Kef 34ds cap's

[Chii]

Hi all

My first post although I've read the forums for a couple of years.

I have 2 pairs of kef 34DS surround speakers that I've bought recently I've found a problem with one set as it looks like the crossovers have been getting very hot in the past. one has blown a cap off the board, its a Bennic 240mfd 25wv Bi-Polar axial. By the looks of it the other board is about to loose one to. in total there are 8 of these in use over the the 4 speakers. I have had real problems trying to track anything similar down and was wondering if any one had any ideas what I can use to replace these?

Thanks for the assist and I hope this is in the right place!

Andy

[wintermute]

Hi Andy, welcome to diyAudio!

I'd say the problem is that the caps are too low a voltage!! Usually crossover caps are rated at 100V or more! For a set of very low powered speakers 25V would be ok, but for anything handling a reasonable amount of power that seems way too low.

What's your location? It will help with recommendations on sources for suitable caps. Places like mouser or digikey in the states. farnell/element14 in other areas of the world, Partsconnexion parts express or madisound would also be places to look.

You might not be able to get a 240uF cap (that is an odd value) you may need to get two (or even three) different caps and wire them in parallel to get the equivalent value.

Tony.

[wintermute]

one of these Bennic 40 mfd Electrolytic Caps: Madisound Speaker Store + one of these Bennic 200 mfd Electrolytic Caps: Madisound Speaker Store

Should get you out of trouble, the only likely problem is the physical size difference...

If you aren't pushing them too hard, you might be able to get away with 50V caps.

Tony.

[Chii]

Thanks for the response's, makes interesting reading. I am in England but also have an address in the states (wifes family) so am able to get parts from all over the place with relative ease.

One noobie question though can 100v capacitors be used instead of 25v with no issues? if this is the case I may just upgrade the entire board and if the caps are too big, etch a new PCB so they will fit. I did notice that all the other capacitors on the board are 50v, but these seem to be part of the high pass filter and are the lower value (25v) for what ever reason.

Long term they will be driven by an arcam alpha 10p each pair I don't see me having them too loud though as I have neighbors to think of.

Once again with thanks
Andy

[wintermute]

Higher voltage should not be an issue, about the only effect I can think of is that higher voltage (electrolytic) caps tend to have a lower esr than lower voltage ones, but the difference should be minimal, and should not affect crossover frequencies.

The reasoning for using the lower voltage caps in the tweeter section may have been that the tweeter does not get the same amount of power as the bass units, however (the first cap at least) will be getting the full voltage and current the amp is outputting thrown at it. If you can find 50V caps then it sounds like they should be fine, A rule of thumb I saw said that 50V caps are good for up to about 70W.

Assuming an 8 ohm load then the absolute max power that can be delivered can be calculated (note that the VW of the cap of 50V may be DC, and AC rating may in fact be quite a bit lower!)

V=IR so 50 = I X 8
I = 6.25
P= VI so P = 312W

doing the same with 25V caps results in P = 78W

Tony.

[Chii]

Great thanks Tony, what you said makes sence. Both of the caps that have either died or are showing signs of heat are the first in line, there is another cap of the same value later down the series which doesn't seem to have ever overheated, so your comments make sence with this issue.

I have noticed that the wax/plastic coating on the coils in the circut has melted off I have to wonder what the last owner of these speakers was doing as there rated at 150w 4ohm's and have 5 drivers (2 uni q tweeters/ 6.5" mids and one of there 6x9 base drivers). They don't take much to fill a room with sound.

I will try and find something at 50v that will do the job, thanks very much for the help.

Andy