music through just the small semi omni cuboid and the high frequencies were definitely too hot. But the enclosure i have for the 5" mid bass is purposefully over sized (over damped) so it does not produce too much bass. The thinking behind that was it would improve the higher frequencies that it produces through lower excursion and therefore lower distortion.

In builds where the enclosure volume is sized for more bass output the padding resistor may (a lower value) or may not still be required. Does this make sense ?

[karatestu]

Time aligning a 12" woofer with magnet facing outwards and a tweeter is quite a challenge. But then with drivers firing in all directions there never was much time alignment to begin with

[karatestu]

Another crazy observation. Since correcting the phase of the big cube's relative to the other drivers i am getting lower cabinet vibration I didn't do anything else. The two tracks on Portishead Dummy which made the cab's audibly rattle don't do it anymore . They still move too much and need bracing .

Track no.8 on Dummy (pedestal) has some very odd deep bass on it. Anyone got it who can play it through some big ass woofers which go low and let me know what they hear ?

[karatestu]

It strikes me that i can play with the box volume (damping) to tune the bass to my room to some extent. It is only an averaged sized room of about 4M square. Doesn't take much bass to pressurise (fill) it.

[karatestu]

Apparently the best height for a woofer is at 25% of the ceiling height from the floor. That is to get the least excitation of floor to ceiling modes. That must be about where most floor standing and stand mount speakers put the woofer. But what about baffle step ? The best place to put a woofer as regards limiting baffle step is as close to the floor as possible.

So you can't have your cake and eat it. Maybe two woofers at differing heights would help ?

[karatestu]

With nva cube speakers you can get the help needed with baffle step by putting the cab right back to the wall - same effect as front firing at floor level.