Mission 770s - what to do with them?

[Bushbaby]

Hi All,
As I have been working from home during the lockdown I have been working in my office (the box room of the house) and have started listening to my hifi again. This comprises of a Arcam Alpha 5 plus (modified) CD player, A&R A60 and Dali Zensor 1 speakers. Its ok but the speakers a a wee bit bright and I really would like to have something more suitable. 

However, I had completely forgotten that I had bought 5 years ago a pair of Mission 770s (I am told these are the worst models of the 770 series). I bought these as I recall in the late 70's, 2 of my friends had Meridian 101/105s, Linn Sondeks and Mission 770s (I cannot recall the cd players?). The sound from the 770 (mk2 I think?), were incredible for their neutrality, clarity and overall definition.

I recall seeing these on a well known auction site and I purchased them for £33 thinking that they were worth a try! However, they have been tweaked around with different drivers (they look new to be honest) BUT still have the original crossovers. I recall that I set them up in my daughters bedroom when it was being decorated and these were incredibly bright/brash and I didnt like them at all. I thought i would then have a dabble with them at the time. I've added a rough schematic for the current crossover and I recall on another hifi forum that someone kindly drew out another crossover which they thought would be better. To be honest, I then lost interest as I had to work away from home for nearly 2 years so the ended up in my workshop (which is insulated/heated and has small dehumidifiers which run all year round so they should be as good as I left them).

Do you think they are worth the effort in trying to get them up and running albeit I think they are a tad too big for my office!!

Best Regards

Rob

                   

[Crackle]

As you get older you may appreciate the brighter sound as your upper hearing gets worse.
I have some Leak 2060 speakers which are very bright, I just turn the treble control on the amp back a notch.
I would think messing with the crossover is going to be an unknown result.
But I am not an expert on these things, I am sure Mark will have some better advice.

Mike

[Mark Hennessy]

It's difficult to know what to advise in this situation.

The crossover circuit is designed around the original drive units. If you change a drive unit, you usually need to change the crossover.

Here, both drive units have been changed.

Also, a box and port tuning is specific to the bass drive unit. A different bass drive unit will have different TS parameters, so will require a different sized box, and a different port. Of course, there is some latitude here.

Really, the enclosure is the easy part.

Start by downloading something like WinISD: http://www.linearteam.org/

Your Vifa bass driver is likely in the supplied database, but if not, you can enter a new driver yourself. The datasheet is here: http://www.sea.vg/vifa/M21WJ-09-08.pdf

Once you've got the driver data, WinISD will suggest the optimum box size and port tuning.

Having done that, measure the inside dimensions of your cabinets to determine the volume. In theory you should knock off a bit to allow for the space occupied by the drivers and crossover, but in practice it's not critical on a larger box like this one. You can then create a new project using the same driver, but change the box volume to yours - leave the first optimum one on the screen to see the differences.

Finally, measure the ports, enter those into WinISD, and that will show you the effect of the existing port. You can play around with the port tuning (and box volume) in real time by just clicking and dragging, which is really interesting and helps to give you a better grasp of how everything interacts.

So, if the port needs changing, WinISD has sorted that for you. But the crossover is the hard thing.

The thing is, the crossover is not just a pair of filters. It is also an equaliser. At the very least, it is doing baffle-step correction, although not a huge amount if the value of the woofer inductor is correct. Other tweaks might be included to compensate for drive unit "funnies". This crossover is pretty simple, which implied either the original drive units were fairly well behaved, or they didn't care about aberrations.

If the replacement drive units are at least as well-behaved as the originals within the range they're being used, and have similar impedances, then they might be OK. But the crossover frequency would have been determined by the behaviour of the original woofer - the balancing act is tricky: as high as possible to protect the tweeter, but as low as possible to get the best power response (off-axis response). An 8" driver becomes very directional above 2kHz as it "beams" - if you take a look at the datasheet, you can see exactly what I mean. Whether the original driver was the same is hard to predict, the Vifa is coated paper whereas the original was polypropylene, I believe, and the two materials can behave differently in the midrange.

With all that said, it's likely that you just need to reduce the tweeter level. Certainly, I'd try that first, because it's easy. If it doesn't give you the results you need, then you'll either have to invest in some measuring equipment, or admit defeat! Without measuring gear, you're basically guessing. There are DIYers who claim to be able to design loudspeakers using nothing but their ears - unfortunately, they're delusional.

Unfortunately, reducing tweeter level is not as simple as just increasing the value of the 1.5 ohm resistor.

The HF filter was designed for a certain value of impedance. If you up the resistor, you up this impedance, and change the frequency. So you need another resistor to bring this back down. Obviously, a designer will do all they can to eliminate components, so having determined how much attenuation a given tweeter needs, they'll add the series resistor then design the filter to suit that - that's why there's only one here.

You'll have to experiment with this, I'm afraid. Without measurements, I don't know how much more efficient the new tweeter is, so can't begin to guess at how much we need to knock off. I recommend disconnecting one end of the 1.5 ohm resistor, and bringing 3 wires out of the enclosure via the bass reflex port - one connected to ground, the other to resistor, and the other to the tweeter. Then get a bunch of resistors and some croc-clips, and experiment. Start by putting 10 ohms in series with the tweeter, which should knock it down significantly, but might cause a midrange bump - perhaps try another 10 ohm resistor between ground and the 1.5 ohm resistor (basically on the filter side of the attenuator). I wouldn't go much lower than 10 ohms here, BTW. For experimentation, any old resistors will do - even low-power ones - but once you've decided on the values, I'd recommend using wire-wound types of at least 5W power rating.

