chartz:

After a few hours listening to this dreadful sound from the MC120.2, the sound seems to have improved a bit somehow. I think the dielectric in those electrolytic capacitors is the problem. The caps seem to "reform" somehow while electricity passes through them... is that possible?

This can certainly be a problem with regular electrolytics if the kit has sat unused for a very long time. In that case the capacitors essentially leak through (as if there was a resistor in parallel) until the oxide layer in the aluminum foil has reformed. The internal construction of bipolar caps is very similar so guess this applies to them as well.

I'm also a firm believer in flat tone control settings. Dunno about other people, but I get used to the sound of a particular setup very quickly, and I prefer to have everything in neutral at that point instead of all over the place...

Hi Jon you sound quite knowlegable with these speakers, I would like to know a little more about the uniphase concept myself if you wouldn't mind spending a moment... Am I right in thinking B&O took a rather unpopular approach to three way speaker design?

Hi,

Step1:

Hi Jon you sound quite knowlegable with these speakers, I would like to know a little more about the uniphase concept myself if you wouldn't mind spending a moment... Am I right in thinking B&O took a rather unpopular approach to three way speaker design?

I would also be interested.

Would anyone be kind enough to publish the S120/MC120.2 crossover diagram with values please? Just out of interest. Thanks.

Jacques

Well, I wouldn't consider myself to be anything approaching "quite knowledgable", but thank you!

As far as I understand it to be, Bang and Olufsen decided on using the filler-driver concept as a way to compensate for the phase shift introduced by a crossover.

Because of the resultant phase shift with a crossover, the driver's polarities usually are reversed so that their acoustic output sums flat at the crossover point. This, however, obviously means the speaker system can never achieve a minimum phase response (zero phase shift being the goal).

What B&O did with the 3-way Uniphase speakers is to design a crossover with symmetrical 2nd order low-pass and high-pass acoustic slopes, but with the drivers connected in phase. Then, in order to counteract the resulting cancellation in response, they used a midrange driver to "fill in" the response - that midrange driver's passband being centered in the middle of the high and low frequency driver's crossover point. Above and below it's passband, the midrange driver's acoustic response rolled off at a shallower rate. This meant that the overall frequency response of the speaker summed flat, yet the drivers were all in phase with each other.

As well, B&O "time-aligned" (physically lined up) the acoustic centers of each respective driver. This is why you see some of the drivers a bit more recessed on the baffle than others. So, the Uniphase designs are "transient-perfect" as well, in that they can (mostly) accurately reproduce a square wave. Notice that "time-aligned" designs which are not also either minimum phase (zero phase shift), or linear phase (the entire passband of the speaker phase-shifted by an equal amount), are not transient perfect.

I believe the 2-way Uniphase speakers have the response of the high-pass and low-pass sections overlapped, and then the hump in response is tuned out.

There are WAY more knowledgeable people than I who discuss this at length. You can probably Google things like "transient perfect" and "filler driver" and find alot of useful, in-depth information. Search for a man named "John Kreskovsky" - he's written a few papers about this subject.

The merits of this design are highly debated, with many people saying you'll never hear the difference. I'm the kind of person that doesn't always trust my ear, but I do believe what I can measure to be true. Within certain limitations, I believe in the merits of these designs.

Many people claim that due to the reflections and phase anomolies introduced just by your listening room, a transient perfect design is a moot point, and I can see where they're coming from.

Also, by nature, these designs have poor vertical polar response, illustrated perfectly by the big difference in sound quality you get when you listen standing up versus sitting down. Even if you could somehow get around the room influences, the Uniphase speakers' transient perfect quality (and really, it would be the same with ANY similar design, not just B&O's) would be evident only through a very narrow listening window. Move up or down off axis out of that listening window, and that phase coherency is lost.

