B&O S75 recap help

crazy-in-az

Uhhhh....
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I'm going to buy some caps for my Bang & Olufsen S75's and I need a little advice. From the looks of the crossover schematic there is a cap and resistor across the input terminal. Would it be better to replace it with a film cap or an electrolytic? I'm pretty sure all of the caps in there now are electrolytic except this one, which looks like a plastic foil cap (a white box).

I'm going to be using Dayton caps since they're affordable and I like the way they sound. Looks like I'll be using electrolytics for both of the 50mf, 37mf and 10mf and poly film for the 3.3mf and 12mf.

Here's a pic of the schematic-

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Any help or advice would be appreciated!
 
Thanks for the reply. I will look into replacing the resistors. I knew this particular crossover was prone to meltdown, but last time I had the speakers open they looked fine.

I'll go ahead and buy a 10mf film cap instead of an electrolytic.
 
OK; have fun! Those are great speakers; I had a pair for a good twenty years, and now my buddy has them still.
 
10 uf? It looks like 2.2 from the schematic. 2.2 is going to be pretty inexpensive in a film cap like a Dayton so no reason to use NPE anyway.

Odd design, I haven't seen a cap/resistor across inputs like that. This is basically a treble contouring circuit but that usually is placed within the tweeter circuit.
 
Oh never mind I see you were talking about a different cap. Yes the 10 uf won't be too huge in a film cap but for the higher ones I too would go NPE.
 
I rebuilt a couple pair of these perhaps 10 years ago. I found that changing the resistors made a bigger difference than changing out the capacitors, although I went ahead and used film capacitors where I could. I have several pair of Beovox (Bang Olufsen) speakers in my house, maybe 8 pair. On a few pair I also found those small resistors got hot, so when I rebuilt the crossovers I used a higher watt part. I do not listen to loud music these days, but after I am gone, the next owners might push them.
 
On mine, being young and dumb, er, etc., I cranked them so much that I blew out two of the tweeters. (I was running them off a 60 WPC Mitsubishi receiver with some heft.) Fortunately, the tweeters were a readily available Philips model, as I recall. Even so, I don't think I ever torched the resistors, but if I had them today I would definitely put in heavier-duty ones. (I have suggested it to my friend who now has that pair, but it's beyond his abilities and I'm not close enough to do it for him.)
 
Wow, thanks for all the info!

I rebuilt a couple pair of these perhaps 10 years ago. I found that changing the resistors made a bigger difference than changing out the capacitors, although I went ahead and used film capacitors where I could. I have several pair of Beovox (Bang Olufsen) speakers in my house, maybe 8 pair. On a few pair I also found those small resistors got hot, so when I rebuilt the crossovers I used a higher watt part. I do not listen to loud music these days, but after I am gone, the next owners might push them.

Madisound has wirewound 25 watt resistors for 75 cents each, so it'll cost me $12 for all of the ones I need.

On mine, being young and dumb, er, etc., I cranked them so much that I blew out two of the tweeters. (I was running them off a 60 WPC Mitsubishi receiver with some heft.) Fortunately, the tweeters were a readily available Philips model, as I recall. Even so, I don't think I ever torched the resistors, but if I had them today I would definitely put in heavier-duty ones. (I have suggested it to my friend who now has that pair, but it's beyond his abilities and I'm not close enough to do it for him.)

I've been powering mine with a Nikko Beta 30 pre and an Audiosource AMP310, which puts out 150 watts/channel. I haven't shaken the walls yet, but I have listened to it kinda loud. The tweeters are still good though. These are probably my favorite speakers because they can sound good with rock, metal and jazz, my three go to music choices.

With more power and a good pre amp I've noticed a lack of highs and very forward mids, especially compared to my recently recapped AMT1 Bookshelf speakers.
 
That's very common in my experience, and a recap usually cleans it up nicely. The caps in the tweeter and mid sections drift off spec, there is too much of the wrong material getting to the drivers, and the response curve is whacked.
 
Okay, so I just got all the resistors and capacitors. I didn't factor in the size of the new resistors. The old ones are the small tube shaped type, but the new ones are the big square ceramic type. They are several times larger, and the leads are very short in comparison.

The crossover is set up kinda strangely with 6 of the 8 resistors standing up on end. The new ones will fit on the board like that, but the top leads aren't long enough to reach the crossover board. Any ideas?

Is it possible to break the ceramic coating without hurting the resistor?
 
Don't mess with the resistor, it will probably wreck it to do that. You can extend the lead by twisting and soldering on a bit of wire. Put some spaghetti tubing over it if you can to keep it from shorting if it's close to other leads.

I was about to say, upon re-skimming the thread, if they were tubular resistors they were probably 2-3 watts max and could be replaced with 5W and get double the heat dissipation capacity. Or maybe 7 or even 10W. But 25W gets pretty big as you see. I wish I had said something about that when you posted about the 25W ones. It's not the first time parts looked way bigger in person than they did in the catalogue, we've all been there man.
 
Yeah, I just did some reading and apparently 5 and 10 watt resistors are still pretty small compared to 25 watt ones . I'll see what I can do about extending the leads, but with these and the new film caps being larger than the originals I don't think I'll have enough room on the board.

Maybe I can send these back to madisound in exchange for some lower wattage ones...
 
Would it be okay to replace a 2.2 ohm resistor with a 2 ohm and 8.2 with 8? Parts-express has the exact values I need, but they are 10 watt ones and they are not much smaller than the 25 watt ones.
 
10 watts is ample. 5 watts is borderline too small. 25 is way overkill.
 
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It wouldn't make a ton of difference to replace 2.2 with 2, etc. That's 10% off and the tolerance on the resistors is probably at least 10% anyway. If you look at where they are in the circuit, you can learn things. The 8.2 is an RC circuit that contours the midrange a bit so the effect would be very subtle. The 2.2 is in series with the tweeter so lowering it will increase the tweeter output just a smidge. But again the tolerances on all the components mean there are error bars on all this stuff anyway. Stay within 10% and you'll be OK.
 
It wouldn't make a ton of difference to replace 2.2 with 2, etc. That's 10% off and the tolerance on the resistors is probably at least 10% anyway. If you look at where they are in the circuit, you can learn things. The 8.2 is an RC circuit that contours the midrange a bit so the effect would be very subtle. The 2.2 is in series with the tweeter so lowering it will increase the tweeter output just a smidge. But again the tolerances on all the components mean there are error bars on all this stuff anyway. Stay within 10% and you'll be OK.

Thanks for the info. Mouser had the correct resistors, and Madisound will let me return the 25W for either a refund or store credit. I'm going to take the latter since I buy a lot of parts from them anyway.

I'll post pics of the job once I have all the correct parts.
 
Updates...

Okay, so I lost the cable for my camera and couldn't find it. Turns out it was underneath a rather large book in the living room...:dunno:

Anyway, I got the first crossover done and installed in the speaker. I left the other one untouched and spent some time comparing the two. The changes are very noticeable, in a good way. There's definitely a lot more treble than there was before. At first it seemed irritating, but when I sat down on axis with all four drivers I was surprised! The sound is much clearer and more defined, and even the bass seems improved. There's a lot more mid-range detail and it makes Roger Daltrey sound like he's right in front of you.

Now that I have the cable for the camera I will take some pics as I recap and re-resistor the second crossover. That will probably have to wait until later in the week. The first crossover was a pain to assemble, but it works, even if it looks cramped and a little messy. The new 12uf film cap is easily two or three times the size of the old one.
 
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