Protecting treble driver from DC?

[AndrewT]

Hi all,
I want to run my Acoustic Energy AE1s as active, driven from 4pole L-R into 100W+100W to treble & bass units.

If the treble unit has an in-line cap to protect it from errant poweramp maladies, how far below the 3kHz crossover should the cap roll off?

I am thinking of using 26u8F (10+10+6.8 propylene).
There are 4 of them for the corners of 5.1 surround.

Any comments?

The bass units will probably get electronic power amp switch off in event of DC output.
Further comment?

 

[cocolino]

If the treble unit has an in-line cap to protect it from errant poweramp maladies, how far below the 3kHz crossover should the cap roll off?

In order that this cap does not "at all" interfere with the alignment of Your active crossover, as a rule of thumb I`d say it should roll off about 1/10th the crossover frequency.

1/10th the crossover fc would lead to rather big cap values.

In practise You might get away with smaller caps (higher roll-off point). You should try if and how much smaller cap values actually have audible impact on Your active crossover alignment.

 

[Bobken]

Hi Andrew,

In my experience, no higher than 300Hz, being a decade lower than the X'over point.

This should at least avoid any phase or roll-off effects, which can commence at a decade lower than the -3dB point with a high pass filter.

Conversely, with a low pass filter, it is safer to ensure that there are no similar potentially adverse effects by using a decade higher than the anticipated -3dB X'over point.

Choice of caps used, however, IMO will also have a marked effect on the sound regardless of the chosen value, but that is another (notably contentious!) story.

Regards,

 

[AndrewT]

Hi,
a decade lower was not the answer I wanted.

I was hoping someone could confirm that about two octaves lower would suffice. i.e. 750Hz for the 26u8F

What symptoms should I expect if the cap rolls off too close to the intended xo frequency?

L-R does not suit 5pole but can I do 3pole and add a 4th pole using the cap and still achieve L-R?

 

[adason]

hi,

in my humble opinion it realy does not have to be that low, as a matter of fact, if you start it roll over just under the tweeters active crossover, you can achieve steeper roll of which can be a great benefit to sound and power handling to the tweeter

ed

 

[cocolino]

What symptoms should I expect if the cap rolls off too close to the intended xo frequency?

The high pass filter function of the cap will "add" to the high pass filter of the active crossover (BTW: as well as does for example the inherent electro-mechanical high-pass filter function of a tweeter).

In other words, when You add another filter stage (the high-pass cap) to an existing (active) crossover, the latter does not exactly behave as it was intended.

As closer the cap high-pass filter fc is to the actual crossover fc, as more influence it has on the latter.
In addition to this, to what extend the added cap has impact also depends of the steepness of the crossover itself. A higher order crossover is less susceptible to this effects.

The symptoms would be phase & frequency variations from the "ideal" target response which would result in "non-ideal" on axis frequency response in the transition range as well as different vertical polar pattern.

a decade lower was not the answer I wanted.
I was hoping someone could confirm that about two octaves lower would suffice. i.e. 750Hz for the 26u8F

The effects described above is what theoretically will happen when You add another filter stage to an existing "perfect" (textbook) one and theoretically You have to stay away a decade from the crossover fc to have negligible impact.
In practice however (acoustically) "perfect" aligned crossovers for speakers do not look like textbook filter designs.

So depending on how "perfect", or not, Your existing crossover is already acoustically aligned, the added HP-filter might even help to improve things, or to "misaligne" further.

Without knowing more about the actual speaker (drivers), it`s difficult to predict what exactly will happen acoustically when adding a 750Hz highpass.
However, also as said the added cap could have negative impact in Your particular case (depending on drivers, mounting positions etc.), I`m inclined to claim that a 750Hz/ 6dB high-pass will not have much (negative) effect on an active 3kHz/4th order LR-filter.

Whatever, the easiest way to find out would probably be to just try in situ and listen to the changes. Just parallel another of Your 26µ8F`s to the existing 26µ8F of one channel (which roughly makes for the theoretical 1 decade requirement) and You`ll see (hear) immediately how much difference it makes in practice.