27-05-2020, 10:26 AM
It's difficult to know what to advise in this situation.
The crossover circuit is designed around the original drive units. If you change a drive unit, you usually need to change the crossover.
Here, both drive units have been changed.
Also, a box and port tuning is specific to the bass drive unit. A different bass drive unit will have different TS parameters, so will require a different sized box, and a different port. Of course, there is some latitude here.
Really, the enclosure is the easy part.
Start by downloading something like WinISD: http://www.linearteam.org/
Your Vifa bass driver is likely in the supplied database, but if not, you can enter a new driver yourself. The datasheet is here: http://www.sea.vg/vifa/M21WJ-09-08.pdf
Once you've got the driver data, WinISD will suggest the optimum box size and port tuning.
Having done that, measure the inside dimensions of your cabinets to determine the volume. In theory you should knock off a bit to allow for the space occupied by the drivers and crossover, but in practice it's not critical on a larger box like this one. You can then create a new project using the same driver, but change the box volume to yours - leave the first optimum one on the screen to see the differences.
Finally, measure the ports, enter those into WinISD, and that will show you the effect of the existing port. You can play around with the port tuning (and box volume) in real time by just clicking and dragging, which is really interesting and helps to give you a better grasp of how everything interacts.
So, if the port needs changing, WinISD has sorted that for you. But the crossover is the hard thing.
The thing is, the crossover is not just a pair of filters. It is also an equaliser. At the very least, it is doing baffle-step correction, although not a huge amount if the value of the woofer inductor is correct. Other tweaks might be included to compensate for drive unit "funnies". This crossover is pretty simple, which implied either the original drive units were fairly well behaved, or they didn't care about aberrations.
If the replacement drive units are at least as well-behaved as the originals within the range they're being used, and have similar impedances, then they might be OK. But the crossover frequency would have been determined by the behaviour of the original woofer - the balancing act is tricky: as high as possible to protect the tweeter, but as low as possible to get the best power response (off-axis response). An 8" driver becomes very directional above 2kHz as it "beams" - if you take a look at the datasheet, you can see exactly what I mean. Whether the original driver was the same is hard to predict, the Vifa is coated paper whereas the original was polypropylene, I believe, and the two materials can behave differently in the midrange.
With all that said, it's likely that you just need to reduce the tweeter level. Certainly, I'd try that first, because it's easy. If it doesn't give you the results you need, then you'll either have to invest in some measuring equipment, or admit defeat! Without measuring gear, you're basically guessing. There are DIYers who claim to be able to design loudspeakers using nothing but their ears - unfortunately, they're delusional.
Unfortunately, reducing tweeter level is not as simple as just increasing the value of the 1.5 ohm resistor.
The HF filter was designed for a certain value of impedance. If you up the resistor, you up this impedance, and change the frequency. So you need another resistor to bring this back down. Obviously, a designer will do all they can to eliminate components, so having determined how much attenuation a given tweeter needs, they'll add the series resistor then design the filter to suit that - that's why there's only one here.
You'll have to experiment with this, I'm afraid. Without measurements, I don't know how much more efficient the new tweeter is, so can't begin to guess at how much we need to knock off. I recommend disconnecting one end of the 1.5 ohm resistor, and bringing 3 wires out of the enclosure via the bass reflex port - one connected to ground, the other to resistor, and the other to the tweeter. Then get a bunch of resistors and some croc-clips, and experiment. Start by putting 10 ohms in series with the tweeter, which should knock it down significantly, but might cause a midrange bump - perhaps try another 10 ohm resistor between ground and the 1.5 ohm resistor (basically on the filter side of the attenuator). I wouldn't go much lower than 10 ohms here, BTW. For experimentation, any old resistors will do - even low-power ones - but once you've decided on the values, I'd recommend using wire-wound types of at least 5W power rating.
Alternatively, you can buy a variable L-pad, but they can be pricey for what they are.
Work on one speaker at a time, and have a reference speaker on the other channel of your amplifier. Play back in mono, and swing the balance control between the Mission and your reference. You need to frequently "reset" your hearing when playing this game
Sorry if all this seems overly complicated. Speakers are complicated. People underestimate just how complicated because superficially, they appear to be very simple. Especially crossovers. It can take weeks or months to optimise a crossover - you usually start out with a complicated circuit and slowly distil it down until you have something that is so finely balanced that even small changes can totally change things - just changing an electrolytic cap for a "better" film type can make things sound worse. Sadly, the internet is full of "experts" telling people to re-cap everything they own - that is not always good advice, sadly.
