Capacitor question for KEF speakers.

[vadimgal]

Hi To all,
I have problem with replacing the caps in my speakers KEF REF. TWO
One months ago I realized that one tweeter sounds bad. I pulled out the x-over which has all NON polar Bennic and ALCAP caps. I tried to replace them with Clarity Cap polypropylene ones and sound is not good.
IS it good Idea to mix NON polar caps with polypropylene ones or all caps has to be the same .
Thanks.

 

[Bobken]

Hi,

Caps made from different materials and by alternative makers usually 'sound' a bit different from each other. As you are talking about all non-polarised film caps here, there is no underlying reason not to 'mix and match' these in a x'over, but they will be likely to give a different overall balance or 'voice' to the results.

Possibly you have struck an unlucky combination here, if you have caps of the correct values, but makers like KEF are careful enough to select their caps where necessary, to suit the application.

Therefore a 10% tolerance Alcap of say nominally 4.7uF, could be anywhere in value between around 5.2uF, or maybe as low as 4.2uF, and this will make lot of difference to the overall 'voicing' in most cases. Component and entire x'over suppliers/manufacturers like Falcon Electronics, which was run by Malcolm Jones the ex-chief engineer from KEF, supplied a lot of high-end speaker manufacturers with their parts. Falcon was adept at this component 'selection' exercise, and I am sure they did this for many of their customers, to satisfy their special needs.

If you have the means, it is always best to measure any caps before replacing them in a x'over like this, and try to obtain something of a similar value, and then you will only have the slight variations in the individual cap's 'sonic' response to worry about.

Regards,

 

[AndrewT]

Hi Vad & Bob,
I think that replacing an electrolytic with all it's non ideal characteristics with a plastic based capacitor of the same value (measured and matched) will sound different.
I further think that saying "only"

then you will only have the slight variations in the individual cap's 'sonic' response to worry about

understates the size of the project involved in changing a well designed combination of crossover and drivers.

Sorry Bob, but this could be another area where we do not see "eye to eye".

 

[KP11520]

Maybe "Burn in" is a factor here. The old caps have hundreds and maybe even thousands of hours on them.

Many caps get better after the first 24 hours and some continue to change (slowly) for hundreds of hours.

Bob would be able to make a better assessment on that but maybe it could be a factor too!

Regards//Keith

 

[AMV8]

Hi

Polypropylene and bi polar caps have different charactoristics which can make it difficult to change over.

Having said that I think you will find that adding some resistence in series with the polypropylene capacitor will help a lot ( Kef recommended this when they changed from bi polar to poly caps ).

Kef recommended, and I also agree, you need IR for each 10uf of capacitance.

Don

 

[Bobken]

Hi Vad & Bob,
I think that replacing an electrolytic with all it's non ideal characteristics with a plastic based capacitor of the same value (measured and matched) will sound different.
I further think that saying "only" understates the size of the project involved in changing a well designed combination of crossover and drivers.

Sorry Bob, but this could be another area where we do not see "eye to eye".

Hi Andrew,

There's no need for any apology here, and there is no reason for us always to see matters exactly the same, as we have had different experiences in life (and in electronics).

You might well be right here, as we are talking about magnitudes of change, which will perhaps impress us slightly differently.

Over the years I have successfully replaced quite a few Alcaps in all kinds of x'overs (Alcaps were all the rage at one time, of course) including a couple of years ago in some monster TDL Reference speakers, which I have mentioned in another thread.

I haven't seen any drastic changes in overall balance which I was able to measure on these occasions, but to some extent it will relate to which 'substitute' caps are used, their individual values, and more importantly whether they are in shunt or in series with the signal.

On no occasion would I have described the subjective results as "not being good" as vadimgal has said, and when used in series I found it will generally 'enhance' the frequency-range being affected, but where used in shunt locations, it has had the opposite effect.

Without knowing anything further about this particular x'over, nor precisely what vadimgal's dissatisfaction is based upon, I wouldn't care to pass any other opinions on this. In other threads quite recently, I spent a lot of time impressing upon others just how difficult and time-consuming 'voicing' speakers and x'overs can be, and to end up with exceptional results is usually quite heavy going.

Regards,

 

[Bobken]

Maybe "Burn in" is a factor here. The old caps have hundreds and maybe even thousands of hours on them.

