converting passive xo to line level passive OR salen key? any advice?

[mondogenerator]

Hi guys. just a bit of a beginner question really.

Ive been looking at trying to make either a passive line level XO or active sallen key type, but i have an finished conventional XO here. How can I translate the filter rolloffs? Im only concerned because I figure that the coil resistance changes rolloff rates, and this is used in the passive design, and im unsure how i can replicate this in a passive line level XO?

If i model each pole of the filter based on the f3 of each component individually, ie for the coil then parallel cap, would i get the right curve?

any advice?

[mondogenerator]

ok maybe a daft question.....i wasnt sure that i could EXACTLY translate the filter from passive to either line level passive or active, since i am not aware if you can alter Q, at least in passive line level stuff

ill go and read some more.

[richie00boy]

Passive line level, unless you use inductors then no you can't get the Q you want. Run the crossover as it should be with the proper drivers connected, then plot the voltage at each drivers terminals. Then you can play about fitting curves of active filters to this response.

[vacuphile]

Mondogenerator, from 2nd order onwards you can shape the Q of a passive filter.

Many passive Xovers do more than just cross over at a certain frequency. They may contain Zobel networks, notch filters, impedance correcting networks etc. Some of these would make sense to translate into an active filter (e.g. notch), whereas others might provide you with surprises.

Any crossover needs a well defined load in order to work. A working passive line level cross-over would most likely go beyond the current capability of most pre-amps.

Throw in a hand of opamps!

[mondogenerator]

Originally Posted by richie00boy

Passive line level, unless you use inductors then no you can't get the Q you want. Run the crossover as it should be with the proper drivers connected, then plot the voltage at each drivers terminals. Then you can play about fitting curves of active filters to this response.

I figured that i still needed inductors for a 2nd order, and I perhaps didnt make the mental leap to that association with filter Q.....seems silly now i THINK about it.....

So if i understand, I have a voltage plot, i just have to test and fiddle until the voltage curve in active/Passive line level matches?

CAN I JUST SAY

DOH!


like this?:

[richie00boy]

Yes that's about it, then on to tweaking by ear.

You will not get a line level signal to look like that using just simple filters, but you should be able to get a good approximation, which may well be better as it's probably impedance anomalies screwing up the passive crossover voltages a bit.