New OBs
Just finished today. Wiring done, after the photos were taken. The upper bass drivers are new so they are connected and getting a 30Hz signal to loosen them up a bit. I'll leave it cooking overnight and have a good listen in the morning.
I'll post some thoughts and describe the design brief in a day or two.
I'll post some thoughts and describe the design brief in a day or two.
Comments
Two primary design goals were:
The structure to be as light and rigid as I could make it. In the absence of unobtanium I went for flooring grade chipboard. I've tried ply in the past and I don't like it's signature, it rings too much for my liking. MDF is just thick cardboard and sounds like it - mushy and soft. Chipboard has the most benign sound to my ears. The flooring grade is made stronger than the usual stuff for obvious reasons. The bracing at the back adds little significant weight but increases the rigidity a lot. Without the drivers each baffle weighs in at less than 10lbs. Nothing has no resonance, nothing at all. Everything has a fundamental resonance, that is to say every material has a point at which it will resonate most. Even air has this. The stiffer the material the higher the resonance - hopefully out of the crucial bass range. Further, the heavier something is the more energy it will store - again no way round that one, there's a physics law that explains it thoroughly but I can't remember which.
Two 15" bass units. Not about creating thunderous bass. It is about decreasing bass distortion. The old adage "no replacement for displacement" is spot on. But rather than use a 6 or 8 inch driver with a a long throw (xmax) I much prefer a stiff short throw driver. The more a driver has to travel the more it distorts, no way round that. Also the further it goes the longer it takes to get back to rest - except it doesn't, it keeps going beyond the centre point and puts back EMF into the amplifier. And this all takes time. A long throw driver will sound slower than a short throw one. A 15" driver has nearly four times the radiating area of an 8". Therefore it can displace the same amount of air by traveling a quarter of the distance. It does this quicker and with less distortion.
These ideas are not new. They're tried and tested, but in one OB design? Not sure about that.
How are you working out how to implement your crossovers? (or are they active?)
Also, are all OBs "Picky" about placement, as Dave is finding?
I don't think they're picky but then I'm used to them. I've been using OBs for over 5 years now, experience is helpful in setting them up. I'm sure it won't take Dave long.
The crossover is a story all on it's own. Short answer, they are active. The Lowthers are driven by Hypex 180 monoblocks. The bass drivers are driven by another pair of the same amps - i.e. one monoblock per two drivers/channel. The longer term plan is to have one mono amp per driver.
A Behringer active crossover sits before the bass amps (the Lowthers are fed direct, no crossover at all). The crossover point is still being settled on. But it's around 130Hz or so. The slope is 48dB per octave.
When I've settled on it I'll explain the process fully.
Speakers look great.
Exactly what I've done on the back and side walls. I may try adding some more.
http://i667.photobucket.com/albums/vv33/mjkempton/2010092238_7792267071.jpg
If you drove Jim I would expect you to drive an American muscle car. Like a 1970 Dodge challenger with a 426 Hemi...
I will remember the way your system does LF for a long time.
BTW, have my amps woken up yet or are they still being whupped by the Hypex's?
Nope, amps still on the floor 'fraid. Tomorrow is supposed to rain again so I'll get the rewiring done mate.
I am thinking of your approach a lot at the moment. My current idle daydream involves Tri-amping actively (like you), bypassing the X-over and limiting the LF of the Royds and adding a single 15" OB. Either on the floor or with a case made so the Royds can sit on it.
Maybe.
Your plan sounds doable mate. You could easily drive the Royds with your existing amps and use the 80s for the LF. No need to tri-amp.
I only tri-amped as I had 4 Hypex amps already. I could drive 2 bass units with one Hypex.
Being realistic, just for a minute, this won't happen in the short term! Bi-amping next, a welcome return to familiar territory. I am hoping for a little more control at volume (read 'cleaner'), and for more life at lower volume in the evening.which is what I want to recapture.
At the moment I have the quality, but need a little more muscle.
There are two key parameters for a bass driver in my view; Qt and Xmax. These two are closely related. They also pretty much define how much bass a driver will produce and what quality it will be - this is a bit of a
simplification TBH but good enough without going on for pages and pages.
