Mission 753 speaker modding

[justblair]

Right I measured the DCR of one of the coils in the bass circuit as Peter described.

I'm getting 1.0ohm-0.4 ohm=0.6ohm

My calculations (and online inductor calculations as well) are telling me I should be getting somewhere in this region if not slightly less (0.5ohm approx) though I guess my own coli winding skills will effect this.

On this basis am I right in saying that the air core should have no ill effect on the flabbiness of the bass?

[petercom]

I've just realised that the crossover version you have has a resistor in series with the bass section! So you don't have to worry. Just wind your air cores and providing the DCR comes to less than 1.6 Ohms each you can reduce the 2.2R resistor accordingly (2.2R + 0.6 + 0.6 = 3.4 Ohms in the current circuit).

You'll just have to be a bit more careful with matching the DCR of the midrange coil reasonably well.

BTW a 2.45mH air core is probably going to be a big beast. Bear in mind that the magnetic field from it is going to spread a long, long way. Best to house these outside the box and play around with the positioning and keep them well away from the midrange crossover before re-housing them!

[justblair]

Originally Posted by petercom

I've just realised that the crossover version you have has a resistor in series with the bass section! So you don't have to worry. Just wind your air cores and providing the DCR comes to less than 1.6 Ohms each you can reduce the 2.2R resistor accordingly (2.2R + 0.6 + 0.6 = 3.4 Ohms in the current circuit).

You'll just have to be a bit more careful with matching the DCR of the midrange coil reasonably well.

BTW a 2.45mH air core is probably going to be a big beast. Bear in mind that the magnetic field from it is going to spread a long, long way. Best to house these outside the box and play around with the positioning and keep them well away from the midrange crossover before re-housing them!

Thanks Peter, I was wondering about why the inductors DCR might have a different effect on the signal than a resistor. I assumed/guessed that there was a relationship going on between the two in the coil that I had not stumbled upon in my research.

I made my first attempt last night at winding a coil.... Not pretty at all. It was meant to be a 2.45, but it came out as 2. I am going to reclaim the copper and try again this time making a rig to help. Maintaining coil density is a real pig to do over so many winds! I have a callous on my thumb! And yes it was huge.

My intention is to split the crossover in the missions, mounting the three as far away from each other as possible, probably the bass on the floor of the speaker, the mid on the upper rear of the lower chamber and the high on the roof of the top chamber. I can organise the orientation of the coils to avoid interference.

Do you happen to know if I should be avoiding proximity to the speaker magnets as well? I am guessing that the speaker motors and baskets will block and stray interference from the air coils reaching the speaker, but is there the possibility of influence in the other direction?

[Ianm2]

Is this simply a straight 2 way design with each pair of bass drivers run in parallel?

Do you know the differences between that and the 753F version, I was quite partial to the latter?

Actually, looking at your original xover?, it seems as its a proper 3 way, there is a rather steep 3rd order electrical on the bass, and the mid is rolled in and rolled out, wheras mission claim its a 2 way...hmm strange I thikn mission claim something like 2 way sealed? with bass assist...strange way to describe it.

anyway, still looking for info on the freedoms, can't quite remember them

[justblair]

Originally Posted by Ianm2

Is this simply a straight 2 way design with each pair of bass drivers run in parallel?

Do you know the differences between that and the 753F version, I was quite partial to the latter?

Actually, looking at your original xover?, it seems as its a proper 3 way, there is a rather steep 3rd order electrical on the bass, and the mid is rolled in and rolled out, wheras mission claim its a 2 way...hmm strange I thikn mission claim something like 2 way sealed? with bass assist...strange way to describe it.

anyway, still looking for info on the freedoms, can't quite remember them

I cant tell you anything about the internals on the freedoms, but the difference on the exteriour was the freedoms had a soft dome tweeter rather than the metal one in my pictures. I was selling HI Fi at the time of the changeover. The sound from the freedoms was as you can imagine less harsh at the top end. I found the freedoms to be a little soft at the time. Funny enough I bought mine a couple of years after they were replaced in the Mission Range. They were store demo models that were being sold off. I snapped them up at a very low price.

I have seen this crossover described as 2.5 way in reviews of the time, but I am learning as I go on crossovers, it looks like 3 to me. My guess is that Mision call it 2 way as that is the number of inputs in.

I intend eventually to split the lower frequency and tri wire the speaker with another set of lm3875 chips. Next time I am ordering from Farnell I will get what I need (Basicly just resistors that will match the feedback to my other 4 amps) Though I reckon my power supplies will run out of oomph running 6 chips. I may need to buy more SMPS units.