Good luck!
Mark
The crossover circuit is designed around the original drive units. If you change a drive unit, you usually need to change the crossover.
Here, both drive units have been changed.
Also, a box and port tuning is specific to the bass drive unit. A different bass drive unit will have different TS parameters, so will require a different sized box, and a different port. Of course, there is some latitude here.
Really, the enclosure is the easy part.
Start by downloading something like WinISD: http://www.linearteam.org/
Your Vifa bass driver is likely in the supplied database, but if not, you can enter a new driver yourself. The datasheet is here: http://www.sea.vg/vifa/M21WJ-09-08.pdf
Once you've got the driver data, WinISD will suggest the optimum box size and port tuning.
Having done that, measure the inside dimensions of your cabinets to determine the volume. In theory you should knock off a bit to allow for the space occupied by the drivers and crossover, but in practice it's not critical on a larger box like this one. You can then create a new project using the same driver, but change the box volume to yours - leave the first optimum one on the screen to see the differences.
Finally, measure the ports, enter those into WinISD, and that will show you the effect of the existing port. You can play around with the port tuning (and box volume) in real time by just clicking and dragging, which is really interesting and helps to give you a better grasp of how everything interacts.
So, if the port needs changing, WinISD has sorted that for you. But the crossover is the hard thing.
The thing is, the crossover is not just a pair of filters. It is also an equaliser. At the very least, it is doing baffle-step correction, although not a huge amount if the value of the woofer inductor is correct. Other tweaks might be included to compensate for drive unit "funnies". This crossover is pretty simple, which implied either the original drive units were fairly well behaved, or they didn't care about aberrations.
If the replacement drive units are at least as well-behaved as the originals within the range they're being used, and have similar impedances, then they might be OK. But the crossover frequency would have been determined by the behaviour of the original woofer - the balancing act is tricky: as high as possible to protect the tweeter, but as low as possible to get the best power response (off-axis response). An 8" driver becomes very directional above 2kHz as it "beams" - if you take a look at the datasheet, you can see exactly what I mean. Whether the original driver was the same is hard to predict, the Vifa is coated paper whereas the original was polypropylene, I believe, and the two materials can behave differently in the midrange.
With all that said, it's likely that you just need to reduce the tweeter level. Certainly, I'd try that first, because it's easy. If it doesn't give you the results you need, then you'll either have to invest in some measuring equipment, or admit defeat! Without measuring gear, you're basically guessing. There are DIYers who claim to be able to design loudspeakers using nothing but their ears - unfortunately, they're delusional.
Unfortunately, reducing tweeter level is not as simple as just increasing the value of the 1.5 ohm resistor.
The HF filter was designed for a certain value of impedance. If you up the resistor, you up this impedance, and change the frequency. So you need another resistor to bring this back down. Obviously, a designer will do all they can to eliminate components, so having determined how much attenuation a given tweeter needs, they'll add the series resistor then design the filter to suit that - that's why there's only one here.
You'll have to experiment with this, I'm afraid. Without measurements, I don't know how much more efficient the new tweeter is, so can't begin to guess at how much we need to knock off. I recommend disconnecting one end of the 1.5 ohm resistor, and bringing 3 wires out of the enclosure via the bass reflex port - one connected to ground, the other to resistor, and the other to the tweeter. Then get a bunch of resistors and some croc-clips, and experiment. Start by putting 10 ohms in series with the tweeter, which should knock it down significantly, but might cause a midrange bump - perhaps try another 10 ohm resistor between ground and the 1.5 ohm resistor (basically on the filter side of the attenuator). I wouldn't go much lower than 10 ohms here, BTW. For experimentation, any old resistors will do - even low-power ones - but once you've decided on the values, I'd recommend using wire-wound types of at least 5W power rating.
Alternatively, you can buy a variable L-pad, but they can be pricey for what they are.
Work on one speaker at a time, and have a reference speaker on the other channel of your amplifier. Play back in mono, and swing the balance control between the Mission and your reference. You need to frequently "reset" your hearing when playing this game
Sorry if all this seems overly complicated. Speakers are complicated. People underestimate just how complicated because superficially, they appear to be very simple. Especially crossovers. It can take weeks or months to optimise a crossover - you usually start out with a complicated circuit and slowly distil it down until you have something that is so finely balanced that even small changes can totally change things - just changing an electrolytic cap for a "better" film type can make things sound worse. Sadly, the internet is full of "experts" telling people to re-cap everything they own - that is not always good advice, sadly.
Good luck!
Mark
