Newbie Question on crossovers

[maheider]

I have a Polk sub/sat system in which I am replacing the front mains and the center channel with Mirage Omni series speakers to get fuller range and provide a better 2-channel listening experience when I just want to listen to music.

On the recommendation of Polk, I set up the sub/sat system by setting the receiver to "Sub=No" (which forces the fronts to "large") and using the speaker level inputs to the sub, then out from the sub using speaker level outs to the front L/R channels. These wires are built-in to the room now, so if at all possible I would like to just swap the Polk speakers out and put the Mirages in their place. Otherwise, I will need to run new wiring to both the front mains and the sub, and it will be difficult to conceal at this point. Further, I am not convinced that the sub operated properly if connected using it's RCA inputs, and it is long out of warranty, so I think I am stuck with the speaker level inputs.

Polk tech support specifically told me when I purchased them that for my Polk speakers, there was no danger to the fronts using this setup. That is to say, the Polk front speakers would not attempt to play frequencies below which they were capable, so the setting of the fronts to large and providing full range signals to them would not cause any trouble.

My question is whether a speaker like my Mirage Omni-50's will protect themselves via an internal crossover network such that if frequencies below their capability are passed to them, that they will not be damaged trying to reproduce those sounds? Is this a common feature of speakers, and if so, what would the advantage be to setting speakers to small at any time?

Sorry for the lack of knowledge on the subject, but I am trying to learn. Thanks for taking time to read this, and I really would appreciate any advice you can provide.

 

[Buckeyefan 1]

Idea behind crossovers and bass management

Polk front speakers would not attempt to play frequencies below which they were capable, so the setting of the fronts to large and providing full range signals to them would not cause any trouble.

My question is whether a speaker like my Mirage Omni-50's will protect themselves via an internal crossover network such that if frequencies below their capability are passed to them, that they will not be damaged trying to reproduce those sounds? Is this a common feature of speakers, and if so, what would the advantage be to setting speakers to small at any time?

Sorry for the lack of knowledge on the subject, but I am trying to learn. Thanks for taking time to read this, and I really would appreciate any advice you can provide.[/QUOTE]

Here's the specs on your speakers:
System Type: Magnetically Shielded Omnipolar Bookshelf Impedance: 8-ohms nominal, 4-ohms minimum
Frequency Response: 55Hz-20KHz, +/- 3dB Recommended
Amplifier Power: Up to 100-watts RMS/Channel
Tweeter: 1" (25mm) PTH™
Pure Titanium Hybrid Inputs: High Quality 5-way Terminals
Woofer: 5-1/2" Polypropylene Titanium Deposit Hybrid Dimensions:
(H x W x D) 14 X 8 X 10 (in)
36 X 20 X 25 (cm)
Crossover points: 2KHz


In a nutshell, the purpose of a crossover is to funnel frequencies to each driver. The best crossovers are closely matched to the response of each driver. The crossover provides several functions. It eliminates low frequencies from reaching the tweeter. A tweeter can operate at very efficient levels, while a woofer needs quite a bit more power. If you have a two way speaker, you should have a one way crossover. A three way speaker should have a two way crossover. The crossover is there to separate the tweeter, midrange if present, and woofer. It does not limit the lowest frequencies sent to the woofer. Those lowest frequencies require the most power to drive them. What that means is your Omnistats which are only rated down to 55Hz (a typical bookshelf) may try to reproduce bass notes down below that limit. Without using your receivers bass management and cutting out the frequencies below 55Hz (or a close setting near 55Hz), your receiver will be using unnecessary power trying to drive a woofer not able to reproduce the lower frequencies. I doubt this will be the case based on the size and sound pressure levels of the Omni's, but that's the theory. Also keep in mind that lack of power is more detrimental to speakers than an excess of power. You are much better driving your Omni's with more power at lower THD levels, than a weaker receiver struggling at higher volume levels. Distortion is the enemy of speakers. Tweeters are usually the first to go from lack of power and an excess of distortion. They can also die from long periods of high volumes and excessive heat. The receivers bass management (setting the receiver to sub on, and setting a crossover point between the mains and subwoofer of, say 40, 60, 80 or 120Hz) provides that extra crossover for the separation between your Omnistat woofer and subwoofer. IMO, you need a dedicated, powered subwoofer with bass management via rca cable, as well as setting your Omni's to small to enjoy what your Omni's can really do. Your speaker level outputs on your sub do have a crossover, but don't provide the same level of bass management your receiver does. I have both an older and newer sub. One is passive and has the internal crossover network. The older sub works terribly with the receiver. There is less flexibility with level outs, relying on a sub to perform all the bass management functions that the receiver should be doing. The newer subs are designed to work in conjunction with the receivers bass management system, thus the reason for updated rca dedicated outs.

 

[maheider]

Thanks for the quick reply Buckeyefan 1. Being an Ohio State fan and an alumnus, I like your handle.

I would like to run something by you. It is safe to say that the designers at Polk understand much better than me the way this should all work. Since they packaged the subwoofer (model is PWS-50) with their satellite system, one would assume that the choice of subwoofer is appropriate for the speakers they paired with it.

Since they recommend using the speaker level connections, and the satellite speakers they supplied are much less capable than the Mirage Omnis (I think they were rated only down to 125 Hz or so), it seems to me that they should have designed that subwoofer to only pass along frequencies above a certain level in order to avoid the situation you described. Now whether they did so is an open question, and if so, what the "high pass" level they chose is. I suppose I could give them a call and see what they have to say.

Would it be common in powered subwoofers to limit the output frequencies on the speaker level outputs when they are made available? If not, it seems like there would really be no use for them (other than maybe paired with full range speakers) since you could not control frequencies while using them.

I probably would be best off to get a new sub, but I don't really have a budget for that right now, so I need to make do with what I have. Also, my receiver (Onkyo 656) is somewhat older but still performs well. It does not support specifically setting any crossovers, however like some of the new ones do. Everything is determined by the size of the speakers you choose in the setup process.