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Capacitor to protect my Tweeters in active speaker

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Super Wammer

I have a pair of KEF 103 that i have converted to active use using MiniDSP DDRC24 unit and a pair of Quad 405 amplifier . I have managed without any difficulty to blow two tweeters in this when running speaker tests by inadvertently putting too much volume through the tweeters.

Due to me currently recovering from a recent operation (TURP if anyone is interested suspect not) it will be six weeks before i will be able to tackle putting these speakers back together again and get them working using my new DIRAC 2.0 software.

Now I have been advised before that I should have a capacitor in line with my tweeter + feed in order to protect them from just the sort of thing i have done. Being a smart arse (for this please read idiot) I ignored this thinking there is no way I will be so clumsy but having had time to think before I replace the tweeters and put the speakers back together I will pay attention to the good advice given and put in the capacitor protection.

Now the reason for this thread , how do I work out what value capacitor should I use for this . The Tweeter is an 8 Ohm unit and the crossover point is at 3000Khz  ? This should be a fairly straight forward bit of advice based on current knowledge .

Now for a more I suspect contentious question , does it matter what type of Capacitor I use ? As musical signal will flow through this capacitor in to each tweeter does it need to be a high quality item the best i can afford or would any electrolytic cap do the job .

I do promise that i will read all responses and give them due consideration as I have decent period of tie both to make up my mind and source the capacitors before I install them .

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Super Wammer

Andrew, no advice on the tweeters but wish you well on recovery from the TURP,  a subject which is close to my heart due to recent experience with a family member.

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Super Wammer
11 minutes ago, Fourlegs said:

Andrew, no advice on the tweeters but wish you well on recovery from the TURP,  a subject which is close to my heart due to recent experience with a family member.

Thanks I do appreciate that but I am really now far more interested in the capacitor issue than what is now in the past. Operation was last Friday and I was discharged around 13.00 on the Saturday which was very uch quicker that the Specialists and Nurses were expecting. All things are going well and the futures so bright I've gotta wear shades .  What I need now is to keep my brain active and get me thinking about getting my music system together again . :nerves:

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Hi Andrew. Also from me, best wishes for a speedy recovery. Not a nice op at all.

With regard to the question, a capacitor will protect the tweeters from DC and from lower frequencies, but it will not protect them from too much power being put through them. There is not really any way to do this passively. Depending on the setup, however, it may well be that they are getting lower frequency input and this is not good for them.

For the value, there are plenty of online calculators. Assuming 8 ohms and 3kHz (I presume you meant 3kHz/3000Hz, not 3000kHz) it should be about 7uF. I'd go lower (frequency) for good practice. A 2kHz crossover with 8 ohms is about 10uF. Even bigger would probably be better, as it's only significantly lower frequencies you want to block.

For the actual caps, it's straight in the signal path, so quality does matter. I'd personally avoid electrolytics. You need non-polar ones anyway, so you might as well use decent quality polypropylene. The Ansar ones are widely used in crossovers and are about £10 a go for 12 or 15uF.

Edited by rabski
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Super Wammer

@rabski Brilliant just the straight forward advice I needed . My lack to technical knowledge often makes my descriptions less accurate that i would like . When using an REW sweep I moved the volume up too high for a second and this cuased the damage . It may well have been to volume and it may well have been that nothing will protect against this but I am going to follow the advice I have been given and fit a 10uF inline to each tweeter just in case will give myself a little projrct now of having a look for the best I can find for the lowest price . At least it keep my mind off going to the loo all the time. May well buy these sooner rather than later and solder them up as this will not need me to lift anyhting heavy which is very much not advised at the moment .

if anyone else wants to add some names for good quality capacitor polypropylene capcitors then add away searching out each one and getting the information on them is just somehting i now have lots of time to do .

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Super Wammer

Okay I think I have contributed enough on this site not to be considered someone who does not do a bit of work and research themselves to get the ball rolling in any list of possible capacitors to use after a little looking round this is what I came up with .

https://www.hificollective.co.uk/components/claritycap_sa.html

Anyone elses suggestions please add them and give me a choice to make .

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Andrew, firstly I would like to add my best wishes for a speedy recovery.

When I looked into this for my actives, the consensus on forums seemed to be that you should choose the capacitor such that the corner frequency of the roll-off from the cap was at least two octaves below the crossover frequency, to minimise any effect on the phase in the pass-band; so in your case you would need a corner frequency of 3000Hz / 4 = 750Hz; using the nominal 8ohms you would need minimum 27uF - in practice the next biggest available size would be 30uF.

With my actives I have done most of the level attenuation on the tweeters with resistors, rather than in the DSP - this is principally because I had a problem with some noise from my multi-channel pre-amp but as a side benefit, the signal level reaching the tweeters is always cut to some degree, giving at least some protection.

--

I'm not a believer in capacitor foo and usually just use basic Solen polyproylenes from Falcon:

https://www.falconacoustics.co.uk/alcap-claritycap-solen-audio-capacitors.html

Physical sizes are given under "additional information" - the large value caps start to get quite sizable physically but I doubt this will be a problem with a cabinet the size of the 103s.

Edited by bobovox

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Super Wammer
3 hours ago, rabski said:

With regard to the question, a capacitor will protect the tweeters from DC and from lower frequencies, but it will not protect them from too much power being put through them. There is not really any way to do this passively. Depending on the setup, however, it may well be that they are getting lower frequency input and this is not good for them.

I’m glad that Richard wrote this, as it was what I was thinking when I read your opening post, Andrew.  It’s very easy to underestimate the power at higher frequencies and these sweep tones are indeed dangerous without great care.  They typically provide equal output at 20kHz as a much more unpleasant and obvious 1kHz.  

As I'm sure you know, music power - by which I mean typical programme/recording content - is massively ‘quieter’ at HF than in mid-range. That’s why you’d rarely blow a tweeter on decent music, unless you’re partying and drunk!  And to add to the problem, many of us cannot hear above 14 or 15kHz any more either, so we’re oblivious!  

A great project for the brain cells, but frankly I cannot see the point.  Stick to no more than, say, one watt and you’ll be fine with your test tones.

Lastly, I had to Google TURP,  but now I know, I send my sympathies and very best wishes too.  Hope you’re soon much better.  

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Hi Andrew - given that you are using a DDRC-24 as your crossover, I would have thought that using the right crossover frequency, coupled with using the "compressor" function (9.3.5 in the manual) to limit the output level of the tweeter channel would give good protection (other than protection from a 0-20K sweep with no crossover set up, of course!). The DDRC's crossover will give you a much sharper cutoff than you would get from a series capacitor. Obviously, the DDRC won't protect against the amp going DC on you, so the cap would help you there, but that wouldn't help the bass drivers...

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