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Active Crossover Tweeter Protection

gabo4au

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Just curious what others do for tweeter protection in active crossover speakers?

My speakers have a ScanSpeak Revelator R2904/7000 tweeter, and at around $350 each, I'm not eager to replace them due to some problem with an amp or active crossover, or even just a mistake by ME.

They crossover at 2500hz.

The typical thought is to put a capacitor in line. The speaker has an Re of 3 ohms, so at 2500hz a cap would be 21uf. But that's a filter that has a 3db down point at 2500hz and is 45 degrees out of phase, so to keep it from interacting and causing issues it really needs to be lower. An octave lower gets it mostly out of the way, but puts the crossover so low (1250) that maybe it's useless. At 6db/octave and increasing phase shift, there really isn't any way to have it completely protective and transparent.

You can go to a 2nd order filter, which does a better job at protection, but has double the phase shift.

I guess I can go to fuses, but they also have issues. Not to mention I have to figure out the current limits on the tweeter and to trust the fuse it has to blow fast and also work as advertised.

There just doesn't seem to be a really good solution. I guess the good news is my tweeters are pretty robust, handling 210w long term so maybe a fuse could be sized effectively. If my math is right, that's sqrt of 210/3, which is about 8.4. So maybe a 5a fuse would offer sufficient protection?

Any comments or help would be appreciated.
 

RayDunzl

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I might start with a really small fast blow fuse and work my way up, only as needed, for the levels and content that you play.

5A (continuous, to blow the fuse) to a tweeter doesn't seem reasonable to me at the moment.

(I get to be wrong)

Or, use a 209W amplifier.
 
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gabo4au

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5A to a tweeter doesn't seem reasonable to me at the moment.

Totally agree with that! I went back to the driver spec and was actually shocked when it listed 210w as the "long term power!" For a tweeter!? Actually made me a lot less concerned about this. Here's the datasheet, https://www.scan-speak.dk/datasheet/pdf/r2904-700000.pdf

But still, for general information, I would like to hear some discussion on this topic. Especially since I read a thread yesterday about why would anyone use a passive crossover these days (paraphrasing, I don't remember the exact thread title).
 

DonH56

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That's a lot of power to a tweeter... Even the 100 hour RMS rating of 160 W is probably much higher than you'd see in the real world. 20 W with 3 ohms yields (P = I^2 * R so I = sqrt(P/R) as you said) 2.6 A so a 2 A to 2.5 A fuse seems more reasonable to me (agree with Scrutinizer).

That said in the primordial past I often skipped the fuse but always included a capacitor. The capacitor's purpose was not to serve as a dynamic frequency impedance but rather as a DC block at start-up and in case the amp died and shorted its rails to the output (and thus the driver). For that purpose, choosing a cap that puts the corner frequency a decade or two below the crossover frequency is appropriate and that way you should be able to neglect the impact in your design.

Disclaimer: I am not a speaker designer... - Don
 

hex168

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gabo4au

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Scan speak goes into more detail on their testing protocol in this pdf: Scan Speak Technical Note

Ah, thanks for that. So the 210w isn't even close to real world. I can see their need for a technical note, but it actually should be included with the specs, easy to overlook that. I understand their testing but it actually makes it a bit confusing for choosing a fuse, if that's the correct way to go.

Maybe a cap at a much lower frequency + a fuse?

Would be nice to hear from the "experts" here as to what is a somewhat standardized procedure for how to go about this with any speaker?
 

DVDdoug

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I would simply go with a higher-value (lower frequency) capacitor. It would only have a slight effect on frequency response and if you're building your own crossover you could (theoretically) compensate for it.

I wouldn't use a fuse unless you're worried about overpowering it under a non-failure conditions. ...And it's virtually impossible to choose a fuse that blows just-before the tweeter fries... So you'd either be lowering the power-rating of the speaker if it blows too soon, or not protecting it if it blows too late.

"Re" is the DC (zero Hz) resistance. You need to use the impedance at the crossover frequency (or the "frequency of interest") which is probably closer to 4 Ohms.
 

mdsimon2

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I run DIY active speakers and do not use any caps as my current equipment has no turn on / off pops but I also use much less expensive tweeters.

In the past I have used caps for DC blocking purposes. I assume if you are using DIY active speakers you have the ability to model the impact of the cap using something like VituixCAD. I use cheap 100 uF electrolytics which push the corner frequency way down to the point that you don't need to worry about it at normal tweeter crossover frequencies.

