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Old 21st April 2009, 12:55 AM   #1
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Default tweeter protection in an active speaker

Let's suppose I want to build speakers for a friend. He will be using an active x-over. But I don't trust him.

Can I put a capacitor in the speaker as a 'passive' line of protection?.

Will his active x-over be affected by a cap in the signal path?.
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Old 21st April 2009, 01:07 AM   #2
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Yes there is no problem with that. Just make sure the capacitor is of good quality and the crossover point is a bit below the intended active crossover point.

I usually use the FS of the tweeter in question as a guideline. Should be somewhere in between the FS and the active crossover point.
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Old 21st April 2009, 02:11 AM   #3
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Hi Billy, I have been using Active XO with no caps for quite sometimes. The amp is Gainclone which I thought is very stable. So far so good !

My worry with caps is the phase shift induced. Frequency response-wise there is no problems.

I found the active lowpass filters out those drones when you forget to turn off the amp and pulling out the rca cables so the tweeter is safe.
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Old 21st April 2009, 04:56 AM   #4
kimbo is offline kimbo  Australia
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I too was going to go the added cap route for my active setup, but when I priced a good quality cap, it wasn't much difference to the cost of the vifa tweeter I was using. So I thought I wouldn't bother adding an extra passive element in the signal path, I just went and bought a spare pair of tweeters.

So far (3 years) so good. The spares are still on my shelf.
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Old 21st April 2009, 11:21 AM   #5
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Thank you for your replies. If I was building the speakers for myself, I wouldn't bother with the cap. Since I am building them for someone else, I wanted the added protection 'just in case'. The tweeter will be an Accuton.

Above Fs, but below the crossover poin,t sounds like a good solution.
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Old 21st April 2009, 12:38 PM   #6
Dr_EM is offline Dr_EM  United Kingdom
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What I am doing to add a degree of protection for my planar tweeters is reducing the active crossover by 1 pole (in my case from 24db to 18db) then adding the capacitor so as to create that extra pole at the crossover point passively.

Of course, it works especially well with a tweeter with flat impedance such as the planar, but should work fine with a normal tweeter providing the corner frequency is away from the tweeter resonance. An 18db Butterworth with added 6db passive creates the same response as a 24db LR, in case that helps
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Old 21st April 2009, 10:08 PM   #7
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Thank you Dr. EM

I will let him know there is a cap in the way. He will then be able to judge his slope accordingly.

What do you mean by 'providing the corner frequency is away from the tweeter resonance'?.
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Old 21st April 2009, 11:39 PM   #8
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I have been using an active crossover for over 20 years and the same amplifiers and speakers for over 30 years for the basement loudspeakers. A7-500 components in a 18cufoot infinite baffle. no problems so far. Depending on what the dc offset of your power amp is or your buddys you will be subjecting the tweeters etc to a slight dc current pushing the tweeter cone slightly out or in depending on polarity. I hope his amplifier has some sort of speaker protection/delay turn on instant turn off. If there is a dc offset at the output or a thump/pop at turn on and turn off then a cap would be strongly advised.

I found this forum a few months back as my 30 year old preamp failed on one channel and I was wondering what's newer in the hi fi world. I have poked around in most of the forums and many discussions are still the same as they were 30 years ago. This time in most cases with a bit more knowledge and experience. The basement loud speakers still impress after nearly 40 years. They look a lot worse for wear. maybe I will rebuild them back to their original glory cabinets in a year or 2. Nice meeting everyone and nice info in the forums.
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Old 22nd April 2009, 12:09 PM   #9
Dr_EM is offline Dr_EM  United Kingdom
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Quote:
Originally posted by Cousin Billy
What do you mean by 'providing the corner frequency is away from the tweeter resonance'?.
Simply that the crossover frequency be suitably higher than the resonance of your driver. At the tweeters resonance it's impedance will change and expecting the filter to operate predictably around this point is unreasonable. Ie, if your tweeter resonance is 800hz, a crossover of 2khz+ would be reccomended. It's never advisable to cross too near the resonance but adding a passive component to the setup will confound the problems
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Old 22nd April 2009, 12:29 PM   #10
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by Dr.EM
What I am doing to add a degree of protection for my planar tweeters is reducing the active crossover by 1 pole (in my case from 24db to 18db) then adding the capacitor so as to create that extra pole at the crossover point passively.

Of course, it works especially well with a tweeter with flat impedance such as the planar, but should work fine with a normal tweeter providing the corner frequency is away from the tweeter resonance. An 18db Butterworth with added 6db passive creates the same response as a 24db LR, in case that helps
I started a similar thread enquiry and that seemed to be the best advice I was given.

But the acoustic response (of tweeter + amp+ active crossover + passive protection cap) seems to be the overwhelming requirement and I've not quite got my head around that part yet.
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