Many caps get better after the first 24 hours and some continue to change (slowly) for hundreds of hours.

Bob would be able to make a better assessment on that but maybe it could be a factor too!

Regards//Keith

Hi Keith,

Burn-in will have some affect, of course, but with this poster's description it seems like a much more serious problem than is likely to be improved much by any burning-in process, in my view. In general (here I go again!) I would expect a fresh film cap to sound more like an old electrolytic in characteristics, with the usual harshness and lack of transparency observed with electrolytics, and a similar expectation with most 'new' film caps.

It would (will?) be interesting to learn more about this problem, whereupon it should be possible for us to give some more relevant and useful advice here.

Regards,

 

[Bobken]

Hi vadimgal,

If you would tell us a bit more about the caps which you replaced, together what you are unhappy with after this substitution, I am sure that we would be able to offer some more-worthwhile advice to you.

You see, so far none of us know the value of the caps, or even which ones they were (unless I am half-asleep), and possibly quite wrongly, I originally assumed that you probably meant the Bennics. You mentioned a problem associated with tweeters, and these are more likely to be the lower-value caps there, which knowing of some KEF x'overs, I would have guessed would probably be plastic films, anyway, just like the majority of yellow film Bennics I have seen before. I felt obliged to guess here as I am not personally familiar with these particular KEFs, whereas others may be so.

It is a slightly different matter 'soundwise' if they are Alcap electrolytics, which some posters here assume you replaced, but, either way, the symptoms you describe appear to be more severe to me than a minor change in the overall 'voicing'.

Regards,

 

[sreten]

Kef recommended, and I also agree, you need IR for each 10uf of capacitance.
Don

hmmm....... so how much for 47uF ? /sreten.

 

[Bobken]

hmmm....... so how much for 47uF ? /sreten.

Actually, I was aghast to read that apparantly KEF recommend *any* resistance in series with *any* drivers except maybe where part of an 'L' pad attenuator, or similar, is concened.

I certainly would not have added anything like 3+ Ohms in series with my ATC mid-domes, when the speakers concerned had passive X'overs, as I know what it would have done to the sound. It would certainly have countered most of the benefits I achieved when replacing polypropylene Chateauroux caps with a bank of all MIT RTXs, these being polystyrene and tin-foils!

Who am I to disagree with KEF, though.

Regards,

 

[vadimgal]

Capacitor question for KEF speakers.

Hi guys,
Thanks a lot your replies.
Here is the story.
When I realized that one of my speakers lost the Highs I pulled out the X-over of the cabinet. All caps were NoN Polar ones made by Bennic and Alcap companies. One of the caps with 6MFD nominal was not good looking( insulation had many gaps). I made measurements and it showed 6.8MFD. So,I decided to change all the caps due the age. Some of them ( with big capacitance such as 80,100,120,150 and 1000MFD) I replaced with the same Bennic NON polar caps , but with 100V (the original ones has 50VW and 25VW)and rest of them with ClarityCap SA type.All the caps were bought from Madison Sound and has the same capacitance as the old ones.
I made changes to one speaker only.In HF I changed 6MFD(3.9 and2.2 in paralell) and 25MFD with Clarity Caps
In MF nad LF almost all caps were changed to new Bennic ones
After these changes bass disappeared, highs were closed and tinny
Mids were not so good as before. It was a BIG ( negative) difference to my NOT updated speaker. I left this speaker for Warm-up period. After almost 1.5months I did not hear any improvement in any region. Yesterday, I installed all all caps back and its sound now much much better but still a little bit dif. in the Bass region. It is not so tight as in not updated speaker.SO , I am just confused. WHat is Wrong???
Thanks in advance.

 

[AMV8]

To all doubters

The question of putting resistence in series with a poly cap that replaces a bipolar does make sense as if you check out the characterisyics of both types of capacitor.

I did discuss this for some time with kef's chief designer as I wanted to change the treble unit on a kef 105 and also replace the old base unit cross over capacitors.

The recommendation from kef was that on the treble unit, where the changed capacitor was 4uf, use 0r4 in series. On the base unit , where the changed capacitor was 100uf, use 10r in series. After trying various combinations of resistor and capacitor I had to conclude that kef were correct.

So doubters I suggest you try this.

