Crossover Question

[cantskienuf]

I'm working on the PE Line Array with the 4" driver and the Onkyo tweeter. In calculating my crossover, do I use the impedance of an individual driver and tweeter, or do I use the impedance at the point that the serial groupings of drivers will meet the serial grouping of tweeters in a parallel connection? For example, the drivers are 8 ohms and the tweeters are 4 ohms. I have two series groups of 6 drivers, each ending at 48 ohms. When these are then connected in parallel they become 24 ohms. The 3 tweeters in series total 12 ohms. When the drivers are then connected in parallel with the tweeters I get 8 ohms. To use the calculator http://www.lalena.com/audio/calculator/xover/ would I enter 8 ohms and 4 ohms or 24 ohms and 12 ohms?

 

[BillFitzmaurice]

First reconfigure your woofers, your load is way too high. Wire twelve woofers as four paralleled sets of three series drivers, that will get the woofers to 6 ohms. If your amp won't handle that go 3 paralleled sets of 4 drivers in series for 10.6 ohms.

Three tweeters at 12 ohms is OK, but why only 3 tweeters? You should have the tweeter and woofer lines approximately the same height, so for twelve woofers you should have at least 18 tweeters.

In any event the crossover is figured on the total load of each respective line, not that of the individual drivers.

 

[cantskienuf]

Thanks Bill. I wanted to use more tweeters, but I couldn't figure out a combination that would leave me at an 8 ohm load and the amps I'm wanting to use to drive these don't have what I would term very robust OPTs. So I put the tweeters at head height and hoped for the best. The same story goes for the driver groupings. I guess you can tell that I'm neither a mathmetician nor a speaker builder, but all of the potential groupings that I looked at appear to drop the load to a low number when the final parallel connections were made. Now I have to admit that I'm simply using serial and parallel resistance calculations for all of the speakers to arrive at what I think the amp will see at the connection point. I guess that leads to the next questions - does the individual load presented by the driver groupings matter most, or is the calculated load seen after connection with the tweeters the number that will challenge the amp? Is the load presented by the tweeters insignificant and not used in calculating a potential final load? Does the signal degrade as it moves through more serial speaker connections (i.e. is that why you recommend 3 groups of 4 in lieu of 2 groups of 6) ? Thanks for any assistance.

 

[BillFitzmaurice]

There are two reasons for the tweeters being the same height as the woofers, arranged in parallel lines. One is to have the same radiation characteristics, especially at the crossover point. The other is sensitivity. The large number of woofers will result in a far higher SPL than the tweeters. In your case four woofers in series keeps a level SPL, but then wiring the three banks in parallel will result in a gain of about 9dB, while wiring the three tweeters in series will give no SPL gain, so your box is going to be very deficient in the high end.

There are reasons for prefering parallel or series configurations, but by far the most important is arriving as close (and low) as you can to the amps best working load without going below it. As far as impedance is concerned it is the total system load that the amp cares about.

It seems that you may think that the impedance of the woofers and tweeters is combined in either a parallel or series connection to determine the final load; it isn't. The crossover maintains the proper load.

 

[cantskienuf]

Thanks again Bill! Thats exactly the type of knowledge I was looking for. I knew I would be loosing high-end gain, but I was mistakenly believing just as you thought regarding the load. I'll start drilling additional 2.5" holes, mount up the tweeters and re-arrange my protoype driver wiring.

How in the world did people find these things before Mr. Gore invented the internet?????