Active crossovers - is it this simple?

[annex666]

I have a seemingly simple question to ask about going active.

If I can open my speakers and look at the crossover (and therefore find values for it's components) and I know the crossover points, can I not just make a miniature (low power) version of the same crossover and place it between my pre-amp and quad mono amps?

I'm guessing this would keep the same frequency dispersion and inherent phase shifts implemented by the original crossover the same?

Is this always a good thing?

Is this a "safe" way of doing things - i.e. not driving the speakers with signals they weren't originaly intended for, as would possibly be the case if I were to make my own crossover points.

Also to (slightly) complicate matters my speakers are 2.5 way B&W603-S3's which use a bass driver (I'm guessing in parallel) with the mid/bass for low frequencies.

Has anyone gone active with these speakers?

Any responce would be good, cheers.

 

[thylantyr]

The component values are based on the speaker impedance.
Using the same components values will not match
the input impedance of the amplifier.

Save yourself the headache and make something
like a linkwitz riley active crossover, 24db/octave.
Make it so you can replace the frequency modules
so you can experiment with or use ganged potentiometers
for variable adjustment. Use some sockets so
you can try various op-amps that meet your
taste in music.

 

[Holger Barske]

I am afraid it is not that simple.
The passive crossover construction is a more or less complex circuit that is matched with the impedance curve of the drivers.
Your power amp input won't have that characteristic impedance - neither concerning ampltude, nor phase. It will most probably look like a simple resistor of a rather high value (compared to the drivers).
The electrical environment is completely different, so you can't directly do a tranfer from the passive filters.
---
Holger

p.s.: ok, mey reply was a little too slow...

 

[annex666]

There's me not thinking about the obvious!

I totaly forgot about impedance matching of the crossover - I was just thinking about it's act as a frequency filter!

I was indeed thinking about a Linkwitz type filter (an ESP variant) as the phase is unchanged through the filter inherantly due to it's design.

If I open my speakers tonight (
) and check out the crossover and maybe get a schematic on here could you guys advise me more on implementation?

I'm thinking it could be awkward due to the 2.5 way design - especially as this is seen more often in manufactured speakers rather than DIY.

Maybe the best option for me is to use the original crossover mid/high pass filter for the tweeter / kevlar midrange and isolate the bass to the aluminium "mushroom" bass driverthrough the active filter.

Cheers again for the responses guys

One last thing now I think of it - will the amp(s) (ESP P3A X2) be (for want of a better word) "upset" by the removal of the impedance-correction network (if there is one present)?

 

[MRehorst]

One more consideration:
A passive, high-level crossover located in the speaker usually provides some protection to the relatively delicate tweeter and midrange drivers. When you go to an active crossover, your amplifier is usually connected directly to the drivers. A small malfunction, DC offset, or even just a turn-on thump from one of your amps can blow a tweeter to pieces. You will have to be careful to protect the drivers against such things.

MR

 

[thylantyr]

I'm paranoid when using active systems, I like to insert
a fuse on the tweeter for sure, perhaps the midrange
too. I have to experiment to get the right value,
typically I start with 3/4 amp and do some tests,
either increase or decrease the value to get the fuse
to blow just beyond the sweet spot. The sweet spot is when you "feel" like the tweeter is just starting to "stress".

cheap fuse = piece of mind

 

[annex666]

Thanks again for the advice all, but unfortunately I can't get to my passive crossover anyway after having a look today

One of the reasons I was thinking of keeping the high-pass circuit was because of the series cap to protect the tweeter although I guess the whole plan has gone out the window now (due to the inability to "adapt" the crossover) so my dreams of perfectly (well relatively at least) phase coherent speakers goes up in smoke.

Thanks for all the posts guys - maybe when I get round to it (and I mean in quite a few years when I finish my degree and get some serious ) I'll build my own hybrid speakers and complete custom home brew setup, but until then...


..cheers again for the posts