I'm making a small 70 watt sonotube subwoofer for my brother. The problem I'm having is that (according to Unibox) at high levels the design I have will bottom out at about 22 Hz. Since my brother is not mechanically inclined and probably won't notice when he's hurting the sub I want to idiot-proof this as much as possible. So I was thinking of putting in a 20 Hz FMOD highpass filter (http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=266-246) into the design.

The only problem is that the FMOD is a line level filter and I just found out my brother has to hook the sub up using the high level inputs. Can I build a high pass filter that goes inbetween the amp and the driver like the one shown here (http://en.wikipedia.org/wiki/High_pass_filter)?

If I can, how do I make sure that I won't put too much power through the capacitor or resistor (mainly the resistor)? Can I make a resistor out of many resistors in parallel to share the load? I figured eight 10 watt resistors in parallel should be able to handle the 70 watt amp.

* Don't use FMODs
* Don't use passive crossovers on subs.

You are substituting one evil for others.

Unless you want to use a real active solution with very steep crossover slope
with adjustable parameters, then education might be the better solution.

After your installation is done, educate the person on how easy it is to overdrive a
subwoofer by showing them the effects - play sweeping test tones and show
them how the woofer responds and how it can damage it. If they break the sub
later, blame it on their stupidity. :p :)

70 watt sonotube

70 watt? lol

As Zoolander would say "Is this a subwoofer for ants ?" :o

Easier said then done. My bro is brilliant . . . on paper. He graduated magna cum laude with a degree in electrical engineering - yet that same year he couldn't install his own car stereo. Great engineer, horrible technician. I've spent 30 years trying to get him to pay attention to "hands on" stuff.

What is the problem with building a highpass or using an FMOD? What is a "real" active solution?

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As I said, it is a little sonotube (about 100 liters). It is small enough I could easily have made an MDF box but I figured a small tube would be fun. It also will fit well near his TV (which is near a corner).

The R is already in there it is the input impedance of the subs amp.If you can determine the value of the input impedance then you can use that to calculate the value of capacitor.This will only give you a shallow 6db roll off so it may not be enough for complete idiot proofing but it should help a bit.

Speaking of which - does anyone know if the 70 watt Dayton amp (http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=300-784) has any kind of bass boost or rumble filter? Most amps seem to have something. I guess I can always get my multimeter and test it out myself.

Speaking of which - does anyone know if the 70 watt Dayton amp (http://www.partsexpress.com/pe/showdetl.cfm?&Partnumber=300-784) has any kind of bass boost or rumble filter? Most amps seem to have something. I guess I can always get my multimeter and test it out myself.They don't mention it having any any boost .They usually do when it an amp has it.

Contact their tech support and ask them if there's any type of 'bass boost' modification for this amp. Since there's no pdf on file, you may be out of luck. But ask anyway, see if they have a schematic for it. With a schematic, it should be easy to figure out what you need to do this actively.

I called them up and they said it had no boost.