These speakers were made back when B&O was more concerned with technical perfection, IMO. It's not an easy thing to do; what they did, that is. It resulted in a more complex and expensive design, but one they considered worth it. Again, many doubt the validity of these designs in the first place, and don't even bother to acheive these design goals. And I've heard many fine speakers that are nowhere near transient perfect in nature. But there are a number of manufacturers that do strive for this, so I guess it just depends on what you choose to believe.

I hope I made sense the way I explained that, and didn't mess anything up.

Jon

Very much appreciated Jon, thank you

Also, there are quite a few differences between the S120's and the MC120.2's, not the least of which is the crossover. If I have a schematic for the MC120.2 somewhere, I'll post it. I don't think the S120 schematic would do you any good, Jacques.

Jon

Thank you Jon for your learned lecture. I did a Google search on John Kreskovsky which indeed returned fascinating articles. I remember building such speakers in my teens, admittedly not knowing exactly what I was doing...

I want to make these speakers sing and I'm pretty sure they will. Let's hope Martin's cap kit will lift that veil that makes them sound like an old AM radiogram—a bit exaggerated then, but after the Arcams, that's what they sound like to my ears.

I could of course take the time to read and take the diagram down myself, but all the better if you have it somewhere!

This is the one of s120's.

Jacques

Thanks Jon that is a very nice explanation - Perhaps you could write an article for Beoworld, I think the more technical subjects (described in leymens terms) like this are missing tbh!

Haha! I have NO business writing a technical article, but thanks for making me feel smart! Boy, have I got you guys fooled!

Jon

Hey Jacques,

I went back to look at this thread again, and the pictures that you posted showed up this time. Previously, they had only been red "X"'s for me.

Anyway, Jaff mentioned earlier about your foam surrounds on the woofers, and you never said anything about them in reply. Are they new, or newER, at least?

The reason I ask; is that when I see the second picture you posted, with the speaker grills off, the foam surround looks a bit haggard. It looks brittle, as if it would start to rip and crumble if you pushed on it. If that's the case, your bass will improve quite a bit by refoaming your drivers. But yes, the Beovox's were meant to have wall/corner reinforcement. They even had optional wall mount brackets. Freestanding far out from a wall will make them (and all passive B&O speakers, even the MS150's) sound thin. This is the effect of the "baffle step" in response. Not a fault of the speaker, just a choice by the designer.

It won't matter whether you use a B&O amp or not. All B&O receivers have a flat frequency response, just like any other quality amplifier. No non-defeatable tone control stuff going on like some of the cheaper, older Beocenters and things.

Remember, these speakers are down 8dB's at 42Hz and 4dB's at 70Hz. There's never going to be alot of bass there. What IS there is clean, quality bass though. I like the way my MC120.2's sound.

Jon

P.S. - You said you're used to a brighter sound.... You need to get un-used to it! Obviously I'm kidding. But the MC120.2's are a very, very neutral speaker. What you're hearing when you listen to these speakers (within their frequency passband, and also when you're done restoring them), is what your amp, source, and recording sound like.

P.P.S. - Remember when I talked about vertical polar response a few replies ago? You should make sure that when you listen to these speakers, they are tilted more or less all the way back by the adjustment screw on the stands, and that you are seated when listening. If you haven't done this, it will help alot.

Hi there,

Well the foam surrounds are absolutely original! They can handle a lot of power, the cones hardly move even when fed hard with lots of bass at high volume!

I do plan to send them over to Martin/Dillen, but the shipping costs are very, very high in France (100€ with return). This is annoying on ebay because the foreign buyers tend to think we overcharge them. 

Anyway, as Martin keeps saying, these woofers are irreplaceable and I won't take any chances fiddling with them.

I'm waiting for my new caps. At the moment they sound pretty awful : little treble definition (as if a thick curtain was laid in front of them) and piercing mids. Bass is okay though, and although there is obviously no deep notes to be heard (organ pipes), what's there is admittedly not bad at all. 

One last comment, the cabinet is quite resonant and I wonder whether this is also wanted or just a cheap cost-cutting measure!

