[chadh]

My current speakers are little 4.5" single drivers, with which I'm generally very happy.  But in another thread, people were encouraging me to supplement these with a pair of stereo subs.  No sooner had this little idea been planted in my head than I sat down to listen to some music, turned the volume up, and found the bass starting to lose its definition and the higher frequencies starting to sound a little brittle as those drivers started to work hard.

In general, though, I love the coherence that the single drivers supply, and would not want to lose that by introducing messy crossovers into the system.  So, while I'm fantasizing about bass augmentation, I may as well fantasize properly and consider a properly active approach.

I don't know the first thing about active crossovers.  I don't even really know what they are.  Are they just the same as a passive, line level crossover, but paired with a gain stage?  I presume not.  Or maybe they can be like that, but the good ones aren't.  I don't know.

Some people like tubed amplifiers, some people like solid state.  Are the differences between tubed and solid state active crossovers similar?

If the active crossover includes some gain, is there a reason not to combine the pre-amp and active crossover into a single unit?

Some active crossovers seem to be fixed at a particular frequency.  Others seem to allow you to adjust this frequency.  Apart from convenience, are there pros and cons with either approach?

How much difference would one really notice between an active crossover and a well built, simple, passive line level crossover?

Albert, from Space Tech Labs, built my pre-amp.  He also makes active and passive crossovers:

Passive 2-way: http://space-tech-lab.com/SP-2WXO-Passive.html

Active, tubed 2-way: http://www.space-tech-lab.com/SP-V-2WXO-SE.html

The price difference here is significant (C$480 vs. C$1880).  What can one get out of the active that the passive doesn't provide?

Thanks for any insight you can provide.

Chad

 

[richidoo]

http://sound.westhost.com/biamp-vs-passive.htm
http://sound.westhost.com/projects-3.htm
http://www.t-linespeakers.org/linx/xolinks.html

Have fun Chad!

[chadh]

http://sound.westhost.com/biamp-vs-passive.htm
http://sound.westhost.com/projects-3.htm
http://www.t-linespeakers.org/linx/xolinks.html

Have fun Chad!

Thanks.

I waded through the first of those articles, and developed some sort of understanding of what was being said.  As I understood it, the benefits proposed in that article came from attaching one channel of amplification to each driver (i.e. "bi-amping" in my single-driver plus sub kind of system).  What this really requires is that the crossover should come before the amp.  But why does that crossover need to be "active?  You can achieve the filtering with a passive circuit.  What's the advantage of doing it actively?

The passive crossover to which I linked in my first post, from Space Tech Labs, would sit between the pre-amp and amp and allow for bi-amplification.  It's also a quarter of the price of the active one the same company makes.  What would a consumer get out of the active solution that one doesn't get from a passive one?

Chad

[planet10]

A well executed PLLXO will smoke an active XO sonically, but are not nearly as versatile.

http://t-linespeakers.org/tech/filters/passiveHLxo.html

$400+ for a boxed PLLXO seems steep... and a 2nd order is a can of worms (not to mention level controls). PLLXO (especially the high pass) needs to be built specifically for the power amp being used (and 2nd order is hard to do if the amp has <50k input impedance)

I have posted a picture of a PLLXO here http://www.diyaudio.com/forums/showthread.php?postid=1827548#post1827548

dave

[chadh]

A well executed PLLXO will smoke an active XO sonically, but are not nearly as versatile.

http://t-linespeakers.org/tech/filters/passiveHLxo.html

$400+ for a boxed PLLXO seems steep... and a 2nd order is a can of worms (not to mention level controls). PLLXO (especially the high pass) needs to be built specifically for the power amp being used (and 2nd order is hard to do if the amp has <50k input impedance)

I have posted a picture of a PLLXO here http://www.diyaudio.com/forums/showthread.php?postid=1827548#post1827548

dave

Thanks Dave.

You'd told me about the PLLXO circuits in another thread.  I'm kind of slow, but I think I'm starting to catch on.

I gather there would be absolutely no problem with building PLLXO just before the outputs from a pre-amp, right?  Then one would just need passive subs, and any old amp to run them!

This kind of solution appeals to me due to its simplicity.

Chad

[richidoo]

What would a consumer get out of the active solution that one doesn't get from a passive one?

Current buffering.

An active crossover provides a current source to maintain the integrity of the signal as it passes through the RC filter. So, when the filtered signal reaches the power amp it will not have lost anything other than the filter's effect. The filter's components raise the current demand on the output stage of the preamp, so the preamp must be stout enough to handle the normal load of amp and cabling, plus the added load of the filter. If it does not have enough current, then more signal than intended in the filter will be lost to current clipping.

I agree that the passive filter is elegant and preferred if the preamp (or volume controlled source) can adequately power the signal through the filter. With multiway speakers, a passive filter might have many filters. A simple tube preamp may not be able to drive that network without clipping.

In your case needing only 2 filters Dave's suggestion is the wtg. But Elliott's technical diatribe answers your basic questions about why use an active CO.

[chadh]

Thanks Richidoo!

Like I said, I'm slow with these things.  But, with help from you guys, I'm starting to understand.

I actually went back to find a thread started by Roger Modjeski in The Lab a while back (see here: http://www.audiocircle.com/index.php?topic=57219.0.  When I read it this time, it made much more sense.  Maybe I'll have to talk to Roger about building me the ideal pre-amp incorporating the passive crossover.

Chad

[richidoo]

How bout trying a DIY? Line level parts are so cheap, and passive filter is so simple, it's worth trying diy! Cut a cheapo interconnect in half and wire a filter into it. Use Dale CMF55 resistors and Vishay MKT1822 caps (from mouser.) Buy some different values so you can experiment with the filter calculator. It can be ugly as sin, it will still work if wired correctly.

[JLM]

While I'm a single driver fan for the coherence, imaging, and inherently active status advantages, I have a struggle with the smaller drivers.  I agree that the midrange is of much more importance than bass or treble and so greatly prefer a 4 inch extended range driver to most 2 or 3 way designs.  But IMO the "little guys" just can't do deep bass or cleanly do high spls.  I know the pros and cons of single driver size.  Some have given up and just used a good extended range driver as an extended mid-range (like the classic single driver designs of decades ago that focused solely on 80 - 6,000 Hz).  Adding a subwoofer and super tweeter is probably the approach I'd take if I ever got the itch to go back to the "dark side".   

The passive crossover is definitely the way to go (if your single driver needs help).  Note that passive attenuators can be used at the power amp inputs to adjust levels between drivers.  When placed at the power amp input (and thus avoiding the passive design issues associated with interconnects) these attenuators can provide world class sound.

Another option is active speakers (that include the passive crossover and amps).  I auditioned passive and active versions of a small 2-way years ago and it was an epiphany (and led me towards single driver designs).  The active speakers were much more dynamic, flatter frequency response, incredibly deep bass, and they played loud.  The advantages of allowing a good designer to pick/design the amps to match to the drivers is not to be underestimated.  Unfortunately most audiophiles shun the whole active concept so most active speakers are designed for professional use.

Another route is to use a digital crossover (that goes between digital source and DAC).  Digitals allow nearly infinite choice of crossover frequencies and slopes with no sonic degradation.  The Behringer DCX2496 is rather inexpensive and designed for professional use (primarily XLR input/outputs).  When modded the built-in DAC is reportedly very good too.