I learned my lesson... how to calculate amperage for fuse or PTC?

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cyberstudio

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Joined 2018
#1
Just blew a tweeter... 😢 So I've decided to add a PTC or a fuse or a fuse lamp to the crossover.

In a 2-way, do you protect both the woofer and the tweeter? Or the tweeter only?

I presume we put it after the crossover/in series with the driver? (But I've seen it before the crossover too, which is more correct?)

I've seen a bypass capacitor in parallel with the PTC. How do I determine the value for that?

The spec says, for the tweeter,

Power handling, long term, IEC 268-5, 2.5 kHz@12dB/oct.65[W]
Power handling, continuous, IEC 268-5, 2.5 kHz@12dB/oct.20[W]

How do I convert this back to an amperage rating? The crossover point is 1.7kHz.

There is already a 3.9 ohm resistor in series with the tweeter, I guess I will just lower the value slightly to compensate.

Thanks in advance!

If you should ask the tweeter is a Wavecor TW022WA04, and the cause of death is the turn on noise of a mixer I added to the system.
 
#3
I'm not aware of any passive protection device which can respond fast enough to protect a tweeter from turn on transients. I generally prefer to use polyswitches for tweeter protection. The problem is always disconnecting the tweeter from the crossover in the shortest time possible.
 
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#4
There are also some FET based designs which don't use slow responding mechanical contacts in the signal path. I don't remember who came out with it first, but I believe EV and JBL have some type of circuit they use in their higher end monitors which disconnect the driver much faster than any other means. Of course, DSP monitored protection has sort of made all that obsolete and avoids tripping protection prematurely. Thats one of the biggest challenges with non-DSP protection solutions.
 
#5
My old Nad from the eighties has a loudspeaker out switch. If something in the chain can pop noises than just turn all on and then release the box. And other way round while turning off first put the amp off then the rest
 
#8
@Michael Bean Your ears won't be able to tell if the amplifier generates DC offset or HF oscillation. In most cases, you'd run a safety coupling cap to block DC when using an active xover. Oscillation only hurts things when the tweeter VC inductance is low enough to allow for sufficient current to flow at higher frequencies. Turn on/off transients are harder to predict, but they're usually an indication of either oscillation or offset, typically not sufficient to do any damage. Distortion itself won't kill a tweeter and in some cases you won't hear the tweeter complain until its too late. Only experience will teach you how loud is too loud. Most 25mm ish HF domes will handle about 10W max before going up in smoke. Even that is a pretty.good figure.
 
#9
cyberstudio said:
How do I convert this back to an amperage rating?
Click to expand...

As others have indicated, PTC cut off can take several seconds which is too slow acting to solve your problem.

However, for reference purposes I will provide a link to a calculator that allows the selection of a suitable PTC current rating according to the power handling and impedance of the driver.

Read the "Input instruction (a guide line according to our experience)" carefully!

https://www.lautsprechershop.de/hifi/ptc_en.htm
 
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#10
profiguy said:
@Michael Bean Your ears won't be able to tell if the amplifier generates DC offset or HF oscillation. In most cases, you'd run a safety coupling cap to block DC when using an active xover. Oscillation only hurts things when the tweeter VC inductance is low enough to allow for sufficient current to flow at higher frequencies. Turn on/off transients are harder to predict, but they're usually an indication of either oscillation or offset, typically not sufficient to do any damage. Distortion itself won't kill a tweeter and in some cases you won't hear the tweeter complain until its too late. Only experience will teach you how loud is too loud. Most 25mm ish HF domes will handle about 10W max before going up in smoke. Even that is a pretty.good figure.
Click to expand...
I've never blown a tweeter in over fifty years of stereo listening...never. The only time I have ever heard of it happening is when it was turned it up to eleven, as I eluded to before. That and use reliable amplifiers and no protective heroics are necessary. I'm not saying it's not a good idea in some cases...but I see speaker protection as just yet more stuff between you and the music, that in most cases, when the equipment is used as directed (sanely that is) really isn't necessary.

Mike
 
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#11
cyberstudio said:
So I've decided to add a PTC or a fuse or a fuse lamp to the crossover.
Click to expand...
My view is that a fuse in series with a tweeter can't be totally transparent, but on the other hand it can save the driver if needed.
Also, I completely realize that losing a driver that way is a bad (still instructive) experience that you do not want to repeat, but to be honest now you wouldn't want to become "over-protective" either.

The choice of the type of protection (no protection included) is also a matter of personal way of thinking.
Just as an example I would never put an active circuit to protect my speakers, but as said that's a personal way of looking (hearing) at (audio) things.
I once read a post on this forum about a protection system that had no physical contact with the signal path, but I didn't find it.

However, it's not an easy task to precisely establish the rating of a fuse for a tweeter and there are no tables or calculators that can really determine it. IMO

From what you described, then a 1A 250V fuse could probably be sufficient, but if you raise the rating too much then it becomes useless.

In the absence of other information (and given the small cost of "normal" fuses) I might suggest you to get a fuse holder and some 6.3x32 mm cartridge 1/4A and 1/2A and 1A 250V fast blow glass fuse and watch/hear what happens when playing your system at your usual levels.
If the 1/4A one burns (too often), switch to the 1/2A one, and so on.

Just for the record, the manufacturer of my 3-way floor loudspeakers recommends 20 to 200W amplifiers and there is a fuse in series with the tweeter rated 2A 250V Fast Blow.
Please note that personally I don't use fuse holders, but (when and if I can) I directly solder the fuse in loco.


A couple of related/older posts

https://www.diyaudio.com/community/threads/funniest-snake-oil-theories.234829/post-7360949

https://www.diyaudio.com/community/threads/funniest-snake-oil-theories.234829/post-7361383
 
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#12
Nominally a 4 ohm tweeter. Assuming one can do a bit of arithmetic I x E = P, I x R = E, therefore I^2 x R = P. Or Sqrt(20/4). So I would try a 2 amp standard fuse.

I is current although properly capital letters mean DC and lower case AC. Measured in amperes.

E is electromotive force measure almost universally in volts.

P is power measured in watts.

And after you get tired of changing fuses you can calculate the maximum voltage across the tweety and use a pair of Zener diodes.
 
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#15
Which of these for a tweeter, the 5 watt type?
Screenshot_20240311_182626.jpg
 
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