Mission 753 Crossover Capacitor Swap

altI love my pair of Mission 753. I have used these speakers for several years now and been very happy with them. However they are of an age now where guarantees have long since become distant memories. Time to tinker with them. This article deals with the crossover, in particular the benefits of a capacitor swap.

The first task in order to begin a crossover upgrade to any speaker, is to find out what you have been supplied inside the speaker to begin with. You can try some internet research, and for a popular model of speaker, you may get lucky. Watch out though, speakers go through revisions in their lifetime. Some revisions are documented, i.e. with an edition change, and others are more subtle. Whether this is an issue, depends on how extensively you wish to modify your crossovers.

Some options that you may consider:

  • Remodelling the crossover in dedicated software in order to improve on the manufacturers design. (more involved, some knowledge required on filters)
  • Removing the crossover from the speaker itself, and re-casing it outside the cabinet. Some have reported improvements this way.
  • Removing the crossover components from a printed PCB and wiring point to point. Again many have reported improvements; the resistance of the connections should be improved this way, as well as their being an opportunity to distance components with stray magnetic fields from each other.
  • Component swapping with higher grade components.

{include_content_item 45}I did a fair bit of research before starting my crossover modifications, and deliberated over what would be the best course of action. In the end I chose a compromise. I did not wish to replace with the most expensive boutique parts, the cost of which would have been overly expensive. At the same time I had aspirations of making some improvements.

 

 

 

 

 

 

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The crossover of the Mission 753 is made up of several components which together make a 2.5 way speaker. For those unfamiliar with the term, a 2.5 way speaker is somewhere between a 2 way (Bass + Tweeter) and a 3 way (Bass + Mid Bass + Tweeter). Another way to describe a 2.5 way speaker is a 2 way speaker with Bass assist.

In the Mission 753 there are 5 drivers (speaker cones) per speaker. The four identical bass drivers are organised in pairs. The lower two, provide the bass assist, the upper two the standard bass and finally the tweeter the high frequencies. The four identical bass drivers are filtered by the crossover so that the upper two produce frequencies up to the crossover point of the tweeter. The lower two do not extend their frequencies this high.

I have read many arguments as to the advantages and disadvantages of 2 way vs. 2.5 way vs. 3 way speaker designs. I suspect the most reasonable to accept is that in a 2.5 way speaker the .5 way (in the mission case the lower two drivers) bolster the bass frequencies at the baffle step frequencies. (Google baffle step if you are unfamiliar with the term)

As part of my research I needed to know the component values and configuration for my speakers. For the Mission speakers, I could not find any published schematics. So time to get out the multi-meter, or to be more exact I needed a new tool.

Resistors and capacitors are easy to determine, the values are generally printed on the component. Inductors on the other hand don’t necessarily give values on the device. The new tool that I invested in was an inductance meter. This allows you to measure those funny coil things you invariably find in speaker crossovers. More of that in another article.

Anyway here is the crossover design for my Mission 753. This may vary from yours; there were apparently around seven revisions through the life of this model.

753scheme

Time to get started then on some modifications. I decided that I wished to have something to compare to in order to evaluate the effect of my tinkering. To do this I used mono signals through each speaker so that I could compare. Once I was satisfied that improvements were forthcoming, I would mod the other to match.

First up was to replace the pair of 1000uf caps on the Bass driver’s crossover.


What was in there was a pair of elecon 1000uf. By using a pair of capacitors back to back, Mission could use a high value of cap in their design, without the added expense of using a bi-polar design (rare at these high values) I opted to replace these with Nichicon capacitors.

I had experience Nichicon Muse series capacitors before in my Gainclone amplifier, this time I bought the Muse KZ. This is the top end of the Nichicon Audio capacitors. Comparatively these are considerably cheaper than using Black Gates, possibly the most raved about electrolytic to have in the signal path.

 

 

 

 

picture 105

The Nichicons are physically a bit larger than the original caps that Mission supplied

basscapYou can see the location of the big caps on the crossover.



The top board was unscrewed, the caps de-soldered with my de-solder pump and the Muse caps inserted. I left the leads long so that I can rebuild the crossover using point to point some time in the near future.

picture 108
The result


I then reassembled and did a comparison between the un-modded and modded speaker. I have to say that it was difficult to tell the difference.

You can see in the schematic that the 1000uf's are not in the signal path.

So I routed through my music collection for music with lots of low down bass. It actually surprises me how little these speakers get worked.

I did manage to hear some small differences on some tracks. However it’s obvious that this is not an area where huge gains can be made. Small gains though all add up!

 

Next up I disassembled and replaced the two 470uf caps in the mid range drivers crossover, as well as the 10uf. Interestingly Mission do not supply these on a circuit board, they are wired point to point. The caps chosen here were more Muse KZ caps for the higher values. To replace the 10 uf cap I used a Nichicon bipolar capacitor.

