Mission 753 speaker modding - World-Designs-Forum

justblair · #1 2nd May 2007, 06:53 PM

Hi all.

I am an overly enthuisiastic noob with a set of Mission 753 speakers. I have owned them for some time and have the itch to improve on the already very good sound.

So I have been researching avidly for suggestions on improving the sound but it seems that 753's just dont get modded that often. In fact the only thread I have found on Mission upgrades has been here.

Here is my state of play so far.

I have mapped out the crossover, including inductor values
I replaced the larger lytics in the crossover with nichicon KZ series
I have a polypropline capacitor on order to replace the tweeter corssover cap
I have applied "no more nails" to the baskets to reduce colouration.
I have removed the metal grill from the tweeter and installed a foam rubber ring in its place

The crossover looks like this

The inductors interest me, I am considering replacing the mid and bass filters with heavier guage air core. Probably home rolled, though I am open to suggestion? The current ones are ferrite cored, guage wise they look around the .7mm region. I have read that ferrite can create distortion?

I am open to the suggestion of replacing the tweeters inductor as well, though with a resistor in the signal line, would increasing the guage (ie lower dcr) make any sense?

What are the thoughts of those that have compared air core to ferrite?

The change over to the nichicon lytics has not made a huge difference to the sound. The mid bass has a little more timbre, which is certainly nice to listen to. Double Bass, chello's and male vocals seem more musical. I am undecided though as to wether this is some extra colouration or an improvement to the quality. I currently have one crossover modded, the other stock. I am breaking in the modded one before doing a critical listen.

The Polyproplene caps I ordered before finding this site. They are mid range (am I allowed to mention the brand?) well reckoned caps. I am looking forward to them arriving. From Peter's advice on an earlier 752 thread here, I am hoping for improvements.

The "No more nails" mod is simple, I have seen various variations on the theme, but basicly i have filled the groove between the basket and the magnet with the stuff, reducing the ring from the pressed metal baskets. The effect was most pleasing. Clarity throughout the range has improved on the speakers. in blind tests everyone heard an improvement in clarity, all but one of my long suffering test subjects felt that it made a significant improvement to the music played. This is a £3 mod that i am so glad i found out about.

I plan to add some further damping to the baskets in the shape of damping sheets on the inside and outside of the legs as the basket legs are currently undamped

Removing the metal grill to the tweeter is an easy mod to do. The speaker is clamped into a plastic housing. Unclipping it, it dissasembles and the metal grill that covers the metal dome jsut pops out. It is a reversible mod, but I dont think that anyone will wish to go back after trying it.

Most recently I attempted to refine this by adding foam rings into the groove lleft behind by the removal of the grill. This seems to make the tweeter sound more like a point source than before, though I have not had a critical comparison before and after.

Here are a couple of bad photos of the completed tweeter mod.

While I had the tweeters apart i had a look at the motor construction.

I have read that some improvements can be had from tweeters by varying the stuffing in the rear chanber, You can see above the tweeters.

My thoughts were to replace the foam plug with lambswool and attach a felt ring around the metal ring that surrounds it. Again does anyone have experiance of such a thing?

My last question is wether enyone has attempted to remove dustcaps from speakers and replace them with aftermarket or DIY phase plugs?

A long post, and hopefully some people here may have some experiances that may be relevant to my project..

Blair

A Stuart · #2 6th May 2007, 07:58 AM

Welcome to the Forum. Apologies for the resounding silence in answer to your first post.

I can only assume the lack of response is because everyone is standing back in amazement at your comprehensive rebuild of the speaker.
Someone normally has something constructive to say on questions like this.
Can't actually add anything useful myself.

Alastair

justblair · #3 6th May 2007, 12:16 PM

Quote:

Originally Posted by A Stuart

Welcome to the Forum. Apologies for the resounding silence in answer to your first post.

I can only assume the lack of response is because everyone is standing back in amazement at your comprehensive rebuild of the speaker.
Someone normally has something constructive to say on questions like this.
Can't actually add anything useful myself.

Alastair

Thanks for the reply Alastair, The silence has been deafening on this one. I think perhaps because I am modding a comercial speaker. When searching around most of the tweaks I have found have related to DIY build speakers. My thinking is that someone who has built their own will spend more time on the details.

I am not sure how comprehensive my rebuild can be considered. I dont have the courage to redesign the crossover.

But I am open to experimentation on anything else. The stuff that i have tried has been picked up from various forums, web pages and such. Intended for use on other systems, I have sort of distilled the ones that seem to make good sense. Some results so far have been pleasing, others have made only little differences.

So if anyone has ideas that have worked in other speaker systems please feel free to describe them and I will investigate them on my own speakers.

Oh incidently. Currently I am using two lm3875's per channel in a sort of biamp setup (All power amps supplied by an array of smps, producing a single current source) I eventually will tri amp this speaker. Moving to bi amp made a really nice difference, I'm hoping tri will bring its own rewards. I have some spare chips, and the gainclone amps are pretty cheap to make, so I dont see a good reason not to do it.

Regards

Blair

Shane · #4 6th May 2007, 12:32 PM

I suspect everyone's waiting for Peter to come back on this one!

justblair · #5 6th May 2007, 02:42 PM

Quote:

Originally Posted by Shane

I suspect everyone's waiting for Peter to come back on this one!

