Getting an external crossover: worth doing?


I'm wondering whether to get an external crossover for my stereo setup, something like the Bryston 10B, and would like to get your opinions.

My setup is made of B&W 804s speakers (that go down to 30Hz -6dB, according to B&W)+ REL Storm III subwoofer, McIntosh MC275 tubed amp, a pre and a cd player.

The purpose of adding a crossover would be to relieve the MC275 and 804s from reproducing the lowest frequencies, which consume so much power, and for which neither of these products are best suited to reproduce. I've read post talking about the midrange opening up, etc.

REL recommends sending the whole-range signal to their subs, and likewise running the speakers full-range.

Then, adding an external crossover means adding more circuits to the path, and Sumiko recommends not crossing over the Storm III above 45 Hz, so I'm wondering if this is worth doing. Or in other words, how much power goes in the range up to 45 Hz?

Sorry if this is not coming up super's me not being clear with my own thoughts.

Your thoughts?
Yes it is worth doing - but only if you implement it properly! The ideal way to do it is to remove the internal crossover in your speaker, get another power amp, and go active. There are a number of theoretical advantages to doing so. In fact I am contemplating doing the same with my speaker - just saving up the pennies for another power amp and the crossovers.
Unless the speaker designer envisioned an external crossover being used, you are altering what they designed. So it's a guess as to what will happen. It may work fine or you may end up with a speaker sounding entirely different from what you had before, and from what the designer intended. It may or may not be an improvement.

Now if you multi-amp with an active crossover, that's a different story. That doesn't seem to be the intention from your post however. It seem that you are still planning to use the single amp.
Sorry my post wasn't clear.

The setup I envision: cdp > pre > crossover > MC275 > speakers
.............................................................|_powered sub

The stereo amp would continue to drive the speakers (with their internal crossovers intact), but the amp would receive a signal from the crossover that would go down to...say 45Hz only. At the same time, out of the crossover would come out another signal, 45Hz and down only, that would drive the powered amp.

So it would be bi-amping in a way, as the sub is powered.
However I'm not talking about a crossover to replace what the speakers have in them.
I assume you mean the 10B SUB crossover since it sounds like you are talking about high passing the main speakers. An advantage of high passing the main speakers is to reduce the distortion they produce in the bass region. The 804 would probably benefit from being high passed. You could email B&W for more complete distortion data.

My understanding is that REL subs are designed to augment full range speakers in the lowest octave. Their frequency response may not be suited to being crossed over at 70 or 80Hz.

So with your system, I'd spend the money on more watts or scrap the REL sub.
After reading your follow-up i'm not so convinced if you will get much benefit from doing this. You will free up some amplifier power by removing the bass frequencies from the MC275, but this is countered by inserting more electronics in the signal path. I would agree with Bob_reynolds above, you would be better off getting a more powerful amplifier.

Your are dead on: I meant 10B sub crossover. And the benefits you describe are exactly what I'm after.

Isn't the bottom octave the 20-40 Hz range? I agree REL subs aren't ideally suited for crossing over at 70 or 80Hz, but then I'm not following you how this is bad news if I'm thinking of crossing over at say 45 Hz. I'm not disagreeing with you; just stating I'm not able to follow your reasoning. More watts would relieve the amp, but then not the speakers.

I do appreciate your input.

Thank you.

With the 804s you'll still produce a lot of distortion if you go as low as 45Hz to accomodate the REL. Check the 804 distortion spec (compare to the 802 just for reference). At 90dB/m from 120Hz to 90Hz the distortion doubles. Look at some of the distortion graphs for subwoofers: You'll see that distortion reaches a point and then it tends to rise very fast. If your goal is to get clean sound from the 804s, then I wouldn't high pass them any lower than 70Hz. As I said you can contact B&W for more distortion data. Note that the subs are tested at 2m not 1m as B&Ws spec.

I agree totally with high passing main speakers and letting the bass chores be handled by a subwoofer. But REL subs aren't designed to be used in this manner. If your goal is to get clean sound from the main speakers, you need a different sub. If your goal is to lighten the power load of your main amp, then REL can still be used. But at 45Hz how much of a load are you removing?