Tweeter protection cap for active build

[mike7877]

So very soon I'll be actively crossing my ATC SCM20 Pro PSL Mk2s (using LA90Ds and 9039Pros)

Regarding, specifically, the tweeter - it's the "S-Spec" version of ATC's in house tweeter (model ATC SH25-76S)

Its performance is absolutely state of the art, so obviously I don't plan to put just any cap in series with it... It must be a quality cap that won't introduce distortion or change over time, or do anything but be as perfect a cap as it can be! (caps like these, aren't just a few dollars...) And it's for this reason I'd like the smallest one feasible...

So the tweeter is rated 6 ohms and doesn't dip below 5.
I plan to cross it 3rd or 5th order Butterworth, probably at 1.6kHz, but it could end up at anywhere between 1.4 and 2.0kHz (whatever sounds best and gives the dispersion I want).

So... how many microfarads is enough to not introduce phase issues and still attenuate 60Hz BZZZZZZZZ! when a cable gets yanked out by goblin, enough that the tweeter's coil doesn't get cooked or blown out the gap (yes, I believe this can happen with this dual suspension tweeter, just like a woofer - I've given it 5-25Hz and it moves over 1cm peak to peak. Yes, this 1 inch dome tweeter moves almost half its diameter...)

Capacitor brand or model recommendations?

 

[voodooless]

Probably 22 uF would work. Gives about 1200 Hz at 6R.

Any halfway decent film cap will probably do.

 

[Devnull]

I try to do my protection caps around an octave under the crossover point. So 700 Hz and 5 ohms is about 44uF. 47 is probably easier and cheaper to find

 

[Dave_Reite]

The specifications for that tweeter show coil length of 2.0mm and a gap of 3.0mm. So, Xmax of 1.0mm peak-to-peak.
If applying 5Hz for some sort of testing purposes, I would recommend you quit doing that.

Regarding a capacitor......I would recommend you select a smaller value that incorporates directly into your 1.6khz crossover frequency.

But, if applying low frequency test signals as you mentioned, you don't appear to be interested in protecting your tweeter anyway.

Dave.

 

[DVDdoug]

It must be a quality cap that won't introduce distortion or change over time, or do anything but be as perfect a cap as it can be! (caps like these, aren't just a few dollars...)

It's not critical except you probably want to avoid electrolytics.

Just get some kind of film capacitor about the right capacitance with an adequate voltage rating.

You don't need low tolerance because it's operating below the crossover frequency so it's not really doing anything or having any effect unless something goes wrong.

 

[sergeauckland]

I use bipolar electrolytics for tweeter protection as any distortion in electrolytics tends to come when there's voltage across the cap. Used for tweeter protection there's hardly any voltage across the cap as the crossover keeps the lower frequencies well away. Bipolar electrolytics are also much smaller and cheaper, so never felt the need to use film caps in that function.

S

 

[MAB]

Its performance is absolutely state of the art, so obviously I don't plan to put just any cap in series with it... It must be a quality cap that won't introduce distortion or change over time, or do anything but be as perfect a cap as it can be! (caps like these, aren't just a few dollars...) And it's for this reason I'd like the smallest one feasible...

Actually, they are just a few dollars:

https://www.parts-express.com/Dayton-Audio-DMPC-20-20uF-250V-Polypropylene-Capacitor-027-436?quantity=1

https://www.parts-express.com/Dayton-Audio-DMPC-40-40uF-250V-Polypropylene-Capacitor-027-442?quantity=1

These are in the range of what you want, about an octave or more below ATC's crossover as a starting point.

So the tweeter is rated 6 ohms and doesn't dip below 5.

'Rated'? Did you measure the impedance, like with a DATs, or some other suitable method? What is the resonance? I ask because:

I plan to cross it 3rd or 5th order Butterworth, probably at 1.6kHz, but it could end up at anywhere between 1.4 and 2.0kHz (whatever sounds best and gives the dispersion I want).

You plan to filter nearly an octave lower than ATC's crossover. Any specific reason? You have some numbers in mind here, how did you come by them?

I've given it 5-25Hz and it moves over 1cm peak to peak.

Oh my! It seems like you need a protection cap for sure.

 

[restorer-john]

Oh my! It seems like you need a protection cap for sure.

Guaranteed he will snap the lead-in wires soon enough.

 

[dfuller]

I'm going to caution hard against crossing a flat baffle tweeter much below 2KHz. There's a reason that's very rare...

 

[Looneybomber]

Depending upon how much phase shift you want at the lower end of the crossover bandwidth, using a 1st order filter a decade below (1/10) your tweeter’s crossover point would not be unheard of.

Looking at the top line, you can see 45deg phase shift at 1khz which results in minimal phase shift at 10khz. If crossing your tweeter over at 2khz, you’ll have minimal phase shift (if that’s important for your needs) if using a capacitor value for a 200hz xo point.

 

[Mr. Widget]

I try to do my protection caps around an octave under the crossover point. So 700 Hz and 5 ohms is about 44uF. 47 is probably easier and cheaper to find

I do this as well. (A series cap calculated to give a first order slope using the actual impedance of the driver at the crossover frequency.)
I have conducted numerous measurements and listening tests and no change is evident as tested by inserting the cap and removing the cap. I use inexpensive film caps.