2-way OB crossing to woofers question:

Viridian · « **on:** 2 Apr 2010, 07:20 pm »

Hi All,
It's been some time since I visited the forum here, I've been too busy enjoying listening to my OB's.

But I've also been thinking that it is time to relieve my 12" coaxials of their bass load. I've been tinkering around with this idea but lack some technical know-how.

I'm presently running tube amplification and would like to keep everything powered by one amp. So, passive XO. What I would like to do is mount 2 15" woofers per side under each 12"driver per baffle.

Here are my thoughts, any input is greatly appreciated:
With my widerangers having 8Z impedance can I run two 15"s in parallel under them? Each of the woofers are also 8Z, which in parallel would be 4Z impedance, correct? But I have noticed in looking at impedance/frequency curves of speakers that often the impedance begins to climb at and below about 100Hz. So, would it be a reasonable guess to say that if I cross to the woofers @ maybe 80Hz that their summed nominal impedance could have risen to 8Z at the point of crossover and therefore be a match for the widerangers? It would certainly help in matching the level of my 12"s, which are very efficient. Also, i have tried active XO and biamping with dissimilar amps but had trouble due to what I am guessing was a messed up power response.

Again, any help would be appreciated in helping me to understand if this idea is workable.

Erik
I'm Viridian

nullspace · « **Reply #1 on:** 3 Apr 2010, 02:51 am »

Going 'Single Amp' and trying to maintain the efficiency of the 12" coax is going to be a tough nut. I've tried that myself, with 98db/1W 12" prosound midbass + compression driver/horn, and results were okay but in the end there's just not enough bass <50hz. Getting a pair of 15" drivers to be very efficient at 40hz is not easy.

Which coax are you using?

As you say, at 80-100hz a pair of 15"s in || should be around 8ohms, so yes that's workable.

It's going to be pretty tough to roll off the coax that low -- it'll take a big, BIG cap. And, even with some measuring gear, it'll totally be a 'suck it and see' type of situation because of the impedence rollercoaster.

Any thought to trying a 2.5way, and rolling on the helper woofers with just an inductor? Depending on Fs of the coax you might have to mind the resulting impedence.

John

Viridian · « **Reply #2 on:** 3 Apr 2010, 05:02 am »

Hi John,
Thanks for replying. I've read your site and am really impressed with how you are so methodical. Are you still using TT12's for your midranges? Still happy with the AH horn? What did you end up doing for your bass needs? AES drivers?

Yes, I think that the challenge of meeting the efficiency of my 12" coax while staying single amp is going to be nearly impossible to surmount. I'm using a Pioneer PAX, which I love. I'm a firm believer that each driver has its own character. I've tried many, sticking mostly to 12" and Alnico magnets. The Pioneer have been in a different universe compared to almost everything else I've tried. Treble to die for. They are rated 103dB/W with an Fs somewhere around 35Hz. Impedance is 8Z.

I like your thought of the 2.5 way. If that means that I run the 12" full range and just bring the 15"s in where it seems to work. Kind of like several guys have done with the B200/EminenceBetaAlpha15?

One more question: If I try to spec a choke for nominal 4ohm load and 80-100Hz crossover point but we agree that at that frequency the actual impedance is likely to be something like double the nominal impedance, doesn't that make it a bit difficult to figure out what value choke is ACTUALLY needed?

Cheers,
Erik

nullspace · « **Reply #3 on:** 3 Apr 2010, 03:24 pm »

I'm still using the TT12 and Beyma drivers. For >125hz, it's really nice. And, I'm a big fan of the AH horns. If I ever move to a bigger house, the AH!300 will move to the top of my shopping list. I'm still using the 4x Eminence Kappa-12A set-up, 3 months now of mono only. On the bigger baffle the ToneTubby rolls off much lower and the low-Q woofers, though lacking <50hz, have a very nice detailed, heftiness to them.

The Pioneers sound like nice drivers. Any independent verification on the 103dB/W? That's a pretty tough number for me to accept at face-value. If that's right, then you'd need a wall of 15" drivers to mate with the coax.

Exactly right. Picking the right choke will likely end up being a trial-and-error thing. I think looking at the numbers will get you in the ballpark, but you'll most likely need to buy a handful of inductors over a range of values and tune by ear. I can imagine that the final value for inductor could be anywhere from .5 to 2x the 'book' value.

I hope that's of some help. For this kind of project there's little substitute for 'suck it and see', unfortunately. The only way to know if it'll work is to try it for yourself. Might be awesome, might be a dead-end. Though my 2.5ways didn't meet my expectations, they were a very educational experiment and I learned some things well outside the original scope of the project. So, for me it was definitely worth the while.

John

Viridian · « **Reply #4 on:** 3 Apr 2010, 04:09 pm »

Yeah, I guess I expected that response. No, I haven't heard of any independent verification of the efficiency number. But, I have to say that it is way louder than anything else I have heard. I would be very surprised to find that it was not very close to published specs. It energizes the room like nothing else I've tried.

Thanks, I do think that that helps. I'll get some cheapo chokes to try things out.

I think it is interesting that you have landed on low Qts woofers. I've decided that I much prefer low Qts woofers in OB as the quality of bass is much better in my subjective opinion. THere are tradeoffs of course.

Cheers,
Erik

DanTheMan · « **Reply #5 on:** 3 Apr 2010, 04:20 pm »

Quote from: Viridian on 3 Apr 2010, 04:09 pm

Thanks, I do think that that helps. I'll get some cheapo chokes to try things out.

