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Old 30th July 2007, 01:19 PM   #11
ssmith is offline ssmith  Kenya
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Originally posted by AndrewT
Does anyone disagree with the two octave rule?
I'd asked various people about this, and read and heard the more the better (withing reason) -- although anything from 'around' 1.5 octaves is just fine if using a 4th order active. At the same time, was also advised that a slightly lower value film cap is better than a more ideally valued bipolar electrolytic cap due to long-term stability issues.
Cannot back this up with any experience or measurements, it's just what I've read and heard from various people (dealers included).
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Old 30th July 2007, 02:15 PM   #12
Svante is offline Svante  Sweden
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Location: Stockholm
Originally posted by AndrewT
Does anyone disagree with the two octave rule?

Svante recommends reducing the active filter slope by one pole and integrating the protection cap into the required overall filter type. I like this, except for the accuracy required and the fixed Q=0.71 that must be compensated by altering the Q of the active filter. Although LR2 can be achieved with an active single pole (q=0.71) and passive single pole (q=0.71). LR4 and Butterworth require more thought.

yes and use hot melt glue or similar to secure the cap to something rigid, otherwise the solid core wire will fatigue and break.
Mja... Dunno if I would propose decreasing the slope of the active filter as a general metod. I just meant that the effects of the passive components should be considered. IMO it is easy to neglect the acoustic effects of the driver, and when it comes to this hybrid type of active/passive filters, it is also easy to neglect the effects of the passive components. Adding a capacitor does not nessecarily result in one extra real pole, it will also result in a tendency to a peak near the driver resonance. It was this effect I had in mind when I commented on the parallel resistor.

Anyway, here is a simulation of a reasonably real tweeter with a rather high fs. The larger capacitor results in a cutoff of 700 Hz electrically according to textbook examples (assuming a purely resistive tweeter). The smaller capacitor results in a cutoff of 1400 Hz. It can be seen on the lowest curve group that the voltage across the tweeter terminals does not follow a standard 1st order HP filter function. In particular, the pink curve has a peak near fs, and it is larger than the light blue peak. This is because the parallel resistor reduces the impedance peak of the driver slightly, even though it does not eliminate it.

It can also be seen in the impedance curves (middle group) that the blue curves (=those with the resistor) presents an unnessecarily low impedance for the amplifier.

So, what is "best" is up to the designer, but IMO all of these effects as a whole should be considered. Changing one component might be just right to compensate for another etc.
In this case it might be easiest to stick with the dark red curve, ie a large enough capacitor in series with the tweeter. Surely it removes low frequencies good enough. However, there will still be a need to integrate the responses of the active filters with the responses of the drivers.

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