I use a Beomaster 6000, but the sound is much the same with my all valve setup—no, not Chinese!

Jacques

Edit: I just saw your post-scriptum and I think their placement is optimal, as per the user's manual's recommendations. Recapping will hopefully help. By the way, are there any differences between s120 (see above) and MC120.2 as regards the crossover then?

Martin if you read this I'm like a child waiting for their Xmas ... I want my caps!

chartz:

Well the foam surrounds are absolutely original! They can handle a lot of power, the cones hardly move even when fed hard with lots of bass at high volume!

The cones aren't seized are they? Mine move quite a bit with lots of bass at high volumes...

Jon

Jon:

The cones aren't seized are they? Mine move quite a bit with lots of bass at high volumes...

No, no, but then I never boost bass frequencies artificially. 

Jacques

chartz:

By the way, are there any differences between s120 (see above) and MC120.2 as regards the crossover then?

Yes. The crossover frequencies are different. I believe they went from 900/3000 Hz, to 800/4000 Hz. I never was able to locate a schematic for the MC120.2's. Sorry Jacques.

Jon

Jon:

chartz:

By the way, are there any differences between s120 (see above) and MC120.2 as regards the crossover then?

Yes. The crossover frequencies are different. I believe they went from 900/3000 Hz, to 800/4000 Hz. I never was able to locate a schematic for the MC120.2's. Sorry Jacques.

Jon

I almost decided to be annoying and suggest that the service manuals are available on site, but apparently they are not!

How about just tracing the existing crossover? It isn't that complicated and the layout will probably be very similar to S120, although the coil values will remain a mystery.

Or if chartz has the front off again he could take a shot of both front and back of the board which we could translate to a diagram :) Would be a good contribution to Beoworld technical too...

Hi,

I will do this when I do the re-cap job just to see whether there are noticeable changes from s120's to MC120.2's. But then I will only have the value of the resistors and capacitors... As I am an electrician with electronic skills I can draw the diagram easily. I will publish photos anyway.

I have nothing to measure the inductors reliably though.

Jacques

I see that more of you are looking for the diagram of a MC 120.2 well here it is

Per H

Wow, thanks Per.

The tweeter filtering is much simplified, now a 6dB/octave one. A bit disappointing actually!

Jacques

chartz:

Wow, thanks Per.

The tweeter filtering is much simplified, now a 6dB/octave one. A bit disappointing actually!

Jacques

What makes you say this Jacques? I think it's pretty cool that B&O was able to achieve their target acoustic slope with a first order filter and a gentle contour circuit. This is when they switched to the Vifa D25 units, from the SEAS tweeters.

Hey Per, did you just happen to have that diagram, or is there a place you went to to get the ".2" version of the Beovox crossovers? I don't think I've ever been able to find them!

Jon

Hey Jon

I have a supplement for the "Beovox 3 diagram".
The supplement includes S 45 - S 80.2 - MC 120.2 - M150.2 - MS 150.2
The number of the supplement is 3538557 and dated 2-83

Per H

Hi,

A few caps from Martin later...

The sound from these speakers is now slightly better—it's not night and day—especially in the highs department. Mids don't shout as much now, and bass performance is the same. A bit more presence altogether. Imaging remains okay.

Happy then? Almost. Still a bit veiled I think, or is it neutral to the point of excess?

Anyway, I still have those woofers to refoam!

Jacques

Nice work on the caps !

Do you use stands when you listen to the speakers ?

Martin

Dillen:

Do you use stands when you listen to the speakers ?

Yes of course I do, as shown on the photos on page 1 here.

Jacques

Must confess to being a little concerned about your woofers! My S120s had a fair amount of movement - as you know I actually replaced the units , not knowing that one could get them repaired. It is the same unit as used in the MS150 and M150 and when I have used these speakers, the bass unit moves pretty well - I use flat controls as well.