If you want to use a smaller cap where it will have an appreciable impact on frequency response it is easy enough to model the response and incorporate it in your active crossover design.

Michael
 
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gabo4au

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I would simply go with a higher-value (lower frequency) capacitor. It would only have a slight effect on frequency response and if you're building your own crossover you could (theoretically) compensate for it.

I wouldn't use a fuse unless you're worried about overpowering it under a non-failure conditions. ...And it's virtually impossible to choose a fuse that blows just-before the tweeter fries... So you'd either be lowering the power-rating of the speaker if it blows too soon, or not protecting it if it blows too late.

"Re" is the DC (zero Hz) resistance. You need to use the impedance at the crossover frequency (or the "frequency of interest") which is probably closer to 4 Ohms.

All good points, and good questions. What would be a procedural method of doing this for any tweeter?

1. Fuse method is potentially unreliable, so is that out for a procedure?

2. The impedance at the crossover frequency would require finding an impedance chart, but of course even then it's a varying thing, so what would a procedure look like for determining what resistance to use? I can measure the impedance of the tweeter, but it's not something everyone has the equipment to do.

3. And, "I would simply go with a higher value," which is great, but what value, how to determine, and is it going to be safe? Seems to me like there is a fine line between it working correctly and not interfering with the sound.

4. Good point about "fixing it" with a DSP crossover. But I would prefer to do as little of that as possible.

Sorry for being so anal, it's just a topic that always comes to my mind when someone says "never use passive crossovers" any more. It would be great to come up with a process for the best way to go about it.
 

MakeMineVinyl

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I simply use an amplifier with low enough power output so that it cannot possibly harm the tweeter. In my case, I use a single ended triode amplifier which can only produce 2 1/2 watts, and this amplifier cannot go into hard clipping. The tweeters (compression drivers) are 107dB/W sensitive so this power is easily enough to cause hearing damage.

It makes no sense to use an amplifier which can produce anything more than perhaps 50 watts with even the most robust tweeter in a home environment, as there is little to no chance that there will be that much power needed in the high frequencies. The real power demands are in the lower frequencies and it makes sense to use a higher power amp there.
 

antennaguru

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I would think that you could use a front panel visible incandescent lamp in one of the leads to the tweeter, mounted right next to the tweeter on the front panel of the loudspeaker enclosure. Assuming you picked the correct lamp rating you would notice the warning lamp glowing on peaks in advance of a blown tweeter.

Several commercial passive crossovers use such an incandescent lamp as protection, so this is nothing new.


 

levimax

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Another perspective that I notice is not popular on these boards. If you are going to the trouble and expense of using an active crossover for better sound quality I would leave the capacitor out as not only will it cost some money for a decent film cap big enough to actually help but any cap in the signal path is going to degrade performance to some extent. Fuses are notoriously difficult to size for this application as well and often times the voice coil ends up protecting the fuse and you can't really "experiment" to find the right size. In addition your tweeters look quite robust and unless there is a catastrophic failure or you are using massively over powered amps for the purpose the odds are low you are going to have any problems. I have been running direct from amp to tweeter for many years and even though I have had some "accidents" I have never damaged a tweeter. Of course other reasonable people will disagree... at the end of the day it depends on your risk tolerance... no protection is cheaper, easier, and higher performance, and you know the downside cost in the unlikely event something goes terribly wrong.
 
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gabo4au

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I would think that you could use a front panel visible incandescent lamp in one of the leads to the tweeter, mounted right next to the tweeter on the front panel of the loudspeaker enclosure. Assuming you picked the correct lamp rating you would notice the warning lamp glowing on peaks in advance of a blown tweeter.

Several commercial passive crossovers use such an incandescent lamp as protection, so this is nothing new.



The whole point of protection is to protect against some "problem" or fault. Either an equipment failure, or a self inflicted problem. Like someone plugging in an input cable to an amp when it's turned on. Or accidently bumping a cable and causing a big noise, or even a power surge/failure. The lamp is a cool idea, but it's not really helpful for protection unless it acts like a fuse and is somehow more reliable/repeatable than a fuse.

Thanks for the suggestions from everyone, apparently there is no tried and true way to do this, It's always a one-off situation where you have to pretty much be an expert in audio to do it.

