My DIY omnidirectional journey.

Take the mid-bass, passive bass radiator and x-over from a pair of Kef Cadenzas, a pair of Vifa tweeters from ebay, one sheet of birch ply....add a weekend in the garage and viola.......



A couple of shots showing the internal bracing. This fixes the sides, back and front at irregular distances. The large "diagonal" brace that connects the sides is there to prevent the back waves from the mid-bass unit from hitting the passive radiator full on, they are deflected towards the bottom of the cabinet then back up to thre passive.





OK then, I have spent the past 2 evenings listening to the "things" so have come to a few conclusions.

Firstly imaging. They dont. Not suprising really and not something that bothers me greatly, but a big plus point is that there is no hot spot. It does not matter where I sit in the room, the sound remains the same. This is a big plus point as my room is not nice at all (and im not on about the wallpaper), bass boom by the bucket load or no bass at all depending where I sat. The "things" in comparison are great in this respect, positioning effects the bass slightly, but not to a huge degree. As they dont image in the normal sense, placement is largely a matter of where they fit in with there surroundings. The Mk.2 version will have the bass radiator on the side so I can get them closer to the rear wall.

I am at a disadvantage in as much as I have no idea how the standard Cadenzas sound but tonaly they are quite uneven, (not suprising as I have changed the tweeter, radicaly altered the cabinet and yet I am useing the standard [30 year old] x-over!). Bass is great. Deep and tunefull and not at all flabby, though it is not particularly fast. Treble is obviously a little curtailed but not to the point that it detracts from the music. The main problem is a large "hole" around the lower midrange which I think is the reason that they sound a little laid back, kick drums seem to waft over you rather than hit you. Tonaly they sound on the warm side.

So I remain quietly confident I can turn them into something I can listen to. Wife likes the shape of them amazingly enough but I am under orders to get them veneered. I need to read up on x-overs as I am certain that is where the problems are, better stock up on headache tablets then!

(Cut and pasted from elsewhere, sorry!)

Paul.

 

Good work there Paul.

Regarding the imaging, or lack of it, if these are omni-directional isn't that to be expected? Or am I missing something?

 

I think it is, but im not quite sure why, i mean the speaker on the left would still be quieter to the right ear... so you should get soundstage and imaging, just not as good.

I tried taking my tweeters out and resting them upwards on top of the speakers and it sounded pretty much like most omni's I have heard. Nice and 'atmospheric' but lacking any engagement and imaging.

Still the birtch ply looks nice!

 

My poor writen English i'm afraid! No, I wasnt expecting them to image, but what I was after was a "big sound", and that is what I have got. I stumbled across a tweek tonight that has made a vast differance for the better. I want to do a bit more listening first but will report back at the weekend.

Paul.

 

Atmospheric, yes thats the word I was looking for!

As for engagement and imaging, I can take or leave the latter, to me personally it doesnt make or break a speaker, its just a matter of presentation to me.
But a speaker (or more importantly the system as a whole) must be engaging. If I listen to a record and want to look at teletext or read a paper then the system has failed.
With the caveat that we can all get a little to over enthusiastic about our latest DIY projects prowers (hence why I want another evening to evaluate the latest change) I am very confient that I can turn this into a more than respectable performer.

PS, please dont expect any groundbreaking revelations, 'tis only a simple change!


Paul.

 

Right then, after tonights fiddling and listening session another update.

First up was reversing the phase on the tweeters. My thinking was that as higher frequencies are more directional than lower ones, is the sound being thrown out of phase by being reflected off the ceiling?? Not got the foggiest to be honest but it certainly worked. Sound became more detailed and a little more dynamic. The soundstage seemed to grow and there was definately a fair bit of foot tapping going on although good old PRaT was a little lacking, I'm wondering if this is down to the passive bass radiator, as while the bass is deep and fairly tunefull it is a bit slow and still seems detached from the lower mid range.

It was getting on a bit when I tried "tweek" no.2. I wired in the x-over from my old Coda's. First thing I had to do was go for the volume controll, these were a lot more efficient than the Cadenza items but were a instant improvement overall. Another increase in detail and treble energy and a bit more PRaT...more deep bass aswell so the black hole in the lower mid remains.

Acording to the Kef litrature the lower bass is "mechanically" crossed over whatever that means. It would seem I have to figure this out next, I can only think that it is done with internal stuffing, so this weekend I will be mostly stuffing and we shall see what that brings.

I have a circuit diagram for the Cadenza x-over so what I need to do is try and draw one for the Coda's and try and work out what the differances are, (as if I am capable of doing that!)

 

some passive crossovers need to have the bass, mid or treble wired out of phase from each other due to their electrical characteristics. for example my ipls have the bass wired out of phase to the mid and tweeters - it's just the way some passive x/o's work. i would imagine from the description that the lower base needs to be connected out of phase from the other drivers.
sorry if i'm teaching gran to suck eggs.
cheers


julian

 

Not at all! I didnt know that...so my findings werent mad after all!

I will try altering the phase of the Coda x-over tonight. Im having a bit of trouble tracing the circuit of the Coda x-over as it has some sticky foam on the back that is pulling the tracks up when I try and remove it. That and the fact that the inductors are not marked

 

Ive been a bit busy over the new year so progress has been a tad slow. The main box is finished, just the grills and the crossover to sort out now.

Main carcase is Birch ply with hardwood quadrant on the corners, finish is 3 coats of Danish Oil. I was very dissapointed with the finish of the birch ply when I started sanding it down, but then it is only cheap stuff from B&Q. The plinth is 30mm MDF with 8mm spikes. At the moment the driver wires just exit form the bottom of the cabinet and the crossovers lay on the floor. When they are finished there is plenty of room in the bottom of the speaker for them and the B139 passive radiator makes access easy.

Sound wise they are much better than the prototype cabs, probably becouse of the care that has gone into bracing and sealing the new cabinets. These are very heavy and very dead indeed but at the momment are relativly lightly lined internally, basically just 2 adjacent sides with 1" dracron and a load of wool in the bottom.

The drivers point towards the ceiling more than the first version and the cabinet is shorter, and this has made them sound far more consistant than the first pair. There is not any differance between sitting and standing now and the soundstage doeas not collapse into the closest speaker like before. Bass is even in all sitting possitions of the room so I am happy that one of my design goals, (no hot spot and an even sound throughout the room) has been reached.

I have finally figured out that the hole in the midrange is a baffle step problem, although the diamond shaped baffle has made things better. I will rebuild the crossover with standard components first and I have bought some SCR capacitors for this purpose then I may have a better idea of where to go from there, (or not!)