bi-(amping) curious....

allknowing001

i'm getting the bi-amping itch and had a couple questions before i began:

- When using the passive crossover with my MMC6500's, my amp "sees" a 4 ohm nominal impedance on each crossover-tweet-mid set. If I were to bi-amp the the tweets and mids what are the individual speaker impedances? These individual speaker impedances are not in the Momo manual nor online, tho I'd assume Polk was smart enough to make them 4 ohms each as well. Just curious so I set up my amp properly.

- The passive crossovers are "bi-ampable" themselves, with both tweeter and woofer input connections. What's the benefit of running wire directly between the speakers and the amp (channels 1&2 to tweeters, channels 3&4 to woofers) or running all four channels to the crossovers (1&3 to crossover 1, 2&4 to crossover 2)? When both tweeter and woofer input connections and the input switch is flipped to "dual" are made does it bypass the crossover network entirely?

I realize the MMC6500 passive crossovers have speaker protection circuits and not using them voids the warranty, but I'm asking these questions from an SQ perspective. Life would be so much easier if I bought the C400.4 in the first place!

candaddy

I'm running a four channel amp with two channels for the tweeters and two for the mids. You run the dual input on the crossover. The dual input just keeps the tweeter and mid portions of the crossover separated so that you can adjust the levels of the mid or tweeter at the amp to suit your tastes. I've found that not only is it more adjustable, but it sounds better.

If you don't use the crossovers at all, you're going to have to run some kind of active filtering to keep lower frequency sounds from getting to the tweeter or you will very quickly destroy it. That's why Polk will void your guarantee if you don't use them. Quality active crossovers can deliver better sound than passives, but it takes a little more knowledge and setup time than simply wiring everything up and driving away.

I imagine the amp will see a nominal 4 ohm load, there will be only a small difference in the impedance with the change in wiring.

MacLeod

I dont know what load a tweeter would carry but it doesnt matter. Theyll have their own gain control so you just set it to where it blends in best with the mids.

If you dont have an active crossover unit youll have to use the passive Momo x-over. The only downside to using the passive is you cant change the crossover points.

allknowing001

candaddy wrote: »

If you don't use the crossovers at all, you're going to have to run some kind of active filtering to keep lower frequency sounds from getting to the tweeter or you will very quickly destroy it.

That's what I was wondering - do I need an outboard/standalone active crossover to replace the passive Momo x-over or will the filtering capabilities of my amp (which meet or exceed those of the C400.4) be sufficient? Again, the amp still has crossover point selections and cutoff slopes to keep the tweets and mids from getting undesirable frequencies...

Basically my question boils down to this: why do I need the Momo passive crossover to filter the signal at 2600 Hz with 12dB slopes when I can accomplish the same thing with my amp settings?

candaddy

If your amp will do that, then you're OK to not use the supplied passives. Most amps only have a crossover that goes to 400hz or something, which is obviously way too low. Keep in mind that you will have no tweeter protection after you remove the passive crossovers, so if you have a turn on thump or wine or anything of that nature, the tweeters could be harmed! Most people use a capacitor on either the positive or negative lead in series with the tweeter to keep DC signals from hurting the tweeter. This will act as a 6db high pass filter and it's customary to size the capacitor such that the filter is one octave or lower than that which you'll actually be using. Do a search on Google or something, there are on-line calculators for sizing capacitors.

MacLeod

If you have an amp that has an active crossover on it and goes high enough then you can use it. Most amps have crossovers but most dont go very high as in no higher than 500 Hz. If you have one like the JL 300/4 or Polk 300.4 that is variable up to 5.5 KHz then that will work fine.

**edit

oops. Didnt mean to repeat your post there candaddy. Didnt see it there.

allknowing001

Thanks for the multiple replies this evening candaddy and MacLeod, it means a lot to get rapid and meaningful responses.:D I don't have any thump or whine issues now, but I'll definitely look into the protective caps since I'll have to reroute my speaker wires a bit to bi-amp and that could very well introduce some new noise.

One thing I noticed on Polk's page for the C400.4 amp is that they actually tell you what the Pre EQ filter settings are! From the product page:

"In the C400.4, Pre-EQ is an auto-preset filter (2nd Order HP, Fc 5.5kHz, Q of 0.707; 2nd Order LP, Fc 3kHz, Q of 0.707; 3rd order HP, Fc 50Hz, Q of 0.707; Notch Filter, Fc 11.7kHz, Q of 5.5, -2dB) that replaces the MMC Series passive Xover, and provides greater dynamic range and efficiency with MMC Series Speakers."

Everything except the notch filter seems somewhat easy to approximate - 1st part sounds like the tweeter HP filter, 2nd and 3rd add up to the woofer bandpass filter. tweaking these will be awesome!