Yes I realise that Peter has the pedigree to answer this question... I saw his CV on another thread...

However I am willing to experiment if anyone else has ideas.

Blair

justblair · #6 7th May 2007, 04:48 PM

I had some time today to give the speakers a critical listen...

I tried first of all the modded speaker (with nichicon lytics) against the unmodded. Like before, the difference depended on the type of music being listened to. Though I felt that it was more cut and dried this time. The modified one had more oomph about it with accoustic drums and double bass, again on tracks with female vocalists the differences were less pronounced.

However 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 today.

Then I had a bit of a suprise. 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, and harder to tell wether the mod was an improvement or not. Running two modded speakers and suddenly things sounded different to before. The replacementcaps 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 seperation of instruments and vocals is more pronounced.

Moving onto my personal favourite, Renauld Garcia Fonz'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 differnece in all tracks, though some more than others. Piano's have become bigger, percussion has more presence.

I'm looking forward to the other seakers caps bedding in, I can hear a bit of a difference between the two that confuses the image a little.

Next i bypassed the 2x 270 lytics with a pair of .1uf polyprop 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 deteriate, so I will leave them in.

I can't wait for my sonicaps to arrive. I am hoping for improvements here as well. After todays listen, I will not do just one speaker at a time.

Another "Mod" has come up on my scope. I have read that earthing the speaker basket has for some made some improvements to clarity. As this is another "free" mod, I'm going to give it a try. However I have some thinking to do because of the setup I have.

The mod involves attaching an earth line to the basket, normally done by attaching to the "-" contact on the speaker wire or copper tape.

i.e

On the 753's though, the drivers are in pairs and in series. So attaching to the negative terminal in one of the pairs at least is not giving a direct link to negative (ground).

Am I right in thinking that I will have to attach a wire from each of the basket to the red speaker binding posts to achieve this?

justblair · #7 7th May 2007, 10:43 PM

I will keep posting the results of my experiments in the hope that someone will join in

Well I tried earthing the baskets... It was a pretty easy job to do, I soldered a wire onto the rivet that holds the terminals on, this (I measured) is attached electrically to the steel basket frame.

I ran this to the earth pin on a 13amp plug and tried a single speaker plug in and plug out.

After 20 minutes of comparison, plug in plug out both I an my flat mate came to the same conclusion.

No discernable difference could be heard between the two. There were a couple of times where I thought I could hear something, but I think this was more I-did-it-itus, as repeating the same stretch of music I could not judge wether the plug was in or out.

A bit of an Edison moment, I know now something that doesn't work!

On the other hand, My multimeter tells me that the basket and the magnet are not electrically linked, I think that they are bonded rather than riveted together. I may try wire linking the two together with the earth wire in place just for completness. I am not expecting though to hear any gains.

petercom · #8 8th May 2007, 01:05 PM

Sorry to be late to advising on this one!

I can only advise on things I know will make a difference. For example earthing the baskets (as is done on high-end Tannoy models) may reduce some RF interference in some situations but most people hear 'no difference'.

1. Changing the ferrite cores to air cores is inadvisable. Unless absolutely enormous the air cores will increase the DCR to the bass units and the bass is likely to become flabby as a result. IMO there is very little wrong with good quality ferrites - and these are UK sourced good quality ferrites. You takes your choice - slightly higher dynamic distortion at high listening levels with good ferrite cores, or flabby, uncontrolled bass with air cores - I know which I prefer. (Of course for speakers designed for the higher resistance of air cores you'd be mad to use ferrites!). AFAIK the treble coil was an air core as standard.

2. Rather than messing around with replacement electrolytics you would be better off bypassing them with small value PPs, say between 2% and 5% of the original value, for the bigger caps. Don't bypass individually, wire a cap right across the back-to-back electrolytics.

3. Replace the original polyester caps with PP caps in the treble circuit.

4. Replace the 10uF cap in the midrange circuit with a PP. http://wduk.worldomain.net/acatalog/...propylene.html

5. Replace the ceramic resistors with Vitreous Wirewound types (IMO this reduces the effect of vibration and enhances dynamic capabilities). http://wduk.worldomain.net/acatalog/Resistors.html

6. You can, by all means, try replacing the foam plug behind the dome but you are doomed to experiment a lot with this one as it will alter the frequency response significantly. As a result it is usually better to stick with the foam that was fitted by the manufacturer - removing the dome time and time again to try out different wool/foam etc runs the risk of damaging that highly fragile metal dome!

7. Replacing the foam inside the speakers should bring a substantial improvement (see our recommendations here http://wduk.worldomain.net/Impspkrs.html)

8. Replace the internal wiring. We use this http://wduk.worldomain.net/acatalog/...Cables_21.html for our speakers which, coincidentally, is similar to that which I designed for the high end Mission speakers (Pilastro and Elegante). If the midrange cables pass into the mid enclosure via pins then run the cables direct through sealed holes in the mid chamber instead.

Finally I wouldn't mess about with the dust caps. Dust caps are usually designed to stiffen the cone and removing them also loses up to 1dB of sensitivity (due to the reduction in active cone area at mid frequencies), so leave well alone.

justblair · #9 8th May 2007, 10:22 PM

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