Active vs Passive

Eric Best · Post by **Eric Best** » Tue Apr 29, 2003 3:18 am

I've been looking into speaker building a lot lately. I think I'm going to be going with a kit as barefoot suggested, but I'm also looking at additional possibilites becasue I want more than one set of speakers.

I have some experience building active filters and have been doing much more reading to refesh my memory because of the need to make a low shelf filter to soffit mount.

In my travels i have read many arguments for using all active systems, cheaper, fewer phase problems, more efficient.

Where do all of you stand on this.

Eric

Post by **John Sayers** » Tue Apr 29, 2003 9:33 am

Eric - barefoot should really answer this - I'd just like to mention that if you do build an active system why not mount the amps separately as Genelec do. At left bank the amps are mounted under the speaker where the controls and power switch are accessible. Genelec provide a mounting kit as an option.

cheers
john

barefoot · Post by **barefoot** » Wed Apr 30, 2003 3:23 am

Eric,

As with anything the final result depends on the quality of the implementation, but with all else equal, active filters are better than passive. Even so, the quality of the drivers and how they are integrated into a total system is much more important than active versus passive.

The primary advantage of active systems is buffering the filters from the drivers which otherwise present complex frequency and power dependent loads. Direct coupling to the power amplifier can also enhance the performance of some drivers that rely more heavily on electrical damping rather than mechanical damping.

On the electronics side, the "cheaper" argument may hold for manufacturers, but not for the do-it-yourselfer. Sounds like you have some experience with electronics, so I'm sure you've found that building your own is rarely cheaper than buying something off the shelf. An active bi-amplified speaker is probably about $100 more expensive than a passive bi-amplified speaker for the do-it-yourselfer - and, of course, a lot more expensive than a passive speaker using just a single amp.

Also, building an active speaker doesn't save you from considering the frequency dependent response, phase, and directivity characteristics of the drivers. It's by no means an easy way out of the challenges of good system integration.

But yeah, if you have the know how, use the same quality drivers, and can afford it, an active system is definitely the way to go.

Thomas

Eric Best · Post by **Eric Best** » Wed Apr 30, 2003 4:29 am

I know with DIY you don't usually get out of it cheaper, but you usually end up with a better product for the same amount of money in most cases(besides, it's more fun). I am also more comfortable repairing what I have built.

What I'm looking at is the filter design from this web page (also attached) http://sound.westhost.com/project09.htm what do you think?

I built active crossovers before for PA systems and still have the power supplies so that would one less thing I would have to build.

Most of the kits that I am looking at, the crossover adds about $400 to the price, and successful active design looks easier than passive. I could build the actives with parts that I already have and spend the $400 on amps for what might be a better product.

Eric

Eric Best · Post by **Eric Best** » Wed Apr 30, 2003 4:31 am

This is his output buffer

Eric Best · Post by **Eric Best** » Wed Apr 30, 2003 4:34 am

John, good idea about the amps thanks.

Eric

barefoot · Post by **barefoot** » Wed Apr 30, 2003 5:41 am

Eric,

Yeah, Rod Elliot's site offers a lot of excellent information. Yet another Australian sharing his wisdom!

This circuit is a 4th order Linkwitz-Riley crossover. It's the sort you typically find in sound reinforcement crossovers. It's also a "generic" circuit and it assumes that the magnitude and phase responses of all the system components are flat - something that's almost never true. So, PA systems usually also incorporate delay circuits and graphic EQ's in order to smooth out all the resulting bumps and dips.

With a truly integrated speaker system the crossover circuits will be designed to work specifically with the response, phase, and dispersion characteristics of the drivers in question. For example, if you see an active studio monitor with the specs Crossover = 4th order L-R @ 2kHz, the circuit will almost never actually look like a 4th order Linkwitz-Riley. More than likely it will be some seemingly strange asymmetric 3rd order alignment chosen such that the resulting acoustic response (the one that really counts) is 4th order Linkwitz-Riley.

Attempting to come up with these crossover alignments by trial and error is like searching for the proverbial needle in the haystack. Some kind of modeling and optimization software like LEAP, Calsod, or LspCAD is invaluable for modern crossover engineering. I use LEAP.

Like I said, active crossovers definitely have their advantages, but from a design perspective they are only just a little easier to pull off.

Thomas

Eric Best · Post by **Eric Best** » Wed Apr 30, 2003 7:22 am

So it's like anything, the better you want it, the more you need to know, and the more you know, the harder you realize that it is going to be to do.

Knowledge is dangerous, it makes you spend money.

Still, something to play with.

Eric

knightfly · Post by **knightfly** » Tue May 06, 2003 9:25 am

Thomas, I've read in a couple of (un-remembered) places that it's not a good idea to mount power amps below soffited speakers, due to "thermals" causing phasing problems (different speed of sound at different temps)

Right on, maybe, or just more bull? Thanks... Steve

barefoot · Post by **barefoot** » Wed May 07, 2003 4:05 am

Steve,

I had never considered this, but yes. "Thermals" could effect the sound. Just as images are distorted by variations in refractive index caused by rising air currents, sound waves can be distorted by variations in density and velocity as well. For a typical studio amp, however, I doubt the effect is very significant. It might be worth the peace of mind not to mount the amps under the speakers, but it's probably not a big worry.

Thomas