OB Design for use with small SETs

[Max N]

I thought I would split this off from the Quasar thread and the 'Economical OB thread'.
My personal interest in building a pair of OBs stems from hearing what James' Quasars can do when driven by SET amps, including Jonathan Noble's little silver wonders as well as NickG's 211 uber-amps.
Just to be clear, one of my motivations for considering something similar, rather than re-building my IPLs, is so I can play with small SET amps.
The other threads contain some interesting stuff about OBs, but the Orion's are active and the DarkStar project seems to major on active crossovers and SS amps. Both are valid approaches and may well be excellent. I am not trying to be prescriptive about what people post, but I do think it will be easier for other people in the future to pick through the various threads and find stuff that is relevent to their plans if we divide the threads.

So this thread is intended for discussion of high(er) efficiency OB speakers with benign impedance curves suitable for use with smallish SE amps.

James, as this topic evolves, I am starting to get the impression that a wide-bandwidth OB with high efficiency is not an easy thing to achieve......
I am starting to appreciate that your Quasars are perhaps quite unusual in what they can do?
I have no experience with OBs or full-range drivers. It would be great for me (and others I hope) if you could pen a few words about how you arrived at the design of your Quasars.
Or, to come at it from another angle, what are the design criteria/compromises/difficulties when trying to design a wide-bandwidth high-efficiency OB?

Cheers

Max

[Paul Barker]

The 108sigma Senon OB which I am using is 75% of the way there.

The story to date seems to be that this Fostex driver is very nice and subtle at low level listening, if I build a low powered set for it there are elements of the depth and delicacy of micro detail you would hear on the Quasars. The problem is that this doesn't continue as the drivers are pushed they take on a different character, harden up. They are probably the inverse of Ariels. Ariels enjoy hard driving, I think at the first eggfest most people quite enjoyed the ariels on the se 212 amp, as Dave Dove said "the cast of Desiderata came into the room". But Ariels midrange hardness and reluctance to do any bass becomes worse the less powerful the amp. They have to be powered through a certain reluctance.

It has cost me far too much to make the 212 se sound and I would prefer these days, since priorities change, to revert back like you into the low powered set approach.

I don't think that this version of OB's is sufficient for that quality of low powered amp. However YMMV so you should wait for EggIV and hear them for yourself. I suspect the drivers are better suited to the Voigt Horn.

So often with these things in the end you buy the right thing which costs you 10 times as much. If you have the money to copy James' Quasars they are the only thing which I have heard that are able to carry the set qualities at low and high level. The STC Oxide coated 4211 demonstartion on them showed that the majic they revealed in Jonothan's unique sub 2 watt 45 amp is not lost at 15 watts when a similar quality valve is used...

I think that at the end of the year when my books are done if there is a load of money left it will be sunk into Quasars.

Not withstanding that Edgar Tightans are my best. If I had the money for a listening room that would accomodate them I could also afford them. One day Dr Edgar won't be with us and we'll be left strugling in the dark to make our own compression horn systems. It is the ultimate, but as Peter pointed out to me you have to also do the bass with a horn, as indeed we heard from Bruce at VSAC, I have photographed a wedding at Westminster Cathedral, the sound of that Organ in that huge space is one listening experience that shall never leave me, the closest thing I have heard to it live or reproduced is Dr Edgar's system in a poxy hotel room with mattresses against the wall, it is vertually there with the Westminster Cathedral organ. Really really lifelike, fast pure perfect deep and delicate.

[James D]

Max,

Ok your on a potted account of how I arrived at the Quasars and what went into designing them coming up!

But it will take a couple of days as I'm really busy to day and tomorrow. But it will happen.

BTW - Paul and you are basically right - it isn't easy to do a wide bandwidth high efficency OB at all. As Peter has repeatedly said to get a driver that does low bass and high efficency isn't easy and results in driver that doesn't want to work on an open baffle... so you end up chasing either expensive drivers like the Supravox, Some PA drivers like the Eminence that just happen to fit the parameters or some cheap drivers like the Senon with too high a Qes from too little magnetic force in the 'gap'... but more on this later...

James

[Clive]

Don't forget the Bastani Prometheus design. This uses an Eminence for the wideband but Robert adds felt pads and lacquer to change the break-up characteristics so the x-over point is 11k. Bass from 100Hz down is by active subwoofer. 100db efficency.

Maybe using the same driver but untreated might give decent results?

[Max N]

James - looking forward to it, thanks.
Paul - I am hoping to be at Egg4, it shouldn't be a problem but you never know what might come up!
Max

[James D]

Pah! Active sub! Pah!

Thats cheating

James

[Clive]

Originally Posted by James D

Pah! Active sub! Pah!

Thats cheating

James

I agree it is. But it's a compromise I can live with, sure it's not purist and if I had the time to be even more obsessive than I am already I'd go down a valve bass route. Maybe with 300B's it's better to run with active bass from the preamp.....

Cheers,

Clive

[Paul Barker]

Though I am hopeless at loudspeakers I will pipe up anyway. The bass method has to be of the same character as the mid trebble method in terms of speed of attack and phase acuracy.

I wander if that is why I have yet to hear a sub which fits in with the rest of the sounds?

Bruce Edgar has made a horn sub which delivers bass at the same rate as his compression horns. You cannot call it a sub because it is integrated seemlessly into the whole sound.

Anything that I can call a sub, I find sounds better turned off.

[Clive]

Hi Paul,

I'm generally very much in agreement with what you say about integration re subs.

When I had 2 REL subs there was no question that they integrated better at speaker level. With the Bastanis I find the bass is just a touch bloated when the subs are connected at speaker level. Connecting to my preamp instead I find that bass character is much more in line with the mid. Being sealed boxes seems to work better than reflex too. The SS amps do have 180 degree phase variation. I think a very important aspect is to have a relatively gentle slope for the integration between subs and mains. It's subs with steep slopes where I hear integration problems.

Anyway that's my theory.

[Paul Barker]

Yes sloap is probably the key. The compression horn guys like to use 6db slopes.