Mission 753 suggested xo mods

[justblair]

Hi all

I have been looking this weekend at my trusty Mission 753 non freedoms. So far with this speaker I have carried out 2 successfull mods.

1. I removed the metal grill that "protects" the metal dome tweeter. A very good result on this one. I always found the freedoms to be too soft for my tastes. The non freedoms a bit harsh. The non freedoms with the tweeter mod gets close to my tastes.

2. I applied "no more nails" between the basket and the magnet. This has made a subtle but worthwhile improvement to the clarity of the speaker.

Next up I plan a rewire and some improvements to the crossover. I took some time to map the crossover.

My schmatic

My plans are as follows

• Replace the coffin style resistors with decent quality wirewounds
• Replace the 6.8uf cap on the tweeter's filter with something nice. suggestions please?
• Replace the back to back 470uf caps on the mid's filter with a decent bipolar cap, suggestions please?
• Replace the back to back 1000uf caps on the lower end filter with a pair of good quality 'lytics, probably nichicon fine gold or similar. Open to suggestions though?
• leave the existing inductors for the moment. I don't have the facility or know how to measure their values, let alone pick replacements.
• Replace the 10mf bipolar cap on the mid range filter with higher quality, again suggestions?
• Split the mid and low end filters and go tri wire!?

The last one I have some questions over. Excuse my ignorance when it comes down to impedence. The reason for wanting to do it is that I recently moved to bi amp (using gainclones) and the difference in clarity was dramatic. Separating the filters looks easy in terms of wiring, I just add a couple of binding posts and wire to them. Impedence though I dont understand fully.

What will be the effect in separating them? Will the volume go out of balance? What would be the impedence be at the inputs to each crossover, or what do I need to know to measure it?

Within my plans have I made any stupid errors?

I don't plan to make any changes that are irreversible, but I would appreciate any guidance that will increase my chances of success.

Thanks in advance

Blair

[sreten]

Hi,

Some notes :

Tri-wiring and tri-amping is a simple as you describe.
You can make the amplifiers gains adjustable slightly for tweaking.
Each amplifier in the bass section will see an easier load.

You should "bypass" the midrange 235uF capacitor with a film type.

Try Falcon components : http://www.falcon-acoustics.co.uk/

/sreten.

[justblair]

Originally posted by sreten
Hi,

Some notes :

Tri-wiring and tri-amping is a simple as you describe.
You can make the amplifiers gains adjustable slightly for tweaking.
Each amplifier in the bass section will see an easier load.

You should "bypass" the midrange 235uF capacitor with a film type.

Try Falcon components : http://www.falcon-acoustics.co.uk/

/sreten.

Thanks for the suggestions Sreten. Couple of more questions

You mention that the amps will see an easier load on the bass sections. The LM3875 I believe aint so hot driving lower impedences. Is there anything that I can do to increase the effective impedance if the amps struggle?

You mention "bypassing" the 235uF capacitor I always took this to mean connecting another cap in parrallel with it. Is this what you mean? I would have thought I would be better replacing it altogether with a better cap? Apologies my terminology is confused.

And thanks for the link, I shall peruse that one later!

[ianpengelly]

Hey Justblair,

Sounds like a fun project, I have always wanted a pair of 753s!

It would be worth looking at madisound for your parts, you'll need to pay overseas shipping, but when I compared the prices between uk parts suppliers and US the difference was more than worth the shipping costs. Plus there was a greater range. They have Eagle Metal Film resistors which are non inductive and may prove superiour to the wire wound versions and at $1.20 each they aren't going to break the bank.

http://www.madisound.com

You could also look at internal damping sheets, upgrade the terminal post (which you'll have to do to tri-wire) and upgrade the sound absorbant material.

Hope this helps.

Ian

[sreten]

Originally posted by justblair


You mention that the amps will see an easier load on the bass sections. The LM3875 I believe aint so hot driving lower impedences. Is there anything that I can do to increase the effective impedance if the amps struggle?

Not really, unless you go for a parallel amplifier topology.
If the bass ampliier is not struggling much now, two of
them will not with only one bass section each to drive.

Originally posted by justblair


You mention "bypassing" the 235uF capacitor I always took this to mean connecting another cap in parrallel with it. Is this what you mean? I would have thought I would be better replacing it altogether with a better cap? Apologies my terminology is confused.

