Here's an idea for some bi-amped monitors. They wouldn't be big powerhouses, but I have a feeling they could be useful for nearfield use. This project is only fiscally possible because Apex Jr. has 25W stereo amps available for $28 a pop. With a little modification, I think they could be used great for bi-amping. The amps are designed to be built in to the speaker cabinet. Two amps would be required for a pair of stereo speakers. Here's what (in theory) should be involved:
If you already have an active crossover, you're one step ahead of the game. Chances are, though, that you don't. There are a couple of options here. The best would probably be to build and active crossover with a fixed crossover point. Adjusting the crossover point is as simple as changing a couple of resistors. This would be a fairly inexpensive project - it would require a few opamps, some capacitors and a few resistors, along with a simple power supply. It could be built into a small box to be hidden behind other components. I would guess the cost for a project like this to be under $50. With a little more thought (and money), I'm sure a system could be designed with a variable crossover point, and built into a nicer enclosure (rack mounted?). Of course, that would be more money, and the point of this project is to keep it cheap. The second option would be a high order (probably 4th) passive crossover before the signal reaches the amps. This would split the signal into high and low channels. This would be the cheaper route. The third alternative would be to run the same signal into both channels of the amp, and have the passive crossovers attached between the amp and speakers, just like a normal loudspeaker arrangement. This might be the simplest arrangement. In all of the above arrangements, the driver levels can be matched by adjusting the balance control on the amp.
So, what's the advantage of a system like this? If mated to some good quality drivers, a nice set of monitors could be the result. If budget is on your mind (for example, computer speakers), these would be great.
Full Range Sub
Here are a couple of design ideas:
Recording monitors - Use some good quality components from a company like Vifa, Audax, Seas or Peerless. Best choices would be a 6-1/2" woofer and 1" textile or metal dome tweeters. If you can find an 8" woofer that has good midrange reproduction and doesn't need to be in a refridgerator-sized enclosure, that might be an option. I would guess that an 8" fiberglass or carbon fiber driver from Audax would do well. The cost of a system of this level could range anywhere from $150 to $250, depending on driver choices. Of course, using higher cost drivers (like Dynaudio, Morel or Focal) will jack the price considerably.
On a side note: I have doubts in my mind that these "premium quality" drivers are worth their price. The most expensive driver I have ever purchased was a Vifa tweeter. I can barely tell the difference between it and the $6 el-crapo variant. Even $10 bargain bin drivers outperform comercial systems I've heard. This leads me to believe that these high dollar units will only offer performance gains noticable only to measurement equipment. Maybe my view stems from the fact that I could never afford $100+ drivers, and I have convinced myself that they are no better than what I have...
Back to the project. The recording monitor would be a great project. It would save having to buy separate amps (which could cost hundreds alone), and allow good adjustability to obtain a smooth frequency output. While 50W per speaker isn't a ton of power, I have a feeling that under near field use, they would perform great.
Computer speakers - This might be a better use for these speakers. The bi-amping here really isn't necessary, except to offer more power. The unfortunate thing is that these amps aren't particularly tiny, so either the speakers themselves would have to be rather large, or the amp would have to be located elsewhere - in a subwoofer cabinet, for example.