Don

 

[Bobken]

Hi vadimgal,

This now helps to explain quite a lot, and I had rather guessed that this was not a question of a simple cap substitution being the cause of such dissatisfaction.

Firstly, you did a very sensible thing in retaining one speaker as original, and I always recommend this to be used as a yardstick for comparison purposes. When carrying out any x'over modifications and for whatever reason, it easy to gradually veer away from the original overall 'voicing' of the unit unwittingly, especially if this happens in small increments, say when changing one cap at a time.

Now that you have relaced the original parts in this speaker, anyway, why not try just substituting one component at a time (which again I would always suggest) and then you will get a much clearer idea of which changes affect the overall sound.

It is not easy (at least for me, anyway!) to diagnose precisely where your problem exists with absolute certainty, and there could be several reasons for your dissatisfaction. For example, you say that one cap nominally 6uF measures at 6.8uF which is quite a difference, depending to some extent where it is in the circuit, but as advised earlier, it is quite possible that KEF chose this value deliberately. If this is a film cap, I would be very surprised if the value has changed much over the years, as in my experience most film caps are extremely stable in value over time, and I have measured many caps which have been in use for 30/40 yrs. If it is an electrolytic, these do tend to vary more over time, though.

Do you have a schematic for this x'over which you can post, as I am amazed at some of the rather high values of caps which you say are in this x'over. It is unusual (for me) to see so many caps which are over 80uF and up to 1000uF in one x'over alone, but after some 40 yrs of playing around with passive x'overs, I am still learning!

Regards,

 

[sreten]

Probably due to KEF "conjugate impedance load matching" in the c/o.

/sreten.

 

[Bobken]

To all doubters

The question of putting resistence in series with a poly cap that replaces a bipolar does make sense as if you check out the characterisyics of both types of capacitor.

I did discuss this for some time with kef's chief designer as I wanted to change the treble unit on a kef 105 and also replace the old base unit cross over capacitors.

The recommendation from kef was that on the treble unit, where the changed capacitor was 4uf, use 0r4 in series. On the base unit , where the changed capacitor was 100uf, use 10r in series. After trying various combinations of resistor and capacitor I had to conclude that kef were correct.

So doubters I suggest you try this.

Don

Hi Don,

I do have just a little experience when comparing the 'sounds' of different capacitors (some 35+yrs) and I, for one, will not be trying out this suggestion.

It would be wrong to fill this thread up with further off-topic comments, but if you are interested in knowing more about this interesting subject, you may find something of value on the following page alone.

http://www.diyaudio.com/forums/showthread.php?s=&threadid=101344&perpage=25&pagenumber=12

If KEF or anyone else sincerely believes that one can replicate similar sonic effects in this manner (or even approach a similar ESR across the audio range) by the addition of a simple series resistor which is not (normally) frequency-dependent no matter what its value may be, then, as I said, who am I to argue with them.

Regards,

 

[richie00boy]

I'm assuming that when you talk of putting a resistor in series, you mean in series with the cap not the bass unit itself?

 

[gfiandy]

Hi,

If the cap you measured was a electrolitic cap (and from your description it sounds like it is) and is measuring 6.8uF when labled as 6.0uF, then is unlikely that there is much of a problem with the capacitors. Electrolitic caps normally lose capacitance as they dry out and this would imply that it is still in fairly good condition.

Have you checked the drive units it may be that one of them is damaged. You initially said that one of the tweeters sounded bad, you could try swaping them between the two cabinets to see if the problem is with the crossover or the drive unit. similarly you could try swaping the other units one at a time to see if it is them that are at fault.

Regards,
Andrew

 

[Bobken]

Hi,

Andrew's comments are very valuable here, and I had also intended to mention that electolytics normally reduce in value, and not increase, after a period in use. Having measured many hundreds of 'old' electrolytics, I have never seen one which had increased in value.

I also wonder about the symptoms too, as initially you mentioned tweeter deficiencies, but after replacing the 'old' components you seem to be more concerned about the bass response now. Assuming that you have put everything back correctly, unless you have unwittingly corrected a previously bad solder-joint or similar, it is hard to understand how this apparent anomaly could come about.

Regards,

 

[vadimgal]

KEF capasitor Question

Hi to all guys,
AI attached the schematic of the Kef speaker

 

[vadimgal]

Kef Capacitor question

I tried to attached the schematic of the KEF speaker , but it is not going.