2x12 high pass cap to alnico speaker

[Guitarist]

I've built a couple 2x12 cabs. Each has a 12" Eminence bass speaker. I'm thinking of pairing them with an lighter Ω AlNiCo speaker and would like to protect them by adding a capacitor in series with it. What value would work to wean out the killer low end a bit?
I got a pair of these but I suspect they're way to small:

[Chuck H]

What???

I don't understand "lighter Ω". Does this mean lower/higher ohm rating?

And if your playing a guitar through the cab I don't know why you would need to use a crossover circuit for a guitar speaker. Some guitar speakers do have big bass while others don't but this isn't usually related to their ability to handle those frequencies, just their ability to reproduce them. As it applies to guitar speakers any how.

It's usually desireable with guitar speakers to get the natural response of the speaker/amp interaction. Even if that means some bass definition is lost. Using a crossover circuit therefore would have unpredictable, and possibly undesireable results.

If the alnico speaker is of a lower watt rating (and in danger even used with the other speaker, which will share the wattage) than the amp then it's possible that trimming the bottom end would protect it. But I've never seen it done this way and there are too many variables to suggest a specific roll off frequency for best and safest performance.

Chuck

[Guitarist]

Oops, my error. Not Ohms but Wattage! Even 160 or 250 Hz would help it from blowing. But then again they sound great approaching that point and it's only a $50 meltdown.
This approach could work with adding a 10" as well. You get some added colour in the important mids spectrum.

Thanks Chuck for the notice.

[olddawg]

I don't feel like calculating the 3db roll off at a specific frequency, but I can tell you that in cheap 3 way stereo speakers that they put a 2-4.7uf cap to the tweeter, a 22uf cap to the mid driver , and run the woofer full range. The slightly better ones use bipolar electrolytics or caps back to back. You might consider a choke and a resistor to ground in parallel to the speaker if you have to do this. Better yet, get an eq pedal. Or.. put the cherished speaker in a box and mount something beefier and less flubby.

[Hardtailed]

Here's a useful RC filter calculator: Guitar Pedals: R-C Filter Calculator

So let's say the speaker is 16 ohm and you want your 3db point at 200Hz, that gives us 50uF. If the speaker is just 8 ohm then that's 100uF (if the capacitor is just before that one speaker, assuming it's paralleled with the other one, then only that one speaker's impedance is to be used in the math.
You don't need a 600V part here, a 100V should give you a lot of headroom (100V into 16 ohm is 625W!)

Effectively, what you want to do is add a crossover. Many cheap 2 voice speakers do just that: full range to the woofer (and use the speaker's inherent high reactance at high frequency to balance out the highs) and a capacitor to block low-frequencies from the tweeter.

Now, two things:
- The speaker's impedance varies with frequency, therefore the exact corner point is hard to determine. It may interact weirdly with the speaker's resonance peak. Try to select a corner frequency a full octave above or below it.
- The reflected impedance to the amp will be impacted. As you move below the corner point of the RC filter, assuming your speakers are paralleled (this wouldn't work in series anyway), the total impedance will move back to that of just one speaker (higher impedance), modifying the power amp's response (a higher reflected impedance can be a problem with pentodes). On a SS amp, that would simply mean that half the power would be developped and dissipated across the remaining speaker for low-frequencies. But tube amps with their high output impedance don't behave the same way, your remaining speaker will now be working double time.

On top of that, your AlNiCo speaker might now not be working hard enough and sound stiffer. You do want to get it moving, and you need low-frequencies for that.

You could also use a high-power AlNiCo speaker (like the WGS 50 and 100W AlNiCo Blue clones)