[Ianm2]

It looks like 3 to me, as well, 2/1/2 means that one rolls off early and the other goes all the way up, but doesn't roll in like a true mid electrically, missions can be weird tho', I think there may be a sealed box loading the 'mids' too. 2.5 way usually saves cost and hassle of designing a 3 way xover, but your design is far from simple, makes you wonder a little...

they are officaly described as 2 way with bass assist

remember xover terminals on the rear tell you nothing about the 'ways' of a speaker, tbh, I think they should have the same no of terminals as drive units, or just 1 pair, I don't believe in biwiring myself, hence the one, but do in actively running, hence multiple, but that would give the makers nitemares imagining what we are doing to their designs personally, I like to keep them as the maker intended them, apart from blatant cost cutting

[justblair]

Originally Posted by Ianm2

It looks like 3 to me, as well, 2/1/2 means that one rolls off early and the other goes all the way up, but doesn't roll in like a true mid electrically, missions can be weird tho', I think there may be a sealed box loading the 'mids' too. 2.5 way usually saves cost and hassle of designing a 3 way xover, but your design is far from simple, makes you wonder a little...

There is a sealed box on the mids. (I'm pretty certain the freedom models were identical cabinate wise.)

I'm wondering wether they went 2.5 way on the freedoms, perhaps to save costs, perhaps to change bass response in line with fashions of the time. The 753 had a reputation for being light sounding, and the competition were making tl speakers that went real low on the bass for less money.

The freedoms certainly had their fans. But I was not one of them, The tweeters were not bright enough for my liking. I would have prefered it if they had retained some of the brightness and gone for a smoother sounding design. I seem to be getting there with mine. They are sounding a lot better at the top end now.

Originally Posted by Ianm2

I personally, I like to keep them as the maker intended them, apart from blatant cost cutting

Hopefully you approve then of my mods. Though I have not spent a fortune, I have certainly replaced the cheap components with mid range. And I am getting a small but very enjoyable lift in quality from this. I would say so far the speakers are more tunefull than before.

[Ianm2]

well, you don't need my approval I hope about the mods, that's your choice, and prerogative.

I agree about the treble of the freedom, it was a bit light and pulled down, bass too, not as deep as expected, but out of a few 5 inchers, you couldn't really expect so and alvin gold I think it was said so, back in the 90s, I found I heard everything he heard, all his choice reviews were spot on imo, too,

they use audax drivers, which were faves of wad, too and I was impressed when I first took them apart with the weight of the drive units

there was an odd arrangement of the drive units, the mids were in series and the bass in parallel, slightly different, no means a cheap implementation

nice speaker, thick heavy cabinets, I think the freedoms would sing actively driven, you could easily adjust levels fed to each driver that way.

that was missions heyday, the 75 series imo, stunning build

[petercom]

Originally Posted by Ianm2

they use audax drivers,

That's news to me!

Originally Posted by Ianm2

there was an odd arrangement of the drive units, the mids were in series and the bass in parallel, slightly different, no means a cheap implementation

All in all I seem to remember uncovering evidence of up to 15 different crossover arrangements for the various implementations of 753. I expect that, when they were brought out, 2-way speakers were the rage so describing them as "2-way with bass assist" is a bit of marketing speak for 2.5 way. Although the mids are in a separate chamber they still assist with all but the lowest bass frequencies.

Originally Posted by Ianm2

nice speaker, thick heavy cabinets, ...
that was missions heyday, the 75 series imo, stunning build

A whole section of Mission's cabinet plant was given up to 75 series, including a fully automated spray and lacquer oven section. The cabinet complexity and build quality went up a notch with the 78 series.

[Ianm2]

yes I seem to recall they were audax, certainly the bass units, and audax championed all those differing materials, tpx, aerogel, carbons, glass, and ceramics, very interesting, I had to get a metal treble for original 752s, that could have been seas, may be wrong about audax's, but seem to recall so.

not a fan of metal domes, but have heard a seas in the harbeths, poss the 752s too, and that was fine for the breed, seas seem rather good imo....???

difficult to imagine any cabinets bettering the 75 series, 753 and 754 astonishing

that's rather depressing news about so many xovers, it makes you wonder a few things....

which one is 'right'

why couldn't they get it right?

did they vary in production?

is it continuous development and improvement, I like to think something would be released as good as it can be, I am always suspicious of tinkering and improving, and for a consumer, changes are disheartening, as you buy something and find its changed, improved and yours is worse...however there is always the argument that things can be improved on, I am not sure which is right, being a perfectionist, perhaps i naively assume it should be right first time? that may not be possible tho'

perhaps another upgrade...send them to peter to rehash the xover?