It has long been accepted practice to try and achieve a Qts (Qt is the driver's Total Q, Qts is the finished
system Q) of 0.707. This is supposed to give the best balance between deep and accurate bass free from
overhang or humps. Small monitors often have a higher Q to give an impression of bass. The LS3/5A does
this, the Linn Kan doesn't - if you're familiar with these speakers you'll know exactly what I'm taking
about. It also explains, largely, why some speakers need to be placed near a wall and some need to be well away from boundaries.
The minute you put a driver in a box whether sealed, ported, transmission line the Qt of the driver will go
up - no way round that. Quarter waves and horns hardly raise the Q at all if any - it's years since I've done
a set of measurements, most of this is from memory. Forgive me any small errors please!
An OB is unlikely to change the Q at all - though I've seen no data regarding this.
When I first tried OBs about 5 years ago (I did have a little go some 20 years ago but it was inconclusive) I
read everything I could find. The accepted wisdom was to use a high Q driver to compensate for the lack of
box and the driver's roll-off at the bass end. The driver of choice seemed to be the Eminence Alpha 15",
this has a Qt of 1.26. This struck me as wrong so I went to the 15 Beta with a Qt of 0.58.
Now, whether it's my room or the position in the room - which although I have the room to myself has limits
on where they can go - the bass is too loose for my liking. I achieved what I wanted in that I reduced bass
distortion and I got closer to the output level of the Lowthers.
But why should I stop?!?!
If I used 'normal' mid and HF drivers I'd probably be OK with the 15 Betas. But I don't. The Lowther spec
and their entire philosophy is about speed and efficiency. The EX4 has a Qt of 0.233 and an Xmax of /-1mm.
These specs translate into midrange speed but no bass! So what I needed were drivers built to a similar
design goal. Precision Devices do just this. Unfortunately they aren't cheap, but what the hell!
I've had a pair of PD107 drivers in storage for years and I've just taken delivery of the PD1550 bass
drivers. Everything about these drivers shouts overkill. Low Qt of 0.21 and an Xmax of 8mm. High efficiency
at 99dB and high power handling suggests very low dynamic compression. I won't repeat the specs here,
they're on the website: http://www.precision-devices.com/ Oh, and it's BL product is massive :-)
My plan is to mount them all on a baffle the same size as the one I have now, i.e. 18" wide and 46" tall.
Bass at the bottom, 10" above it and EX4 at the top. I will then use the active crossover to blend them all
together but that comes later.
I've got to build the damn things yet!
That's an LP sleeve for scale!
Edit: sorry about the formatting - CRs all over the place.
Loved reading that jim. It's a cracking project, doubtless with impressive results to come. Thanks for the update.
But why are you working out of a Moroccan boutique...?
Cost also. The drivers would cost close to £2,300.00 - that equates to at least £10k at retail. Minus amps of course.
Words and pictures a bit later
I'm playing with the crossover points and will do so for a while I expect. ATM the bass driver is going up to 80Hz, the mid is 80Hz to 320Hz and the Lowther is 320Hz and up.
Certainly removing any bass from the Lowther has allowed them to play louder with less break up. They sound far, far cleaner. An example is a jazz group really going at it; drums, bass, horns and the drummer is hitting the cymbal. The cymbal is as clean as the proverbial now - it exists in its own space and no matter what else is going on it can't be disturbed and shifted from that. Drums sound as they should. It's all to easy to forget that apart from tympani drums have no real bass but so often they sound like a thud. Far more important IMV to hear the skin and the smack of the transient.
The 15" driver is brand new so will need breaking in over the next few days. I'll leave a 30Hz tone running through - when I can stop listening to it.
So two thumbs up thus far.
I love that you're focussing on drum kit in your written description. That bodes well. The resolution of individual components in a kit is one of my personal bench marks when I'm choosing speakers. Sounds like you're achieving something rather special!
TBH drums were the first thing that was obvious. Since then I've heard a lot more stuff.
I'll wait a few days to report back though.
As you know, I'm a percussion nut.