DIY Audio Projects Forum

I'm confused. Sorry if this is a basic question but I'd really like to avoid damaging my amp!

If I have a bass unit and a treble unit each with an impedance of 8 ohms then when I put them together in a speaker cabinet my overall impedance must be either 4 ohms (parallel) or 16 ohms (series). With three drivers or more it becomes more complicated. But I want the impedance of the speaker 'cluster' to be 8 ohms. How do I acheive this? Is there some circuitry in the crossover design other than high pass/low pass filtering that compensates? What am I missing?

Thanks.

First, don't worry about damaging your amp. Even with the load impedance cut in half, unless you really drive up the volume, you're not going to hurt anything.

What you are missing here is the frequency dependance of the crossover.

Think about your two drivers (woofer and tweeter) in parallel. First, the woofer impedance is going to be quite high at tweeter frequencies. As such, the 8Ω impedance of the tweeter in parallel with the woofer (at high frequencies) means that the parallel combination is fairly close to just the impedance of the tweeter. But, bass frequencies can really hurt a tweeter due to it's "relatively" lighter construction. So what is generally done is the tweeter is put in series with a capacitor. The capacitor blocks the lower frequencies from the getting through and tweeter only sees the high frequencies. In many inexpensive 2-way speakers, the entire crossover is just this capacitor. This is the "poor man's" solution.

Now, in reality, things are not quite that simple. The impedance of the driver works in conjunction with the crossover components and forms a frequency selective filter. So the series capacitor and the tweeter form a high pass filter. The impedance is very high at frequencies below its lower corner frequency, so at these frequencies, the amp only "sees" the woofer. But at high frequencies the amps sees the two in parallel. And even though the woofer impedance get pretty high at higher frequencies (and it's audio response is very low) you can still do better.

The solution in this case is to now put an inductor in series with the woofer. Then the impedance of the inductor and the woofer form a low pass filter, and the woofer only sees low frequencies. Together (if designed correctly) they form a matched pair which has reasonable impedance across the entire frequency band. This is an example of a basic single pole, 2 way crossover.

There are lots of variations depending on what you are attempting to achieve, but the basic idea remains the same. Take a look at this website (http://www.audioholics.com/loudspeaker-design/crossover) and see if it answers your questions. If not, come back and ask some more.

_________________
Matt
It's all about the Glass!
http://www.CascadeTubes.com
Cascade Tubes Blog

Thanks, extremely helpful.

I wonder if you wouldn't mind confirming that my crossover design is OK. I am a little concerned in that it has evolved rather than being designed from scratch.

I started with a Fostex FE126en which is a full range driver - and very impressive. With this driver, after searching the internet, I learned that a baffle step correction circuit is recommended. Using one of the available calculators I decided on a 1mH inductor in parallel with a 5R resistor. Results were very good and the harsh top end was tamed. I also learned that a zobel impedance correction circuit was also recommended. Calculators revealed that I needed a 0.4uF capacitor in parallel with a 9R resistor. All seemed to work well.

Then I decided to add a Fostex FT17H super tweeter to add 'air' and space. I was recommended to let the FE126en roll off at the top and add a high pass filter to the tweeter circuit. Research suggested that 10MHz or higher was the best low end for the tweeter. An online calculator revealed that a 1uF capacitor in series and a 0.27mH inductor in parallel would acheive this with the side effect of reversing the polarity. Next I needed to attenuate the tweeter to allow for higher efficiency and baffle effects. Trial and error revealed -6dB to be about right. This was achieved with a 3R9 resistor in series and an 8R2 resistor in parallel.

As this is the first time I have built anything like this I am concerned that I have taken the right steps. Is it right to let the FE126en roll off at the top end or should I add a low pass filter to the circuit? Will the amplifier see a consisten 8 ohm load? Also I don't want to keep adding components in my signal path (partly becaue they are expensive).

I've attached a drawing of my crossover just in case my explanation is not very clear.

Thanks.