Mission 753 speaker modding

[petercom]

Originally Posted by justblair

I am interested in your suggestion here Peter. I thought that since it was out of the signal path this would not be so critical. Hence I ordered a nichicon muse (ES I think). Can you tell me more as to why this cap is important?

In a speaker crossover all components are in the signal path in one way or another. For example the 10uF helps to form the crossover slope (in simple terms it bypasses the driver, so if its ESR is rising with frequency the driver will get more high frequency 'hash' than is desirable). Reducing its ESR should reduce treble harshness.

Originally Posted by justblair

I had planned to upgrade the coffin resistors. I think I may have found my choice. Do wirewounds not have their own inductance? Is this in the slightest bit relevant, or too small a value to worry about? .

The inductance of the wirewounds we supply is in the fraction of a microHenry range. So it might trouble you if you were using them for RF, or had a treble unit going up to 100KHz, but not audio frequencies.

Originally Posted by justblair

mmmmmm. I know you are talking sense here.. My curiousity is burning me, I have to admit to being influenced by planet10's site. I suspect that aesthetically the idea of solid wood phase plugs (in rosewood of course) is what appeals the most (with felt plugs on the bass driver). STUPID I KNOW! I really must try and stop thinking about this.

As for the stiffening of the cone, I am guessing that is why those that do this on fostex cones treat the cone with puzzlecoat? The 1db drop in sensitivity doesn't strike me as being a problem, I'm going to bet that I am being niave?

The crossovers AFAIK include resistors in the signal path to attenuate the different sensitivities of each section. Can the values not be adjusted to counter this?

The cost of failure on modding the drivers will probably put me off. I do wish that someone had pioneered here so I could make a more informed decision.

The reason I said it is because I actually have done that. When I joined Mission there was talk of 'upgrading' the 753. I fitted the bass units with phase plugs and the MF response went haywire.

Don't try and draw parallels with Full Range Units. These have a different set of characteristics - whizzer cones and the like - and the cone shape is designed for gradual breakup, not stiffness.

As for sensitivity the bass and mid units have no series attenuation, only the treble unit does. You could, I suppose, increase the treble attenuation to make up for the loss of MF, but you may have to adjust the crossover as well to take account of the change in response of the drive units.

I just don't think it's worth doing.

[Scottmoose]

FWIW (not much), I'm with Peter all the way on these. Particularly in re phase-plugs. Properly designed plugs, like Dave's, can have a dramatic beneficial effect on an FR unit, especially those with whizzers etc., killing the resonance of the hollow pole-piece, and smoothing the midrange / treble, but conceptually such units are a world apart from a mid-bass driver, which have an entirely different set of design goals. The fact that he's tried it on the 753 himself & confirms it wrecked the response underlines the fact.

If memory serves, the 753s bass response wasn't much to write home about -I'd save the money & put it toward a pair of good TL loaded woofers, if you haven't already got some help down there.

[justblair]

Originally Posted by petercom

In a speaker crossover all components are in the signal path in one way or another. For example the 10uF helps to form the crossover slope (in simple terms it bypasses the driver, so if its ESR is rising with frequency the driver will get more high frequency 'hash' than is desirable). Reducing its ESR should reduce treble harshness.

Thanks for the explanation. I will give this a try in the near future

Originally Posted by petercom

The inductance of the wirewounds we supply is in the fraction of a microHenry range. So it might trouble you if you were using them for RF, or had a treble unit going up to 100KHz, but not audio frequencies.

NP Thats good to know, I'll order a few values soon

Originally Posted by petercom

The reason I said it is because I actually have done that. When I joined Mission there was talk of 'upgrading' the 753. I fitted the bass units with phase plugs and the MF response went haywire.

Don't try and draw parallels with Full Range Units. These have a different set of characteristics - whizzer cones and the like - and the cone shape is designed for gradual breakup, not stiffness.

As for sensitivity the bass and mid units have no series attenuation, only the treble unit does. You could, I suppose, increase the treble attenuation to make up for the loss of MF, but you may have to adjust the crossover as well to take account of the change in response of the drive units.

I just don't think it's worth doing.

That is just the sort of advice I needed. I dont think I want to get into a redesign of the crossover. I will put this thought behind me... Though it would have looked good.

Thanks Scottmoose for joining in. A sub may be on the cards in the future, though my next target is a new DAC. I am running an origional Dacmagic from Cambridge Audio. I want a god DIY usb dac.

My 6.8uf caps arrived today. Tonight though is gf night. I will try them tomorrow.

Also I received 2kg of 1.25mm mag wire. Is 0.5ohm DCR (per inductor) going to produce that flabby bass that Peter thinks may be produced? It will be a while till I have time to try it out.

[planet10]

The only comments i'd add to Peter's are:

1/ it is a rare commercial box that can't benefit from some extra bracing.

