Two 750s instead of One 770

What do you guys think of getting 2 750's instead of 1 770? With 10 channels of power, could you bi-amp the front 3 speakers and then have the remaining 4 channels reserved for the rears speakers in a 7.1 configuration? It would cost $401 more than the 770 alone, and you could take your time buying the second amp. I have never owned an separate amp(s) before so if this is impossible, forgive me.

You can bi-amp your fronts if they are bi-ampable; you can't bi-amp just any old speaker. You can tell if it is bi-amp ready if it has two sets of binding posts, connected by a jumper (you remove the jumper to bi-amp).

My mains are bi-amp capable, but my center is not. Centers don't usually have big woofers in them so I'm not sure that there would be too many out there that can be bi-amped. I haven't decided if I'm going to bi-amp the mains, yet, but it's definitely a down-the-road project if I do.

Also, those of you who do bi-amp: do you use a crossover in the signal cable to the two amps, or do you use a Y-adapter and let the speaker's crossovers do the work?

My mains and center are bi-ampable. (Paradigm Reference) I really don't know much about amps, but wouldn't you be sending 165x2 to each of the mains and 165x1 to the rears in the situation I described above? Or would difference in impedience get you close enough to the same power that you could even it out using the preamp?

My mains are triampable (Linn 5140); my rears are biampable (Mirage); and my center is biampable (Linn 5120). Also, the center channel produces a large amount of bass. I think the center channel is rated to 50 Hz or so, and I believe it gets there.

Is the 750 bridgable?

Charlie

No, sir.

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Gonk

What does an amp need to be bridgable, anyway? As I understand it, you basically just connect two channels in series with each other and invert the phase on one of them. Couldn't you do that with some clever external connections?

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Matthew J. Hill
matt@idsi.net

i wonder whether the y adapter should be at the amp end or at the pre/pro end when one tries to bi-amp? i havent read anything about that. any suggestion? thx. luke.

Isn't the benefit of bi-amping realized by having separate power supplies, i.e. monoblocks? Unless each channel is driven by a different power supply, I don't see what the potential gain is. I don't recall how many transformers the multi-channel amps have, but I'm sure it's not more than 2.

[This message has been edited by alfack (edited February 17, 2002).]

You need a crossover before the amps in order to realize the full benefits of bi-amping. It is different then bi-wiring.

How do you know what frequencies to cross over at? Do you just have to get specs from the speaker manufacturer, or is it trial and error?

Also, do the speaker's built-in passive crossovers get taken out of the signal path when you bi-amp?

If you biamp or triamp, there's generally a crossover module used in the amplifiers (one for each frequency range) to force the frequency to the correct drivers. If you biwire or triwire, the internal crossovers are still used. I'm not sure how you bypass the internal crossovers when you bi/triamp, but there's a way to do it.

Bi/triamping is ridiculously expensive. For instance, for my Linn 5140s, I'd have to have 6 amplifiers and 6 active crossovers. For my center channel, I'd have to have two amplifiers and two active crossovers. It's not worth it to me. I'd rather just have two amps to each speaker and let the internal crossovers do their stuff. Or, have one kick butt amp that goes to all of the inputs on each of the front speakers.

Hmm. Maybe the 950's successor will have dual outputs for the front three speakers and built-in active crossovers. Doesn't seem like it would be too hard to do if you've got everything in the digital domain anyway, and would probably sound better than external analog crossovers.

Actually, on a decently designed set of loudspeakers, the analog crossovers have a much better chance of sounding "good" compared to a "generic" active crossover. Crossovers do more than just send high frequencies to the tweeters, and low frequencies to the bass drivers. They are also used to "shape" the response from each driver. Many drivers have peaks and dips in their frequency response, and there are special filters used in a well designed crossover that can account for those abnormalities, to provide a smoother frequency response. There are a host of other frequency response abnormalities that can be corrected in the crossover, but it would take all day to list them all. The analog crossover also will be designed to account for the relative offset of the drivers (the woofer's acoustic center is further "behind" that of the tweeter). This insures proper phase alignment, which drastically improves imaging. The crossover can also be used to shape the impedence and phase profile of a speaker, which is particularly important for speakers that are to be used with tube amps, to allow an easier load for the amplifier.

