Simple (stupid?) question

[kevinp.]

Does it matter if driver impedences don't match? (Assuming solid state amp)
What are the implications of using an 8ohm woofer with a 4 ohm tweeter? I understand drivers don't represent a simple load over their range (except planars/ribbons?), but is there a general rule here?

thanks!

[cjd]

It only matters to the crossover, assuming your amp (solid state or not) is stable at the lowest impedance it ends up being presented.

Both of the 5-driver (WWMTM and WMTMW) designs I've done run dual 8ohm woofers in parallel (4ohm), dual 8ohm mids in series (16ohm) and a 4ohm tweeter. Net impedance on the WWMTM drops to 2.8ohm and on the WMTMW's to 3.2ohm IIRC.

C

[joecarrow]

I'd suggest checking out the threads in the Mission Accomplished area- there is a lot of information about how the crossovers here were designed.

In general, you have the right idea- the impedance of a driver changes with frequency, and it does have an effect on crossover behavior. The crossovers in the Mission Accomplished section take that into account, as well as a few other less obvious things.

The best of these designs will show you a designer who has measured the actual response of the drivers, mounted in their final spot, and then simulated the combined frequency response using software to model the net effect of the crossover (which does more than just cross-over, it also shapes the response, corrects for baffle step, and sometimes more), including the effect of small changes- like using a resistor that's 2% off, or an inductor that's increased its resistance by some percent due to heating.

There's an awful lot to learn, and this website holds it to the highest standard I've seen anywhere. I encourage you in your quest to learn more, but please don't be discouraged if we don't all step forward with the attitude of "Yes, you two can design like this, right now!". No, some of the folks here have been seriously designing and building speakers for 20 or 30 years, and work as engineers.

I'd also recommend checking out zaphaudio.com, linkwitzlab.com, audioheuristics.org, and sound.westhost.com/

Sorry if someone's already pointed this kind of stuff out to you- I hope it sounds welcoming.

[kevinp.]

thanks very much for the info. I am familiar with some of those sites, and have read Ray Alden's Loudspeaker 201 and have the Loudspeaker Design Cookbook at home, but am still hazy on how impdence fits into the crossover picture.

[joecarrow]

Maybe this will help-

The sound coming out of the driver is controlled by how much current passes through the voice coil. Look to Kirchoff's and Maxwell's laws for how to determine that. Sorry I have to be brief- I'm about to catch a flight.

[Dennis H]

It only matters to the crossover, assuming your amp (solid state or not) is stable at the lowest impedance it ends up being presented.

Both of the 5-driver (WWMTM and WMTMW) designs I've done run dual 8ohm woofers in parallel (4ohm), dual 8ohm mids in series (16ohm) and a 4ohm tweeter. Net impedance on the WWMTM drops to 2.8ohm and on the WMTMW's to 3.2ohm IIRC.

C

Yeah, what Chris said.

Digging into it a bit more, he did what he did (series vs. parallel) to get the SPL of the woofer and mid sections to be about the same. He doesn't really care what the impedance is as long as it doesn't dip too low for the amp to handle. The XO design software uses the actual measured SPL and impedance vs. frequency curves of the drivers so the modeled response is accurate no matter what the nominal driver impedance is.