Alternatively, you can buy a variable L-pad, but they can be pricey for what they are.

Work on one speaker at a time, and have a reference speaker on the other channel of your amplifier. Play back in mono, and swing the balance control between the Mission and your reference. You need to frequently "reset" your hearing when playing this game

Sorry if all this seems overly complicated. Speakers are complicated. People underestimate just how complicated because superficially, they appear to be very simple. Especially crossovers. It can take weeks or months to optimise a crossover - you usually start out with a complicated circuit and slowly distil it down until you have something that is so finely balanced that even small changes can totally change things - just changing an electrolytic cap for a "better" film type can make things sound worse. Sadly, the internet is full of "experts" telling people to re-cap everything they own - that is not always good advice, sadly.

Good luck!

Mark

[Bushbaby]

Mike & Mark,
Given your kindly responses I am thinking that this one is out of my league to deliver and I am now unsure whether it is worth the time and effort to do any work on. Bottom line is that I am convinced that I need the right speakers to suit the amp/CD player and the environment (a small office/box room). This is potentially a challenge as potential speakers my sound great in a show room or large room and sound awful in my small office (2.5m x 2.5m) and that I can only really use smaller floorstand/bookshelf speakers.

I would love to try Heybrook HB2 which work really well with the A&R A60 but the chances of finding any are virtually zero.

Best Regards

[Mark Hennessy]

Hi Rob,

I've just found a set of measurements for your current speakers (just to check, are they Zensor 1, as mentioned above, or Spektor 1, as mentioned in an email?). I'll assume that they are Zensor 1s.

https://www.stereophile.com/content/dali...asurements

They seem quite respectable to me, especially for the price. A little bit forward, and perhaps a bit sharp at the top end, but they are hi-fi speakers, so you'd expect that. But your A&R has tone controls. Are you able to get something close to acceptable with fine adjustments to the treble control?

Mark

[Bushbaby]

HUGE MISTAKE ON MY PART.........I looked at the Dali manual and realised in my small office that these were NOT positioned close to the wall as recommended. They are now siting about 2 inches from the wall and facing full out (not a slight turn in) - WOW, what a difference. The brightness has remarkably reduced and the bass midrange has improved. I've even flattened the controls on the treble and bass on the A&R A60. The are not impressive but more 'flat' sounding which is what I expect from a reasonable quality speaker. Still could be a bit more detailed!
Oh dear, I've got the bug again and wondering what the next level up speakers will be like?

Best Regards

[Mark Hennessy]

Curious - I've been reading the manual myself, and they suggest the speaker can be hung from a wall using the installed keyhole, but they are rear ported. So how is that supposed to work?

The manual mentions "rubber bumpers". Presumably these space the cabinet away from the wall sufficiently to allow the air from the port to escape around the edges of the cabinet?

There's a tear-down video on YouTube: https://www.youtube.com/watch?v=ueNnWewcENw - I haven't listened to it (ironically, perhaps) because my computer amplifier is currently being refurbished, but the pictures show a perfectly respectable construction standard for the price. The crossover is obviously built to a price, but I'd accept it for what it is. The Stereophile review is very complimentary, for what that's worth (actually, they write better reviews than many, and are one of the few to include measurements). They apparently don't lack detail - and why should they?

My advice at this point: forget about the gear and listen to the music. Simple to say, but honestly the best advice you'll get from me. Worrying about the gear will lead you down the carefully prepared path that is designed to extract money from those who want to believe. The best upgrade for any hi-fi system? A pile of new CDs

Any idea what you'll do about those Missions? At the very least, you should experiment with reducing the tweeter level.

[Bushbaby]

Hi Mark,
You are indeed correct instead of listening for faults and weaknesses the whole point is to listen to the music! Given that I was not working from home for 2.5 years (I was in Sellafield, Warrington, Anglesey and Gloucester - nuclear construction projects) I don't even think these speakers are even run in yet!

Yes, I will have a play with the mission speakers to see what the outcome will be.

Best Regards

[Diabolical Artificer]

I had a pair of the early 770's with the clear Peerless woofers, they were my first "proper" speakers. the Seas tweeters in these are pretty damn good, you can find the specs on Seas website who have all the specs for vintage drivers. I sold mine and not have Mission 720's, Mission's top of the range speakers at the time, also have a pair of 730's that have had the tweeters and very hard to find Celestion MF mid drivers removed. I rebuilt the 720's from scratch, the tweeters were blown and the mid/woofer drivers surrounds had gone. also recapped the X-overs which if like mine are glued to the speaker cabs and take applied violence to get em unstuck.

Your drivers look original, there's a lot of threads out there that state these tweeters are ferrofluid, they're not. Have you checked the fuses Rob to see if they have the correct rated fuse and not a six inch nail or similar? From memory mine had loose fuse holders. You can't beat big speakers but the size can be a problem obviously especially with wives.

Andy.

[Craig]

Oh dear, I've got the bug again and wondering what the next level up speakers will be like?

Best Regards

Welcome to the rabbit hole. There is no return.

Craig