Tri-amping speakers - Which amps

Depending on the efficiency of the speakers, the crossover points and slopes used, the type of music being played, etc... this will determine how much power you'll need for each frequency range being amplified. Having said that, i've always found that "more is better", so long as quality isn't compromised in order to obtain the greater quantity.

One thing that should be noted is that system and amplifier efficiency will go up, so you'll be drawing less power from any given amp at any given volume. Most of this is due to limiting the bandwidth that each amp sees, but some of it will be due to getting rid of the "power sucking" passive crossovers too. Those "power sucking" crossovers also "cover up" low level resolution, so expect to hear greater transparency too.

Due to the lower power demands and greater resolution, crossover distortion from the amps may become more apparent to you. In this area, it helps to have amps that run in Class A longer than most commercially produced amps do. This increases signal purity and moves the point that the amp crosses over into Class B operation above the point of average power consumption. The only downside to this is that there is greater heat to deal with.

As was mentioned, it may make things easier for you by using amplifiers from one manufacturer. In doing so, you can have them increase the bias levels to where you want them and gain match all of the channels at one time. Depending on if the amps are new or used, and what the policies of the company are, they may / may not charge for this service to be done. As such, you might want to talk to the service department of the manufacturer's that you are considering using.

You should be aware that there is quite a bit of "fine tuning" that you'll have to do with a system of this nature. If you are someone that wants to maintain a high level of accuracy, you will at least have to take some minor electrical measurements using a multi-meter. If you're not worried about that kind of stuff, you can simply adjust things by ear until they sound "good" to you. I'm primarily talking about gain settings on the electronic crossover that you use, but you can make this as simple or complicated as you would like to carry it.

One more thing. Running multiple amps will require greater electrical resources. Are your AC resources up to the task? If not, factor that into the cost of such an installation too. Sean
>

Jerry / Onplane: I would beg to differ as i've yet to hear even a 6 dB crossover ( a simple capacitor in series with the tweeter ) that sounds as good as a "naked" tweeter ( no capacitor ) actively crossed. This not only has to do with removing the capacitor from the high current area of the circuit, but also from the electrical changes that take place prior to the amplification stage.

My Brother made the same assumption that you did here, as he was running his system as you described for quite some time. When i finally got him to try using an antique "Pro Sound" active 3 way that i had laying around, he was flabbergasted at the difference. He couldn't believe he had wasted all those listening sessions with reduced fidelity when even a 30 year old active crossover ( $40 on Ebay ) could have provided so much better results. Bare in mind that this was using cheap plastic Radio Shack 1/4" to RCA adaptors on every cable in and out of the crossover too. Sean
>

As Jerry mentioned I have considered bi-amping due to the advantages such as cost, space, & simplicity to name a few. But that would seem to be a compromise. Think I would always wonder if bi-amping was an improvement, wouldn't tri-amping be better. Have not totally discarded the idea however.
Sean, you mentioned most of the things I have had to consider if I go this route. I have thought about Class A or tubes for the tweeters and mids and bipolar amps for the woofer. Obviously you cannot have this with a multi channel amp. With 3 stereo amps you have to consider compatibility issues with the system. I am not totally familiar with an electronic crossover, but as I understand it cannot totally correct for all mismatches between power sources. Any suggestions on actual amps to consider? (I am serious about doing this but don't want to just throw money at it. I like a noticable improvement, and not to quickly reach the diminishing return category.)
In your last section you mentioned 'electrical resources'. Were you talking about available power from my wall socket? If so I think the increased efficiency of tri-amping will allow amps of about 50% the usual power requirements.
The monitors with mids and tweeters are an MTM design with 4 ohm impedance and 90 db sensitivity. The wooofer cabinets have a 10" active driver & 12" passive, 8 ohm and 84 db. The sensitivity of the woofer cabinet is low but the designer has told me that 100 quality watts should do fine.
Any more discussion would be greatly appreciated.

One can typically raise the bias of ANY amp, so long as there is adequate heatsinking and air-space available. As such, you can use a multi-channel amp and have the manufacturer adjust the bias setting to operate in Class A up to X amount of power, switching over to Class B above that point.

In my experience, depending on speaker efficiency and impedance, somewhere between 6 - 10 watts of Class A power @ 8 ohms seems to work quite well with Class B operation kicking in above that point. Due to the lower impedance of your MTM section and low efficiency of your woofer section, i would recommend going higher on the bias scale rather than lower. That is, 6 watts of Class A bias @ 8 ohms is only going to net you 3 watts of Class A @ 4 ohms. While this would still be better than most, moving up to 10 watts @ 8 ohms would give you 5 watts of Class A @ 4 ohms. By the time you switch over to Class B, your listening level is so high that any form of crossover distortion becomes negligible.

As far as the woofer section goes, 100 watts wouldn't make an 84 dB woofer flinch, even if actively crossed and bandwidth limited. Besides that, passive radiator systems generate more reflected EMF than most other designs, meaning that you need even more power to keep the driver under control. I don't know how big your room is, how loud you like to listen, etc... but i would be looking at some very high powered, high current amps for that section if i were you. That is, unless you listen to smooth jazz or chamber music at background levels.

With the above info taken into account, you might want to think about a medium to high powered ( 100 - 200 wpc @ 8 ohms ) four channel amp for the highs / mids and a very robust ( 350+ wpc @ 8 ohms ) two channel amp for the woofers. Larger amps typically have more output devices and heatsinking, which allows them to share the higher load of a richer Class A bias with out as mech thermal stress on any given part. As mentioned above, sticking with the same manufacturer would have several advantages.

Before investing in any of this, do a LOT of homework. What you are attempting to do is neither inexpensive nor simple in most cases. Obviously, it can be done using less power and in a very simple & convenient manner ( one six channel amp ), but why go to all of this trouble unless you're really trying to obtain max performance? Sean
>

For all the bother, you're probably better off looking at a pair of ATC active 50's or 100's. They're triamped internally and you'll avoid all the hassle of cabling, shelf space, etc. I'll bet that the results will be very similar for less money too.