Will you let us know where you find some cheap chokes?

Thanks,

Dan

nullspace · « **Reply #6 on:** 3 Apr 2010, 04:27 pm »

RE: Low Qts woofers, I liked the AE OB15 a lot on open baffle, but the Kappa-12A blend in with the TT and compression driver + horn better. I can't say if it's strictly the drivers, or if it's because I'm using just one amp now, or that the rolled-off bass creates the illusion of detail and 'speed', but I do like the Eminence drivers better.

There are several people around eschewing the high-Qts driver approach. Among others, CLS over on diyAudio has a thread on high output impedence amps and low-Qts woofers on OB. I have the parts to put together a SS amp with variable output impedence but haven't gotten to that project yet. I'll also have to build a new active crossover, so I've been putting it off.

John

Viridian · « **Reply #7 on:** 3 Apr 2010, 04:54 pm »

Hmm, I see that I've been ignorant about the cost of chokes.

Quote from: DanTheMan on 3 Apr 2010, 04:20 pm

Will you let us know where you find some cheap chokes?

Thanks,

Dan

DanTheMan · « **Reply #8 on:** 3 Apr 2010, 05:00 pm »

Where have you looked?

nullspace · « **Reply #9 on:** 3 Apr 2010, 05:44 pm »

Erse makes decent stuff. Here's their inexpensive line of steel-core inductors: IQ Coils.

John

neobop · « **Reply #10 on:** 3 Apr 2010, 06:36 pm »

Sorry, but it's hard to imagine how this could possibly work out. Is you tube amp going to drive this load? Woofer impedance might drop to 3 or so ohms at some frequencies. Probably even lower, as the Pioneers will be doubling in the bass. They are in parallel with the subs.

Viridian · « **Reply #11 on:** 3 Apr 2010, 06:55 pm »

I had just looked at parts express.

Quote from: DanTheMan on 3 Apr 2010, 05:00 pm

Where have you looked?

Viridian · « **Reply #12 on:** 3 Apr 2010, 06:59 pm »

You could be right. If I run all this off of the 4ohm taps wouldn't that give me a greater chance of success? Would that detrimentally affect the way the amp drives the 8ohm coaxes?

Erik

Quote from: neobop on 3 Apr 2010, 06:36 pm

Sorry, but it's hard to imagine how this could possibly work out. Is you tube amp going to drive this load? Woofer impedance might drop to 3 or so ohms at some frequencies. Probably even lower, as the Pioneers will be doubling in the bass. They are in parallel with the subs.

nullspace · « **Reply #13 on:** 3 Apr 2010, 07:00 pm »

Quote from: neobop on 3 Apr 2010, 06:36 pm

Sorry, but it's hard to imagine how this could possibly work out. Is you tube amp going to drive this load?

It can work, though no guarantee that it will. The attached impedence curve is for FIVE 8ohm 12" drivers wired in parallel. I dare say any amplifier could drive this load.

John

painkiller · « **Reply #14 on:** 3 Apr 2010, 07:45 pm »

Wow, Nullspace, that curve is really interresting! I had no idea that five drivers in parallel would produce such an impedance curve. Do you have a reasonable explanation for this result?

It never ceases to amaze me how practical results always seem to differ from theory.

Viridian · « **Reply #15 on:** 3 Apr 2010, 07:55 pm »

John,

You just messed with my head. What you just posted isn't at all intuitive to me. I understand that impedance is a total of resistive, capacitive and inductive load but my knowledge doesn't go much further than that. To put it simply, I'm shocked by what you just related. Can it be true?

Quote from: nullspace on 3 Apr 2010, 07:00 pm

It can work, though no guarantee that it will. The attached impedence curve is for FIVE 8ohm 12" drivers wired in parallel. I dare say any amplifier could drive this load.

John

Viridian · « **Reply #16 on:** 3 Apr 2010, 08:00 pm »

Cheers! I'm checking them out now.

Quote from: nullspace on 3 Apr 2010, 05:44 pm

Erse makes decent stuff. Here's their inexpensive line of steel-core inductors: IQ Coils.

John

panomaniac · « **Reply #17 on:** 3 Apr 2010, 08:03 pm »

Maybe John meant series parallel?

nullspace · « **Reply #18 on:** 3 Apr 2010, 09:05 pm »

Nope. Under about 120hz, there's 5 8ohm drivers in parallel. It's not completely up-to-date, but here's the network that goes with the impedence:

neobop · « **Reply #19 on:** 3 Apr 2010, 10:26 pm »

LOL, and without the network.... Viridian is talking about using a coil only for the subs. In that case you'll have 3 - 8 ohm drivers in parallel playing bass. At least 2 will have nearly identical impedance curves, so dips will coincide. The 12" Pioneer might have a similar curve in the bass region. I don't think your amp, will like it, but more importantly, I doubt if it will sound good. IMO you need drivers that add up to the same efficiency as Pioneers, and roll off the bass in the main spks, to make that work.

If you try biamping again, that could work out. Even if you run the main spks full range, you could augment the bass with regular sub amps. I've done it that way, and it's pretty easy to work out. A big advantage is that you' don't have to mess with your Pioneers or tube amp. You have low pass crossover frequency and volume on the subs. It's not that hard to dial in. The only problem is the same one with all subs, deep bass content varies with program.