Mission 700 LE Speakers - Rebuild/upgrade

[ubergeeknz]

So my faithful Mission 700 Leading Edge speakers have a bit of foam rot going on, time to refoam. While I'm in there I thought why not upgrade/refresh the factory crossovers a bit.

The crossover is very simple ; a 670uH ferrite core inductor in series with the woofer, and a second order filter on the tweeter consisting of a 1 ohm ceramic resistor, 3.3uF and 2.2uF Bipolar in paralel to make 5.5uF, and a 160uH air core across the tweeter. It's all held together with hot glue onto the terminals, which are simple spring-clamp type.

I'm intending to replace the ferrite core inductor with a homemade air core inductor, replace the Bipolar electroytics with polypropylene, and 3d print a new terminal block which will use gold plated brass binding posts. I'll put the crossover on a custom PCB. I've also got some heavy foam material which I can stick to the panels, probably enough for the sides only.

I do want to ask what people think about the following

Should I go all out and add 2 sets of terminals to allow for bi-wire/bi-amp? I don't have such a set up presently but I'm interested to see what people think.

Should I try to brace the box any? These are pretty small and the chipboard isn't too bad resonance wise but I wonder if a simple brace on the sides will improve matters slightly?

Where's the best place/way to install the crossover board?

Any other suggestions while I have these apart?

Thanks in advance! And yes, I'll post some photos of progress/before and after etc.

 

[ubergeeknz]

Some before pics. The gaskets are all pretty stuffed, I'll need to make new ones.

 

[ubergeeknz]

I already got these 3u3 caps because that's what I could see when I had a quick look last time. So I'll be using them with some matching 2u2 caps. I've also done a preliminary board layout. I'm using 2 x 2R because RS, who worked out cheapest for the magnet wire (by a good margin), only had 3W metal film resistors. The original was rated 5W.

I'll be using 18GA wire for the new inductor, on a 3d printed former.

 

[leesonic]

I think I would be inclined to stick with the stock inductors, and use some better caps. You could use a terminal cup with four posts if you fancy some bi-wiring, this might also give some room to glue the components to the back of it. Failing that, you could mount them onto a piece of wood, and screw it to the back panel.

Lee.

 

[ubergeeknz]

I was going to leave the tweeter bypass inductor alone, as it seems fine.

The woofer inductor just seems like it could be improved on, however. It's not even 20GA wire and I understood that ferrite core inductors can saturate and cause distortion, but I'm not sure how much of an issue this is in practice.

As for caps, what would you recommend that's still budget friendly?

 

[leesonic]

You would probably have to be pushing some serious volume through these to get the ferrite core to saturate. Going from iron cored to air cored is tricky, because you don't want to introduce too much resistance into the system, as this would make the woofer quieter.

I've always used Erse Audio caps in my restorations.

 

[ubergeeknz]

Thanks for the advice, of course I've already ordered the parts now.

But it's got me thinking I should actually test the inductor and see how it behaves. Could be informative.

Might test the caps too. They ought to perform well, according to the datasheet at least, since they have a low ESR and low ESL.

 

[ubergeeknz]

Reworked the PCB design for Bi-wire/Bi-amp operation, and beefed things up a bit.

 

[ubergeeknz]

Designed terminal cups and printed a rough proto for test fit. For some reason it's horribly underextruded, and of course the finished item will be in black.

Undecided if I will go to the additional trouble of printing in ABS, I've found PLA to be fine for most applications where there isn't too much heat or bending stress involved.

 

[ubergeeknz]

These aren't the binding posts I plan to use, but similar. It's a tight squeeze to get 4 in. I may need to go deeper and reduce the angle, which will make wire binding tricky, find different shorter binding posts, or ... Gasp... Make a bigger hole (not keen on that last one tbh)

 

[homebrew]

I wouldn't worry about bi-wiring the Missions. Kind of like putting lipstick on a pig. As far as the inductors, leave them as is. Iron core inductors can saturate with enough power but it's unlikely you'd be able to saturate the inductors without incurring other damage to the drivers. Just upgrade the caps and replace the sandcast tweeter resistor with a wire wound like Eagle, Lynx, or Mills (if you can find them). It wouldn't hurt to mount the crossovers on separate boards. It would also be a good idea to use a 3/4" diameter dowel as a brace, mounting it from side to side,

 

[ubergeeknz]

Starting to come to the same conclusion on the biwire. It seemed simple enough as a concept but in practice it's turning out to be more effort than it's worth especially with the small opening for the terminal cup. I can make it work but I will need to order smaller terminals, it will be fiddly to use, and I really don't want to cut the back to fit larger terminals.

For resistors I have Vishay metal film on the way, 2 x 2 ohm 3W in parallel, I think it should do the job.

I am keen on winding an inductor, and doing some experiments on the bench to see how the original small ferrite rod inductor performs next to an air core, even though it may be unnecessary. I've not wound a coil since my obligatory childhood crystal radio kit (just a one layer on cardboard tube) and buzzer for a "steady hand" toy, and it's a skill/technique I'd like to try out for other projects down the line.

As for bracing, that's probably the ticket, as it has to be done through the woofer hole, I am sure I have some suitable dowel, maybe a bit smaller, I can always put in 2 of them.

Thanks for the input, much appreciated.

 

[homebrew]

I used to wind my own inductors but I found it's cheaper and easier to buy them and unwind if necessary. In addition, perfect lay inductors do not have the internal resonances that the inductors wound in a more haphazard manner have.

Regarding speaker wire. Want to avoid the fiddling with wrapping bare wires around the binding posts? Use banana jacks.

 

[ubergeeknz]

I intend to use banana jacks But still it's handy to have the option.

The factory inductor is anything but carefully wound by the look of it. I'm sure I can't do worse...

 

[homebrew]

Just make sure it's the correct inductance AND as close to the original resistance as possible. That usually means 14 ga. or smaller wire.

 

[theDG]

Hi, on my pair the capacitor s are 3.3uf and 4.7uf?

 

[theDG]

I think they must have changed the crossover on later(?) units, also inductors: 832uH & 320uH

 

[RCA Plugger]

I think they must have changed the crossover on later(?) units, also inductors: 832uH & 320uH

That is like the scheme that is on Hifiengine. Different crossover it seems. Maybe it is a regional thing. The woofer is also second order on there and the tweeter has only one cap (the 3.3). The 4.7 goes to ground on the woofer.

 

[innkeeper]

Hi, on my pair the capacitor s are 3.3uf and 4.7uf?

whats your derial number(s)?
mine:
serial number 5535 - 2.2uf + 3.3 uf design - btw my inductors measured 621uh and 160uh

maybe we can figure out by serial numbers when they changed from a first order and second order crossover to 2 second order filters.
or confirm the regional theory.

 

[RCA Plugger]

I have a serial 008987 2,2 3.3 uf design an 043573 with de 3.3/4.7 uf design, both made in England and bought in the Netherlands.