Tweeter protection in active setup

[krassyg]

I need to protect a very expensive Focal Be tweeter in an active crossover setup with a MiniDsp 2x8 digital crossover. The crossover frequency is 2200Hz. What is the absolute best capacitor for this purpose and what is the value that you guys would recommend. I am new to this hoby and would really appreciate your help.

Thanks in advance,
Chris

[CopperTop]

I pondered on tweeter protection when I blew one up with an oscillating amplifier but still haven't done anything about it. Are you thinking about protecting against damage from HF oscillation or something else?

[krassyg]

The power amp would be an Onkyo PA-MC5500 . It is a four way setup and I just want to protect the tweeter against problems with the amp during the on/off cycle. I don't even know if this is necessary with this amp, I am a noob when it comes to speaker building.

[Loren42]

Try this link here.

The point is to protect the tweeter from DC, pops, or really low frequencies that usually come from equipment turning on or off.

First, you need to select a frequency for the capacitor that is below your intended crossover point for the active crossover. Since caps can vary as much as 20% for their stated value, you should account for any tolerances in that selection point. Check the specs for the capacitor you select.

At or above the tweeter's lowest usable frequency is a good point for the protection cap. Going lower is probably not going to matter since there should not be any program material down there anyway.

Second the quality of the cap is not really going to be super critical, so don't go wild, but do select a good one and do not use electrolytic caps.

Madisound sells some good caps and Parts-Express is another source.

[krassyg]

This is the tweeter I am using:
Focal Grande Utopia technologies: tweeter Beryllium IAL2 generation.

From what I understand, the lowest safe operating frequency is 528Hz. The operating crossover setting is going to be 2200Hz. I don't know the impedance of the tweeter though, in order to calculate the capacitor value. How can I measure it?

[chops]

On my last big loudspeaker project, I was running totally active. On the Altec compression driver I was using, I had a Solen 51uF cap on the drivers for a 400Hz point. The driver was actively crossed at 600Hz, 4th order. I just had the cap there for any possible turn on/off thump the amp might have put out, though it never did.

[Henry8]

To put a cap in an active setup is going backwards.
Test the amps with the actual drivers and squarewave at a low level-this should reveal any amp instablities with that driver.
Use a protection circuit with a delay at switch-on and fast turn off as psu falls - mine also incorporates dc detection.

[18Hurts]

Originally Posted by Henry8

To put a cap in an active setup is going backwards.

It sure is! That is why capacitors are used with active amplification for safety purposes. Safety means it will ALWAYS work! Fuses always work--over current sensors don't.

I was in medical equipment for 25 years--the safety stuff was almost always mechanical in nature. Philips built a mammography machine with an automatic compression paddle--it was computer controlled hydraulic entertainment. GE made one that used the same computer controller but used air for the compression paddle. The air lines would physically blow off when they hit 35 PSI. The Philips had a massive recall when the computer controlled stuff failed and ripped the breasts off a patient with a hydraulic crusher paddle. The hydraulic paddle broke and then bent the frame of the unit. That one cost Philips 20 million dollars (here is a blank check)

Fuses and capacitors for active systems make sense. Everything fails eventually so throwing a poly cap that will outlast the electronics makes sense. Not exotic by any means but neither are fuses, mechanical stops, bumpers and seat belt pendulums... but they work!

Throw a cap on that tweeter and don't worry about it.

[Loren42]

Originally Posted by Henry8

To put a cap in an active setup is going backwards.
Test the amps with the actual drivers and squarewave at a low level-this should reveal any amp instablities with that driver.
Use a protection circuit with a delay at switch-on and fast turn off as psu falls - mine also incorporates dc detection.

That will not protect you if something goes wrong with a cable on the input side of the amp or the crossover has a pop or worse yet, fails. I had the latter happen with an Ashley that produced a loud pop. This was after the unit was running for a time.

[Loren42]

Originally Posted by krassyg

This is the tweeter I am using:
Focal Grande Utopia technologies: tweeter Beryllium IAL2 generation.

From what I understand, the lowest safe operating frequency is 528Hz. The operating crossover setting is going to be 2200Hz. I don't know the impedance of the tweeter though, in order to calculate the capacitor value. How can I measure it?

That would require you to use some form of tester to get the impedance of the driver and care must be taken to not damage it.

The alternative is to try to get the manufacture to give you that data at maybe 1 kHz.