2/ a rule-of-thumb for bypassing the big caps (if you can't afford to replace them completely with poly) is 10% bypass, and then bypass, the bypass with 10%. The caps in the XO are really monsters (500 uF & 230 uF) and when values get that large one starts to think about an active XO (6 200-some cheap Solens would set you back on the order of $300 CAD)

By the time you start thinking of going all-out thou it is time to start thinking of just starting over from scratch.

dave

PS: are the surrounds on the midbasses foam?

[justblair]

Originally Posted by planet10

1/ it is a rare commercial box that can't benefit from some extra bracing.

The bracing looks pretty solid to my newb eyes. The cabinate is split in to two chambers with a thick mdf brace in each. There are bitumen pads bonded in and some green foam stuff at the back of each chamber.

Originally Posted by planet10

2/ a rule-of-thumb for bypassing the big caps (if you can't afford to replace them completely with poly) is 10% bypass, and then bypass, the bypass with 10%. The caps in the XO are really monsters (500 uF & 230 uF) and when values get that large one starts to think about an active XO (6 200-some cheap Solens would set you back on the order of $300 CAD)

I can try bypassing the mids with the 6.8uf caps I have in my grubby mitts just now before I use them on the tweeter.

I may in the end go active on the bass part of the crossover. But other stuff in the system I think will give me higher returns in the short term

Originally Posted by planet10

By the time you start thinking of going all-out thou it is time to start thinking of just starting over from scratch.

dave

I do fancy building my own speakers, but have no experiance so far. My Gf is interested in a better system, so I might start with a simple design for her and then try a more complicated design for myself. The 753's are very good to my ear, I am uncertain as to what level or design would give me a significant improvement.

Originally Posted by planet10

PS: are the surrounds on the midbasses foam?

The surrounds on the mids and bass are rubber the four drives are identical.

[planet10]

Originally Posted by justblair

I do fancy building my own speakers, but have no experiance so far. My Gf is interested in a better system, so I might start with a simple design for her and then try a more complicated design for myself. The 753's are very good to my ear, I am uncertain as to what level or design would give me a significant improvement.

Probably less than you think would be required...

dave

[Scottmoose]

Agreed. I wouldn't spend too much money on the 753s. They were decent mid-level speakers in their day, and with that array of drivers, they certainly looked impressive (still do), but it won't be difficult to better them for relatively little cash or effort. The WD kits for example would be a huge step forward. You'd be surprised just what gains you can get -speakers are usually the weak link in most systems. Assuming the source & amplification are adequte, IMO you'll get greater benefits uprading the speakers than spending the same money on the source or amps. You need to make sure you've got a synergistic match of course -partnering a bright amp / source (whatever the price) to a pair of forward-balanced speakers is just looking for trouble. But that's not really a price issue.

[justblair]

My Sonicaps arrived and I had the chance to try them out this weekend.

The first thing I noticed upon trying the new caps was that the tweeter gained some volume. This was not good to the overall sound. I could certainly hear more detail.

Fortunately the solution was pretty simple. The tweeter has two 1ohm resistors in parrallel to attenuate the volume. Removing 1 resistor took the resistance from 0.5 ohm to 1ohm. This brought the tweeter volume more or less back in line with the rest of the drivers.

The sonicap modded crossover gives me a bit more detail, and a smoother sound.

I then swapped out the caps in the other speaker.

I am hearing a bit more detail from the music than before, the top end is definately smoother, more liquid and I am getting a more natural sound overall. There is more "air" in the sound

The best difference is the hardest to discern but has a very influencial effect on my enjoyment of the music. With the sonicaps there is something I can only describe as sparkle in the top frequencies. I hear it most on high hats, particularly in Jazz recordings where the drummer uses a brush on them. The resonance in the metal is now present. I can hear the hats ring.

The effect this has is to make the music more engaging to listen to. It is more vibrant in the top end and I would say more enjoyable.

I will make it clear, this mod does not make a night and day difference to the speaker, but makes a subtle but definate improvement to the sound.

As I have the wire now, I am still more than likely going to try making some air core coils. If I am right in the way i am measuring the existing coils,(Getting around 5ohm when I connect a multimeter accross them) My calculated DCR is less than this. Does this mean that I might get less flabbiness in the bass?

I just know that I am wrong here, but I dont know why. Its great being a noob!

Am I right in saying that DCR is the figure that you get with a std multimeter measuring resistance?

[petercom]

It sounds as though you are measuring across the coils in circuit and your multimeter is 'seeing' the impedance of the bass or midrange units.

To measure the DCR of the coils accurately you will have to remove them from the crossover. Then check the resistance shown when you touch the ends of your multimeter probes together (it ought to be 0 but sometimes the probe leads add 0.1 or 0.2 Ohms).

Now measure the resistance of the coil and, if necessary, subtract the resistance of the probe leads.

[justblair]

[quote=petercom;38983]It sounds as though you are measuring across the coils in circuit and your multimeter is 'seeing' the impedance of the bass or midrange units.
QUOTE]

Thanks Peter for the advice.... I will see what the real values are. I searched on measuring DCR, but it is suprising how little relevant info google threw up.

Thanks again

Blair