Cross-over in LSiM705?

Ocezam

I've got a friend who's will lend me a dbx 223 active crossover, and another friend who'll lend me a pair of Emotiva XPA-100 amps. I'm going to do some experiments with bi-amping my 705's. I'll set the dbx 223 to send the lower freqs to the lower binding posts and the rest of the signal to the upperbinding posts.

My question is, where do I set the crossover point on the dbx? I can fudge this some for the best sound, but where to start?

The factory Polk specs for the internal crossovers in the 705 state:
Midrange/midbass crossover Frequency 280 Hz
Midbass/woofer crossover Frequency 100 Hz

So where in that chain do the lower binding posts come in? In other words, do the lower binding posts just drive the two oval woofers, or do they also drive the mid-bass diver? The answer would be 100hz if only the woofers are connected to the lower binding posts, or 280hz if the woofers AND mid-bass are connected to the lower binding posts.

Anybody know for sure?

Thanks

WLDock

Just remove the jumpers and see what drivers are tied to what terminals when power is applied. There you go!

Ocezam

Yeah. Good Idea. Simple as that. Thanks.

falconcry72

There is no point in using external crossovers if you are still using the speaker's internal crossovers... you'll just make it sound worse.

Ocezam

falconcry72 wrote: »

There is no point in using external crossovers if you are still using the speaker's internal crossovers... you'll just make it sound worse.

Yeah, thanks.

WLDock

falconcry72 wrote: »

There is no point in using external crossovers if you are still using the speaker's internal crossovers... you'll just make it sound worse.

Ocezam, falconcry72 brings up a a point that maybe you are not thinking about? Removing the jumpers if I had to guess will net...
-One terminal for the TWEETER/MID 280Hz UP
-One terminal for the MIDBASS/SUBASS 280Hz DOWN.

Its a 4-way passive crossover similar in layout to this:


Removing the jumper allows PASSIVE BI-AMPING...therefore no active crossover is needed as the passives are still inline. Polk did not offer any ACTIVE Bi-AMPING options or terminals. If one wants to do that you would have to remove the crossovers and configure a bypass yourself.

Ocezam

WLDock wrote: »

Ocezam, falconcry72 brings up a a point that maybe you are not thinking about? Removing the jumpers if I had to guess will net...
-One terminal for the TWEETER/MID 280Hz UP
-One terminal for the MIDBASS/SUBASS 280Hz DOWN.

Its a 4-way passive crossover similar in layout to this:


Removing the jumper allows PASSIVE BI-AMPING...therefore no active crossover is needed as the passives are still inline. Polk did not offer any ACTIVE Bi-AMPING options or terminals. If one wants to do that you would have to remove the crossovers and configure a bypass yourself.

I am aware of that, thanks.

aboroth00

The passive crossovers will still filter the frequencies. There's no point in applying an active crossover without removing the passive ones. You'll effectively be filtering out certain frequencies with the active crossover then the passive crossover would filter those frequencies again. Biamping MIGHT be beneficial if the speakers are so inefficient, you need to apply power to the bass independently, but this isn't the case with your speaker. However, leaving the passive crossovers in there, all the extra energy will just be burnt off as heat.

To get the benefits of an active crossover, you have to remove the passive crossover, and apply amp power to each driver and use an active XO to filter the frequencies to each driver. However, if a speaker with a passive XO sounds fine, I wouldn't mess with it by yanking out the XO.

Ocezam

Doing what I'm getting ready to do here isn't any more complicated (or wrong) then using the bass management feature found on many preamps. There is no need to "tear out all the crossovers" and amplify each driver. To do so wouldn't be bi-amping. In this case it would be quad-amping. The passive internal crossovers are still very much needed.

With a 24 dB/Octave 2-Way Linkwitz-Riley Electronic Crossover used to split off the lowest freqs the only critical concern is to nail the crossover point of the external crossover VERY close to the mid point of the overlapping frequency curves created by the internal passive crossover at the lower binding posts. This isn't difficult to do with a well made active crossover.

As to weather doing this is "needed" or not...
The "need" doesn't arise based on the specs of the speakers, but on the specs of the amps. My tubes are rated at 140 watts each, but being tubes, their dampening factor is significantly lower than SS amps. As I hope most of you know, a high dampening factor is fairly critical in controlling woofers, and fairly insignificant in controlling mid and high freq drivers. By ACTIVELY bi-amping I'll get the benefits of more power, but just as importantly, the benefits of a higher damping factor solid state amp on the woofers.