Keep in mind that when someone designed your $1000 pair of speakers, they did more than just slap a couple of drivers in a box with some old crossover they had sitting on a shelf. *One would hope* they specifically chose a set of drivers, and used some very sophisticated software and measurement equipment to get the best possible performance while staying within their budget constraints. By comparison, an active crossover is a shot in the dark!

my 2¢
Jason

Hmm, didn't know it was so complex. Thanks for the description. I'd always subscribed to the notion that any sort of passive crossover in the signal path was "bad" and that pulling it out would make things sound better.

To be fair, I'll also point out that it is possible to make an active crossover that behaves the same as an analog crossover. In this regard, you *could* be better off. It's easier (i.e. cheaper) to create steeper crossover slopes with an active crossover, and you can do all of the same phase and impedence adjustments using active crossovers. In fact, I've heard of people using DSP chips to create "digital filters", and the result is unbelievable sounding speakers. However, the important thing is that those design decisions have to be made for a specific set of drivers.

Using a GENERIC active crossover is where you could run into trouble. Typically an active crossover will only allow you to set the crossover frequency and probably provide you with a gain control. With the aid of a parametric or graphic EQ, you might be able to gain back some of the response shaping capabilities of the analog crossover.

Now for the scary part: All of what I have said to this point assumes that we are talking about WELL DESIGNED speakers. How much did you spend for your speakers? If it was less than $1000/pr, there's probably a good chance that a lot of compromises were made in the design of your speaker, and probably includes sub-par crossover components. The crossover is likely to include electrolytic capacitors, small gauge iron core inductors, and wire wound resistors, as opposed to more expensive poly caps, large gauge air core inductors, and non-inductive resistors. Also, many of the specific response shaping circuits have probably been omitted, and the overall crossover topology may be insufficent to properly control the drivers. The result is that your speaker may have more "character" than they should. In cases like this, it's quite possible that even that standard electronic crossover will sound better.

This is why I'm such a big advocate of building your own speakers. To get a commercially built speaker with high quality components and that have been well designed will likely cost you several thousand dollars. To build a comparable speaker yourself may only cost a few hundred.

I'll get off my soapbox now.
Jason

[This message has been edited by JasonA (edited February 26, 2002).]

Jason, thanks for the info. Very informative.

My speakers were $1100/pr, and they do sound awesome. I'm not really a speaker "techie," though, so I have no idea what is inside them, except for the number of drivers and their diameters. I also wouldn't even know what questions to ask if I had one of the speaker's designers in my living room.

I do know they're bi-ampable, and I thought I knew the general principles of bi-amping and what the advantages were. It's nothing I'll do soon, both due to time & money constraints, and because they really do sound very good right now. But it's nice to know that I have somewhere to go if I ever become more of an audiophile.

DIY would have been cool, but seeing as I consider myself lucky to be able to change the oil in my car, I think anything involving cabnetry & complex wiring would have been beyond my ability. Plus, I don't know if my ear would have been trained enough to tell the difference. The speakers I have now are the best I've ever heard and I'm quite happy with them. They're Axioms, BTW.

Who manufactures your speaker will also determine whether your speaker is biwireable/ampable. Linn, the manufacturer of my speakers, is really into biamping, and they make the active crossovers and amps specifically for their speakers. Others think that the biamping philosophy is hogwash, like Thiel, and Wilson. These manufactures provide "great" internal crossovers and only one set of speaker connections. (Speaking of great, if you can, you should go listen to some Thiels or Wilsons through a nice system -- You'll be amazed at how good these sound.)

Which one is better? I personally don't know. I've never heard an active system before, but I have heard some really nice sounding passive systems. Personally, I'm opting for buying the best amp, preamp, and cd player that I can afford and hoping for the best. My speakers (Linns, which I bought used) sound pretty darn good through good equipment, although my friend's sound better (VIENNA BEETHOVEN -- sorry for the caps; I copied it from a website) through the same equipment. These two speakers show yet another difference in manufactures -- driver sizes and quantities. My speakers have a single tweeter, midrange, and woofer (8 inch), while the Viennas have a single tweeter, two midranges, and two woofers (6 inch). My speakers have much more ooomph for rock and movies, but the Viennas have much more midrange clarity and have a more laid back but clear bass sound. For stereo, I'd choose the Viennas; for movies, I'd choose the Linns.

I bi-amped my system and am very happy with the results. Now it sounds even cleaner than before. It's very simple to do, and "affordable" with Outlaw amps. I suggest bi-amping 6 channels with a 770 plus a 755. (You really only need a 6.1 system with one rear center speaker.) You don't need any more crossovers, just your Y or piggyback adapters on the amp inputs.