The Against Protocol 3-Way, or AP3 for short, is a small and slim, bookshelf mounted 3-way that can play down to 40 Hz with low midrange and treble distortion.
Initially this project started with a desire to use the overachieving Vifa TC9 in a build. It is a great little driver with an easy to work with response and very nice distortion performance all at the low low cost of $12. I picked up a pair of the Vifa XT25SC90 a little bit later on. So then it came down to what woofer to use to make a nice 3-way. I debated for a while on what I wanted to do. I didn't really want to go with something pricey, since the rest of the build was so cheap. I also wanted something that could be stand-mounted to use in my bedroom (the eventual home of these). I looked around quite a bit and ended up deciding on another overachiever, the Dayton ND105-4.
lot of people tend to urge against going with small woofers for a 3-way, saying that you should either go with a good size woofer or just pick a pricier woofer for a 2-way due to costs. I definitely understand that logic, but here was my rational: The midrange on the TC9 is great and it is super cheap, but it definitely can't be used too low. The ND105 has crazy output potential for a 4" woofer due to its extremely large xmech and it plays down to 40hz in a roughly 7.5 liter cab. To get the combination of low extending bass and the midrange performance of the two, I would have had to go with a much more expensive, and probably larger driver, and a much larger cab in most cases. So going against the conventional wisdom on this, I decided to name them the Against Protocol 3-way, or AP3 for short. Total cost of the project using all poly caps except for the two largest values can be had for just under $200 in parts. My total cost was about $150 since I got the ND105 on sale and the other drivers used.
Paul Carmody mentioned in a thread on the Tech Talk message board that during a design he was working on, he had found it was difficult to work with small woofers in a 3-way due to the higher Fs interfering with the crossover between the mid and woofer. I noticed this problem as well, and it took a bit of finagling to get it to play nice. The crossover ended up fairly straight forward in the end. Second order electrical on the woofer LP and on the mid HP and LP, and third order on the tweeter.
These were voiced in a rush and I messed up bit when I modeled these and ended up with the tweeter a bit hot, so you probably want to start with a larger resistor in the tweeter circuit and see what you like. In my bedroom, with a large down comforter and carpeted floors, using them mainly for TV watching, they sounded fine. I've demoed them at two events with this crossover and gotten good feedback but that is also in quick listening sessions. I have noticed in some settings or comparing back to back with a smoother measuring pair of speakers I've built, you can hear a bit of edginess sometimes in the lower treble.
Measurements show the real benefits of using this setup. Upper midrange through treble distortion is extremely low for a speaker using less than $200 in parts. You can see here how the treble region beyond about 4.5 kHz gets a bit hot. If you listen off-axis, a lot of this will be minimized, as the XT series ring radiators fall off rather quickly off-axis.
Construction on this was a bit weird because both woofers have frames that are terrible for flush mounting. I wanted to rear mount them, but the round over on the front of the baffle made it difficult to screw the drivers in from behind without the screws coming through the front of the baffle. Instead, I ended up using a back plate to basically clamp the drivers into place.