Dumb impedence questions (sub plus top for bass rig)

[kesslari]

If I understand correctly:

1. If I set up a passive system, with a 4 ohm sub and an 8 ohm top (and yes, assume a 2 ohm capable amp) - the sub would receive double the power that the top gets - correct?

2. And in theory, this wouldn't necessarily be a bad thing (and might be a good thing), because the sub needs more power than the top - also correct?

Details - I'm considering putting a 4 ohm fEarful sub under an 8 ohm BFM omni 10.5. If it works, it *should* sound great. But there's a fair bit of "if"...

[Downunderwonder]

You're going to do a passive crossover? Theoretically you'll need one.

[Tim1]

Don't want to disagree with a fellow kiwi, but I wouldn't worry about a passive crossover unless you are really cranking it. If you were looking at having to design a custom crossover I would flag the idea. The BFM Omni seems to get a fair bit of respect and the combination could work fine in theory. Best idea is to try it and see, but I am always a bit wary of mixing and matching different brands.

[wcriley]

Originally Posted by kesslari

I'm considering putting a 4 ohm fEarful sub under an 8 ohm BFM omni 10.5.

Do you plan on building a 1515/66 or 1212/6 without the mid drivers? Intersting idea!

[Foz]

Might work OK - try it!

But there is a lot potential for problems - not the least of which is the +/- 3 octave overlap between the two mismatched cabs - nor the potential for wild variance in sensitivity.

[kesslari]

Sorry, to clarify, the fEarful Sub design includes a passive crossover. I would probably start with an active crossover that I could use to figure out a good crossover point for those two cabs, and then build it (or have someone smarter than me build it) and put it in the sub.

[mystic38]

some notes..

if you are going to do a crossover, then your impedance does not fall to 2R, though reactively this may be the measured case...(in the same way an 8R speaker is often 4-5R at some frequency).
Note that if you use an active crossover to identify the crossover frequency, then note that a passive crossover will have markedly different phase response... and hence sound very different.. phase is key at low frequencies.

[CL400Peavey]

The only 4 Ohm fEARful sub is the 1212sub.

[T-Bird]

Hi.

Originally Posted by kesslari

1. If I set up a passive system, with a 4 ohm sub and an 8 ohm top (and yes, assume a 2 ohm capable amp) - the sub would receive double the power that the top gets - correct?

Yes.

Originally Posted by kesslari

2. And in theory, this wouldn't necessarily be a bad thing (and might be a good thing), because the sub needs more power than the top - also correct?

Yes.
But twice is nowhere near the balanced amount.

Originally Posted by kesslari

Details - I'm considering putting a 4 ohm fEarful sub under an 8 ohm BFM omni 10.5. If it works, it *should* sound great. But there's a fair bit of "if"...

Mixing and matching different cabs of the same design is a crapshoot, especially with a passive cross-over, but mixing and matching cabs with totally different designs is damn near impossible with a regular passive cross-over, and still very hard even with an active one.

Can be done, of course, and plenty of people (me included) have experimented with all imaginable -and some unimaginable- combinations. The results I for example have had, hasn't been as good as the initial thought was .

Since You're going with a BFM design for the top, why not build one of his subs that are designed to work well with his tops?

Not that there's nothing wrong with fEarfuls, quite the opposite, but I'd say even the wise men behind those two said designs would advice You to choose either one, not the mix.
It won't be easy to force the drivers to be in phase +/- an octave around the cross-over frequency.

BTW, on a passive cross-over scenario, the sub determines the impedance.
Unless You're bi-amping of course.

Regards
Sam

[kesslari]

Originally Posted by CL400Peavey

The only 4 Ohm fEARful sub is the 1212sub.

No, because Avatar makes a 4 ohm 3015lf.

[kesslari]

Originally Posted by T-Bird

Hi.

Mixing and matching different cabs of the same design is a crapshoot, especially with a passive cross-over, but mixing and matching cabs with totally different designs is damn near impossible with a regular passive cross-over, and still very hard even with an active one.

Can be done, of course, and plenty of people (me included) have experimented with all imaginable -and some unimaginable- combinations. The results I for example have had, hasn't been as good as the initial thought was .

Since You're going with a BFM design for the top, why not build one of his subs that are designed to work well with his tops?

Not that there's nothing wrong with fEarfuls, quite the opposite, but I'd say even the wise men behind those two said designs would advice You to choose either one, not the mix.
It won't be easy to force the drivers to be in phase +/- an octave around the cross-over frequency.

BTW, on a passive cross-over scenario, the sub determines the impedance.
Unless You're bi-amping of course.

Regards
Sam

Sam, thanks for the good insights.
The BFM subs are simply too big for me to want to haul them. You are probably right, it's a risky, maybe-good-maybe-not proposition.

[Bassmec]

Originally Posted by kesslari

Sam, thanks for the good insights.
The BFM subs are simply too big for me to want to haul them. You are probably right, it's a risky, maybe-good-maybe-not proposition.

You know the rules!, Don't go off half cocked, you need a digital system controller with micro delay phase correcting ability and a separate amp for the sub.
Those are the rules in PA, what makes these heretics feel that just because its a bass rig, we can afford to be a huge inductor out of phase with everything else and not even bother to properly time or phase align the mid.

[will33]

Word from the fearful folks is the Avatar 4ohm LF gives up a fair bit of performance to achieve it's lower impedance. Though they haven't given any details, it doesn't matter in this case. I agree with the above...very likely you'd end up with quite a bit poorer performance than expected combining those 2.


There is the Tuba24, though, the answer there is, you'd give up spl for lowend extension you really don't need, though I think you'd certainly notice it's presence with the Omni rolling off around 100hz...30hz extension is a waste of time and money in a bass rig, IMO.

Might want to rethink this deal. If you like your Omni, just want a little more depth, consider a pair of Jack12's, one with woofer only, one with switchable array.

Also be a pretty good idea to ask over at Bill's forum. Folks over there are plenty helpful, not to mention getting input straight from the source. There maybe some guys who've already accomplished what you're looking to do and Bill isn't the type to tell people to just buy/build more stuff if he doesn't think it'll get you where you want to go.

He does give passive XO details in his Titan plans....with the huge caveat that it is only for backline or extremely small PA applications, and only then when you're out of other options. IMO, when you can get an active crossover and a poweramp off craigslist for $200 or less, I wouldn't bother with the passive.