Protection Cap on Dome Mid ?

[Ugg10]

I am just about to start a build using the Morel EM1308 and ET448 mid dome and tweeter but will be using active crossovers (Minidsp Nanodigi and Rotel RMB1066 amp @ 60w per channel into 8 ohms). I understand that I need a protection cap on the tweeter (18uF seems to be Ok), but do I also need one on the Mid Dome which is basically a large tweeter? Looks like I will need 68uF for this is this correct if needed. For info crossover points will be approx 700-750Hz and 3500-4000Hz.

Thoughts and comments welcome and thanks in advance for replies.

[fpitas]

Just like with the tweeter, it depends on your amp. The big danger is turn-on and turn-off thump. The amp I use for my horns (First Watt J2) has no turn-on or turn-off thump, so I don't use a protection cap. Most amps do have a thump, unless they use an output relay. Another related issue is thumps from turning electronics like crossovers etc. on and off.

[billfitzmaurice]

Weighing the cost of a cap versus the cost of a driver use the cap. Size it as a first order high pass with the knee frequency one octave down from the crossover frequency.

[fpitas]

Unless you positively know the amp has no thump (I measured mine), I completely agree.

[Ugg10]

Thanks for the comments and confirmation that I am on the right track. Using this quick calculator -

https://www.omnicalculator.com/physics/crossover

Crossover point 700hz (350hz -1octave) for two 8ohm speakers is 56uF
Crossover point 3600hz (1800hz -1octave) for two 8ohm speakers is 11uF

Will edit my model to suit and use these figures.

[fpitas]

Thanks for the comments and confirmation that I am on the right track. Using this quick calculator -

https://www.omnicalculator.com/physics/crossover

Crossover point 700hz (350hz -1octave) for two 8ohm speakers is 56uF
Crossover point 3600hz (1800hz -1octave) for two 8ohm speakers is 11uF

Will edit my model to suit and use these figures.

That's the idea; of course the actual speaker resistance may not be exactly 8 ohms etc.

[Ugg10]

fpitas Many thanks, just checked and the EM1308 at 350hz is right around the resonant frequency so has a nominal impedance is 12ohms and the ET448 is around 7ohms at 1800hz. This changes the values to 39uF and 12uF.

Any particular make/model of Cap I should use, was looking at Jantzen CrossCap for the bigger one and Standard Z for the smaller or is this overkill ?

[fpitas]

I'm an active crossover sort. When I need caps I use Panasonic industrial polypropylene, but I'm sure they aren't audiophile or whatever ;)

[johnnyrichards]

fpitas Many thanks, just checked and the EM1308 at 350hz is right around the resonant frequency so has a nominal impedance is 12ohms and the ET448 is around 7ohms at 1800hz. This changes the values to 39uF and 12uF.

Any particular make/model of Cap I should use, was looking at Jantzen CrossCap for the bigger one and Standard Z for the smaller or is this overkill ?

For purposes of low-cut during testing? A cheap NPE will suffice. I use those Chinese "Audiofiler" caps for testing and almost all of my cheaper builds these days. They measure spot-on (measured around a hundred of them at this point - they should be advertised as 1%, and not 3%), are affordable, and look nice in a circuit. I suspect they are the core windings in many, many other brands.

For what its worth, I did full testing on a smaller 3-way this last summer. It used the Dayton waveguide tweeter, the cheap HiVi 2" dome, and a generic 6.5" kevlar woofer. I did multiple sweeps on each driver including several at pretty high volume. I did not use any protection circuit on either dome and they both survived. YMMV.