After.

picture 106

Again nothing here has been nailed down, and leads were left untrimmed. I will rebuild properly after when I have settled on all of the mods. I even left the original caps glued in case I wanted to reverse the mod.

This time I fired up the speaker for a comparison to the un-modded one. I put on Norah Jones to start with. It sounded poor. I was horrified to realise the tweeter looked like it had barely survived a round with a 5 year old! A bit of speaker snogging (sucking out the bent metal dome with my mouth!) later things were better. But still the mod had little effect, I could hear little difference.

Fortunately moving away from "Norah" and onto "The Muse" things started looking up. Well at least I could here a difference. Male vocals in the modded speaker had more "life" to them and percussion had more drive and purpose. I at this point hauled my then flatmate through and used him to do a blind test. He reported the same as I had noted, and without prompting.

{include_content_item 57}I have since listened to a few more pieces of music. My favourite so far has been Renauld Garcia Fons, the double bass maestro. On the album Arcoluz, The timbre from the double bass and classical guitars is more pronounced. The instruments have gained some life about them and I find it enjoyable to listen to. Switching back to un-modded, the high end seems more pronounced certainly, but the rest of the sound seems thinner. My first thought was that I had lost some detail in the mod, but that’s not the case.

Drums on Arcoluz seem more engaging. The percussionist plays traditional Arabic drums which punch the mid range. They seem to have more presence.

I was tempted to go ahead and mod the second speaker, but I did not do so immediately. I wanted to give the Muse caps some playing time to loosen up. Capacitors are reported by many hobbyists to improve once in use for a while. Black gates from instance it is said take a couple of hundred hours to reach their optimum

picture 106

 

 

After about 48 hours playing time, I repeated the mono comparison of unmodified speaker against modified. The differences between them were more pronounced now, the modifications to bass and bass assist capacitors making a small but significant improvement.

 

 

I decided that I liked the modified better and went on to modify the other speaker.

First things first I checked the newly modded with the previously modded speaker. There was quite a difference between the two. I think the caps do take a bit of time to bed in. The previously modded speaker sounded quite a bit more refined than the one I did later.

Then I had a bit of a surprise. I plugged both in and tried some Norah Jones-I'll be your baby tonight. When I had tried one speaker against the other it was hard to discern a difference with her music, and harder to tell whether the mod was an improvement or not. Running two modded speakers and suddenly things sounded different to before. The replacement caps have made the speakers sound richer, the bass seems more extended and there seems more depth to the music.

I then cycled through some other tracks from different artists. "Muse-Butterflies and Hurricanes" had more presence than before and the separation of instruments and vocals is more pronounced.

Moving onto my personal favourite, Renauld Garcia Fons's album "Arcoluz" and I was really chuffed. The difference is more striking still. The double bass sounds more lifelike, the drums had more presence, the Spanish guitar more melody.
I went through the corners of my collection and could hear a difference in all tracks, though some more than others. Pianos have become bigger, percussion has more presence.

Next I bypassed the 2x 270 ‘lytics with a pair of .1uf polypropylene caps that I found in my odds and sods box. I think I can hear a small difference in the mid range, but I really should have let my ears bed in to the new sound before trying this. The sound didn't deteriorate, so I left them in.

 

At this point, I received some Sonicap Gen I capacitors which replace the capacitor on the tweeter. These are polypropylene capacitors that come with plenty of recommendations behind them. Humblehomemadehifi reviewed these against many other choices. The magic words here are “Value for money”. There are many top end caps, but when you get to these values, we are talking some serious cash. The sonicaps rate well against the top end caps and cost significantly less.

The sonicap capacitors replace the square white capacitor at the fore of the following image

 

sonicapsideI bypassed the sonicaps with a .015uf Vishay cap as recommended by the vendor and humblehomemadehifi.


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 parallel 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.

Next I was not getting much difference from the sonicap and the original cap when I played them back to back. I bypassed some decoupling caps in my chip amps and this improved the sound. I could now hear differences in the two speakers.

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 definitely 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 influential 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.


The modifications that I have detailed in this article should be viewed in context. Each of the Capacitor swaps within the crossover has made some improvement to the sound of the speakers. Mission like any manufacturer is fighting against costs when they design their speakers. Thus the components of the crossover reflect this battle.

 

 

 

For not a lot of money, it is easy to improve on the stock capacitors and subsequently improve on the sound of your speakers. I have made other attempts to improve on the speakers, some successful and others most definitely not.

 

If I was to travel back in time, and had to make the decisions again on capacitors, I would not hesitate to complete the modifications. At every frequency the sound of the Missions has been improved. Not earth shatteringly so, I would not go as far as to call them a “new set of speakers”, but enough that I consider the capacitor swaps to be worthwhile.

I will in other articles give examples of modifications that just didn’t work out. This I can happily say is not one of them!

 

I would recommend this process to all speaker owners. The Nichicon and the Sonicap capacitors I think are good mid range components that offer an improvement to the stock items with out costing the bank.