This isn't the first time I've worked with active bi-amping. I'm not speaking theoretically. I'm not interested in anyone's THEORY of why this will or will not work. In fact I've done this, mostly successfully, at least a half dozen times in the last 35 years. I've had friends do it DOZENS of times. Success is based almost entirely on setting up the active crossover well. The degree of success is also based on the specific speaker/amp combo you start with. I've never had any "failure" biamping. If you start with over powered solid state amps, you are unlikely to see a significant improvement, but that isn't failure. Starting with mid powered tubes, I expect a fairly significant upgrade of woofer control.

There are any number of other forums where MANY enthusiasts regularly actively bi-, tri-, and quad-amp.

Here's a decent article on how active bi-amping is accomplished and why. It should be obvious after reading this why the signals aren't "double filtered", and the obvious need to leave the passive crossovers in place It also briefly touches on why passive bi-amping does VERY little. And why passive bi-amping doesn't free up, or add, any amp power.

http://sound.westhost.com/bi-amp.htm

Face

A little knowledge can be dangerous is a dangerous thing.

F1nut

As I hope most of you know, a high dampening factor is fairly critical in controlling woofers

Actually the difference between a damping factor of 50 and 1000 is so insignificant that it merits no consideration. It's only when the damping factor is below 10 that it seriously affects the woofer control.

aboroth00

Ocezam wrote: »

Doing what I'm getting ready to do here isn't any more complicated (or wrong) then using the bass management feature found on many preamps. There is no need to "tear out all the crossovers" and amplify each driver. To do so wouldn't be bi-amping. In this case it would be quad-amping. The passive internal crossovers are still very much needed.

As to weather doing this is "needed" or not...
The "need" doesn't arise based on the specs of the speakers, but on the specs of the amps. My tubes are rated at 140 watts each, but being tubes, their dampening factor is significantly lower than SS amps. As I hope most of you know, a high dampening factor is fairly critical in controlling woofers, and fairly insignificant in controlling mid and high freq drivers. By ACTIVELY bi-amping I'll get the benefits of more power, but just as importantly, the benefits of a higher damping factor solid state amp on the woofers.



http://sound.westhost.com/bi-amp.htm

The same article you cited states:

The Most Common Question About Biamping
The most common question I get is ...

"Do I need to disconnect the passive crossover in my speakers?"
The answer is ... Yes, otherwise you are not really biamping at all.

The article also states that installing an active crossover before a passive one can create all types of hell.

So yes we are giving you the correct information. To actively drive your speakers you need to remove the passive crossover. And you cite that you need a high damping factor for tube amps. The thing standing in your way of a high damping factor is the passive crossover. Here's an article by the same guy you cited earlier demonstrating what the passive crossover does to reduce damping factor.

http://sound.westhost.com/biamp-vs-passive.htm

I think you're sorely mistaken about what biamping or amplifying each driver independently.

FTGV

Ocezam wrote: »

I'm not speaking theoretically. I'm not interested in anyone's THEORY of why this will or will not work.

Ok then I won't mention that with the passive crossovers still in place you negate the advantages of active crossovers.

megasat16

FTGV wrote: »

Ok then I won't mention that with the passive crossovers still in place you negate the advantages of active crossovers.

I also won't mention that I fully agreed with you!

WLDock

I agree with the others...Not sure what can be gained with all the passives still inline at about the same freqs as the active?
I do however get the idea that some might want to use a tube amp for the high/mid and a SS amp for the midbass/sub.

As far as damping factor...best not to go there. I started reading about that over twenty years ago in articles by Tom Nousaine and Kenneth Pohlmann stating that its an often debated and sometimes misunderstood spec. The number that some think is high enough is all over the place, 50, 100, 200+,etc. There is more to speaker damping than that one spec alone. The debate has not changed much.

You asked so just sharing my knowledge and experience. Spent much more time passively and actively bi-amping several car/home audio 2-way, 3-way, 4-way raw drivers, component sets, AND SUBS(My first sub crossover was passive!) in the home lab and noting the differences myself. Anyway, your speakers, your gear, your room, your ears, your opinion, your beliefs. Go for it! That's audio!