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Hi Everyone, I'm showing my ignorance here.... In a typical L-R 12 db crossover at 1000HZ and 8 ohms the values are about 10uf and 2.5mh. What happens if you use other combinations.... like say 10uf and 2 mh or 2.5mh and 15uf. What does this do the response. My guess is that it would give something other than 12 db / octave (a different slope) and probably have phase implications.
That's right, exactly. Manipulation of the values (or the load) changes the transfer function and phase. You can slightly overdamp or underdamp the filter to provide a little bit of a peak or dip at the crossover frequency, and do other little adjustments like that. I don't know of any optimized crossover that uses textbook values because it is useful to dial-in the transfer function and phase by modifying the values a little bit. You can do this by trial-and-error (solder/measure) or you can use Spice to model the circuit, to get it close first. I prefer a combined approach, possible with the Smith & Larson WTPro system. It allows me to essentially build an active crossover using properties configured by a Spice model. I can change crossover values just by typing them into the Spice model. Once the crossover is perfected, I can build a physical model and it measures exactly like the simulated one did. It's really a very useful tool. All you have to do is to measure the drivers impedance, and type up a Spice (nodelist) model. Spice is kind of cumbersome at first, I suppose, but I've been doing it so long it seems very natural to me. But crossovers are pretty simple circuits, so they're easy to model in Spice. You may already have experience with it modeling tube amps, and if so, this is cake to you. Once you have the Spice model and the driver impedance measurements, you run the ICD in WTPro. It sends test signals out to the right and left outputs, which you run through amps to the woofer and tweeter. Set the microphone out in front of the speaker, wherever you want to measure from. The WTPro becomes active crossover and test platform, providing acoustic measurements of the loudspeaker using the crossover defined in the Spice model. Here's a writeup of how I do that when I design a loudspeaker. There's even a video showing me testing for the position of the vertical nulls, which I set by adjusting the crossover values in my Spice model. I can get them to be precisely where I want them to be using this approach. Crossover optimization for DI-matched two-way speakers When the loudspeaker design is fully optimized, this is an example of the (amplitude and phase) response I get: four π loudspeaker amplitude and phase response
Hi Wayne, thanks it confirms what I suspected. I got the H290Cs and put them in my enclosures and that did a lot to clean up the sound. Then I started to work on getting the cross over from the woofer right. I finally settled on a L-R 12 db/oct at 1600 from it. Still wasn't quite right. So I started to investigate as to why ... that prompted the posting. But since then I discovered that the spread sheet in the LD7 program was not computing the Zobel network correctly. It gave a value of 112 for the capacitor and even the formula in the book itself came up with 175. Oops. I checked with two other on line calculators, all said 175uf. Plugged that into the crossover and now things are rather nice overall. I'll run some tests and sweeps probably tomorrow (birthday party today - me 66 on Monday). I'll look for funky stuff in the 1000-2000 range where the drivers crossover. I had already determined that they were OK outside that range.
Happy Birthday, Bruce!