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Topic: Do high-frequency speakers deteriorate over time? Or is it just my ears?
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Chuck McGregor
Film Handler
Posts: 47
From: Bremen, ME, USA
Registered: Mar 2012
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posted 01-29-2013 10:58 AM
quote: Paul Stevens
blasting past all limiting factors that keeps parts from flying through the screen.
As they are bi-amplified, are there "protection" capacitors in line (i.e. in series) with the HF driver on the amplifier outputs?
Typically HF diaphragms shattering is almost always the result of the diaphragm bottoming out against the phase plug. HF oscillations or excess higher frequencies will burn out the coil not shatter the diaphragm.
It would seem some kind of low frequency energy got to the drivers. This could be from a dc pulse, a low frequency oscillation, SW or LF signal bleed. Very hard to say.
All it takes is one good slam to the phase plug to cause damage. This is especially true for titanium diaphragms, not so much with softer aluminum ones, and very rarely a problem with phenolic-type (like polyamide), or carbon fiber diaphragms. One such HF driver made of the latter was tested for around 24 hours of hard slamming against the phase plug using an LF sine wave. The result? An abraded poly-carbonate phase plug. But I digress.
Bottom lines:
You almost assuredly had an "LF event" that occurred in or from something between the output of the electronic crossover and the drivers.
If you don't have them you (anyone actually who is bi-amplified) should have protection capacitors on the HF drivers to protect against both dc voltage and below crossover LF from getting to the drivers for whatever reason.
After the electronic crossover there is usually no such protection, hence the use of "protection" capacitors. This likely would not have happened had they been in place.
Such capacitors must be non-polarized "by nature," like metallized polypropylene (i.e not electrolytics back-to-back), have a low ESR (equivalent series resistance), and an ac voltage rating 3 times (for reliability) the maximum voltage capability of the amplifier (not driver). This should be the peak voltage aka clip voltage, not rms. A 250 V rating is usually sufficient and is a common specification.
Their value must be small enough to block as much low frequency energy as possible without substantially affecting the crossover response. This means measuring or modeling the crossover response with and without the capacitor. For a 500 Hz electronic crossover this would end up in the rough ballpark of 50 uF to 75 uF, but it depends. Higher crossover points mean a smaller value. Lastly they should be soldered in in-line right at the driver. I normally connect one lead of the capacitor directly to the driver.
This all means they could be big (inches in size) and cost way more than your your "average" capacitor. Sources are limited and after much research I settled on "Pulse-X" from ERSE Audio. It's "audiophile" oriented ($$) but the product has the right characteristics and thus worth the price.
It cost me about $60 each (with shipping) to outfit the 3 screen channels. Several times since then a pulse has been accidently sent to the drivers at the full output voltage of the amplifiers > 55 V, which was generated AFTER the electronic crossover. The driver is rated for 40 V peak, but only ABOVE 500 Hz. Because excursion quadruples for each halving of frequency this pulse with a lot of LF energy in it should have caused instant catastrophe = diaphragm dust. They're fine.
Just my $0.02
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