Ok. Something like 220uF bipolar with 10uF film bypass will be fine.

/sreten.

[justblair]

Originally posted by ianpengelly
Hey Justblair,
Sounds like a fun project, I have always wanted a pair of 753s!

Its just one of the fun projects that I have been inspired to by sites like this and the helpfulness of the site dwellers

Originally posted by ianpengelly

It would be worth looking at madisound for your parts, you'll need to pay overseas shipping, but when I compared the prices between uk parts suppliers and US the difference was more than worth the shipping costs. Plus there was a greater range. They have Eagle Metal Film resistors which are non inductive and may prove superiour to the wire wound versions and at $1.20 each they aren't going to break the bank.

Great tip on the site. The metal films are defo on the shopping list.

They also seem to have a nice selection on the parts. I am looking at the mundorf supreme caps. I can get a 6.8uf for the tweeter no probs. Though it costs a bit... Not sure wether its good value for money?

Still lost a bit on the choice for the caps to replace the mid range pair of lytics. The value is much higher than I can find in the good caps.

The pair of 1000uf I will probably replace with Nichicon fine golds. The choice of these is simple. I use this cap on the 4 power amps that I have built already. If I am making more amps I will have to buy another 4. Might as well buy 8. Is this a good choice?

Originally posted by ianpengelly
You could also look at internal damping sheets, upgrade the terminal post (which you'll have to do to tri-wire) and upgrade the sound absorbant material.
[/B]

The missions are already fitted with internal damping sheets on the side walls (not full sized) I don't know if that will make a difference in this case. The MDF is also pretty thick and heavy... Maybe I will look at it when I get to fine tuning

The sound absorbing material looks pretty basic. Its just foam cut to cover the back wall of the speaker case. I will certainly be looking at this in the future

Originally posted by sreten

Not really, unless you go for a parallel amplifier topology.
If the bass ampliier is not struggling much now, two of
them will not with only one bass section each to drive.

Thanks for clarifying. I think I will "suck it and see" on this. My belief was that the lm3875 struggles into low impedances, looking at the schematic, the bass drivers are paired up in series. That (excuse my newbness) will produce a high impedance. I am still confused, but I am sure that all will reveal itself in the testing. I will start by disconnecting the low pass filter and listening. If it works I will go ahead and order parts for a third pair of amps.

Originally posted by sreten

Ok. Something like 220uF bipolar with 10uF film bypass will be fine.

Again thanks for clarifying... I feel a bit dumb now for not getting that straight away. Can you point me at good quality 220uf caps?

[justblair]

Right I have been doing some searching on the internet. And I have a parts list sorta sorted out. I would appreciate some constructive criticism

Per speaker
Tweeter

Sonic Craft Sonicap MKP 6.8uf 220VDC – 5% tolerance
Vishay Roederstein MKP1837 0,01mF MKP 100VDC – 1% tolerance
Eagle metal oxide resitors

Mid

2x Black Gate standard 470uf polar cap 50v
Vishay Roederstein MKP1837 0,01mF MKP 100VDC – 1% tolerance
Eagle metal oxide resitors

Bass
2x Black Gate standard 1000uf polar cap 50v
Vishay Roederstein MKP1837 0,01mF MKP 100VDC – 1% tolerance
Eagle metal oxide resitors

My maths tells me this will cost about £80 for all the parts.

I would welcome suggestions for bringing down the costs or getting better results for the same sort of budget?

[Inductor]

Originally posted by justblair
I would welcome suggestions for bringing down the costs or getting better results for the same sort of budget?

You don't need MKP's on the bass section.

Nice website about Engineer Malcolm Jones and Falcon Acoustics, UK, sreten.

[justblair]

Originally posted by Inductor


You don't need MKP's on the bass section.

Is that because the caps are out of the signal path, or because they are insignificant compared to the 500uf caps that are there?

Other than that. Am I on the right lines?

Regards

Blair

[Sonusthree]

Originally posted by justblair


Is that because the caps are out of the signal path, or because they are insignificant compared to the 500uf caps that are there?

Bypassing is usually done to enhance the mid and treble so is fairly redundant in the bass section. The thinking is that small bypass caps give better response with higher frequencies.

You are on the right path here but I cannot comment on your choice of Black Gates since I have only ever used them in amps and CD players.

P.S. I have used sonicaps and found them to be good value for money and it seems that others agree. Very similar to clarity caps IMO.