[3dit0r]

Hi all

OK sorry for the newbie post and possibly some questions which will be considered old hat, but I have done some considerable reading, both on this site and on quarter-wave, etc. and I still need some help deciding which way to go, if I may?

First of all, although I'm a long-time audiophile, this will be the first time I'll be actually attempting to build speakers. I'm generally pretty good theory-wise and pretty good at building things and soldering, etc.

OK well first off I should say that I was attracted to these OB designs because of the claims of lack of box coloration, and particularly because of the more directional dipole bass. I've always found that the one thing I can't quite ever solve in my system is that the bass doesn't quite feel like it's integrated in the soundstage like the midband and treble, and I'm hoping this will help. Second, I have always had a problem with room modes in the bass, and read that OB/dipole designs help alleviate this due to the directionality of the bass and less excitement of width/height modes in the room? My small-ish/square-ish room is bad in this regard.

I would really like to stay with a passive design, and stay as efficient as possible - I like the rest of my system and don't really want to add more components into the chain, so I'm trying to find out what's really possible within that goal.

Two broad designs have intrigued me in particular so far; MJK's H-frame passive design, and the CS2 clones in passive mode that BrassEar started a thread on, and has kindly been telling me about in more detail.

I know that BrassEar is using multiple active subs to fill in the bass and would pursue this if neccessary, but would still prefer to stay passive if possible - I'm also unsure as to whether the subs would be dipole/directional? MJK's H-frame appears to be better in terms of passive bass, but does the H-frame introduce coloration and interfere with the directional/dipole bass?

In the midband and treble, it would seem a good idea to avoid crossover in the midband, but I'm unsure as to whether a 3-way design would therefore be warranted. BrassEar seems to be very happy indeed with his midband/treble though.

A general question - why does one look for a bass driver with high qts? I understand from the reading I've done that this boosts bass response in passive designs, but does this not introduce a lack of control and responsiveness in the bass - always before I'd looked for speakers with large, but well controlled woofers.

OK, no more silly questions for now!

TIA
James

[BrassEar]

Just for the record (assuming anyone really cares)...

After months of work and dozens of prototypes I have settled (yes, really settled, DONE) on the following:

Pair B&C DE250 compression drivers
Pair 15-inch Geddes waveguides
Geddes Summa xover for high pass
ONE 15-inch Alpha driver
Simple choke low pass at 900 Hz.
All of this is on a 19 inch wide by 38 inch OPEN baffle

Following the Geddes approach to good sound, I am currently using a Martin Logan Grotto sub and Energy 10inch powered sub fed from my sub out on an integrated amp. I intend to obtain a third sub in the future.

[3dit0r]

I think it's interesting to hear where you ended up after following the thread you started.

If one were to go down the OB + sub route, presumably a stiffer, lower qts (i.e. less than the qts 1 or so normally recommended for passive bass in OB/H frame designs) woofer might be utilized? The thinking being that this might be able to increase bass control - the powered sub then adding the lower registers.

- James

[3dit0r]

BTW, can anyone identify these drivers? They're off my speakers but curiously they and the crossover are glued in as well as being screwed, so I can't see the make! Since I'm really happy with the midrange/treble, one option is to buy these drivers for use on OB, then use a larger bass driver and/or sub, etc. for the low end...



Ignore the felt ring around the tweeter, it's an accessory not part of the original driver. Tweeter is a ribbon. Woofer is 12".

- James

[Rudolf]

A general question - why does one look for a bass driver with high qts? I understand from the reading I've done that this boosts bass response in passive designs,...

That's right. There is a system Qt, which is sort of the sum of the driver Qt and the enclosure Qt. For best transient response (or steepest rise to the linear part of the frequency response curve) you want this system Qt to be 0.7 or even up to 1. For OBs the "enclosure Qt" is almost zero. So you need a driver Qt of at least 0.7 to get the proper system Qt.

but does this not introduce a lack of control and responsiveness in the bass ...

Knowledgable people don´t talk of "Control", but of "underdamped" and "overdamped" drivers or systems. Systems with Qt<0.7 tend to be overdamped, systems with Qt>0.7 tend to be underdamped. You may decide for yourself, where you want your system Qt to be.

Looking at "Control" from another point of view: Deep bass in a BR box is not controlled by the driver, but mostly by a simple plastic pipe called BR-channel. Deep bass in a closed box IS controlled by the driver, who has to fight against an increasingly stiffer pneumatic spring inside the box. Luckily an OB lacks both of these control "features".

[3dit0r]

Thanks Rudolf, that's a nice clarification of that issue. So I'd need to look for a woofer with a Qt of 0.7 to 1 for decent OB bass response.

I'm definitely leaning toward BrassEar's OB design with powered subs. It sounds like a sensible approach. Can one make decent powered subs, or is it neccessary to buy them? This is not an area I've investigated much. I like extended bass, but I can't stand any flabbyness or boomyness, so I'd need to make sure a sub wasn't going to do this.

[Badwater]

Build one.  This is a design that I did 2005. Very nice, still using it.  I painted it to match the walls, high WAF

http://www.diyaudio.com/forums/showthread.php?s=&threadid=50778

in particular Posts 10, 39 and 44

[nodiak]

Tweeter looks like a Focal TC 120 inverted dome (not a ribbon). What make and model speakers? Interesting frame on the woofer. Rythmik sub kits are nice.

[BrassEar]

Can one make decent powered subs, or is it neccessary to buy them? This is not an area I've investigated much. I like extended bass, but I can't stand any flabbyness or boomyness, so I'd need to make sure a sub wasn't going to do this.

Parts Express makes very good kit subs. Geddes prefers sealed subs (as opposed to ported). I have found (as he has) that the more subs you use, the less important the qualities of each sub, to a point. I would not use a sub with a driver smaller than 10-inches.

[3dit0r]

Thanks guys, looking into all this now. Yes, I would greatly prefer sub to be sealed anyway, I've never much liked ported designs, just a personal preference.

What would you recommend in terms of amplification for the subs, and does this require a HT amp, or digital x-over, etc.?

[3dit0r]

Can one make decent powered subs, or is it neccessary to buy them? This is not an area I've investigated much. I like extended bass, but I can't stand any flabbyness or boomyness, so I'd need to make sure a sub wasn't going to do this.

Parts Express makes very good kit subs. Geddes prefers sealed subs (as opposed to ported). I have found (as he has) that the more subs you use, the less important the qualities of each sub, to a point. I would not use a sub with a driver smaller than 10-inches.

How are you running multi-subs? Are you using the sub-out on a HT amp and then splitting the low-level signal wires? If you're doing it this way, how do you control the amount of delay for each sub when they're in different parts of the room? What are my options here?