Interaction between Sub XO and Amp?

I want to use my sub's crossover to cut off the lowest frequencies from my main (bookshelf) speakers. The concern I have is that the sub's amp will process the frequencies above the filter, resulting in a degredation of the sound quality.

Optimally, I'd like to use the line-level in/outs to run preamp out1 to the sub, then the line-level outs on the sub to the amp (and amp to the main speakers.) If the crossover works "before" the amp, and simply passes the frequencies above the cut-off to the amp, life is good. But if the amp gets involved and "corrupts" the clean, accurate signal my preamp is sending I need to decide whether to buy a sub whose amp won't degrade the signal (much) or just go with floorstanders despite a fairly small room (12x17).

Thanks for your help!


If your preaqmp has 2 oputputs, run one to the fulrange amp and the other to the sub. That way, there is no risk to the full range signal. This of course, assumes your satellites can handle full range sound well. This will work best if your preamp outs are buffered from each other. This means the 2 outputs are electrically isolated form each other. I have a similar situation, running Entec subwoofers and a Behringer EQ from 1 output, and full range speakers from the others. My outputs are not buffered from each other, so if I make changes to the EQ, it affects the sound of the main speakers. I think only the volume of the sound changes, not the fundamental character of the sound, but it is worrisome to me nonetheless. But the system sounds good, so I won't get too worked up over it. The only way I know of to tell if the outputs are buffered from each other is to call the manufacturer and ask.

What subs are you using? Maybe someone else here has them and can answer more specifically.
Re: your question, the signal will certaqinly pass thru the subs crossover, which could cause degradation. The more complicatred the high-pass portion of th crossover, the more likely there will be degradation. There may be some line-level preamp type gain stages involved, particularly if the sub allows you to do any processing of the high pass signal, such as boosting it's level, changing phase, whatever. I think most subs, though, have this type of control working on the bass, not the rest of the signal.
in any case, running your signal through the sub's crossover will degrade the signal. Explore dedicated crossover between the preamp out-amp in. I believe paradigm makes one, and there are some other choices out there.
The high pass (to your main amp) is typically applied before the sub's amp. However, the high pass is still another device between the preamp and main amp, so you're dependent on it's quality (although it's just a capacitor, but how good?.) Further, you will have four cable connections between you preamp and main amp instead of two, plus some internal wiring (in the sub) of unknown origin.

If your preamp has two pairs of "main outs", and depending on your main speakers (you didn't say) the ideal way IMO is to run them full range from one set of preamp outs, and run the sub (again, you didn't say what kind) from the other set of preamp outs, just enough to fill in the very bottom that your main speakers miss.
Nsgarch: thanks for your thoughts. The way you describe is how I had my system set up. A friend told me that cutting the lowest frequencies from my mains would provide better clarity in the mids. I tried it (reluctantly) and was very surprised at how much more "slam" my system had, and how much better it sounded in general. It also helped my mains "get right" on piano and some male vocals, and removed a "haze" or slight midrange glare that exists when I have the mains reproduce the full range of sound.

I'm looking at upgrading my sub (currently a Velodyne CT-120, originally purchased more movies) and my speakers are nOrh mini 9.0's (website claims it reaches 45Hz at -3dB, but that's a stretch in my room based on measurements with my Radio Shack tester.)
This might be your ticket!!! check it out.
Hi Howard,

It's surprising that you haven't figured it out or that no one mentioned more about the Velodyne to you, especially that you stated you use one, I believe! In any case this is what I feel you should do. Firstly, as good as the Intech's supposed to be, best results with them are in most cases, usally in a large room, you need a pair for best results and they are expensive, however, now that prices in high end have decreased substantially, so if you must have Intech, maybe you could get a good enough savings. One thing to remember though, is that there are a lot of veriables when it comes to using subs, and you may find that one sub can be and sound better than two and be more flexiable, in certain situations! Now, all I know is this, I own a Velodyne FSR 18 and man does it ever kick F***'n Ass. I won't go into all of my system, but I'll tell you this much. The Velodyne FSR (earlier HGA) is so flexiable, it has at least 20 features and controls, the low pass is 40 to 120 Hz., there's internal crossover switch In/Out for external crossover use, at the flip of a switch, O" /180" phase adjustment, High pass crossover switch 80/100Hz., you could use line level like you're inquiring about and also speaker level, just to name few. It goes on and on. I own an original pair of Mission 770's from 1978 (1977-'79) whenMission changed the placement of the tweeter from '77, which made a world of difference. They were the first speaker on the market to offer the public polypropelyne! I have a Jolida tube preamp which is a surprising joy. I have a very powerful Amp, and all I can say is that the Velodyne is amazingly fast(for an 18"), coherent and seemlessly blends in with everything in my system that you don't even know it's there. If you could pick one up with the cheap prices around I don't think you'd be doing wrong. Stereophile rated the Velodyne 18" subs over many years as one of three best subs in the world. They also rated it class A year after year for many years. These earlier edition 18's were so much more flexible than their smaller models,with all the bells and whistles (that are very important with a sub) however their new 12's and 15's that they are making now supposed to be pretty good and might have a lot of the features and controls that you looking for and need! Check it out and let me know how you make out? Sincerely,