Fortunately I have enough expertise to come up with a good solution, I just wondered if there was a somewhat standard way that worked for every case. I guess the answer to that is no. Thanks.
 

gene_stl

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I have expensive and almost unreplaceable Be tweeters in a four way active system.
When I first started using it I fused the tweeters and midranges at 250 ma. When I bought those fuses I expected they would blow and had at the ready some 500 ma fuses (Fast blow in both cases) The 250s never blew despite being played loudly from time to time.

I intend to add some oversized capacitors in series. I consider it unlikely that you could hear a difference. (just an opinion)

Tom Christensen at Neurochrome has some speaker protection modules that use power FETs to protect the tweeters. (iirc)
 

gnarly

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4. Good point about "fixing it" with a DSP crossover. But I would prefer to do as little of that as possible.

Sorry for being so anal, it's just a topic that always comes to my mind when someone says "never use passive crossovers" any more. It would be great to come up with a process for the best way to go about it.
With DSP i don't think there is anything to fix concerning driver protection, i think it's a matter of extra implementations........ limiters.
A good DSP, will have voltage based limiters, above and beyond the common set of EQ's, xovers, delays, levels, polarity, etc ......
An even better DSP will have both RMS and peak limiters, where thermal protection and peak excursion limiting can be set independently.

Since using a DSP implies multi-way active, each driver can have it's own limiter set.
I feel safe using a 2000W amp on a 150W tweeter, when limiters are properly set.
 
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gabo4au

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With DSP i don't think there is anything to fix concerning driver protection, i think it's a matter of extra implementations........ limiters.
A good DSP, will have voltage based limiters, above and beyond the common set of EQ's, xovers, delays, levels, polarity, etc ......
An even better DSP will have both RMS and peak limiters, where thermal protection and peak excursion limiting can be set independently.

Since using a DSP implies multi-way active, each driver can have it's own limiter set.
I feel safe using a 2000W amp on a 150W tweeter, when limiters are properly set.

The previous poster was talking about fixing any frequency roll-off that might be caused by a passive device in front of your speaker via the DSP crossover. It's a valid point.

And DSP limiters and protection are of limited help as they are before the amps. They can help with any issue that's upstream from the DSP, but not with an amp/cable/downstream issue. That's the concern with active, your crossover is before the input to your amps and your amps are directly connected to your speakers. Any issue with an amp, such as a power related "pop" or an amp input connector, or an amp failure issue, and it will deliver whatever power it can muster up to your speaker. A "pop" from that 2000w amp and it'll lay your 150w tweeter out on the floor!
 

gnarly

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The previous poster was talking about fixing any frequency roll-off that might be caused by a passive device in front of your speaker via the DSP crossover. It's a valid point.

Understood his point. And agreed.
I have to believe the HF rolloff could be compensated for with further passive work...but that's out of my league...i don't mess with passives at all.
And DSP limiters and protection are of limited help as they are before the amps. They can help with any issue that's upstream from the DSP, but not with an amp/cable/downstream issue. That's the concern with active, your crossover is before the input to your amps and your amps are directly connected to your speakers. Any issue with an amp, such as a power related "pop" or an amp input connector, or an amp failure issue, and it will deliver whatever power it can muster up to your speaker. A "pop" from that 2000w amp and it'll lay your 150w tweeter out on the floor!

Well, if we are using amps that can produce pops on their own, I say ditch the junk.
I think it's far more likely to get line level pops or overdrives, and there DSP limiters are perfectly appropriate.

Since i moved to pro audio amps, XLR line level gear(interconnect reliability), and DSP limiters, I've simply quit worrying about driver protection.
Plus, drivers are tougher than we think...most survive over excursion better than I would have imagined. Not so with thermal overdrive, despite the so called 'continuous' ratings.
.
I've heard absolute "rifle cracks" go off, with no damage. Although, it's been with compression drivers.
 

dc655321

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With DSP i don't think there is anything to fix concerning driver protection, i think it's a matter of extra implementations

That’s an overly broad statement and disregards where the dsp resides. Safeguards as you describe require instrumentation.
 

sergeauckland

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There are two issues here.
Firstly, protecting tweeters against overload. That is by far best done by limiters in the DSP crossover. I've set mine to around 40 watts which is similar to what B&W used for their APOC protection with the same tweeters. Fast-acting iimiters cause no additional distortion, and don't allow clipping.


Secondly, there's protecting the tweeters against an amplifier fault that could put 50v or more DC on the tweeters, and that's most easily done with a series capacitor sized so it doesn't affect the crossover point. I used 47uF for the tweeters, and 250uF for the midrange, if I remember correctly.

S
 
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